The DIFFERENTIAL GROUP DELAY t ( l ) is the variation in propagation delay that occurs because of the different group velocities of each wavelength in an optical fiber.
The RANGE OFWAVELENGTHS over which meaningful chromatic dispersion data is obtained depends on the wavelength range of optical source(s) used.
FIBER GEOMETRY MEASUREMENTS are performed by end users to reduce system attenuation and coupling loss resulting from poor fiber fabrication.
The CLADDING DIAMETER is the average diameter of the cladding.
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CLADDING NONCIRCULARITY, or ellipticity, is the difference between the smallest radius of the fiber (Rgmin) and the largest radius of the fiber (Rgmax) divided by the average cladding radius (Rg).
The CORE-CLADDING CONCENTRICITY ERROR for multimode fibers is the distance between the core and cladding centers divided by the 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 radius of the core (Rcmin) and the largest radius of the core (Rcmax) divided by the core radius (Rc).
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.
The NEAR-FIELD REGION is the region close to the fiber-end face.
The CORE DIAMETER is derived from the normalized output near-field radiation pattern. The core diameter (D) is defined as the diameter at the 2.5 percent (0.025) level.
The NUMERICAL APERTURE (NA) is a measurement of the ability of a multimode optical fiber to capture light.
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 FAR-FIELD REGION is the region far from the fiber-end face.
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.
INSERTION LOSS is composed of the connection coupling loss and additional fiber losses in the fiber following the connection.
REFLECTANCE is a measure of the portion of incident light that is reflected back into the source fiber at the point of connection.
RETURN LOSS is the amount of loss of the reflected light compared with the power of the incident beam at the interface.
OPTICAL FIBER and OPTICAL CONNECTION FIELD MEASUREMENTS measure only the transmission properties affected by component or system installation or repair.
OPTICAL TIME-DOMAIN REFLECTOMETRY is recommended for conducting field measurements on installed optical fibers or cable plants of 50 meters or more in length.
An OPTICAL LOSS TEST SET (OLTS) combines the power meter and source functions into one physical unit.
An OPTICAL TIME-DOMAIN REFLECTOMETER (OTDR) measures the fraction of light that is reflected back because of Rayleigh scattering and Fresnel reflection.
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.
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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 fiber breaks. Point defects, or faults, can be reflective or nonreflective.
A DEAD-ZONE fiber is placed between the test fiber and OTDR to reduce the influence of the initial pulse resulting from Fresnel reflection at the OTDR connection.
The AMOUNT OF OPTICAL POWER BACKSCATTERED because of Rayleigh scattering at one point depends on the forward optical power and the fibers backscatter capture coefficient.
The EFFECTS OF BACKSCATTER VARIATIONS can be eliminated by test personnel by performing the OTDR attenuation measurement in each direction along the test fiber and averaging (bidirectional readings).
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.
ToMEASURE FIBER ATTENUATION and TRANSMISSION LOSS in the field, test personnel use an optical power meter and stabilized light source.
OPTICAL POWERMETERMEASUREMENTS are recommended when the length of an installed optical fiber cable or cable plant is less than 50 meters.
ANSWERS TO QUESTIONS Q1. THROUGH Q38. A1. Cladding diameter, core diameter, numerical aperture, and mode field diameter. A2.
A3. Yes.
A4. Launch spot size and angular distribution.
A5. Low-order modes.
A6. Overfilled. A7. Second-order mode.
A8. Free-form loop and mandrel wrap. A9. Depressed-clad.
A10. Higher.
A11. The -3 decibel (dB) is the lowest frequency at which the magnitude of the fiber frequency response has decreased to one half its zero-frequency value.
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A12. Differential group delay t (l ). A13. The wavelength range of the optical source(s) used. A14. To reduce system attenuation and coupling loss resulting from poor fiber fabrication. A15. The cladding diameter is the average diameter of the cladding. A16. Multimode core-cladding concentricity error is the distance between the core and cladding
centers expressed as a percentage of core diameter while the single mode core-cladding concentricity error is just the distance between the core and cladding centers.
A17. The near-field region is the region close to the fiber-end face. A18. The core diameter is defined as the diameter at which the near-field intensity is 2.5 percent of the
maximum intensity.
A19. The far-field region is the region far from the fiber-end face. A20. Increase.
A21. By disturbing the fiber's mode power distribution (MPD). A22. Reduce the stability of the system source and increase the signal noise present at the optical
detector.
A23. The ratio of reflected optical power to incident optical power. A24. No.
A25. No.
A26. When installed optical fiber cables or links are 50 meters or more in length. A27. Rayleigh scattering and Fresnel reflection. A28. Optical fiber cables, connectors, splices, mounting panels, jumper cables, and other passive
components.
A29. A point defect. A30. To reduce the effect of the initial reflection at the OTDR. A31. Forward optical power and backscatter capture coefficient. A32. By performing the OTDR attenuation measurements in each direction along the test fiber. A33. A point defect. A34. (a) Reflective, (b) nonreflective, and (c) nonreflective. A35. 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. A36. An exaggerated loss.
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A37. When an installed optical fiber cable or cable plant is less than 50 meters in length. A38. With an OTDR unless it is less than 50 meters in length. If it is less than 50 meters in length,
continuity should be verified with a flashlight.
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Upon completion of this chapter, you should be able to do the following:
1. Explain the principal properties of an optical source and fiber optic transmitter.
2. Discuss the optical emission properties of semiconductor light-emitting diodes (LEDs) and laser diodes (LDs).
3. Describe the operational differences between surface-emitting LEDs (SLEDs), edge-emitting LEDs (ELEDs), superluminescent diodes (SLDs), and laser diodes.
4. Describe typical fiber optic transmitter packages.
Chapter 1 taught you that a fiber optic data link has three basic functions. One function is that a fiber optic data link must convert an electrical signal to an optical signal permitting the transfer of data along an optical fiber. The fiber optic device responsible for that signal conversion is a fiber optic transmitter.
A fiber optic transmitter is a hybrid device. It converts electrical signals into optical signals and launches the optical signals into an optical fiber. A fiber optic transmitter consists of an interface circuit, a source drive circuit, and an optical source. The interface circuit accepts the incoming electrical signal and processes it to make it compatible with the source drive circuit. The source drive circuit intensity modulates the optical source by varying the current through the source.
An optical source converts electrical energy (current) into optical energy (light). Light emitted by an optical source is launched, or coupled, into an optical fiber for transmission. Fiber optic data link performance depends on the amount of optical power (light) launched into the optical fiber. This chapter attempts to provide an understanding of light-generating mechanisms within the main types of optical sources used in fiber optics.
