Navy systems require that fiber optic cables meet stringent environmental conditions. The types of cable designs considered by the Navy include the optical fiber cable component (OFCC), stranded, and ribbon cable designs. The cable must meet minimal levels of performance in safety (low smoke, low toxicity, low halogen content, etc.), durability (able to withstand shock, vibration, fluids, etc.), and optical performance. The cable must also be easy to install and repair. These factors greatly influence the design of the cables.
(OFCCs).  The OFCC outer diameter is typically 2 millimeters (mm). The fiber is typically buffered with a polyester elastomer to a total diameter of 900 m m. Figure 3-14 illustrates the design of the OFCCs. The size of the OFCCs limits the amount of fibers contained within an OFCC cable. An OFCC cable generally contains less than 36 fibers (OFCCs). An OFCC cable of 0.5-inch cable outer diameter can accommodate about 12 fibers.
Figure 3-15 shows an isometric view of a four-fiber shipboard OFCC cable. In this multifiber cable design, the OFCCs surround a flexible central member in a helical manner. The central member may add to cable strength or only support the OFCCs. For additional protection, two layers of arimid yarn strength members encase the OFCC units. These strength members are stranded in opposing lays to minimize microbending of the fibers. The arimid yarn strength members may be treated with polymers that are water absorbing, blocking, and sealing. This treatment eliminates the need for additional water blocking protection. Finally, a low-halogen, flame-resistant outer jacket is extruded over the strength members.
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OFCC cables are easy to handle because each cable contains its own subcable, the OFCC. These OFCC subcables make it easy to reconfigure systems and handle individual fibers. Rugged OFCC cable design permits cable use in harsh environments, including Navy applications. OFCC-type cable is recommended for use in low-density (less than 24 fibers) Navy applications. OFCC-type cable is also being evaluated for use in Navy applications with fiber counts up to 36 fibers.
A stranded cable is a fiber optic cable consisting of buffered fibers stranded down the center of the cable surrounded by strength members and a protective jacket. Figure 3-16 shows a cross-sectional view of the stranded cable. The fiber is typically buffered with a polyester elastomer to a total diameter of 900 m m. The recommended use of stranded cables is in medium-density (24 to 72 fibers) Navy applications. However, this recommendation is preliminary. Further test and evaluation of prototype stranded cable designs is continuing. Final approval of the stranded cable will occur only after prototype cables have passed all tests.
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Stranded cable designs increase fiber counts without greatly increasing cable size. Stranded cables are used when fiber counts exceed the limits of OFCC-type cables. For example, the stranded cable design can accommodate about 48 fibers in a O.5-inch cable. The OFCC cable design can accommodate around 12 fibers. The individual fiber is not protected as well in the stranded design as in the OFCC design. For this reason more care is required in handling the individual fibers in the stranded design. The primary problem of the stranded cable design is in meeting the waterblocking requirements. Once manufacturers correct this design problem, the Navy expects that the stranded cable design will meet Navy performance requirements.
A ribbon cable consists of optical fiber ribbons stranded down the center of the cable surrounded by a protective tube, strength members, and an outer jacket. The fiber optic ribbon consists of multiple- coated, 250 µ m diameter fibers sandwiched in a plastic material. Figure 3-17 shows a cross-sectional view of a 12-fiber ribbon. Cable manufacturers stack these ribbons to form a rectangular cross-sectional array of fibers. Stacked ribbons are the basic building blocks of the ribbon cable. Figure 3-18 illustrates this cross-sectional array of ribbons. Manufacturers introduce a controlled twist to the stacked ribbons to minimize fiber stress when the cable is bent. An inner plastic tube, strength members, and an outer protective jacket surround the stacked ribbons, providing environmental protection.
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The ribbon cable design has the highest fiber capacity. Ribbon cables can hold 204 fibers in a 0.5-inch cable. However, ribbon cables have worse bend performance than OFCC and stranded cables. Ribbon cables also have the poorest waterblocking capabilities of the three cable designs. The bend performance is expected to worsen if manufacturers add appropriate compounds to increase waterblocking capabilities.
Ribbon cables are also hard to handle. Individual fibers are highly susceptible to damage when separated from the ribbon. This susceptibility to fiber damage during fiber breakout makes it necessary to perform multifiber connections. Multifiber connections can introduce single points of failure in multiple systems. The use of multifiber terminations also introduces maintenance, reconfiguration, and repair problems. Currently, the Navy does not recommend the use of ribbon cables in shipboard systems.
Q30. List the three types of cable designs being considered by the Navy. Q31. Describe an optical fiber cable component (OFCC).
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Q32. Two layers of arimid yarn strength members encase the OFCC units. Why are these strength members stranded in opposing directions?
Q33. Why do cable manufacturers introduce a controlled twist to the stacked ribbons during the cabling process?
Q34. OFCC, stranded, and ribbon cables have different fiber capacities. What is the approximate number of fibers that each cable can accommodate in a 0.5-inch cable?
Q35. Which fiber optic cable (OFCC, stranded, or ribbon) has the worst bend performance?
Now that you have completed this chapter, let's review some of the new terms, concepts, and ideas that you have learned. You should have a thorough understanding of these principles before moving on to chapter 4.
OPTICAL FIBER CLASSIFICATION depends on more than the number of modes that a fiber can propagate. The optical fiber's refractive index profile and core size further distinguish different types of single mode and multimode fibers.
The REFRACTIVE INDEX PROFILE describes the value of the fiber's refractive index as a function of axial distance at any fiber diameter.
In STEP-INDEX fibers, the refractive index of the core is uniform and undergoes an abrupt change at the core-cladding boundary.
In GRADED-INDEX fibers, the refractive index of the core varies gradually as a function of radial distance from the fiber center.
MULTIMODE STEP-INDEX FIBERS have a core of radius (a), and a constant refractive index n1. A cladding of slightly lower refractive index n2 surrounds the core.
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D ) is the difference in the core and cladding refractive index. The ability of the fiber to accept optical energy from a light source is related to D .
MULTIMODE STEP-INDEX FIBERS have relatively large core diameters and large numerical apertures. Unfortunately, multimode step-index fibers have limited bandwidth capabilities and poor bend performance. Short-haul, limited bandwidth, low-cost applications use multimode step-index fibers.
MULTIMODE GRADED-INDEX FIBERS have a core of radius (a). Unlike step-index fibers, the value of the refractive index of the core (n1) varies according to the radial distance (r). The value of n1 decreases until it approaches the value of the refractive index of the cladding (n2). Like the step-index fiber, the value of n2 is constant and has a slightly lower refractive index than n1.
The PROFILE PARAMETER ( a ) determines the shape of the core's refractive index profile. As the value of a increases, the shape of the core's profile changes from a triangular shape to a step.
The gradual decrease in the core's refractive index from the center of the fiber causes propagating modes to be refracted many times.
Multimode graded-index fibers have lessMODAL DISPERSION than multimode step-index fibers. Lower modal dispersion means that multimode graded-index fibers have higher bandwidth capabilities than multimode step-index fibers.
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SOURCE-TO-FIBER COUPLING EFFICIENCY and INSENSITIVITY TO MICROBENDING ANDMACROBENDING LOSSES are distinguishing characteristics of multimode graded-index fibers. 62.5 m m fibers offer the best overall performance for multimode graded-index fibers.
Coupled power increases with both core diameter and D , while bending losses increase directly with core diameter and inversely with D . However, a smaller D improves fiber bandwidth.
MATCHED-CLAD and DEPRESSED-CLAD are two types of single mode step-index fibers. Matched cladding means that the fiber cladding is a single homogeneous layer of dielectric material. Depressed cladding means that the fiber cladding consists of two regions: an inner and outer cladding region.
SINGLE MODE FIBER CUTOFF WAVELENGTH is the smallest operating wavelength where single mode fibers propagate only the fundamental mode. At this wavelength, the 2nd-order mode becomes lossy and radiates out of the fiber core.
SINGLE MODE FIBERS have low attenuation and high-bandwidth properties. Present applications for single mode fibers include long-haul, high-speed telecommunication systems.
VAPOR PHASE OXIDATION and DIRECT-MELT PROCESS are two methods of fabricating multimode and single mode optical fibers.
CABLE STRUCTURES include buffers, strength members, and the jacket, or sheath. TIGHT-BUFFERED, LOOSE-TUBE, and GEL-FILLED LOOSE-TUBE are types of fiber optic
buffering techniques.
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FIBER OPTIC CABLES use strength members to increase the cable's strength and protect the optical fibers from strain.
JACKET MATERIAL should have low smoke generation, low toxicity, low-halogen content, flame retardance, fluid resistance, high abrasion resistance, and stable performance over temperature.
Navy systems require that fiber optic cables meet stringent environmental conditions. The types of cable designs considered by the Navy include the OPTICAL FIBER CABLE COMPONENT (OFCC), STRANDED, and RIBBON cable designs.
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ANSWERS TO QUESTIONS Q1. THROUGH Q35. A1. Refractive index profile describes the value of refractive index as a function of radial distance at
any fiber diameter. A2. Step-index.
A3. Multimode graded-index fiber. A4. Multimode step-index fibers: 50 m m and 100 m m.  Single mode fibers: between 8 m m and 10 m m. A5. Cladding.
A6. Most modes in multimode step-index fibers propagate far from cutoff. A7. Make it easier to couple light from a light-emitting diode (LED) into the fiber. A8. From a triangular shape to step.
A9. When the angle of incidence becomes larger than the critical angle of incidence. A10. Numerical aperture (NA), relative refractive index difference (D ), profile parameter (a ), and
normalized frequency (V). A11. Decreases the time difference between light rays, which reduces modal dispersion and increases
fiber bandwidth. A12. 62.5/125 m m multimode graded-index fiber. A13. Source-to-fiber coupling efficiency and insensitivity to microbending and macrobending losses. A14. Coupling efficiency increases with both core diameter and D , while bending losses increase
directly with core diameter and inversely with D .
A15. Smaller.
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A16. Matched-clad and depressed-clad.
A17. Depressed.
A18. Cladding material.
A19. The fundamental mode becomes increasingly lossy. A20. To increase performance and reduce losses caused by bending and splicing. A21. To increase performance and reduce cost. A22. High NA, low bandwidth, tight bend radius, short length, and low cost.
A23. Heavy-metal fluorides, chalcogenide glasses, and crystalline materials. A24. Vapor phase oxidation and direct-melt process.
A25. Vapor phase oxidation.
A26.
a. Protect optical fibers from damage or breakage during installation and over the fiber's lifetime.
b. Provide stable fiber transmission characteristics compared with uncabled fibers. c. Maintain the physical integrity of the optical fiber.
A27. To provide additional mechanical protection and preserve the fiber's inherent strength. A28. Tight-buffered, loose-tube, and gel-filled loose-tube. A29. Low smoke generation, low toxicity, low halogen content, flame retardance, fluid resistance, high
abrasion resistance, and stable performance over temperature. A30. Optical fiber cable component (OFCC), stranded, and ribbon cables designs. A31. OFCCs are tight-buffer fiber surrounded by arimid yarn and a low-halogen outer jacket. A32. To minimize microbending of the fibers. A33. To minimize fiber stress when the cable is bent. A34. OFCC (12 fibers), stranded (48 fibers), ribbon (204 fibers). A35. Ribbon.
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Upon completion of this chapter, you should be able to do the following:
1. Describe a fiber optic splice, connector, and coupler and the types of connections they form in systems.
2. List the types of extrinsic and intrinsic coupling losses.
3. Understand the degree to which fiber alignment and fiber mismatch problems increase system loss.
4. Detail the score-and-break cleaving process for fiber-end preparation.
5. Identify the types of fiber optic mechanical and fusion splices. Outline the basic splicing techniques for each type of fiber optic splice.
6. List the types of fiber optic connectors. Detail the procedure for installing a fiber optic connector on an optical fiber.
7. Discuss the types of fiber optic passive couplers.
Chapter 1 states that a fiber optic data link performs three basic functions. First, the data link transmitter converts an electrical input signal to an optical signal. Then, the optical fiber transmits this optical signal. Finally, the data link receiver converts the optical signal back to an electrical signal identical to the original input. However, chapter 1 does not describe how optical power transfers from one optical component to another.
This chapter describes how optical power is transferred from one fiber optic component to another. It describes how an optical source launches optical power into a fiber as well as how one optical fiber couples light into another fiber. In fiber optic system design, this launching or coupling of optical power from one component to the next is important.
Fiber optic connections permit the transfer of optical power from one component to another. Fiber optic connections also permit fiber optic systems to be more than just point-to-point data communication links. In fact, fiber optic data links are often of a more complex design than point-to-point data links.