Q1. What are the three parts of a fiber optic transmitter? Q2. Which part of a fiber optic transmitter converts the processed electrical signal to an optical
signal?
The development of efficient semiconductor optical sources, along with low-loss optical fibers, led to substantial improvements in fiber optic communications. Semiconductor optical sources have the physical characteristics and performance properties necessary for successful implementations of fiber optic systems. It is desirable that optical sources:
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• Emit light at wavelengths that minimize optical fiber loss and dispersion. Optical sources should have a narrow spectral width to minimize dispersion
Semiconductor optical sources suitable for fiber optic systems range from inexpensive light-emitting diodes (LEDs) to more expensive semiconductor lasers. Semiconductor LEDs and laser diodes (LDs) are the principal light sources used in fiber optics.
Fiber optic communication systems operate in the 850-nm, the 1300-nm, and the 1550-nm wavelength windows. Semiconductor sources are designed to operate at wavelengths that minimize optical fiber absorption and maximize system bandwidth. By designing an optical source to operate at specific wavelengths, absorption from impurities in the optical fiber, such as hydroxyl ions (OH -), can be minimized. Maximizing system bandwidth involves designing optical fibers and sources that minimize chromatic and intermodal dispersion at the intended operational wavelength.
Initially, the material properties of semiconductor optical sources provided for optical emission in the 850-nm wavelength region. An 850-nm operational wavelength avoids fiber absorption loss from OH- impurities near the 900-nm wavelength. Light sources for 850-nm systems were originally semiconductor LEDs and lasers. Currently, most 850-nm systems use LEDs as a light source. LEDs operating at 850-nm provide sufficient optical power for short-distance, low-bandwidth systems. However, multimode fiber dispersion, the relatively high fiber attenuation, and the LED's relatively low optical output power prevent the use of these devices in longer-distance, higher bandwidth systems.
The first development allowing the operational wavelength to move from 850 nm to 1300 nm was the introduction of multimode graded-index fibers. Multimode graded-index fibers have substantially lower intermodal dispersion than multimode step-index fibers. Systems operating at 850 nm cannot take full advantage of the fiber's low intermodal dispersion because of high chromatic dispersion at 850 nm. However, the use of multimode graded-index fibers allow 850-nm LEDs to operate satisfactorily in short-distance, higher bandwidth systems.
Following the enhancements in multimode fiber design, next generation LEDs were designed to provide optical emission in the 1300-nm region. Multimode graded-index fiber systems using these LEDs can operate over longer distances and at higher bandwidths than 850-nm systems. Longer distances and higher bandwidths are possible because fiber material losses and dispersion are significantly reduced at the 1300-nm region.
Advances in single mode fiber design and construction sped the development of semiconductor LEDs and LDs optimized for single mode fibers. Single mode fibers have very low dispersion values. However, existing LEDs were unable to focus and launch sufficient optical power into single mode fibers
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for long-haul, very high-bandwidth communication systems. New semiconductor LEDs and LDs capable of operating with single mode fibers at 1300 nm were developed to take advantage of single mode fiber's very low value of dispersion. Additionally, LEDs and LDs operating at 1550 nm were developed to take advantage of the fiber's lowest loss.
Q3. LEDs operating at 850 nm provide sufficient optical power for short-distance, low-bandwidth multimode systems. List three conditions that prevent the use of LEDs in longer distance, higher bandwidth multimode systems.
Q4. Why can multimode graded-index fiber 1300-nm systems using LEDs operate over longer distances and at higher bandwidths than 850-nm systems?
Semiconductor LEDs emit incoherent light. Spontaneous emission of light in semiconductor LEDs produces light waves that lack a fixed-phase relationship. Light waves that lack a fixed-phase relationship are referred to as incoherent light. Spontaneous emission of light is discussed in more detail later in this chapter. The use of LEDs in single mode systems is severely limited because they emit unfocused incoherent light. Even LEDs developed for single mode systems are unable to launch sufficient optical power into single mode fibers for many applications. LEDs are the preferred optical source for multimode systems because they can launch sufficient power at a lower cost than semiconductor LDs.
Semiconductor LDs emit coherent light. LDs produce light waves with a fixed-phase relationship (both spatial and temporal) between points on the electromagnetic wave. Light waves having a fixed- phase relationship are referred to as coherent light. Stimulated emission of light is discussed later in this chapter. Since semiconductor LDs emit more focused light than LEDs, they are able to launch optical power into both single mode and multimode optical fibers. However, LDs are usually used only in single mode fiber systems because they require more complex driver circuitry and cost more than LEDs.
Optical power produced by optical sources can range from microwatts (ÿW) for LEDs to tens of milliwatts (mW) for semiconductor LDs. However, it is not possible to effectively couple all the available optical power into the optical fiber for transmission.
The amount of optical power coupled into the fiber is the relevant optical power. It depends on the following factors:
• The coupling characteristics of the fiber (such as the NA and the refractive index profile) Typically, semiconductor lasers emit light spread out over an angle of 10 to 15 degrees.
Semiconductor LEDs emit light spread out at even larger angles. Coupling losses of several decibels can easily occur when coupling light from an optical source to a fiber, especially with LEDs.
Source-to-fiber coupling efficiency is a measure of the relevant optical power. The coupling efficiency depends on the type of fiber that is attached to the optical source. Coupling efficiency also depends on the coupling technique. Source-to-fiber coupling involves centering a flat fiber-end face over
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the emitting region of the light source. If the fiber end face is directly placed over the source emitting region, it is referred to as butt coupling. If the source's output light pattern is larger than the fiber's acceptance pattern, source-to-fiber coupling efficiency may be improved by placing a small lens between the source and fiber. Lensing schemes improve coupling efficiency when coupling both LEDs and LDs to optical fibers.
Q5. Semiconductor LEDs emit incoherent light. Define incoherent light. Q6. Which semiconductor sources (LD or LED) emit more focused light and are capable of launching
sufficient optical power into both single mode and multimode fibers? Q7. The amount of optical power coupled into an optical fiber depends on what four factors?
Understanding optical emission in semiconductor lasers and LEDs requires knowledge of semiconductor material and device properties. Providing a complete description of semiconductor properties is beyond the scope of this introductory manual. In this chapter we only discuss the general properties of semiconductor LEDs and LDs.
Semiconductor sources are diodes, with all of the characteristics typical of diodes. However, their construction includes a special layer, called the active layer, which emits photons (light particles) when a current passes through the layer. The particular properties of the semiconductor are determined by the materials used and the layering of the materials within the semiconductor. Silicon (Si) and gallium arsenide (GaAs) are the two most common semiconductor materials used in electronic and electro-optic devices. In some cases other elements, such as aluminum (Al), indium (In) and phosphorus (P), are added to the base semiconductor material to modify the semiconductor properties. These elements are called dopants.
Current flowing through a semiconductor optical source causes it to produce light. An in-depth description of either of the two processes by which this occurs is beyond the scope of this module. However, we discuss elementary descriptions in the following paragraphs.
LEDs generally produce light through spontaneous emission when a current is passed through them. Spontaneous emission is the random generation of photons within the active layer of the LED. The emitted photons move in random directions. Only a certain percentage of the photons exit the semiconductor and are coupled into the fiber. Many of the photons are absorbed by the LED materials and the energy dissipated as heat. This process causes the light output from an LED to be incoherent, have a broad spectral width, and have a wide output pattern.
Laser diodes are much more complex than LEDs. Laser is an acronym for light amplification by the stimulated emission of radiation. Laser diodes produce light through stimulated emission when a current is passed through them. Stimulated emission describes how light is produced in any type of laser. In the laser diode, photons, initially produced by spontaneous emission interact with the laser material to produce additional photons. This process occurs within the active area of the diode called the laser cavity. The process does not affect the original photon. The stimulated photon has many of the same properties (wavelength, direction, phase) as the original photon.
As with the LED, not all of the photons produced are emitted from the laser diode. Some of the photons are absorbed and the energy dissipated as heat. The emission process and the physical characteristics of the diode cause the light output to be coherent, have a narrow spectral width, and have a narrow output pattern.
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It is important to note that in both LED and laser diodes all of the electrical energy is not converted into optical energy. A substantial portion is converted to heat. Different LED and laser diode structures convert differing amounts of electrical energy into optical energy.
Q8. What are the two most common semiconductor materials used in electronic and electro-optic devices?
Q9. What is a laser? Q10. .Describe stimulated emission.
A light-emitting diode (LED) is a semiconductor device that emits incoherent light, through spontaneous emission, when a current is passed through it. Typically LEDs for the 850-nm region are fabricated using GaAs and AlGaAs. LEDs for the 1300-nm and 1550-nm regions are fabricated using InGaAsP and InP.
The basic LED types used for fiber optic communication systems are the surface-emitting LED (SLED), the edge-emitting LED (ELED), and the superluminescent diode (SLD). LED performance differences help link designers decide which device is appropriate for the intended application. For short-distance (0 to 3 km), low-data-rate fiber optic systems, SLEDs and ELEDs are the preferred optical source. Typically, SLEDs operate efficiently for bit rates up to 250 megabits per second (Mb/s). Because SLEDs emit light over a wide area (wide far-field angle), they are almost exclusively used in multimode systems.
For medium-distance, medium-data-rate systems, ELEDs are preferred. ELEDs may be modulated at rates up to 400 Mb/s. ELEDs may be used for both single mode and multimode fiber systems. Both SLDs and ELEDs are used in long-distance, high-data-rate systems. SLDs are ELED-based diodes designed to operate in the superluminescence mode. A further discussion on superluminescence is provided later in this chapter. SLDs may be modulated at bit rates of over 400 Mb/s.
The surface-emitting LED (shown in figure 6-1) is also known as the Burrus LED in honor of C. A. Burrus, its developer. In SLEDs, the size of the primary active region is limited to a small circular area of 20 m m to 50 m m in diameter. The active region is the portion of the LED where photons are emitted. The primary active region is below the surface of the semiconductor substrate perpendicular to the axis of the fiber. A well is etched into the substrate to allow direct coupling of the emitted light to the optical fiber. The etched well allows the optical fiber to come into close contact with the emitting surface. In addition, the epoxy resin that binds the optical fiber to the SLED reduces the refractive index mismatch, increasing coupling efficiency.
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The demand for optical sources for longer distance, higher bandwidth systems operating at longer wavelengths led to the development of edge-emitting LEDs. Figure 6-2 shows a typical ELED structure. It shows the different layers of semiconductor material used in the ELED. The primary active region of the ELED is a narrow stripe, which lies below the surface of the semiconductor substrate. The semiconductor substrate is cut or polished so that the stripe runs between the front and back of the device. The polished or cut surfaces at each end of the stripe are called facets.
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In an ELED the rear facet is highly reflective and the front facet is antireflection-coated. The rear facet reflects the light propagating toward the rear end-face back toward the front facet. By coating the front facet with antireflection material, the front facet reduces optical feedback and allows light emission. ELEDs emit light only through the front facet.
ELEDs emit light in a narrow emission angle allowing for better source-to-fiber coupling. They couple more power into small NA fibers than SLEDs. ELEDs can couple enough power into single mode fibers for some applications. ELEDs emit power over a narrower spectral range than SLEDs. However, ELEDs typically are more sensitive to temperature fluctuations than SLEDs.
A laser is a device that produces optical radiation by the process of stimulated emission. It is necessary to contain photons produced by stimulated emission within the laser active region. Figure 6-3 shows an optical cavity formed to contain the emitted photons by placing one reflecting mirror at each end of an amplifying medium. One mirror is made partially reflecting so that some radiation can escape from the cavity for coupling to an optical fiber.
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Only a portion of the optical radiation is amplified. For a particular laser structure, there are only certain wavelengths that will be amplified by that laser. Amplification occurs when selected wavelengths, also called laser modes, reflect back and forth through the cavity. For lasing to occur, the optical gain of the selected modes must exceed the optical loss during one round-trip through the cavity. This process is referred to as optical feedback.
The lasing threshold is the lowest drive current level at which the output of the laser results primarily from stimulated emission rather than spontaneous emission. Figure 6-4 illustrates the transition from spontaneous emission to stimulated emission by plotting the relative optical output power and input drive current of a semiconductor laser diode. The lowest current at which stimulated emission exceeds spontaneous emission is the threshold current. Before the threshold current is reached, the optical output power increases only slightly with small increases in drive current. However, after the threshold current is reached, the optical output power increases significantly with small changes in drive currents.
Many types of materials including gas, liquid, and semiconductors can form the lasing medium. However, in this chapter we only discuss semiconductor laser diodes. Semiconductor laser diodes are the
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primary lasers used in fiber optics. A laser diode emits light that is highly monochromatic and very directional. This means that the LD's output has a narrow spectral width and small output beam angle.