A system connection may require either a fiber optic splice, connector, or coupler. One type of system connection is a permanent connection made by splicing optical fibers together. A fiber optic splice makes a permanent joint between two fibers or two groups of fibers. There are two types of fiber optic splices--mechanical splices and fusion splices. Even though removal of some mechanical splices is possible, they are intended to be permanent. Another type of connection that allows for system reconfiguration is a fiber optic connector. Fiber optic connectors permit easy coupling and uncoupling of
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optical fibers. Fiber optic connectors sometimes resemble familiar electrical plugs and sockets. Systems may also divide or combine optical signals between fibers. Fiber optic couplers distribute or combine optical signals between fibers. Couplers can distribute an optical signal from a single fiber into several fibers. Couplers may also combine optical signals from several fibers into one fiber.
Fiber optic connection losses may affect system performance. Poor fiber end preparation and poor fiber alignment are the main causes of coupling loss. Another source of coupling loss is differences in optical properties between the connected fibers. If the connected fibers have different optical properties, such as different numerical apertures, core and cladding diameters, and refractive index profiles, then coupling losses may increase.
Q1. Which fiber optic component (splice, connector, or coupler) makes a permanent connection in a distributed system?
Q2. What are the main causes of coupling loss?
Ideally, optical signals coupled between fiber optic components are transmitted with no loss of light. However, there is always some type of imperfection present at fiber optic connections that causes some loss of light. It is the amount of optical power lost at fiber optic connections that is a concern of system designers.
The design of fiber optic systems depends on how much light is launched into an optical fiber from an optical source and how much light is coupled between fiber optic components, such as from one fiber to another. The amount of power launched from a source into a fiber depends on the optical properties of both the source and the fiber. The amount of optical power launched into an optical fiber depends on the radiance of the optical source. An optical source's radiance, or brightness, is a measure of its optical power launching capability. Radiance is the amount of optical power emitted in a specific direction per unit time by a unit area of emitting surface. For most types of optical sources, only a fraction of the power emitted by the source is launched into the optical fiber.
The loss in optical power through a connection is defined similarly to that of signal attenuation through a fiber. Optical loss is also a log relationship. The loss in optical power through a connection is defined as:
For example, Po is the power emitted from the source fiber in a fiber-to-fiber connection. Pi is the power accepted by the connected fiber. In any fiber optic connection, Po and Pi are the optical power levels measured before and after the joint, respectively.
Fiber-to-fiber connection loss is affected by intrinsic and extrinsic coupling losses. Intrinsic coupling losses are caused by inherent fiber characteristics. Extrinsic coupling losses are caused by jointing techniques. Fiber-to-fiber connection loss is increased by the following sources of intrinsic and extrinsic coupling loss:
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Intrinsic coupling losses are limited by reducing fiber mismatches between the connected fibers. This is done by procuring only fibers that meet stringent geometrical and optical specifications. Extrinsic coupling losses are limited by following proper connection procedures.
Some fiber optic components are modular devices that are designed to reduce coupling losses between components. Modular components can be easily inserted or removed from any system. For example, fiber optic transmitters and receivers are modular components. Fiber optic transmitters and receivers are devices that are generally manufactured with fiber pigtails or fiber optic connectors as shown in figure 4-1. A fiber pigtail is a short length of optical fiber (usually 1 meter or less) permanently fixed to the optical source or detector. Manufacturers supply transmitters and receivers with pigtails and connectors because fiber coupling to sources and detectors must be completed during fabrication. Reduced coupling loss results when source-to-fiber and fiber-to-detector coupling is done in a controlled manufacturing environment. Since optical sources and detectors are pigtailed or connectorized, launching optical power is reduced to coupling light from one fiber to another. In fact, most fiber optic connections can be considered fiber-to-fiber.
Q3. Define the loss in optical power through a connection. Q4. Fiber-to-fiber coupling loss is affected by intrinsic and extrinsic coupling losses. Can intrinsic
coupling losses be limited by limiting fiber mismatches?
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When optical fibers are connected, optical power may be reflected back into the source fiber. Light that is reflected back into the source fiber is lost. This reflection loss, called Fresnel reflection, occurs at every fiber interface. Fresnel reflection is caused by a step change in the refractive index that occurs at the fiber joint. In most cases, the step change in refractive index is caused by the ends of each fiber being separated by a small gap. This small gap is usually an air gap. In Fresnel reflection, a small portion of the incident light is reflected back into the source fiber at the fiber interface. The ratio (R), shown below, approximates the portion of incident light (light of normal incidence) that is reflected back into the source fiber.
R is the fraction of the incident light reflected at the fiber n1 is the refractive index of the fiber core. n 0 is the refractive index of the medium between the two fibers.
Fresnel refraction occurs twice in a fiber-to-fiber connection. A portion of the optical power is reflected when the light first exits the source fiber. Light is then reflected as the optical signal enters the receiving fiber. Fresnel reflection at each interface must be taken into account when calculating the total fiber-to-fiber coupling loss. Loss from Fresnel reflection may be significant. To reduce the amount of loss from Fresnel reflection, the air gap can be filled with an index matching gel. The refractive index of the index matching gel should match the refractive index of the fiber core. Index matching gel reduces the step change in the refractive index at the fiber interface, reducing Fresnel reflection.
In any system, index matching gels can be used to eliminate or reduce Fresnel reflection. The choice of index matching gels is important. Fiber-to-fiber connections are designed to be permanent and require no maintenance. Over the lifetime of the fiber connection, the index matching material must meet specific optical and mechanical requirements. Index matching gels should remain transparent. They should also resist flowing or dripping by remaining viscous. Some index matching gels darken over time while others settle or leak out of fiber connections. If these requirements are not met, then the fiber-to-fiber connection loss will increase over time. In Navy applications, this variation in connection loss over time is unacceptable. In Navy systems, index matching gels are only used in fiber optic splice interfaces.
Q5. In fiber-to-fiber connections, Fresnel reflection is one source of coupling losses. Light is reflected back into the source fiber and is lost. What causes Fresnel reflection?
Q6. Reduction of Fresnel reflection is possible by reducing the step change in the refractive index at the fiber interface. What material reduces the step change in refractive index at a fiber interface?
A main source of extrinsic coupling loss in fiber-to-fiber connections is poor fiber alignment. The three basic coupling errors that occur during fiber alignment are fiber separation (longitudinal misalignment), lateral misalignment, and angular misalignment. Most alignment errors are the result of mechanical imperfections introduced by fiber jointing techniques. However, alignment errors do result from installers not following proper connection procedures.
With fiber separation, a small gap remains between fiber-end faces after completing the fiber connection. Figure 4-2 illustrates this separation of the fiber-end faces.
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Lateral, or axial, misalignment occurs when the axes of the two fibers are offset in a perpendicular direction. Figure 4-3 shows this perpendicular offset of the axes of two connecting fibers.
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Angular misalignment occurs when the axes of two connected fibers are no longer parallel. The axes of each fiber intersect at some angle ( Q ). Figure 4-4 illustrates the angular misalignment between the core axes.
Coupling loss caused by lateral and angular misalignment typically is greater than the loss caused by fiber separation. Loss, caused by fiber separation, is less critical because of the relative ease in limiting the distance of fiber separation. However, in some cases, fiber optic connectors prevent fibers from actual contact. These fiber optic connectors separate the fibers by a small gap. This gap eliminates damage to fiber-end faces during connection. For connectors with an air gap, the use of index matching gel reduces the coupling loss.
Most newer connectors are designed so that the connector ferrule end faces contact when the connector is mated. The connector can be assembled onto the fiber so that the fibers also contact when mated. However, they also can be assembled so that the fibers do not. Whether or not the fibers contact is determined by whether the fiber sticks out slightly from the ferrule or is recessed inside the ferrule. The fiber position can be controlled by the connector polishing technique. The physical contact (PC) polish technique was developed for most connectors so that the fibers would touch when mated. In these types of connectors, index gel is not needed to reduce reflections.
While index matching gel reduces coupling loss from fiber separation, it does not affect loss in lateral misalignment. Additionally, index matching gel usually increases the fiber's coupling loss sensitivity to angular misalignment. Although angular misalignment involves fiber separation, index matching gel reduces the angle at which light is launched from the source fiber. Index matching gel causes less light to be coupled into the receiving fiber. To reduce coupling loss from angular misalignment, the angle Q should be less than 1º.
Coupling losses due to fiber alignment depend on fiber type, core diameter, and the distribution of optical power among propagating modes. Fibers with large NAs reduce loss from angular misalignment and increase loss from fiber separation. Single mode fibers are more sensitive to alignment errors than multimode fibers because of their small core size. However, alignment errors in multimode fiber
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connections may disturb the distribution of optical power in the propagating modes, increasing coupling loss.
Q7. List the three basic errors that occur during fiber alignment. Q8. When the axes of two connected fibers are no longer in parallel, the two connected fibers are in
what kind of misalignment? Q9. How does index matching gel affect the amount of coupling loss caused by (a) fiber separation,
(b) lateral misalignment, and (c) angular misalignment? Q10. Which are more sensitive to alignment errors, single mode or multimode fibers?
In fiber-to-fiber connections, a source of extrinsic coupling loss is poor fiber end preparation. An optical fiber-end face must be flat, smooth, and perpendicular to the fiber's axis to ensure proper fiber connection. Light is reflected or scattered at the connection interface unless the connecting fiber end faces are properly prepared. Figure 4-5 shows some common examples of poor fiber ends. It illustrates a fiber- end face tilt, lip, and hackle. Quality fiber-end preparation is essential for proper system operation.
Fiber-end preparation begins by removing the fiber buffer and coating material from the end of the optical fiber. Removal of these materials involves the use of mechanical strippers or chemical solvents. When using chemical solvents, the removal process must be performed in a well-ventilated area. For this reason mechanical strippers are used for buffer and coating removal in the shipboard environment. After removing the buffer and coating material, the surface of the bare fiber is wiped clean using a wiping tissue. The wiping tissue must be wet with isopropyl alcohol before wiping.
The next step in fiber-end preparation involves cleaving the fiber end to produce a smooth, flat fiber- end face. The score-and-break, or scribe-and-break, method is the basic fiber cleaving technique for
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preparing optical fibers for coupling. The score-and-break method consists of lightly scoring (nicking) the outer surface of the optical fiber and then placing it under tension until it breaks. A heavy metal or diamond blade is used to score the fiber. Once the scoring process is complete, fiber tension is increased until the fiber breaks. The fiber is placed under tension either by pulling on the fiber or by bending the fiber over a curved surface.
Figure 4-6 shows the setup for the score-and-break procedure for fiber cleaving. Under constant tension, the score-and-break method for cleaving fibers produces a quality fiber end. This fiber end is good enough to use for some splicing techniques. However, additional fiber-end preparation is necessary to produce reliable low-loss connections when using fiber optic connectors.
Polishing the fiber ends removes most surface imperfections introduced by the fiber cleaving process. Fiber polishing begins by inserting the cleaved fiber into the ferrule of a connector assembly. A ferrule is a fixture, generally a rigid tube, used to hold the stripped end of an optical fiber in a fiber optic connector. An individual fiber is epoxied within the ferrule. The connector with the optical fiber cemented within the ferrule can then be mounted into a special polishing tool for polishing.
Figure 4-7 shows one type of fiber polishing tool for finishing optical fibers in a connector assembly. Various types of connector assemblies are discussed later in this chapter. In this type of polishing tool, the connector assembly is threaded onto the polishing tool. The connector ferrule passes through the center of the tool allowing the fiber-end face to extend below the tool's circular, flat bottom. The optical fiber is now ready for polishing.
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Fiber polishing involves a step-down approach. The first step is to give the surface of the fiber end a rough polish. Rough-polishing occurs when the fiber, mounted to the polishing tool, moves over a 5m to 15 m grit abrasive paper. The mounted fiber moves over the abrasive paper in a figure-eight motion. The next step involves giving the surface of the fiber end a fine polish. Fine-polishing occurs when the mounted fiber moves over a 0.3 m to 1m grit abrasive paper in the same figure-eight motion. Fiber inspection and cleanliness are important during each step of fiber polishing. Fiber inspection is done visually by the use of a standard microscope at 200 to 400 times magnification.
A standard microscope can be used to determine if the fiber-end face is flat, concave, or convex. If different parts of the fiber-end face have different focus points, the end face is not flat. If all parts of the fiber-end face are in focus at the same time, the end face is flat.
Q11. Quality fiber-end preparation is essential for proper system operation. What properties must an optical fiber-end face have to ensure proper fiber connection?
Q12. What is the basic fiber cleaving technique for preparing optical fibers for coupling? Q13. Using a standard microscope to inspect a fiber-end face, you observe that all parts of the fiber-
end face are in focus at the same time. Is the fiber-end face flat, concave, or convex? FIBER MISMATCHES
Fiber mismatches are a source of intrinsic coupling loss. As stated before, intrinsic coupling loss results from differences (mismatches) in the inherent fiber characteristics of the two connecting fibers. Fiber mismatches occur when manufacturers fail to maintain optical or structural (geometrical) tolerances during fiber fabrication.