A semiconductor LD's geometry is similar to an ELED with light-guiding regions surrounding the active region. Optical feedback is established by making the front facet partially reflective. This chapter provides no diagram detailing LD structures because they are similar to ELEDs in design. The rear facet is typically coated with a reflective layer so that all of the light striking the facet is reflected back into the active region. The front facet is typically left uncoated so that most of the light is emitted. By increasing the drive current, the diode becomes a laser.
At currents below the threshold current, LDs function as ELEDs. To optimize frequency response, laser diodes are often biased above this laser threshold. As a result, in an LD fiber optic system, light is modulated between a high power level and a lower power level, but never shut off. LDs typically can be modulated at frequencies up to over 2 gigahertz (GHz). Some lasers are capable of being modulated at frequencies over 20 GHz.
There are several important differences between LDs and LEDs. One is that LEDs usually lack reflective facets and in some cases are designed to suppress reflections back into the active region. Another is that lasers tend to operate at higher drive currents to produce light. A higher driver current results in more complicated drive circuits and more heat dissipation in the device.
LDs are also much more temperature sensitive than either SLEDs or ELEDs. Increases in the laser temperature significantly reduce laser output power. Increases in laser temperature beyond certain limits result in the loss of lasing. When lasers are used in many applications, the temperature of the laser must be controlled. Typically, electronic coolers, called thermo-electric (TE) coolers, are used to cool LDs in system applications.
Superluminescence occurs when the spontaneous emissions of an ELED experience gain due to higher injected currents and reflections from facets. Superluminescent diodes (SLDs) are differentiated from both conventional LEDs and LDs. Although the output is not fully coherent, SLDs emit light that consists of amplified spontaneous emissions. The spectral width and beam angle of SLDs are narrower than that of conventional LEDs and wider than that of LDs.
An SLD is, in essence, a combination of a laser and an ELED. SLDs are similar in geometry to lasers but have no built-in optical feedback mechanism required by laser diodes for stimulated emission to achieve lasing. SLDs have structural features similar to those of ELEDs that suppress the lasing action by reducing the reflectivity of the facets. SLDs are essentially highly optimized ELEDs.
While SLDs operate like ELEDs at low current levels, their output power increases superlinearly and the spectral width narrows at high currents. Optical gain resulting from the higher injection currents causes the superlinear power increase and narrowing of the spectral width.
The advantages of SLDs over conventional LEDs include higher coupled power, narrower spectral width, and greater bandwidths. The disadvantages include nonlinear power-current characteristics, higher temperature sensitivity, and lower reliability.
Q11. What are the three basic LED types? Q12. Which types of LEDs are the preferred optical sources for short-distance, low-data-rate fiber
optic systems?
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Q13. What are facets? Q14. What is lowest current at which stimulated emission exceeds spontaneous emission in a
semiconductor laser called?
Q15. Describe the output of a laser diode. Q16. Which type of optical source usually lacks reflective facets and in some cases are designed to
suppress reflections back into the active region? Q17. Which type of optical source tends to operate at higher drive currents to produce light? Q18. Are the effects of temperature changes on LDs more or less significant than for LEDs? Q19. Specify the mechanism that SLDs lack that is required by laser diodes to achieve lasing.
As stated previously, a fiber optic transmitter is a hybrid electro-optic device. It converts electrical signals into optical signals and launches the optical signals into an optical fiber. A fiber optic transmitter consists of an interface circuit, a source drive circuit, and an optical source. The interface circuit accepts the incoming electrical signal and processes it to make it compatible with the source drive circuit. The source drive circuit intensity modulates the optical source by varying the current through it. The optical signal is coupled into an optical fiber through the transmitter output interface.
Although semiconductor LEDs and LDs have many similarities, unique transmitter designs result from differences between LED and LD sources. Transmitter designs compensate for differences in optical output power, response time, linearity, and thermal behavior between LEDs and LDs to ensure proper system operation. Nonlinearities caused by junction heating in LEDs and mode instabilities in LDs necessitate the use of linearizing circuits within the transmitter in some cases.
Fiber optic transmitters using LDs require more complex circuitry than transmitters using LEDs. The basic requirement for digital systems is for drive circuitry to switch the optical output on and off at high speeds in response to logic voltage levels at the input of the source drive circuit. Because LDs are threshold devices, LDs are supplied with a bias just below the threshold in the off state. This bias is often referred to as prebias. One reason for prebiasing the LD is to reduce the turn-on delay in digital systems.
Most LD transmitters contain output power control circuitry to compensate for temperature sensitivity. This circuitry maintains the LD output at a constant average value by adjusting the bias current of the laser. In most cases LED transmitters do not contain output power control circuitry. LD and LED transmitters may also contain cooling devices to maintain the source at a relatively constant temperature. Most LD transmitters either have an internal thermo electric cooler or require a relatively controlled external temperature. Because LDs require more complex circuitry than LEDs, fiber optic transmitters using LDs are more expensive. For more information concerning fiber optic transmitters and their drive circuitry, refer to the reference material listed in appendix 2.
Transmitter output interfaces generally fall into two categories: optical connectors and optical fiber pigtails. Optical pigtails are attached to the transmitter optical source. This pigtail is generally routed out of the transmitter package as a coated fiber in a loose buffer tube or a single fiber cable. The pigtail is either soldered or epoxied to the transmitter package to provide fiber strain relief. The buffer tube or single fiber cable is also attached to the transmitter package to provide additional strain relief.
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The transmitter output interface may consist of a fiber optical connector. The optical source may couple to the output optical connector through an intermediate optical fiber. One end of the optical fiber is attached to the source. The other end terminates in the transmitter optical output connector. The optical source may also couple to the output optical connector without an intermediate optical fiber. The optical source is placed within the transmitter package to launch power directly into the fiber of the mating optical connector. In some cases lenses are used to more efficiently couple light from the source into the mating optical connector.
Q20. How does the source drive circuit intensity modulate the source? Q21. What is a prebias? Q22. Is the drive circuitry generally more complex for an LED or a laser diode? Why? Q23. What are the two types of output interfaces for fiber optic transmitters?
Fiber optic transmitters come in various sizes and shapes. The least complex fiber optic transmitters are typically packaged in transistor outline (TO) cans or hybrid microcircuit modules in dual inline packages (DIPs). These simple transmitters may require separate circuitry in the system equipment to provide an acceptable input signal to the transmitter. More complex fiber optic transmitters are available that have some or all of the signal conditioning circuitry integrated into the package. These transmitters typically are packaged in hybrid microcircuit modules in either DIP or butterfly lead packages, circuit cards, or complete stand-alone fiber optic converters. Stand-alone fiber optic converters and circuit cards generally contain sources in either TO cans or one of the hybrid microcircuit packages. For commercial applications, the most popular transmitter packages are the TO can and the DIP hybrid microcircuit.