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Fiber mismatches are the result of inherent fiber characteristics and are independent of the fiber jointing techniques. Types of fiber mismatches include fiber geometry mismatches, NA mismatch, and refractive index profile difference. Fiber geometry mismatches include core diameter, cladding diameter, core ellipticity, and core-cladding concentricity differences. Figure 4-8 illustrates each type of optical and geometrical fiber mismatch. Navy fiber specifications tightly specify these parameters to minimize coupling losses from fiber mismatches.
Core diameter and NA mismatch have a greater effect on intrinsic coupling loss than the other types of fiber mismatches. In multimode fiber connections, the coupling loss resulting from core diameter mismatch, NA mismatch, and refractive index profile difference depends on the characteristics of the launching fiber. Coupling loss from core diameter mismatch results only if the launching fiber has a larger core radius (a) than the receiving fiber. Coupling loss from NA mismatch results only if the launching fiber has a higher NA than the receiving fiber. Coupling loss from refractive index profile difference results only if the launching fiber has a larger profile parameter ( a ) than the receiving fiber. Q14. List six types of fiber mismatches. Q15. Does coupling loss from refractive index profile difference result when the receiving fiber has a
larger profile parameter (a ) than the transmitting fiber?
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A fiber optic splice is a permanent fiber joint whose purpose is to establish an optical connection between two individual optical fibers. System design may require that fiber connections have specific optical properties (low loss) that are met only by fiber-splicing. Fiber optic splices also permit repair of optical fibers damaged during installation, accident, or stress. System designers generally require fiber splicing whenever repeated connection or disconnection is unnecessary or unwanted.
Mechanical and fusion splicing are two broad categories that describe the techniques used for fiber splicing. Amechanical splice is a fiber splice where mechanical fixtures and materials perform fiber alignment and connection. A fusion splice is a fiber splice where localized heat fuses or melts the ends of two optical fibers together. Each splicing technique seeks to optimize splice performance and reduce splice loss. Low-loss fiber splicing results from proper fiber end preparation and alignment.
Fiber splice alignment can involve passive or active fiber core alignment. Passive alignment relies on precision reference surfaces, either grooves or cylindrical holes, to align fiber cores during splicing. Active alignment involves the use of light for accurate fiber alignment. Active alignment may consist of either monitoring the loss through the splice during splice alignment or by using a microscope to accurately align the fiber cores for splicing. To monitor loss either an optical source and optical power meter or an optical time domain reflectometer (OTDR) are used. Active alignment procedures produce low-loss fiber splices.
Q16. Define a fiber optic splice. Q17. Fiber splicing is divided into two broad categories that describe the techniques used for fiber
splicing. What are they?
Mechanical splicing involves using mechanical fixtures to align and connect optical fibers. Mechanical splicing methods may involve either passive or active core alignment. Active core alignment produces a lower loss splice than passive alignment. However, passive core alignment methods can produce mechanical splices with acceptable loss measurements even with single mode fibers.
In the strictest sense, a mechanical splice is a permanent connection made between two optical fibers. Mechanical splices hold the two optical fibers in alignment for an indefinite period of time without movement. The amount of splice loss is stable over time and unaffected by changes in environmental or mechanical conditions.
If high splice loss results from assembling some mechanical splices, the splice can be reopened and the fibers realigned. Realignment includes wiping the fiber or ferrule end with a soft wipe, reinserting the fiber or ferrule in a new arrangement, and adding new refractive index material. Once producing an acceptable mechanical splice, splice realignment should be unnecessary because most mechanical splices are environmentally and mechanically stable within their intended application.
The types of mechanical splices that exist for mechanical splicing include glass, plastic, metal, and ceramic tubes; and V-groove and rotary devices. Materials that assist mechanical splices in splicing fibers include transparent adhesives and index matching gels. Transparent adhesives are epoxy resins that seal mechanical splices and provide index matching between the connected fibers.
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Mechanical splicing may involve the use of a glass or ceramic alignment tube, or capillary. The inner diameter of this glass or ceramic tube is only slightly larger than the outer diameter of the fiber. A transparent adhesive, injected into the tube, bonds the two fibers together. The adhesive also provides index matching between the optical fibers. Figure 4-9 illustrates fiber alignment using a glass or ceramic tube. This splicing technique relies on the inner diameter of the alignment tube. If the inner diameter is too large, splice loss will increase because of fiber misalignment. If the inner diameter is too small, it is impossible to insert the fiber into the tube.
Mechanical splices may also use either a grooved substrate or positioning rods to form suitable V-grooves for mechanical splicing. The basic V-grooved device relies on an open grooved substrate to perform fiber alignment. When inserting the fibers into the grooved substrate, the V-groove aligns the cladding surface of each fiber end. A transparent adhesive makes the splice permanent by securing the fiber ends to the grooved substrate. Figure 4-10 illustrates this type of open V-grooved splice.
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V-grooved splices may involve sandwiching the butted ends of two prepared fibers between a V-grooved substrate and a flat glass plate. Additional V-grooved devices use two or three positioning rods to form a suitable V-groove for splicing. The V-grooved device that uses two positioning rods is the spring V-grooved splice. This splice uses a groove formed by two rods positioned in a bracket to align the fiber ends. The diameter of the positioning rods permits the outer surface of each fiber end to extend above the groove formed by the rods. A flat spring presses the fiber ends into the groove maintaining fiber alignment. Transparent adhesive completes the assembly process by bonding the fiber ends and providing index matching. Figure 4-11 is an illustration of the spring V-grooved splice. A variation of this splice uses a third positioning rod instead of a flat spring. The rods are held in place by a heat-shrinkable band, or tube.
In a rotary splice, the fibers are mounted into a glass ferrule and secured with adhesives. The splice begins as one long glass ferrule that is broken in half during the assembly process. A fiber is inserted into each half of the tube and epoxied in place using an ultraviolet cure epoxy. The endface of the tubes are then polished and placed together using the alignment sleeve. Figure 4-12 is an illustration of a rotary splice. The fiber ends retain their original orientation and have added mechanical stability since each fiber is mounted into a glass ferrule and alignment sleeve. The rotary splice may use index matching gel within the alignment sleeve to produce low-loss splices.
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In shipboard applications, the Navy recommends using the rotary splice. The rotary splice is a low- loss mechanical splice that provides stable environmental and mechanical performance in the Navy environment. Stable performance means that splice loss does not vary significantly with changes in temperature or other environmental or mechanical conditions. Completing a rotary splice also requires only a small amount of training, or expertise. This shorter training time is another reason why the Navy recommends using the rotary splice over other mechanical or fusion splicing techniques.
Q18. Describe a transparent adhesive. Q19. The Navy recommends using the rotary splice for what two reasons?
The process of fusion splicing involves using localized heat to melt or fuse the ends of two optical fibers together. The splicing process begins by preparing each fiber end for fusion. Fusion splicing requires that all protective coatings be removed from the ends of each fiber. The fiber is then cleaved using the score-and-break method. The quality of each fiber end is inspected using a microscope. In fusion splicing, splice loss is a direct function of the angles and quality of the two fiber-end faces.
The basic fusion splicing apparatus consists of two fixtures on which the fibers are mounted and two electrodes. Figure 4-13 shows a basic fusion-splicing apparatus. An inspection microscope assists in the placement of the prepared fiber ends into a fusion-splicing apparatus. The fibers are placed into the apparatus, aligned, and then fused together. Initially, fusion splicing used nichrome wire as the heating element to melt or fuse fibers together. New fusion-splicing techniques have replaced the nichrome wire with carbon dioxide (CO2) lasers, electric arcs, or gas flames to heat the fiber ends, causing them to fuse together. The small size of the fusion splice and the development of automated fusion-splicing machines have made electric arc fusion (arc fusion) one of the most popular splicing techniques in commercial applications.
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Arc fusion involves the discharge of electric current across a gap between two electrodes. By placing the fiber ends between the electrodes, the electric discharge melts or fuses the ends of each fiber. Figure 4-13 shows the placement of the fiber ends between tungsten electrodes during arc fusion. Initially, a small gap is present between the fiber ends. A short discharge of electric current is used to prepare the fiber ends for fusion. During this short discharge, known as prefusion, the fiber ends are cleaned and rounded to eliminate any surface defects that remain from fiber cleaving. Surface defects can cause core distortions or bubble formations during fiber fusion. A fusion splice results when the fiber ends are pressed together, actively aligned, and fused using a longer and stronger electric discharge. Automated fusion splicers typically use built-in local optical power launch/detection schemes for aligning the fibers.
During fusion, the surface tension of molten glass tends to realign the fibers on their outside diameters, changing the initial alignment. When the fusion process is complete, a small core distortion may be present. Small core distortions have negligible effects on light propagating through multimode fibers. However, a small core distortion can significantly affect single mode fiber splice loss. The core distortion, and the splice loss, can be reduced by limiting the arc discharge and decreasing the gap distance between the two electrodes. This limits the region of molten glass. However, limiting the region of molten glass reduces the tensile strength of the splice.
Fusion splicing yields typically vary between 25 and 75 percent depending on the strength and loss requirements for the splice and other factors. Other factors affecting splice yields include the condition of the splicing machine, the experience of the splice personnel, and environmental conditions. Since fusion splicing is inherently permanent, an unacceptable fusion splice requires breakage and refabrication of the splice.
In general, fusion splicing takes a longer time to complete than mechanical splicing. Also, yields are typically lower making the total time per successful splice much longer for fusion splicing. Both the yield and splice time are determined to a large degree by the expertise of the fusion splice operator. Fusion splice operators must be highly trained to consistently make low-loss reliable fusion splices. For these reasons the fusion splice is not recommended for use in Navy shipboard applications.
Q20. What fiber property directly affects splice loss in fusion splicing? Q21. List two reasons why fusion splicing is one of the most popular splicing techniques in commercial
applications.
Q22. What is a short discharge of electric current that prepares the fiber ends for fusion called?
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Q23. Do small core distortions formed by arc fusion's self-alignment mechanism have more of an affect on light propagating through multimode or single mode fibers?
Normally, multifiber splices are only installed on ribbon type fiber optic cables. Multifiber splicing techniques can use arc fusion to restore connection, but most splicing techniques use mechanical splicing methods. The most common mechanical splice is the ribbon splice.
A ribbon splice uses an etched silicon chip, or grooved substrate, to splice the multiple fibers within a flat ribbon. The spacing between the etched grooves of the silicon chip is equal to the spacing between the fibers in the flat ribbon. Before placing each ribbon on the etched silicon chip, each fiber within the ribbon cable is cleaved. All of the fibers are placed into the grooves and held in place with a flat cover. Typically, an index matching gel is used to reduce the splice loss. Figure 4-14 shows the placement of the fiber ribbon on the etched silicon chip.
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A fiber optic connector is a demateable device that permits the coupling of optical power between two optical fibers or two groups of fibers. Designing a device that allows for repeated fiber coupling without significant loss of light is difficult. Fiber optic connectors must maintain fiber alignment and provide repeatable loss measurements during numerous connections. Fiber optic connectors should be easy to assemble (in a laboratory or field environment) and should be cost effective. They should also be reliable. Fiber optic connections using connectors should be insensitive to environmental conditions, such as temperature, dust, and moisture. Fiber optic connector designs attempt to optimize connector performance by meeting each of these conditions.
Fiber optic connector coupling loss results from the same loss mechanisms described earlier in this chapter. Coupling loss results from poor fiber alignment and end preparation (extrinsic losses), fiber mismatches (intrinsic loss), and Fresnel reflection. The total amount of insertion loss for fiber optic connectors should remain below 1 dB. Fiber alignment is the critical parameter in maintaining the total insertion loss below the required level. There is only a small amount of control over coupling loss resulting from fiber mismatches, because the loss results from inherent fiber properties. Index matching gels cannot be used to reduce Fresnel losses, since the index matching gels attract dust and dirt to the connection.
Fiber optic connectors can also reduce system performance by introducing modal and reflection noise. The cause of modal noise in fiber optic connectors is the interfering of the different wavefronts of different modes within the fiber at the connector interface. Modal noise is eliminated by using only single mode fiber with laser sources and only low-coherence sources such as light-emitting diodes with multimode fiber. Fiber optic connectors can introduce reflection noise by reflecting light back into the optical source. Reflection noise is reduced by index matching gels, physical contact polishes, or antireflection coatings. Generally, reflection noise is only a problem in high data rate single mode systems using lasers.