Fiber optic transmitters can be classified into two categories: digital and analog. Digital transmitters produce two discrete optical power levels. These levels are essentially on and off with the exception that some light is emitted in the off state by some transmitters. Analog transmitters continuously vary the output optical power level as a function of the input electrical signal.
Different types of fiber optic transmitters are used for different digital applications. For each specific application, the link data rate, transmission length, and operating environment influence the source type, center wavelength, spectral width, and package type chosen.
For low-data-rate applications, fiber optic transmitters generally use LEDs operating in either the 850-nm or 1300-nm window as their source. For the lowest data rates (0 to 20 megabits per second (Mbps)), sources tend to operate in the 850-nm window. For moderate data rates (50 to 200 Mbps), sources tend to operate in the 1300-nm window. Laser sources are almost never used in low-data-rate applications. Laser sources are only used when extremely high transmitter output powers are required in the application. The packages found in low-data-rate applications include all of the package types discussed earlier.
For high-data-rate applications, most fiber optic transmitters use laser diodes as sources. The sources typically operate in either the 1300-nm or 1550-nm windows. Most high-data-rate applications use LDs as the optical source and operate in the 1300-nm region. Almost all 1550-nm systems use an LD as the optical source. 1550-nm transmitters are usually only used in the extremely long distance high-data-rate
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applications (undersea links, etc.). High-data-rate transmitters are generally hybrid microcircuit modules or complete circuit cards. Almost all high-data-rate transmitters contain power control circuitry. Depending upon the application, high-data-rate transmitters may contain TE coolers.
Q24. List five common fiber optic transmitter packages. Q25. What type of source is typically used in low-data-rate digital applications? Q26. Why would a laser diode be used in a low-data-rate digital application? Q27. What type of source is generally used in high-data-rate digital applications?
Different types of fiber optic transmitters are also used for different analog applications. For each specific application, analog signal type, transmission length, and operating environment influence the source type, center wavelength, spectral width, and package type chosen.
For low-frequency applications, analog fiber optic transmitters generally use LEDs operating in either the 850-nm or 1300-nm window. Typical low frequency applications are analog audio and single channel video systems. For these systems, sources tend to operate in the 850-nm window. For moderate frequency applications, sources tend to operate in the 1300-nm window. These types of systems include multi-channel analog audio and video systems as well as frequency modulated (FM) systems. Laser sources are almost never used in low- or moderate-frequency analog applications. The main reason for this is the added circuit complexity that laser sources require. Laser sources are only used if extremely high transmitter output powers are required in the application. Most low-frequency analog transmitters are hybrid microcircuit modules, circuit cards, or stand-alone boxes.
For high-frequency applications, analog fiber optic transmitters use laser diodes as sources. Typical high frequency applications are cable television trunk line and raw radar remoting applications. The LDs typically operate in either the 1300-nm or 1550-nm windows. 1550-nm transmitters are typically used in cable television trunk line applications. Other applications may use either 1300-nm or 1550-nm LDs. High frequency transmitters are predominately circuit cards, but some hybrid microcircuit modules are also used. All high frequency analog transmitters contain TE coolers as well as linearization and power control circuitry.
Q28. Why are LEDs preferred over laser diodes for low- and moderate-frequency analog applications?
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 7.
 It converts electrical signals into optical signals and launches the optical signals into an optical fiber.
An OPTICAL SOURCE converts electrical energy (current) into optical energy (light). The principal LIGHT SOURCES used in fiber optics are semiconductor light-emitting diodes
(LEDs) and laser diodes (LDs).
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SEMICONDUCTOR LD's emit coherent light. Light waves having a fixed-phase relationship are referred to as coherent light.
SEMICONDUCTOR LED'S emit incoherent light. Light waves that lack a fixed-phase relationship are referred to as incoherent light.
The RELEVANT OPTICAL POWER is the amount of optical power coupled into the fiber. It depends on the angle over which the light is emitted, the size of the source's light-emitting area relative to the fiber core size, the alignment of the source and fiber, and the coupling characteristics of the fiber (such as the NA and the refractive index profile).
SOURCE-TO-FIBER COUPLING EFFICIENCY is a measure of the relevant optical power.
SILICON (Si) and GALLIUM ARSENIDE (GaAs) are the two most common semiconductor materials used in electronic and electro-optic devices.
In a semiconductor device, PHOTONS (LIGHT) are emitted when current flows through the active area.
SPONTANEOUS EMISSION occurs when photons are emitted in a random manner. Spontaneous emission produces incoherent light.
STIMULATED EMISSION occurs when a photon interacts with the laser material to produce additional photons.
A LIGHT-EMITTING DIODE (LED) is a semiconductor device that emits incoherent light, through spontaneous emission, when a current is passed through it. The basic LED types used for fiber optic communication systems are the SURFACE-EMITTING LED (SLED), the EDGE-EMITTING LED (ELED), and the SUPERLUMINESCENT DIODE (SLD).
In SURFACE-EMITTING LED'S (SLEDs), the size of the primary active region is limited to a small circular area of 20 m to 50 m in diameter. The active region is the portion of the LED where photons are emitted. SLEDs usually emit more total power into the air gap at the fiber interface than an ELED, but they do not launch as much power into the fiber. SLEDS also tend to emit power over a wider spectral range than ELED.
EDGE-EMITTING LED'S (ELEDs) emit light in a narrow emission angle allowing for better source-to-fiber coupling. They couple more power into small NA fibers than SLEDs. The polished or cut surfaces at each end of the ELED active stripe are called FACETS.
SUPERLUMINESCENCE occurs when the spontaneous emissions of an ELED experience gain due to higher injected currents and reflections from facets.
SUPERLUMINESCENT DIODES (SLDs) are similar in geometry to lasers but have no built-in optical feedback mechanism required by laser diodes for stimulated emission to achieve lasing. Although the output is not fully coherent, superluminescent diodes (SLDs) emit light that consists of amplified spontaneous emissions. The spectral width and beam angle of SLDs are narrower than that of conventional LEDs and wider than that of LDs.
The ADVANTAGES of SLDs over conventional LEDs include higher coupled power, narrower spectral width, and greater bandwidths. The DISADVANTAGES include nonlinear power-current characteristics, higher temperature sensitivity, and lower reliability.