Butt-jointed connectors and expanded-beam connectors are the two basic types of fiber optic connectors. Fiber optic butt-jointed connectors align and bring the prepared ends of two fibers into close contact. The end-faces of some butt-jointed connectors touch, but others do not depending upon the connector design. Types of butt-jointed connectors include cylindrical ferrule and biconical connectors. Fiber optic expanded-beam connectors use two lenses to first expand and then refocus the light from the transmitting fiber into the receiving fiber. Single fiber butt-jointed and expanded beam connectors normally consist of two plugs and an adapter (coupling device). Figure 4-15 shows how to configure each plug and adapter when making the connection between two optical fibers.
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Ferrule connectors use two cylindrical plugs (referred to as ferrules), an alignment sleeve, and sometimes axial springs to perform fiber alignment. Figure 4-16 provides an illustration of this basic ferrule connector design. Precision holes drilled or molded through the center of each ferrule allow for fiber insertion and alignment. Precise fiber alignment depends on the accuracy of the central hole of each ferrule. When the fiber ends are inserted, an adhesive (normally an epoxy resin) bonds the fiber inside the ferrule. The fiber-end faces are polished until they are flush with the end of the ferrule to achieve a low- loss fiber connection. Fiber alignment occurs when the ferrules are inserted into the alignment sleeve. The inside diameter of the alignment sleeve aligns the ferrules, which in turn align the fibers. Ferrule connectors lock the ferrules in the alignment sleeve using a threaded outer shell or some other type of coupling mechanism.
As stated before, fiber alignment depends on an accurate hole through the center of the ferrule. Normally, ferrule connectors use ceramic or metal ferrules. The center hole is generally drilled in a metal ferrule. Drilling an accurate hole through the entire metal ferrule can be difficult. To improve fiber alignment, some metal ferrule connectors use precision watch-jeweled centering. In precision watch- jeweled centering, a watch jewel with a precision centered hole is placed in the tip of the ferrule. The
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central hole of the watch jewel centers the fiber with respect to the axis of the cylindrical ferrule. The watch jewel provides for better fiber alignment, because regulating the hole tolerance of the watch jewel is easier than maintaining a precise hole diameter when drilling through an entire ferrule.
The center hole in a ceramic ferrule is created by forming the ferrule around a precision wire, which is then removed. This method produces holes accurately centered in the ferrule. Most cylindrical ferrule connectors now use ceramic ferrules. The Straight Tip (STÒ connector is an example of a ceramic ferrule connector. (ST is a registered trademark of AT&T.)
Other cylindrical ferrule connectors have a ferrule that contains both metal and ceramic. For these connectors a ceramic capillary is placed within the tip of a metal ferrule to provide for precision fiber alignment. The ceramic capillary is a ceramic tube with a small inner diameter that is just larger than the diameter of the fiber. Figure 4-17 shows the placement of the ceramic capillary within the metal ferrule.
Another type of butt-jointed connector is the biconical connector. Biconical connectors use two conical plugs, a double conical alignment sleeve, and axial springs to perform fiber alignment. Figure 4-18 is an illustration of this basic biconical connector design. Formation of the plugs and alignment sleeve involves transfer molding. Transfer molding uses silica-filled epoxy resin to mold the conical plug directly to the fiber or around a cast (precision wire). After connecting the conical plugs to the optical fibers, the fiber-end faces are polished before the plugs are inserted into the molded alignment sleeve. During fiber insertion, the inside surface of the double conical sleeve performs fiber alignment, while the axial springs push the fiber ends into close contact. If the alignment sleeve permits the fibers to actually become in contact, then the axial spring provides enough force to maintain fiber contact but prevent damage to the fiber-end faces. Normally, biconical connectors lock the fibers in alignment using a threaded outer shell.
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Multifiber connectors join and align multifiber cables to reduce the time it takes to connect multiple fibers. One type of multifiber connector is the array connector. The array connector is used to connect individual ribbons of ribbon-type cables. The array connector is similar to the ribbon splice. In the array connector, the fibers of each ribbon are epoxied into grooves of a silicon chip so that the fiber ends protrude from the end of the chip. The chip and the protruding fibers are polished flat for connection. Each half of the connector is prepared separately before being butt-jointed. A spring clip and two grooved metal-backed plates are used to align and connect the stacked ribbons of the two ribbon cables. Array connectors may also use an alignment sleeve with V-grooved silicon chips and metal springs to align and connect stacked ribbons. Figure 4-19 shows the spring clip method of array connector alignment. The multifiber array connector is only one example of a multiple connector. Many types of multiple connectors exist that connect different types of multifiber cables.
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Figure 4-20 shows how an expanded-beam connector uses two lenses to expand and then refocus the light from the transmitting fiber into the receiving fiber. Expanded-beam connectors are normally plug- adapter-plug type connections. Fiber separation and lateral misalignment are less critical in expanded- beam coupling than in butt-jointing. The same amount of fiber separation and lateral misalignment in expanded beam coupling produces a lower coupling loss than in butt-jointing. However, angular misalignment is more critical. The same amount of angular misalignment in expanded-beam coupling produces a higher loss than in butt-jointing. Expanded-beam connectors are also much harder to produce. Present applications for expanded-beam connectors include multifiber connections, edge connections for printed circuit boards, and other applications.
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Q24. What connection properties result in fiber optic connector coupling loss? Q25. Which is the more critical parameter in maintaining total insertion loss below the required level,
fiber alignment or fiber mismatch? Q26. Fiber optic connectors can reduce system performance by increasing what two types of noise? Q27. Which type of fiber optic connector (butt-jointed or expanded beam) brings the prepared ends of
two optical fibers into close contact? Q28. Is coupling loss from fiber separation and lateral misalignment more critical in expanded-beam
or butt-jointed connectors? Q29. Is coupling loss from angular misalignment more critical in expanded beam or butt-jointed
connectors?
Light-duty connectors and heavy-duty connectors are two ways that the Navy classifies fiber optic connectors. Light-duty connector shipboard applications include locations that protect the connectors from the environment, such as in a junction box or equipment enclosure. Heavy-duty applications require a very rugged, stand-alone, sealed connector. A heavy-duty connector must also withstand pulls and tugs on the fiber cable without disrupting system operation. Light-duty connectors can be of the ferrule, biconical, or expanded-beam designs. Ferrule-type ST® connectors are becoming the commercial connector of choice for local area network (LAN) and data transfer links and are the standard connector for Navy light duty applications. This connector is described in specification sheets 16, 17, and 18 of MIL-C-83522. Figure 4-21 shows the ST type of light-duty connector.
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Figure 4-22 shows one type of heavy-duty connector designed for use in harsh Navy environments. This connector is described by the military specification MIL-C-28876. This connector comes in various sizes capable of terminating 2, 4, 6, or 8 fibers. Each fiber termination, called a terminus, is of the cylindrical ferrule type. Two slightly different termini are used to form a connection; a pin terminus and a socket terminus. The pin terminus consists of a terminus body, which holds the terminus within the connector shell and a ceramic ferrule. The socket terminus consists of a terminus body, a ceramic ferrule, and an alignment sleeve, which attaches to the ceramic ferrule. Fiber alignment occurs when the pin terminus slides into the alignment sleeve of the socket terminus. The termini are held within an insert in the connector shell. When the connector halves are mated, the connector inserts align the mating termini, which then align the mating fibers. The connector shell and backshell protect the termini from the surrounding environment and provide strain relief for the multifiber cable.
Q30. The Navy classifies fiber optic connectors in what two ways?
Some fiber optic data links require more than simple point-to-point connections. These data links may be of a much more complex design that requires multi-port or other types of connections. Figure 4-23 shows some example system architectures that use more complex link designs. In many cases these types of systems require fiber optic components that can redistribute (combine or split) optical signals throughout the system.
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One type of fiber optic component that allows for the redistribution of optical signals is a fiber optic coupler. A fiber optic coupler is a device that can distribute the optical signal (power) from one fiber among two or more fibers. A fiber optic coupler can also combine the optical signal from two or more fibers into a single fiber. Fiber optic couplers attenuate the signal much more than a connector or splice because the input signal is divided among the output ports. For example, with a 1 · 2 fiber optic coupler, each output is less than one-half the power of the input signal (over a 3 dB loss).
Fiber optic couplers can be either active or passive devices. The difference between active and passive couplers is that a passive coupler redistributes the optical signal without optical-to-electrical conversion. Active couplers are electronic devices that split or combine the signal electrically and use fiber optic detectors and sources for input and output.
Figure 4-24 illustrates the design of a basic fiber optic coupler. A basic fiber optic coupler has N input ports and M output ports. N and M typically range from 1 to 64. The number of input ports and output ports vary depending on the intended application for the coupler. Types of fiber optic couplers include optical splitters, optical combiners, X couplers, star couplers, and tree couplers.
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An optical splitter is a passive device that splits the optical power carried by a single input fiber into two output fibers. Figure 4-25 illustrates the transfer of optical power in an optical splitter. The input optical power is normally split evenly between the two output fibers. This type of optical splitter is known as a Y-coupler. However, an optical splitter may distribute the optical power carried by input power in an uneven manner. An optical splitter may split most of the power from the input fiber to one of the output fibers. Only a small amount of the power is coupled into the secondary output fiber. This type of optical splitter is known as a T-coupler, or an optical tap.
An optical combiner is a passive device that combines the optical power carried by two input fibers into a single output fiber. Figure 4-26 illustrates the transfer of optical power in an optical combiner.
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An X coupler combines the functions of the optical splitter and combiner. The X coupler combines and divides the optical power from the two input fibers between the two output fibers. Another name for the X coupler is the 2 · 2 coupler.
Star and tree couplers are multiport couplers that have more than two input or two output ports. A star coupler is a passive device that distributes optical power from more than two input ports among several output ports. Figure 4-27 shows the multiple input and output ports of a star coupler. A tree coupler is a passive device that splits the optical power from one input fiber to more than two output fibers. A tree coupler may also be used to combine the optical power from more than two input fibers into a single output fiber. Figure 4-28 illustrates each type of tree coupler. Star and tree couplers distribute the input power uniformly among the output fibers.
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· 1) tree coupler designs.
Fiber optic couplers should prevent the transfer of optical power from one input fiber to another input fiber. Directional couplers are fiber optic couplers that prevent this transfer of power between input fibers. Many fiber optic couplers are also symmetrical. A symmetrical coupler transmits the same amount of power through the coupler when the input and output fibers are reversed.
Passive fiber optic coupler fabrication techniques can be complex and difficult to understand. Some fiber optic coupler fabrication involves beam splitting using microlenses or graded-refractive-index (GRIN) rods and beam splitters or optical mixers. These beamsplitter devices divide the optical beam into two or more separated beams. Fabrication of fiber optic couplers may also involve twisting, fusing, and tapering together two or more optical fibers. This type of fiber optic coupler is a fused biconical taper coupler. Fused biconical taper couplers use the radiative coupling of light from the input fiber to the output fibers in the tapered region to accomplish beam splitting. Figure 4-29 illustrates the fabrication process of a fused biconical taper coupler.
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Q31. What is the difference between passive and active fiber optic couplers? Q32. Which type of optical splitter (Y-coupler or T-coupler) splits only a small amount of power from
the input fiber to one of the output fibers? Q33. Describe a directional coupler.
Now that you have completed this chapter, let's review some of the new terms, concepts, and ideas you have learned. You should have a thorough understanding of these principles before moving on to chapter 5.
FIBER OPTIC CONNECTIONS transfer optical power from one component to another. Fiber optic connections also permit fiber optic systems to be more than just a point-to-point data link.
A FIBER OPTIC SPLICE is a permanent joint between two fibers or two groups of fibers. FIBER OPTIC CONNECTORS permit easy coupling and uncoupling of optical fibers.
FIBER OPTIC COUPLERS distribute or combine optical signals between fibers.
POOR FIBER END PREPARATION and POOR FIBER ALIGNMENT are the main causes of coupling loss.
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RADIANCE is the amount of optical power emitted by a unit area of emitting surface per unit time in a specified direction. An optical source's radiance, or brightness, is a measure of its optical power launching capability.
FIBER-TO-FIBER COUPLING LOSS is affected by intrinsic and extrinsic coupling losses. INTRINSIC COUPLING LOSSES are caused by inherent fiber characteristics. EXTRINSIC COUPLING LOSSES are caused by jointing techniques.
A FIBER PIGTAIL is a short length of optical fiber (usually 1 meter or less) permanently fixed to a fiber optic component, such as an optical source or detector.
FRESNEL REFLECTION occurs twice in a fiber-to-fiber connection. A portion of the optical power is reflected when the light first exits the source fiber. Light is then reflected as the optical signal enters the receiving fiber.
INDEX MATCHING GEL eliminates or reduces the step change in the refractive index at the fiber interface, reducing Fresnel reflection.
POOR FIBER ALIGNMENT is a main source of coupling loss in fiber-to-fiber connections. The three basic coupling errors that occur during fiber alignment are fiber separation (longitudinal misalignment), lateral misalignment, and angular misalignment.
In FIBER SEPARATION a small gap remains between fiber-end faces after completing the fiber connection. LATERAL, or AXIAL,MISALIGNMENT is when the axes of the two fibers are offset in a perpendicular direction. ANGULARMISALIGNMENT is when the axes of the two fibers are no longer parallel.
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SINGLE MODE FIBERS are more sensitive to alignment errors than multimode fibers because of their small core diameters and low numerical apertures.
TheMODE POWER DISTRIBUTION (MPD) is the distribution of radiant power among the various modes propagating along the optical fiber.
Poor FIBER END PREPARATION is another source of extrinsic coupling loss. An optical fiber end face must be flat, smooth, and perpendicular to the fiber's axis to ensure proper fiber connection.
The SCORE-AND-BREAK method is the basic fiber cleaving technique for preparing optical fibers for coupling.
POLISHING the fiber ends removes most surface imperfections introduced by the fiber cleaving or cutting process. Fiber polishing involves a step-down approach. The first step is to give the surface of the fiber end a rough polish. The next step involves giving the surface of the fiber end a fine polish.
FIBER MISMATCHES are a source of intrinsic coupling loss. Types of fiber mismatches include fiber geometry mismatches, NA mismatch, and refractive index profile difference.
FIBER GEOMETRY MISMATCHES include core diameter, cladding diameter, core ellipticity, and core-cladding concentricity differences.
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CORE DIAMETERMISMATCH causes coupling loss only if the launching fiber has a larger core radius than the receiving fiber.
NAMISMATCH causes coupling loss only if the launching fiber has a higher NA than the receiving fiber.
A REFRACTIVE INDEX PROFILE DIFFERENCE causes coupling loss only if the launching fiber has a larger profile parameter than the receiving fiber.
MECHANICAL and FUSION SPLICING are two broad categories that describe the techniques used for fiber splicing. A mechanical splice is a fiber splice where mechanical fixtures perform fiber alignment and connection. A fusion splice is a fiber splice where localized heat fuses or melts the ends of two lengths of optical fiber together.
InMECHANICAL SPLICING, mechanical fixtures hold the two optical fibers in alignment for an indefinite period of time without movement. The amount of splice loss is stable over time and unaffected by changes in environmental or mechanical conditions.
ARC FUSION involves the discharge of electric current across a gap between two electrodes. By placing the fiber end between the electrodes, the electric discharge melts or fuses the ends of the fibers.
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PREFUSION involves a short discharge of electric current across the gap between the electrodes. In prefusion the fiber ends are cleaned and rounded to eliminate any surface defects that remain from fiber cleaving.
A FIBER OPTIC CONNECTOR is a demateable device that permits the coupling of optical power between two optical fibers or two groups of fibers.
FIBER ALIGNMENT in a fiber optic connector is the critical parameter in maintaining total insertion loss below the required level.
FIBER OPTIC CONNECTORS can affect system performance by increasing modal and reflection noise.
MODAL NOISE is eliminated by using only single mode fiber with laser sources and only low- coherence sources such as light-emitting diodes with multimode fiber.
REFLECTION NOISE is reduced by index matching gels, physical contact polishes, or antireflection coatings.
BUTT-JOINTED and EXPANDED BEAM CONNECTORS are two ways to classify fiber optic connectors. Butt-jointed connectors bring the prepared ends of two fibers into close contact. Expanded beam connectors use two lenses to first expand and then refocus the light from the transmitting fiber into the receiving fiber.
LIGHT-DUTY and HEAVY-DUTY CONNECTORS are two ways that the Navy classifies fiber optic connectors. Light-duty connector shipboard applications include locations that protect the connectors from the environment such as in a junction box. Heavy-duty applications require a very rugged, stand-alone, sealed connector.
A PASSIVE COUPLER redistributes an optical signal without optical to electrical conversion.
An OPTICAL SPLITTER is a passive device that splits the optical power carried by a single input fiber into two output fibers.
An OPTICAL COMBINER is a passive device that combines the optical power from two input fibers into a single output fiber.
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A STAR COUPLER is a passive device that distributes optical power from more than two input ports among several output ports.
A TREE COUPLER is a passive device that splits the optical power from one input fiber to more than two output fibers. A tree coupler may also be used to combine the optical power from more than two input fibers into a single output fiber.
DIRECTIONAL COUPLERS are fiber optic couplers that prevent the transfer of optical power from one input fiber to another input fiber.
A SYMMETRICAL COUPLER transmits the same amount of power through the coupler when the input and output fibers are reversed.
ANSWERS TO QUESTIONS Q1. THROUGH Q33. A1. Splice.
A2. Poor fiber end preparation and poor fiber alignment. A3.
A4. Yes.
A5. A step change in refractive index that occurs at fiber joints, caused by fiber separation. A6. Index matching gel.
A7. Fiber separation (longitudinal misalignment), lateral misalignment, and angular misalignment. A8. Angular misalignment.
A9. (a) Reduces coupling loss, (b) does not change coupling loss, and (c) increases coupling loss. A10. Single mode.
A11. Be flat, smooth, and perpendicular to the fiber axis. A12. Score-and-break.
A13. Flat.
A14. Core diameter mismatch, cladding diameter mismatch, core ellipticity, core and cladding concentricity differences, NA mismatch, and refractive index profile differences.
A15. No.
A16. A permanent fiber joint whose purpose is to establish an optical connection between two individual optical fibers.
A17. Mechanical and fusion splicing.
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A18. An epoxy resin that seals mechanical splices and provides index matching between the connected fibers.
A19. It is a low-loss mechanical splice that provides stable environmental and mechanical performance in the Navy environment, and it requires only a small amount of training.
A20. The angles and quality of the two fiber-end faces. A21. The small size of the fusion splice and the development of automated fusion-splicing machines. A22. Prefusion. A23. Single mode fibers. A24. Poor fiber alignment and end preparation, fiber mismatches, and Fresnel reflection. A25. Fiber alignment.
A26. Modal and reflection. A27. Butt-jointed connectors. A28. Butt-jointed connectors. A29. Expanded beam connectors.
A30. Light-duty and heavy-duty connectors.
A31. Passive couplers redistribute optical signals without optical-to-electrical conversion.
A32. T-coupler.
A33. A fiber optic coupler that prevents the transfer of power between input fibers.
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Upon completion of this chapter, you should be able to do the following:
1. Identify the prime reasons for conducting fiber optic manufacturing laboratory and field measurements.
2. Describe the optical fiber and optical connection laboratory measurements performed by the Navy to evaluate fiber optic component and system performance.
3. Describe the near-field and far-field optical power distribution of an optical fiber.
4. Describe optical fiber launch conditions and modal effects that affect optical fiber and optical connection measurements.
5. Understand the term optical time-domain reflectometry and the interpretation of an optical time- domain reflectometer (OTDR) trace.
6. Describe the procedure for locating a fiber fault using an OTDR.
Fiber optic data links operate reliably if fiber optic component manufacturers and end users perform the necessary laboratory and field measurements. Manufacturers must test how component designs, material properties, and fabrication techniques affect the performance of fiber optic components. These tests can be categorized as design tests or quality control tests. Design tests are conducted during the development of a component. Design tests characterize the component's performance (optical, mechanical, and environmental) in the intended application. Once the component performance is characterized, the manufacturer generally only conducts quality control tests. Quality control tests verify that the parts produced are the same as the parts the design tests were conducted on. When manufacturers ship fiber optic components, they provide quality control data detailing the results of measurements performed during or after component fabrication.
End users (equipment manufacturers, shipbuilders, maintenance personnel, test personnel, and so on) should measure some of these parameters upon receipt before installing the component into the fiber optic data link. These tests determine if the component has been damaged in the shipping process. In addition, end users should measure some component parameters after installing or repairing fiber optic components in the field. The values obtained can be compared to the system installation specifications. These measurements determine if the installation or repair process has degraded component performance and will affect data link operation.
Whenever a measurement is made, it should be made using a standard measurement procedure. For most fiber optic measurements, these standard procedures are documented by the Electronics Industries Association/Telecommunications Industries Association (EIA/TIA). Each component measurement
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procedure is assigned a unique number given by EIA/TIA-455-X. The X is a sequential number assigned to that particular component test procedure. System level test procedures are assigned unique numbers given by EIA/TIA-526-X. Again the X is a sequential number assigned to that particular system test procedure.
Providing a complete description of every laboratory measurement performed by manufacturers and end users is impossible. This chapter only provides descriptions of optical fiber and optical connection measurements that are important to system operation. The list of optical fiber and optical connection laboratory measurements described in this chapter includes the following:
End users routinely perform optical fiber measurements to measure fiber power loss and fiber information capacity. End users may also perform optical fiber measurements to measure fiber geometrical properties. Optical fiber power loss measurements include attenuation and cutoff wavelength. Optical fiber information capacity measurements include chromatic dispersion and bandwidth. Fiber geometrical measurements include cladding diameter, core diameter, numerical aperture, and mode field diameter. Optical connection measurements performed by end users in the laboratory include insertion loss and reflectance or return loss.
Q1. List the fiber geometrical measurements performed in the laboratory. Attenuation
Attenuation is the loss of optical power as light travels along the fiber. It is a result of absorption, scattering, bending, and other loss mechanisms as described in chapter 3. Each loss mechanism contributes to the total amount of fiber attenuation.
End users measure the total attenuation of a fiber at the operating wavelength (l ). The total attenuation (A) between an arbitrary point X and point Y located on the fiber is
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Px is the power output at point X. P y is the power output at point Y. Point X is assumed to be closer to the optical source than point Y. The total amount of attenuation will vary with changes in wavelength l .
L is the distance between points X and Y. a is a positive number because Px is always larger than Py. The attenuation coefficient will also vary with changes in l .
CUTBACK METHOD.—In laboratory situations, end users perform the cutback method for measuring the total attenuation of an optical fiber. The cutback method involves comparing the optical power transmitted through a long piece of test fiber to the power present at the beginning of the fiber.
The cutback method for measuring multimode fiber attenuation is EIA/TIA-455-46. The cutback method for measuring single mode fiber attenuation is EIA/TIA-455-78. The basic measurement process is the same for both of these procedures. The test method requires that the test fiber of known length (L) be cut back to an approximate 2-m length. This cut back causes the destruction of 2-m of fiber. This method requires access to both fiber ends. Each fiber end should be properly prepared to make measurements. EIA/TIA-455-57 describes how to properly prepare fiber ends for measurement purposes.
Figure 5-1 illustrates the cutback method for measuring fiber attenuation. The cutback method begins by measuring, with an optical power meter, the output power P1 of the test fiber of known length (L) (figure 5-1, view A). Without disturbing the input light conditions, the test fiber is cut back to an approximate 2-m length. The output power P2 of the shortened test fiber is then measured (figure 5-1, view B). The fiber attenuation AT and the attenuation coefficient a are then calculated.
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LAUNCH CONDITIONS.—Measurement personnel must pay attention to how optical power is launched into the fiber when measuring fiber attenuation. Different distributions of launch power (launch conditions) can result in different attenuation measurements. This is more of a problem with multimode fiber than single mode fiber. For single mode fiber, optical power must be launched only into the fundamental mode. This is accomplished using a mode filter on the fiber. For multimode fiber, the distribution of power among the modes of the fiber must be controlled. This is accomplished by controlling the launch spot size and angular distribution.
The launch spot size is the area of the fiber face illuminated by the light beam from the optical source. The diameter of the spot depends on the size of the optical source and the properties of the optical elements (lenses, and so on) between the source and the fiber end face. The angular distribution is the angular extent of the light beam from the optical source incident on the fiber end face. The launch angular distribution also depends on the size of the optical source and the properties of the optical elements between the optical source and the fiber end face.
Multimode optical fiber launch conditions are typically characterized as being underfilled or overfilled. An underfilled launch concentrates most of the optical power in the center of the fiber. An underfilled launch results when the launch spot size and angular distribution are smaller than that of the fiber core. Underfilling the fiber excites mainly low-order modes. Figure 5-2 illustrates an underfilled launch condition.
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Overfilling the fiber excites both low-order and high-order modes. Figure 5-3 illustrates an overfilled launch condition. An overfilled launch condition occurs when the launch spot size and angular distribution are larger than that of the fiber core. Incident light that falls outside the fiber core is lost. In addition, light that is incident at angles greater than the angle of acceptance of the fiber core is lost.
In attenuation measurements, cladding-mode strippers and mode filters eliminate the effects that high-order modes have on attenuation results. A cladding-mode stripper is a device that removes any cladding mode power from the fiber. Most cladding-mode strippers consist of a material with a refractive
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index greater than that of the fiber cladding. For most fibers, the fiber coating acts as an excellent cladding-mode stripper.