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A LASER is a device that produces optical radiation using stimulated emission rather than spontaneous emission. Laser is an acronym for light amplification by the stimulated emission of radiation.
The LASING THRESHOLD is the lowest drive level at which the output of the laser results primarily from stimulated emission rather than spontaneous emission.
The THRESHOLD CURRENT is the lowest current at which stimulated emission exceeds spontaneous emission.
A LASER DIODE is a semiconductor diode that emits coherent light by lasing. The LD's output has a narrow spectral width and small output beam angle.
TRANSMITTER OUTPUT INTERFACES fall into two categories: optical connectors and optical fiber pigtails.
FIBER OPTIC TRANSMITTERS using LDs require more complex circuitry than transmitters using LEDs.
Because LDs are threshold devices, LDs are supplied with a bias just below the threshold in the off state. This bias is often referred to as a prebias.
The least complex FIBER OPTIC TRANSMITTERS are typically packaged in transistor outline (TO) cans or hybrid microcircuit modules in dual inline packages (DIPs).
More complex FIBER OPTIC TRANSMITTERS typically are packaged in hybrid microcircuit modules in either DIP or butterfly lead packages, circuit cards, or complete stand-alone fiber optic converters.
FIBER OPTIC TRANSMITTERS can be classified into two categories: digital and analog.
DIGITAL TRANSMITTERS modulate the fiber optic source between two discrete optical power levels. These levels are essentially on and off with the exception that some light is emitted in the off state by some transmitters.
ANALOG TRANSMITTERS continuously vary the output optical power level as a function of the input electrical signal.
For LOW-DATA-RATE APPLICATIONS (0 to 20 Mbps), fiber optic transmitters generally use LEDs operating in either the 850-nm or 1300-nm window.
ForMODERATE-DATA-RATE APPLICATIONS (50 to 200 Mbps), fiber optic transmitters generally use LEDs operating in the 1300-nm window.
For HIGH-DATA-RATE APPLICATIONS, most fiber optic transmitters use laser diodes as sources.
LASER SOURCES are almost never used in low- or moderate-frequency analog applications because LED sources require much less complex circuitry.
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ANSWERS TO QUESTIONS Q1. THROUGH Q28. A1. Interface circuit, source drive circuit, and an optical source. A2. The optical source.
A3. Multimode fiber dispersion, the relatively high fiber attenuation, and the LED's relatively low optical output power.
A4. Longer distances and higher bandwidths are possible because fiber material losses and dispersion are significantly reduced at the 1300-nm region.
A5. Light waves that lack a fixed-phase relationship. A6. LDs.
A7. (1) The angles over which the light is emitted. (2) The size of the source's light-emitting area relative to the fiber core size. (3) The alignment of the source and fiber. (4) The coupling characteristics of the fiber (such as the NA and the refractive index profile).
A8. Silicon and gallium arsenide.
A9. A laser is a device that produces optical radiation using stimulated emission rather than spontaneous emission.
A10. A photon initially produced by spontaneous emission in the active region interacts with the laser material to produce additional photons.
A11. Surface-emitting LEDs (SLEDs), edge-emitting LEDs (ELEDs), and superluminescent diodes (SLDs).
A12. SLEDs and ELEDs.
A13. Cut or polished surfaces at each end of the narrow active region of an ELED. A14. Threshold current.
A15. The LD's output has a narrow spectral width and small output beam angle.
A16. LED.
A17. Laser.
A18. More.
A19.  A20. By varying the current through the source.
A21. A current applied in the laser off state just less than the threshold current. A22. For a laser diode. The laser diode transmitter generally contains output power control circuitry
and may contain a TE cooler and some circuitry associated with the TE cooler.
A23. Optical connectors and optical fiber pigtails.
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A24. TO can, DIP, butterfly lead microcircuits, circuit cards, and stand-alone optical fiber converters. A25. LED.
A26. When extremely high transmitter output powers are required.
A27. Laser diode.
A28. LEDs require less complex circuitry than lasers.
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Upon completion of this chapter, you should be able to do the following:
1. Explain the principal properties of an optical detector and fiber optic receiver.
2. Detail semiconductor optical detector performance and capability requirements necessary for the successful implementation of fiber optic systems.
3. List the main components of a fiber optic receiver.
4. Discuss receiver sensitivity, dynamic range, and other key operational parameters used to define receiver performance.
Chapter 6 taught you that a fiber optic transmitter is an electro-optic device capable of accepting electrical signals, converting them into optical signals, and launching the optical signals into an optical fiber. The optical signals propagating in the fiber become weakened and distorted because of scattering, absorption, and dispersion. The fiber optic device responsible for converting the weakened and distorted optical signal back to an electrical signal is a fiber optic receiver.
A fiber optic receiver is an electro-optic device that accepts optical signals from an optical fiber and converts them into electrical signals. A typical fiber optic receiver consists of an optical detector, a low-noise amplifier, and other circuitry used to produce the output electrical signal (see figure 7-1). The optical detector converts the incoming optical signal into an electrical signal. The amplifier then amplifies the electrical signal to a level suitable for further signal processing. The type of other circuitry contained within the receiver depends on what type of modulation is used and the receiver electrical output requirements.
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Receiver spectral response, sensitivity, frequency response, and dynamic range are key receiver performance parameters that can affect overall system operation. The choice of optical detector materials and structures determines the spectral response. Silicon (Si), gallium arsenide (GaAs), and gallium aluminum arsenide (GaAlAs) are typical detector materials used for receiver operation in the 850-nm wavelength region. germanium (Ge), indium phosphide (InP), and indium gallium arsenide (InGaAs) are examples of detector materials used for receiver operation in the 1300-nm and 1550-nm wavelength regions.
The receiver sensitivity is the minimum amount of optical power required to achieve a specific receiver performance. For digital transmission at a given data rate and coding, this performance is described by a maximum bit-error rate (BER). In analog systems, for a given modulation and bandwidth, it is described by a minimum signal-to-noise ratio (SNR). Dynamic range refers to the range of optical power levels over which the receiver operates within the specified values. It usually is described by the ratio of the maximum input power to the sensitivity. Before discussing receiver sensitivity, bandwidth, dynamic range, and frequency response in more detail, we discuss the main types of optical detectors used in fiber optics.
Q1. What is a fiber optic receiver? Q2. Which part of the receiver amplifies the electrical signal to a level suitable for further signal
processing?