Amode filter is a device that attenuates specific modes propagating in the core of an optical fiber. Mode filters generally involve wrapping the test fiber around a mandrel. For multimode, tight bends tend to remove high-order modes from the fiber. This type of mode filter is known as amandrel wrap mode filter. For multimode fibers, mode filters remove high-order propagating modes and are individually tailored and adjusted for a specific fiber type.
For single mode fibers, a mode filter is used to eliminate the second-order mode from propagating along the fiber. The propagation of the second-order mode will affect attenuation measurements. Fiber attenuation caused by the second-order mode depends on the operating wavelength, the fiber bend radius and length.
The two most common types of mode filters are free-form loops and mandrel wraps. Figure 5-4 illustrates the free-form loop and mandrel-wrap types of mode filters. Mandrel wraps for multimode fibers consist of several wraps (approximately 4 or 5) around a mandrel. A 20-mm diameter mandrel is typically used for 62.5 m m fiber. Mandrel wraps for single mode fibers consist of a single wrap around a 30-mm diameter mandrel. Another common mode filter for single mode fibers is a 30-mm diameter circular free-form loop. Additional information on multimode and single mode filters (and launch conditions) is available in EIA/TIA-455-50 and EIA/TIA-455-77, respectively.
Launch conditions significantly affect the results of multimode fiber attenuation measurements. If the fiber is underfilled, high-order-mode power loss has minimal effect on the measurement results. If too much power is launched into high-order modes, the high-order-mode power loss will dominate the attenuation results. Generally, fiber attenuation measurements are performed using an underfilled launch
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condition. Power in high-order modes is eliminated by either controlling the input spot size and angular distribution or using mode filters to remove high-order mode power.
Q2. End users measure the total attenuation of a fiber at the operating wavelength ( l ). Write the equation for total attenuation (A), between an arbitrary point X and point Y located on an optical fiber.
Q3. Will an optical fiber's attenuation coefficient vary with changes in wavelength? Q4. What two properties of the launch condition may affect multimode fiber attenuation
measurements?
Q5. Does underfilling a multimode optical fiber excite mainly high-order or low-order modes? Q6. Multimode optical fiber launch conditions are typically characterized as being overfilled or
underfilled. Which of these optical launch conditions exists if the launch spot size and angular distribution are larger than that of the fiber core?
Q7. A mode filter is a device that attenuates specific modes propagating in the core of an optical fiber. What mode propagating along single mode fibers do mode filters eliminate?
Q8. What are the two most common types of mode filters? Cutoff Wavelength
The wavelength at which a mode ceases to propagate is called the cutoff wavelength for that mode. However, an optical fiber is always able to propagate at least one mode, the fundamental mode. The fundamental mode can never be cut off. The cutoff wavelength of a single mode fiber is the wavelength above which the fiber propagates only the fundamental mode.
Determining the cutoff wavelength of a single mode fiber involves finding the wavelength above which the power transmitted through the fiber decreased abruptly. This power decrease occurs when the second-order mode propagating in the fiber is cut off. The cutoff wavelength of single mode fibers depends on the fiber length and bend conditions. The effects of length and bending are different on different fibers depending on whether they are matched-clad or depressed-clad in design. The cutoff wavelength of matched-clad fibers is more sensitive to bends than the cutoff wavelength of depressed- clad fibers. The cutoff wavelength of depressed-clad fibers is more sensitive to length than the cutoff wavelength of matched-clad fibers.
Cutoff wavelength may be measured on uncabled or cabled single mode fibers. A slightly different procedure is used in each case, but the basic measurement process is the same. The test method for uncabled single mode fiber cutoff wavelength is EIA/TIA-455-80. The test method for cabled single mode fiber cutoff wavelength is EIA/TIA-455-170. The fiber cutoff wavelength (l ) measured under EIA/TIA-455-80 will generally be higher than the cable cutoff wavelength ( l ) measured under EIA/TIA- 455-170. The difference is due to the fiber bends introduced during the cable manufacturing process.
Each test method describes the test equipment (input optics, mode filters, and cladding-mode strippers) necessary for the test. Cutoff wavelength measurements require an overfilled launch over the full range of test wavelengths. Since the procedures for measuring the cutoff wavelength of uncabled and cabled single mode fibers are essentially the same, only the test method for measuring the cutoff wavelength of uncabled fiber is discussed.
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Measuring the cutoff wavelength involves comparing the transmitted power from the test fiber with that of a reference fiber at different wavelengths. The reference fiber can be the same piece of single mode fiber with small bends introduced or a piece of multimode fiber. If the same fiber with small bends is used as the reference fiber, the technique is called the bend-reference technique. If a piece of multimode fiber is used as the reference fiber, the technique is called the multimode-reference technique.
For both techniques, the test fiber is loosely supported in a single-turn with a constant radius of 140 mm. Figure 5-5 shows this single-turn configuration. The transmitted signal power Ps ( l ) is then recorded while scanning the wavelength range in increments of 10 nm or less. The launch and detection conditions are not changed while scanning over the range of wavelengths. The wavelength range scanned encompasses the expected cutoff wavelength.
The reference power measurement is then made. For the bend-reference technique, the launch and detection conditions are not changed, but an additional bend is added to the test fiber. The test fiber is bent to a radius of 30 mm or less to suppress the second-order mode at all the scanned wavelengths. For the multimode-reference technique, the single mode fiber is replaced with a 2-m length of multimode fiber. The transmitted signal power Pr ( l ) is recorded while scanning the same wavelength range in the same increments of 10 nm or less. The attenuation A(l ) at each wavelength is calculated as follows:
Figure 5-6 shows an example attenuation plot generated using the bend-reference technique. The longest wavelength at which A( l ) is equal to 0.1 dB is the fiber cutoff wavelength (l cf). l cf is marked on figure 5-6.
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Figure 5-7 shows an attenuation plot generated using the multimode-reference technique. A straight line is fitted to the long-wavelength portion of A( l ). This straight line is then displaced upward by 0.1 dB. The point at which the straight line intersects the A( l ) plot defines the fiber cutoff wavelength ( l cf).
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Dispersion reduces the bandwidth, or information-carrying capacity, of an optical fiber. Dispersion causes the spreading of the light pulse as it travels along the fiber (see figure 2-20). Fiber dispersion mechanisms include intramodal (chromatic) dispersion and intermodal (modal) dispersion. Multimode fiber bandwidth is a measure of the intermodal dispersion of the multimode fiber.
Intermodal dispersion is maximum when all fiber modes are excited. The source used for intermodal dispersion measurements must overfill the fiber. The optical source must also have a narrow spectral width to reduce the effects of chromatic dispersion in the measurement.
There are two basic techniques for measuring the modal bandwidth of an optical fiber. The first technique characterizes dispersion by measuring the impulse response h(t) of the fiber in the time domain. The second technique characterizes modal dispersion by measuring the baseband frequency response H(f) of the fiber in the frequency domain. H(f) is the power transfer function of the fiber at the baseband frequency (f). H(f) is also the Fourier transform of the power impulse response h(t). Only the frequency response method is described here.
The test method for measuring the bandwidth of multimode fibers in the frequency domain is EIA/TIA-455-30. Signals of varying frequencies (f) are launched into the test fiber and the power exiting the fiber at the launched fundamental frequency measured. This optical output power is denoted as Pout(f). The test fiber is then cut back or replaced with a short length of fiber of the same type. Signals of the same frequency are launched into the cut-back fiber and the power exiting the cut-back fiber at the launched fundamental frequency measured. The optical power exiting the cutback or replacement fiber is denoted as Pin (f). The magnitude of the optical fiber frequency response is
The fiber bandwidth is defined as the lowest frequency at which the magnitude of the fiber frequency response has decreased to one-half its zero-frequency value. This is the -3 decibel (dB) optical power frequency (f3dB). This frequency is referred to as the fiber bandwidth.
Bandwidth is normally given in units of megahertz-kilometers (MHz-km). Converting the - 3 dB fiber bandwidth to a unit length assists in the analysis and comparison of optical fiber performance. For long lengths of fiber (>1km), the method for normalization is to multiply the length times the measured bandwidth.
Q9. The cutoff wavelength of matched-clad and depressed-clad single mode fibers varies according to the fiber's radius of curvature and length. The cutoff wavelength of which single mode fiber type is more sensitive to length?
Q10. Will the cutoff wavelength of uncabled fibers (l cf) generally have a value higher or lower than the cutoff wavelength of cabled fibers (l cc)?
Q11. Describe the -3 decibel (dB) optical power frequency (f3dB). Chromatic Dispersion
Chromatic, or intramodal, dispersion occurs in both single mode and multimode optical fibers. Chromatic dispersion occurs because different colors of light travel through the fiber at different speeds.
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Since the different colors of light have different velocities, some colors arrive at the fiber end before others. This delay difference is called the differential group delay t ( l ) per unit length. This differential group delay leads to pulse broadening.
Chromatic dispersion is measured using EIA/TIA-455-168 in the time domain. Chromatic dispersion is also measured in the frequency domain using EIA/TIA-455-169 and EIA/TIA-455-175. These methods measure the composite optical fiber material and waveguide dispersion. To understand the contribution that material and waveguide dispersive mechanisms have on multimode and single mode fiber dispersion, refer to chapter 2. In this chapter we limit the discussion on chromatic dispersion to the time domain method described in EIA/TIA-455-168.
The chromatic dispersion of multimode graded-index and single mode fiber is obtained by measuring fiber group delays in the time domain. These measurements are made using multiwavelength sources or multiple sources of different wavelengths. A multiwavelength source could be a wavelength-selectable laser.
The pulse delay for both a long test sample fiber and a short reference fiber are measured over a range of wavelengths. The pulse delay for the reference fiber as a function of wavelength is t in( l ). The pulse delay for the test fiber as a function of wavelength is t out( l ). The group delay t ( l ). per unit length at each wavelength is
where Ls is the test sample fiber length in kilometers (km) and Lref is the reference sample length in km. The fiber chromatic dispersion is defined as the derivative, or slope, of the fiber group delay curve
with respect to wavelength. Generally, the group delay as a function of wavelength is fit to a simple mathematical function and the derivative calculated. The range of wavelengths over which meaningful data is obtained depends on the wavelength range of optical source(s) used. The zero-dispersion wavelength ( l 0) and the zero-dispersion slope (S0) are determined from the chromatic dispersion curve. Q12. Delay differences between the source wavelengths occur as the optical signal propagates along
the fiber. What is this called? Q13. What determines the range of wavelengths over which meaningful data is obtained for
calculating the chromatic dispersion?
End users perform fiber geometry measurements to reduce system attenuation and coupling loss resulting from poor fiber fabrication. Fiber attenuation and intrinsic coupling loss result from mismatches in the inherent fiber characteristics of two connecting fibers. Fiber mismatches occur when manufacturers fail to maintain optical or structural (geometrical) tolerances during the fiber fabrication process. Fiber geometry measurements performed in the laboratory identify fiber mismatches before the optical fiber is installed.
The procedure for measuring multimode and single mode fiber geometry is detailed in EIA/TIA-455- 176. The fiber-geometrical parameters measured include cladding diameter, cladding noncircularity, core- cladding concentricity error, and core noncircularity. Figure 4-8 (chapter 4) illustrates core noncircularity
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(ellipticity) and core-cladding concentricity error. The core noncircularity measurement is for multimode fibers only.
Other test methods are available for measuring other multimode and single mode fiber core parameters. Additional test methods exist for measuring multimode fiber core diameter and NA. For single mode fibers, the mode field diameter measurement replaces core diameter and NA measurements. Core diameter, numerical aperture, and mode field diameter measurements are identified and explained later in this chapter.
To make fiber geometry measurements, the input end of the fiber is overfilled and any cladding power stripped out. The output end of the fiber is prepared and viewed with a video camera. Generally the fiber is less than 10 m in length. An objective lens magnifies the output image (typically 20 · ) going to a video camera. The image from the video camera is displayed on a video monitor and is also sent to the computer for digital analysis.
The computer analyzes the image to identify the edges of the core and cladding. The centers rc and rg of the core and cladding, respectively, are found. The cladding diameter is defined as the average diameter of the cladding. The cladding diameter is twice the average radius (Rg). The core diameter is defined as the average diameter of the core. The core diameter is twice the average core radius (Rc).
Cladding noncircularity, or ellipticity, is the difference between the smallest radius of the fiber (Rgmin) and the largest radius (Rgmax) divided by the average cladding radius (Rg). The value of the cladding noncircularity is expressed as a percentage.
The core-cladding concentricity error for multimode fibers is the distance between the core and cladding centers divided by the core diameter. Multimode core-cladding concentricity error is expressed as a percentage of core diameter. The core-cladding concentricity error for single mode fibers is defined as the distance between the core and cladding centers.
Core noncircularity is the difference between the smallest core radius (Rcmin) and the largest core radius (Rcmax) divided by the core radius (Rc). The value of core noncircularity is expressed as a percentage. Core noncircularity is measured on multimode fibers only.