Q3. Which performance parameter is the minimum amount of optical power required to achieve a specific bit-error rate (BER) in digital systems or a given signal-to-noise ratio (SNR) in analog systems?
Q4. Define receiver dynamic range.
A transducer is a device that converts input energy of one form into output energy of another. An optical detector is a transducer that converts an optical signal into an electrical signal. It does this by generating an electrical current proportional to the intensity of incident optical radiation. The relationship
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between the input optical radiation and the output electrical current is given by the detector responsivity. Responsivity is discussed later in this chapter.
Fiber optic communications systems require that optical detectors meet specific performance and compatibility requirements. Many of the requirements are similar to those of an optical source. Fiber optic systems require that optical detectors:
• Be compatible in size to low-loss optical fibers to allow for efficient coupling and easy packaging.
• Have a high sensitivity at the operating wavelength of the optical source.
• Have a sufficiently short response time (sufficiently wide bandwidth) to handle the system's data rate.
• Contribute low amounts of noise to the system.
• Maintain stable operation in changing environmental conditions, such as temperature.
Optical detectors that meet many of these requirements and are suitable for fiber optic systems are semiconductor photodiodes. The principal optical detectors used in fiber optic systems include semiconductor positive-intrinsic-negative (PIN) photodiodes and avalanche photodiodes (APDs).
Q5. Describe the operation of an optical detector. Q6. For efficient operation, should a detector have a high or low responsivity at the operating
wavelength?
Q7. List the two principal optical detectors used in fiber optic systems.
Semiconductor photodiodes generate a current when they absorb photons (light). The amount of current generated depends on the following factors:
The optical fiber is coupled to semiconductor photodiodes similarly to the way optical sources are coupled to optical fibers. Fiber-to-photodiode coupling involves centering the flat fiber-end face over the photodiode active area. This is normally done directly by butt coupling the fiber up to the photodiode surface. As long as the photodiode active area is larger than that of the fiber core, fiber-to-detector coupling losses are very low. In some cases a lens may be used to couple the fiber end-face to the detector. However, this is not typically done.
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The mechanism by which optical detectors convert optical power into electrical current requires knowledge of semiconductor material and device properties. As stated in chapter 6, providing a complete description of these properties is beyond the scope of this manual. In this chapter we only discuss the general properties of semiconductor PINs and APDs.
Semiconductor detectors are designed so that optical energy (photons) incident on the detector active area produces a current. This current is called a photocurrent. The particular properties of the semiconductor are determined by the materials used and the layering of the materials within the device. Silicon (Si), gallium arsenide (GaAs), germanium (Ge), and indium phosphide (InP) are the most common semiconductor materials used in optical detectors. In some cases aluminum (Al) and indium (In) are used as dopants in the base semiconductor material.
Responsivity is the ratio of the optical detector's output photocurrent in amperes to the incident optical power in watts. The responsivity of a detector is a function of the wavelength of the incident light and the efficiency of the device in responding to that wavelength. For a particular material, only photons of certain wavelengths will generate a photocurrent when they are absorbed. Additionally, the detector material absorbs some wavelengths better than others. These two properties cause the wavelength dependence in the detector responsivity. Responsivity is a useful parameter for characterizing detector performance because it relates the photocurrent generated to the incident optical power.
Q8. What are the four most common materials used in semiconductor detector fabrication? Q9. What is a photocurrent? Q10. Define responsivity.
A PIN photodiode is a semiconductor positive-negative (p-n) structure with an intrinsic region sandwiched between the other two regions (see figure 7-2). It is normally operated by applying a reverse-bias voltage. The magnitude of the reverse-bias voltage depends on the photodiode application, but typically is less than a few volts. When no light is incident on the photodiode, a current is still produced. This current is called the dark current. The dark current is the leakage current that flows when a reverse bias is applied and no light is incident on the photodiode. Dark current is dependent on temperature. While dark current may initially be low, it will increase as the device temperature increases.
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Q11. How are PIN photodiodes usually biased? Q12. What is the dark current? Q13. Will dark current increase or decrease as the temperature of the photodiode increases?
There are several factors that influence the response time of a photodiode and its output circuitry (see figure 7-3). The most important of these are the thickness of the detector active area and the detector RC time constant. The detector thickness is related to the amount of time required for the electrons generated to flow out of the detector active area. This time is referred to as the electron transit time. The thicker the detector active area, the longer the transit time will be.
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The capacitance (C) of the photodiode and the resistance (R) of the load form the RC time constant. The capacitance of the photodetector must be kept small to prevent the RC time constant from limiting the response time. The photodiode capacitance consists mainly of the junction capacitance and any capacitance relating to packaging. The RC time constant is given by tRC = RC.
Trade-offs between fast transit times and low capacitance are necessary for high-speed response. However, any change in photodiode parameters to optimize the transit time and capacitance can also affect responsivity, dark current, and coupling efficiency. A fast transit time requires a thin detector active area, while low capacitance and high responsivity require a thick active region. The diameter of the detector active area can also be minimized. This reduces the detector dark current and minimizes junction capacitance. However, a minimum limit on this active area exists to provide for efficient fiber-to-detector coupling.
Q14. Should the capacitance of the photodetector be kept small or large to prevent the RC time constant from limiting the response time?
Q15. Trade-offs between competing effects are necessary for high speed response. Which competing effect (fast transit time, low capacitance, or high quantum efficiency) requires a thin active area?
Reverse-biased photodetectors are highly linear devices. Detector linearity means that the output electrical current (photocurrent) of the photodiode is linearly proportional to the input optical power. Reverse-biased photodetectors remain linear over an extended range (6 decades or more) of photocurrent before saturation occurs. Output saturation occurs at input optical power levels typically greater than 1
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milliwatt (mW). Because fiber optic communications systems operate at low optical power levels, detector saturation is generally not a problem.
Q16. Why is detector saturation not generally a problem in fiber optic communications systems? AVALANCHE PHOTODIODES
An avalanche photodiode (APD) is a photodiode that internally amplifies the photocurrent by an avalanche process. Figure 7-4 shows an example APD structure. In APDs, a large reverse-bias voltage, typically over 100 volts, is applied across the active region. This voltage causes the electrons initially generated by the incident photons to accelerate as they move through the APD active region. As these electrons collide with other electrons in the semiconductor material, they cause a fraction of them to become part of the photocurrent. This process is known as avalanche multiplication. Avalanche multiplication continues to occur until the electrons move out of the active area of the APD.