Q14. Why do end users perform fiber geometry measurements in the laboratory? Q15. Define cladding diameter. Q16. Explain the difference between multimode and single mode core-cladding concentricity errors.
Core diameter is measured using EIA/TIA-455-58. The core diameter is defined from the refractive index profile n(r) or the output near-field radiation pattern. Our discussion is limited to measuring the core diameter directly from the output near-field radiation pattern obtained using EIA/TIA-455-43.
The near-field power distribution is defined as the emitted power per unit area (radiance) for each position in the plane of the emitting surface. For this chapter, the emitting surface is the output area of a fiber-end face. Near-field power distributions describe the emitted power per unit area in the near-field region. The near-field region is the region close to the fiber-end face. In the near-field region, the distance between the fiber-end face and detector is in the micrometers ( m m) range.
EIA/TIA-455-43 describes the procedure for measuring the near-field power distribution of optical waveguides. Output optics, such as lenses, magnify the fiber-end face and focus the fiber's image on a
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movable detector. Figure 5-8 shows an example setup for measuring the near-field power distribution. The image is scanned in a plane by the movable detector. The image may also be scanned by using a detector array. Detector arrays of known element size and spacing may provide a display of the power distribution on a video monitor. A record of the near-field power is kept as a function of scan position.
The core diameter (D) is derived from the normalized output near-field radiation pattern. The normalized near-field pattern is plotted as a function of radial position on the fiber-end face. Figure 5-9 shows a plot of the normalized near-field radiation pattern as a function of scan position.
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The core diameter (D) is defined as the diameter at which the intensity is 2.5 percent of the maximum intensity (see figure 5-9). The 2.5 percent points, or the 0.025 level, intersects the normalized curve at radial positions - a and a. The core diameter is simply equal to 2a (D=2a). Q17. Near-field power distributions describe the emitted power per unit area in the near-field region.
Describe the near-field region. Q18. How is the core diameter defined?
The numerical aperture (NA) is a measurement of the ability of an optical fiber to capture light. The NA can be defined from the refractive index profile or the output far-field radiation pattern. Our discussion is limited to measuring the NA from the output far-field radiation pattern.
The NA of a multimode fiber having a near-parabolic refractive index profile is measured using EIA/TIA-455-177. In EIA/TIA-455-177, the fiber NA is measured from the output far-field radiation pattern. The far-field power distribution describes the emitted power per unit area in the far-field region. The far-field region is the region far from the fiber-end face. The far-field power distribution describes the emitted power per unit area as a function of angle ÿ some distance away from the fiber-end face. The distance between the fiber-end face and detector in the far-field region is in the centimeters (cm) range for multimode fibers and millimeters (mm) range for single mode fibers.
EIA/TIA-455-47 describes various procedures, or methods, for measuring the far-field power distribution of optical waveguides. These procedures involve either an angular or spacial scan. Figure 5-10 illustrates an angular and spacial scan for measuring the far-field power distribution.
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Figure 5-10 (method A) illustrates a far-field angular scan of the fiber-end face by a rotating detector. The fiber output radiation pattern is scanned by a rotating detector in the far-field. The detector rotates in a spherical manner. A record of the far-field power distribution is kept as a function of angle ÿ.
 In a far-field spacial scan, lens L1 performs a Fourier transform of the fiber output near-field pattern. A second lens, L2, is positioned to magnify and relay the transformed image to the detector plane. The image is scanned in a plane by a movable detector. The scan position y in the Fourier transform plane is proportional to the far-field scan angle ÿ. A record of the far-field power distribution is kept as a function of the far-field scan angle.
The normalized far-field pattern is plotted as a function of the far-field scan angle ÿ. Figure 5-11 shows the plot of the normalized far-field radiation pattern as a function of scan angle.
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Fiber NA is defined by the 5 percent intensity level, or the 0.05 intensity level, as indicated in figure 5-11. The 0.05 intensity level intersects the normalized curve at scan angles ÿ A and ÿ B. The fiber NA is defined as
where ÿ 5 is the 5 percent intensity half angle. ÿ 5 is determined from ÿ A and ÿ B as shown below:
Q19. Far-field power distributions describe the emitted power per unit area as a function of angle ÿ in the far-field region. Describe the far-field region.
The mode field diameter (MFD) of a single mode fiber is related to the spot size of the fundamental mode. This spot has a mode field radius w0. The mode field diameter is equal to 2w 0. The size of the mode field diameter correlates to the performance of the single mode fiber. Single mode fibers with large mode field diameters are more sensitive to fiber bending. Single mode fibers with small mode field diameters show higher coupling losses at connections.
The mode field diameter of a single mode fiber can be measured using EIA/TIA-455-167. This method involves measuring the output far-field power distribution of the single mode fiber using a set of
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apertures of various sizes. This far-field power distribution data is transformed into the near-field before using complex mathematical procedures. The mode field diameter is calculated from the transformed near field data. The mathematics behind the transformation between the far-field and near-field is too complicated for discussion in this chapter. Refer to EIA/TIA-455-167 for information on this transformation procedure.
Q20. Will fiber coupling loss generally increase or decrease if the mode field diameter of a single mode fiber is decreased?
Insertion loss is composed of the connection coupling loss and additional fiber losses in the fiber following the connection. In multimode fiber, fiber joints can increase fiber attenuation following the joint by disturbing the fiber's mode power distribution (MPD). Fiber joints may increase fiber attenuation because disturbing the MPD may excite radiative modes. Radiative modes are unbound modes that radiate out of the fiber contributing to joint loss. In single mode fibers, fiber joints can cause the second-order mode to propagate in the fiber following the joint. As long as the coupling loss of the connection is small, neither radiative modes (multimode fiber) or the second-order mode (single mode fiber) are excited.
Insertion loss of both multimode and single mode interconnection devices is measured using EIA/TIA-455-34. For Navy applications, an overfill launch condition is used at the input fiber. For other applications a mandrel wrap may be used to strip out high-order mode power. The length of fiber before the connection and after the connection may be specified for some applications. Power measurements are made on an optical fiber or fiber optic cable before the joint is inserted and after the joint is made. Figure 5-12 illustrates the mandrel wrap method of measuring the insertion loss of an interconnecting device in EIA/TIA-455-34.
Initial power measurements at the detector (P0) and at the source monitoring equipment (PM0) are taken before inserting the interconnecting device into the test setup. The test fiber is then cut at the location specified by the end user. The cut results in a fiber of lengths L1 and L2 before and after the interconnection device that simulates the actual system configuration. After interconnection, the power at
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the detector (P 1) and at the source monitoring equipment (PM1) is measured. The insertion loss is calculated as shown below:
If the source power is constant, then the calculation of the insertion loss is similar to that of fiber attenuation.
Q21. In multimode fibers, how do fiber joints increase fiber attenuation following the joint? Return Loss and Reflectance
Reflections occur at optical fiber connections. Optical power may be reflected back into the source fiber when connecting two optical fibers. In laser-based systems, reflected power reaching the optical source can reduce system performance by affecting the stability (operation) of the source. In addition, multiple reflections occur in fiber optic data links containing more than one connection. Multiple reflections can reduce data link performance by increasing the signal noise present at the optical detector.
Reflectance is a measure of the portion of incident light that is reflected back into the source fiber at the point of connection. Reflectance is given as a ratio (R) of the reflected light intensity to the incident light intensity. The reflectance ratio (R) for Fresnel reflection is discussed in chapter 4.
Return loss and reflectance are measured using EIA/TIA-455-107. They are measured using an optical source connected to one input of a 2 · 2 fiber optic coupler. Light is launched into the component under test through the fiber optic coupler. The light reflected from the component under test is transmitted back through the fiber optic coupler to a detector connected to the other input port. The optical power is measured at the output of the device under test (Po) and at the input port of the coupler where the detector is located (Pr). Po is corrected to account for the loss in power through the device under test. Pr is corrected to account for the loss in power through the coupler and any other connection losses in the path. The reflectance is then given by the ratio Pr/Po.
Return loss is the amount of loss of the reflected light compared with the power of the incident beam at the interface. The optical return loss at the fiber interface is defined as
Return loss is only the amount of optical power reflected and does not include power that is transmitted, absorbed, or scattered.
Q22. List two effects that reflections can have on a fiber optic data link. Q23. Reflectance is given as what ratio? Q24. Does return loss include power that is transmitted, absorbed, and/or scattered?
Field measurements differ from laboratory measurements because they measure the transmission properties of installed fiber optic components. Laboratory measurements can only attempt to simulate the actual operating conditions of installed components. Fiber optic component properties measured in the
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laboratory can change after the installation of these components on board ship. End users must perform field measurements to evaluate those properties most likely affected by the installation or repair of fiber optic components or systems.
The discussion on field measurements is limited to optical fiber and optical connection properties. Optical fiber and optical connection field measurements evaluate only the transmission properties affected by component or system installation or repair. Because optical fiber geometrical properties, such as core and cladding diameter and numerical aperture, are not expected to change, there is no need to remeasure these properties. The optical fiber properties that are likely to change include fiber attenuation (loss) and bandwidth. Bandwidth changes in the field tend to be beneficial, so field bandwidth measurement is generally not performed. If field bandwidth measurements are required, they are essentially the same as laboratory measurements so they will not be repeated. The optical connection properties that are likely to change are connection insertion loss and reflectance and return loss.
The installation and repair of fiber optic components on board ship can affect system operation. Microbends introduced during installation can increase fiber attenuation. Modal redistribution at fiber joints can increase fiber attenuation in the fiber after the joint. Fiber breaks or faults can prevent or severely disrupt system operation. Poor fiber connections can also increase insertion loss and degrade transmitter and receiver performance by increasing reflectance and return loss. End users should perform field measurements to verify that component performance is within allowable limits so system performance is not adversely affected.
There are additional differences in measuring optical fiber and optical connection properties in the field than in the laboratory. Field measurements require rugged, portable test equipment, unlike the sophisticated test equipment used in the laboratory. Field test equipment must provide accurate measurements in extreme environmental conditions. Since electrical power sources may not always be available in the field, test equipment should allow battery operation. In addition, while both fiber ends are available for conducting laboratory measurements, only one fiber end may be readily available for field measurements. Even if both fiber ends are available for field measurements, the fiber ends are normally located some distance apart. Therefore, field measurements may require two people.
The main field measurement technique involves optical time-domain reflectometry. An optical time- domain reflectometer (OTDR) is recommended for conducting field measurements on installed optical fibers or links of 50 meters or more in length. An OTDR requires access to only one fiber end. An OTDR measures the attenuation of installed optical fibers as a function of length. It also identifies and evaluates optical connection losses along a cable link and locates any fiber breaks or faults.
End users can also measure fiber attenuation and cable plant transmission loss using an optical power meter and a stabilized light source. End users use this measurement technique when optical time-domain reflectometry is not recommended. Measurements obtained with a stabilized light source and power meter are more accurate than those obtained with an OTDR. Measuring fiber attenuation and transmission loss using a power meter and light source requires access to both ends of the fiber or link. An optical loss test set (OLTS) combines the power meter and source functions into one physical unit. Q25. Is it essential for end users to remeasure optical fiber geometrical properties after installation in
the field? Q26. When is an OTDR recommended for conducting field measurements on installed optical fibers or
links?
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End users use optical time-domain reflectometry to characterize optical fiber and optical connection properties in the field. In optical time-domain reflectometry, an OTDR transmits an optical pulse through an installed optical fiber. The OTDR measures the fraction of light that is reflected back due to Rayleigh scattering and Fresnel reflection. By comparing the amount of light scattered back at different times, the OTDR can determine fiber and connection losses. When several fibers are connected to form an installed cable plant, the OTDR can characterize optical fiber and optical connection properties along the entire length of the cable plant. A fiber optic cable plant consists of optical fiber cables, connectors, splices, mounting panels, jumper cables, and other passive components. A cable plant does not include active components such as optical transmitters or receivers.
The OTDR displays the backscattered and reflected optical signal as a function of length. The OTDR plots half the power in decibels (dB) versus half the distance. Plotting half the power in dB and half the distance corrects for round trip effects. By analyzing the OTDR plot, or trace, end users can measure fiber attenuation and transmission loss between any two points along the cable plant. End users can also measure insertion loss and reflectance of any optical connection. In addition, end users use the OTDR trace to locate fiber breaks or faults.
Figure 5-13 shows an example OTDR trace of an installed cable plant. OTDR traces can have several common characteristics. An OTDR trace begins with an initial input pulse. This pulse is a result of Fresnel reflection occurring at the connection to the OTDR. Following this pulse, the OTDR trace is a gradual downsloping curve interrupted by abrupt shifts. Periods of gradual decline in the OTDR trace result from Rayleigh scattering as light travels along each fiber section of the cable plant. Periods of gradual decline are interrupted by abrupt shifts called point defects. A point defect is a temporary or permanent local deviation of the OTDR signal in the upward or downward direction. Point defects are caused by connectors, splices, or breaks along the fiber length. Point defects, or faults, can be reflective or nonreflective. An output pulse at the end of the OTDR trace indicates the end of the fiber cable plant. This output pulse results from Fresnel reflection occurring at the output fiber-end face.
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ATTENUATION.—The fiber optic test method for measuring the attenuation of an installed optical fiber using an OTDR is EIA/TIA-455-61. The accuracy of this test method depends on the user entering the appropriate source wavelength, pulse duration, and fiber length (test range) into the OTDR. In addition, the effective group index of the test fiber is required before the attenuation coefficient and accurate distances can be recorded. The group index (N) is provided by fiber manufacturers or is found using EIA/TIA-455-60. By entering correct test parameters, OTDR fiber attenuation values will closely coincide with those measured by the cutback technique.
Test personnel can connect the test fiber directly to the OTDR or to a dead-zone fiber. This dead- zone fiber is placed between the test fiber and OTDR to reduce the effect of the initial reflection at the OTDR on the fiber measurement. The dead-zone fiber is inserted because minimizing the reflection at a fiber joint is easier than reducing the reflection at the OTDR connection.
Figure 5-14 illustrates the OTDR measurement points for measuring the attenuation of the test fiber using a dead-zone fiber. Fiber attenuation between two points along the test fiber is measured on gradual downsloping sections on the OTDR trace. There should be no point defects present along the portion of fiber being tested.
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OTDRs are equipped with either manual or automatic cursors to locate points of interest along the trace. In figure 5-14, a cursor is positioned at a distance zo on the rising edge of the reflection at the end of the dead-zone fiber. Cursors are also positioned at distances z1 and z2. The cursor positioned at z1 is just beyond the recovery from the reflection at the end of the dead-zone fiber. Since no point defects are present in figure 5-14, the cursor positioned at z2 locates the end of the test fiber. Cursor z2 is positioned just before the output pulse resulting from Fresnel reflection occurring at the end of the test fiber.
The attenuation of the test fiber between points z1 and z2 is (P1 - P 2) dB. The attenuation coefficient ( a ) is
If fiber attenuation is measured without a dead-zone fiber, z 0 is equal to zero (z0 = 0). At any point along the length of fiber, attenuation values can change depending on the amount of
optical power backscattered due to Rayleigh scattering. The amount of backscattered optical power at each point depends on the forward optical power and its backscatter capture coefficient. The backscatter capture coefficient varies with length depending on fiber properties. Fiber properties that may affect the backscatter coefficient include the refractive index profile, numerical aperture (multimode), and mode-field diameter (single mode) at the particular measurement point. The source wavelength and pulse width may also affect the amount of backscattered power.
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By performing the OTDR attenuation measurement in each direction along the test fiber, test personnel can eliminate the effects of backscatter variations. Attenuation measurements made in the opposite direction at the same wavelength (within 5 nm) are averaged to reduce the effect of backscatter variations. This process is called bidirectional averaging. Bidirectional averaging is possible only if test personnel have access to both fiber ends. OTDR attenuation values obtained using bidirectional averaging should compare with those measured using the cutback technique in the laboratory.
POINT DEFECTS.—Point defects are temporary or local deviations of the OTDR signal in the upward or downward direction. A point defect, or fault, can be reflective or nonreflective. A point defect normally exhibits a loss of optical power. However, a point defect may exhibit an apparent power gain. In some cases, a point defect can even exhibit no loss or gain. Refer back to figure 5-13; it illustrates a reflective fault and a nonreflective fault, both exhibiting loss. Figure 5-15 shows a nonreflective fault with apparent gain and a reflective fault with no apparent loss or gain.
Point defects are located and measured using EIA/TIA-455-59. Test personnel must enter the appropriate input parameters including the source wavelength, the pulse duration, and the fiber or cable group index into the OTDR. The nature of fiber point defects depends on the value of each parameter entered by the end user. The pulse duration usually limits the length of the point defect while other input parameters, such as the wavelength, can vary its shape.
If the length of the fiber point defect changes with the pulse duration, then the OTDR signal deviation is in fact a point defect. If the length remains the same, then the OTDR signal deviation is a region of high fiber attenuation. Regions of high fiber attenuation are referred to as attenuation non-uniformities.
Fiber point defects occur from factory fiber splices or bends introduced during cable construction or installation. For shipboard applications, manufacturers are not allowed to splice fibers during cable construction. Fiber joints are natural sources of OTDR point defects. However, fiber breaks, cracks, or microbends introduced during cable installation are additional sources of point defects.
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Point defects that occur at fiber joints are relatively easy to identify because the location of a fiber joint is generally known. A reflective or nonreflective fault occurs at a distance equal to fiber joint location. In most circumstances, an optical connector produces a reflective fault, while an optical splice produces a nonreflective fault.
Reflective and nonreflective faults occurring at distances other than fiber joint locations identify fiber breaks, cracks, or microbends. A fiber break produces a reflective fault because fiber breaks result in complete fiber separation. Fiber cracks and microbends generally produce nonreflective faults.
A point defect may exhibit apparent gain because the backscatter coefficient of the fiber present before the point defect is higher than that of the fiber present after. Test personnel measure the signal loss or gain by positioning a pair of cursors, one on each side of the point defect. Figure 5-16 illustrates the positioning of the cursors for a point defect showing an apparent signal gain. The trace after the point defect is extrapolated as shown in figure 5-16. The vertical distance between the two lines in figure 5-16 is the apparent gain of the point defect.
Point defects exhibiting gain in one direction will exhibit an exaggerated loss in the opposite direction. Figure 5-17 shows the apparent loss shown by the OTDR for the same point defect shown in figure 5-16 when measured in the opposite direction. Bidirectional measurements are conducted to cancel the effects of backscatter coefficient variations. Bidirectional averaging combines the two values to identify the true signal loss. Bidirectional averaging is possible only if test personnel have access to both ends of the test sample.
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OTDRs can also measure the return loss of a point defect. However, not all OTDRs are configured to make the measurement. To measure the return loss of a point defect, the cursors are placed in the same places as for measuring the loss of the point defect. The return loss of the point defect is displayed when the return loss option is selected on the OTDR. The steps for selecting the return loss option depend upon the OTDR being used.
Q27. An OTDR measures the fraction of light that is reflected back from the fiber or link under test. What causes light to be reflected back into the OTDR?
Q28. List the types of fiber optic components considered part of a fiber optic cable plant. Q29. What is a temporary or permanent local deviation of the OTDR signal in the upward or
downward direction called?
Q30. Why is a dead-zone fiber placed between the test fiber and OTDR when conducting attenuation measurements?
Q31. The amount of backscattered optical power at each point depends on what two properties? Q32. How can test personnel eliminate the effects of backscatter variations? Q33. If the length of the fiber point defect changes with pulse duration, is the OTDR signal deviation a
point defect or a region of high fiber attenuation? Q34. Give the type of fault (reflective or nonreflective) normally produced by: (a) fiber breaks, (b)
fiber cracks, and (c) fiber microbends. Q35. Explain how a point defect may exhibit an apparent gain. Q36. A point defect exhibiting an apparent gain in one direction will exhibit what, when measured in
the opposite direction?
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Test personnel also use an optical power meter and stabilized light source to measure fiber attenuation and transmission loss in the field. Optical power meter measurements are recommended when the length of an installed optical fiber cable or cable plant is less than 50 meters. A test jumper is used to couple light from the stabilized source to one end of the optical fiber (or cable plant) under test. An additional test jumper is also used to connect the other end of the optical fiber (or cable plant) under test to the power meter. Optical power meter measurements may be conducted using an optical loss test set (OLTS). An OLTS combines the power meter and source functions into one physical unit. When making measurements, it does not matter whether the stabilized source and power meter are in one physical unit or two.
Power meter measurements are conducted on individual optical fiber cables installed on board ship. The installed optical fiber cable must have connectors or terminations on both ends to make the measurement. If the installed optical fiber cable does not have connectors or terminations on both ends, an OTDR should be used to evaluate the cable. If the cable is too short for evaluation with an OTDR, cable continuity can be verified using a flashlight.
Power meter measurements for cable assembly link loss require that test personnel clean all optical connections at test jumper interfaces before performing any measurement. Test personnel should use cotton wipes dampened with alcohol to clean connectors and blow dry before making connections. End users should also ensure that test equipment calibration is current.
Power meter measurements connecting a test reference cable between the light source and power meter. The test reference cable has the same nominal fiber characteristics as the cable under test. The optical power present at the power meter is the reference power (P1). Disconnect the test reference cable and connect the optical fiber cable under test between the light source and power meter using test jumpers. If possible, the test reference cable should be used as the input jumper cable for the test cable measurement. The test jumper fiber properties, such as core diameter and NA, should be nominally equal to the fiber properties of the cable being tested. The optical power present at the power meter is test power (P2).
Test personnel use P1 and P2 to calculate the cable assembly link loss. The cable assembly link loss (BCA) of optical fiber installed with connectors or terminations on both ends is
The cable assembly link loss should always be less than the specified link loss for that particular link.
Besides measuring individual cables, test personnel measure the transmission loss of installed fiber optic cable plants. The transmission loss of fiber optic cable plants is measured using EIA/TIA-526-14 method B (multimode fiber) or EIA/TIA-526-7 (single mode fiber). The procedure measures the internal loss of the cable plant between points A and B, plus two connection losses. Figure 5-18 (A) illustrates the method described in EIA/TIA-526-14 method B for measuring the reference power (P1). Figure 5-18 (B) shows the final test configuration for measuring the cable plant test power (P2).
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The procedure is exactly the same as described for measuring the link loss of an individual cable assembly. The total optical loss between any two termination points, including the end terminations, of the optical fiber cable plant link is measured. The measured cable plant link loss should always be less than the specified cable plant link loss.
Test personnel should conduct cable assembly link loss, and cable plant transmission loss measurements in both directions and at each system operational wavelength. By performing these measurements in each direction, test personnel can better characterize cable and link losses. Unlike optical time-domain reflectometry, bidirectional readings are always possible when performing power meter measurements. In power meter measurements, by definition, end users have access to both ends of the cable or cable plant.
Q37. When is an optical power meter measurement recommended for conducting field measurements on installed optical fiber cables or cable plants?
Q38. If an installed optical fiber cable does not have connectors or terminations on both ends, how should the cable be tested?
Now that you have completed this chapter, let's review some of the new terms, concepts, and ideas that you have learned. You should have a thorough understanding of these principles before moving on to chapter 6.
END USERS (equipment manufacturers, shipbuilders, maintenance personnel, test personnel, and so on) should measure some component parameters upon receipt before installing the component into the fiber optic data link. In addition, they should measure some component parameters after installing or repairing fiber optic components in the field.
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chromatic dispersion, fiber geometry, core diameter, numerical aperture (multimode), mode field diameter (single mode), insertion loss, and reflectance and return loss.
ATTENUATION is the loss of optical power as light travels along the fiber. It is a result of absorption, scattering, bending, and other loss mechanisms.
The LAUNCH SPOT SIZE and the ANGULAR DISTRIBUTION may affect multimode fiber attenuation measurement results by affecting modal distributions.
An UNDERFILLED launch results when the launch spot size and angular distribution are smaller than that of the fiber core.
An OVERFILLED launch condition occurs when the launch spot size and angular distribution are larger than that of the fiber core.
A CLADDING-MODE STRIPPER is a device that removes any cladding mode power from the fiber.
AMODE FILTER is a device that attenuates a specific mode or modes propagating in the core of an optical fiber.
The CUTOFF WAVELENGTH of a single mode fiber is the wavelength above which the fiber propagates only the fundamental mode. The cutoff wavelength of a single mode fiber varies according to the fiber's radius of curvature and length. The fiber cutoff wavelength ( l cf) will generally be higher than the cable cutoff wavelength ( l cc).
PULSE DISTORTION is the spreading of the light pulse as it travels along the fiber caused by dispersion. It reduces the bandwidth, or information-carrying capacity, of an optical fiber.
Two BASIC TECHNIQUES are used for measuring the modal bandwidth of an optical fiber. The first characterizes dispersion by measuring the IMPULSE RESPONSE h(t) of the fiber in the time domain. The second characterizes modal dispersion by measuring the BASEBAND FREQUENCY RESPONSE H(f) of the fiber in the frequency domain.
The LOWEST FREQUENCY at which the magnitude of the fiber frequency response has decreased to one half its zero-frequency value is the -3 decibel (dB) optical power frequency (f3dB).
CHROMATIC DISPERSION occurs because different colors of light travel through the fiber at different speeds. Since the different colors of light have different velocities, some colors arrive at the fiber end before others.