The gain of the APD can be changed by changing the reverse-bias voltage. A larger reverse-bias voltage results in a larger gain. However, a larger reverse-bias voltage also results in increased noise levels. Excess noise resulting from the avalanche multiplication process places a limit on the useful gain of the APD. The avalanche process introduces excess noise because every photogenerated carrier does not undergo the same multiplication. The noise properties of an APD are affected by the materials that the
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APD is made of. Typical semiconductor materials used in the construction of low-noise APDs include silicon (Si), indium gallium arsenide (InGaAs), and germanium (Ge).
Trade-offs are made in APD design to optimize responsivity and gain, dark current, response time, and linearity. This chapter does not attempt to discuss trade-offs in APD design in more detail. Many aspects of the discussion provided on responsivity, dark current, and response time provided in the PIN photodiodes section also relate to APDs. The response time of an APD and its output circuitry depends on the same factors as PIN photodiodes. The only additional factor affecting the response time of an APD is the additional time required to complete the process of avalanche multiplication. To learn more about APD design trade-offs and performance parameters, refer to the reference material listed in appendix 2.
Q17. Describe avalanche multiplication. Q18. How can the gain of an APD be increased?
In fiber optic communications systems, optical signals that reach fiber optic receivers are generally attenuated and distorted (see figure 7-5). The fiber optic receiver must convert the input and amplify the resulting electrical signal without distorting it to a point that other circuitry cannot use it.
As stated previously, a fiber optic receiver consists of an optical detector, an amplifier, and other circuitry. In most fiber optic systems, the optical detector is a PIN photodiode or APD. Receiver performance varies depending on the type of detector used. The amplifier is generally described as having two stages: the preamplifier and the postamplifier. The preamplifier is defined as the first stage of amplification following the optical detector. The postamplifier is defined as the remaining stages of amplification required to raise the detector's electrical signal to a level suitable for further signal processing. The preamplifier is the dominant contributor of electrical noise in the receiver. Because of this, its design has a significant influence in determining the sensitivity of the receiver.
The output circuitry processes the amplified signal into a form suitable for the interfacing circuitry. For digital receivers, this circuitry may include low-pass filters and comparators. For analog receivers, this circuitry may also include low-pass filters.
Receiver sensitivity, bandwidth, and dynamic range are key operational parameters used to define receiver performance. One goal in designing fiber optic receivers is to optimize receiver sensitivity. To increase sensitivity, receiver noise resulting from signal-dependent shot noise and thermal noise must be kept at a minimum. A more detailed discussion of receiver shot and thermal noise is provided later in this chapter.
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In addition to optimizing sensitivity, optical receiver design goals also include optimizing the bandwidth and the dynamic range. A receiver that has the ability to operate over a wide range of optical power levels can operate efficiently in both short- and long-distance applications. Because conflicts arise when attempting to meet each goal, trade-offs in receiver designs are made to optimize overall performance.
Q19. Which amplifier stage (the preamplifier or the postamplifier) is a dominant contributor of noise and significantly influences the sensitivity of the receiver?
Q20. List the key operational parameters used to define receiver performance. RECEIVER NOISE
Noise corrupts the transmitted signal in a fiber optic system. This means that noise sets a lower limit on the amount of optical power required for proper receiver operation. There are many sources of noise in fiber optic systems. They include the following:
Because the intent of this chapter is to discuss optical detector and receiver properties, only noise associated with the photodetection process is discussed. Receiver noise includes thermal noise, dark current noise, and quantum noise. Noise is the main factor that limits receiver sensitivity.
Noise introduced by the receiver is either signal dependent or signal independent. Signal dependent noise results from the random generation of electrons by the incident optical power. Signal independent noise is independent of the incident optical power level.
Thermal noise is the noise resulting from the random motion of electrons in a conducting medium. Thermal noise arises from both the photodetector and the load resistor. Amplifier noise also contributes to thermal noise. A reduction in thermal noise is possible by increasing the value of the load resistor. However, increasing the value of the load resistor to reduce thermal noise reduces the receiver bandwidth. In APDs, the thermal noise is unaffected by the internal carrier multiplication.
Shot noise is noise caused by current fluctuations because of the discrete nature of charge carriers. Dark current and quantum noises are two types of noise that manifest themselves as shot noise. Dark current noise results from dark current that continues to flow in the photodiode when there is no incident light. Dark current noise is independent of the optical signal. In addition, the discrete nature of the photodetection process creates a signal dependent shot noise called quantum noise. Quantum noise results from the random generation of electrons by the incident optical radiation. In APDs, the random nature of the avalanche process introduces an additional shot noise called excess noise. For further information on the excess noise resulting from the avalanche process, refer to the avalanche photodiode section.
Q21. List the main types of receiver noise. Q22. What is the main factor that determines receiver sensitivity? Q23. For a reduction in thermal noise, should the value of the detector's load resistor be increased or
decreased?
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Q24. What are two types of noise that manifest themselves as shot noise? RECEIVER DESIGN
The simplest fiber optic receivers consist of only the optical detector and a load resistor. However, the output signal of these simple receivers is not in a suitable form for most types of interfacing circuitry. To produce a suitable signal, a preamplifier, a post amplifier, and other circuitry are generally included in the receiver.
The choice of an optical detector and the design of the preamplifier help determine the operational characteristics of the receiver. Fiber optic receivers using APDs have greater sensitivity than those using PIN photodiodes. In addition, trade-offs are made in preamplifier designs to increase sensitivity while optimizing bandwidth and dynamic range. The two basic types of amplifiers used in fiber optic receivers are the high-impedance amplifier and the transimpedance amplifier.
The high-impedance preamplifier is generally used with a large load resistor to improve sensitivity. The large load resistor is used to reduce thermal noise. Although the high-impedance preamplifier achieves high sensitivity, receiver bandwidth and dynamic range are limited. The transimpedance preamplifier uses a low-noise, high-input impedance amplifier with negative feedback. This design provides improvements in bandwidth and dynamic range with some degradation in sensitivity from an increase in noise. For more information on receiver performance and design, refer to the reference material listed in appendix 2.
Q25. What are the two basic types of preamplifiers used in fiber optic receivers? Q26. Which preamplifier design (high-impedance or transimpedance) provides improvements in
bandwidth and greater dynamic range with some degradation in sensitivity from an increase in noise?
Fiber optic receivers come in packages similar to those for fiber optic transmitters. For information on fiber optic receiver packages, refer back to the fiber optic transmitter packages section of chapter 6.
Fiber optic receivers can be classified into two categories: digital and analog. Digital receivers detect the input optical signal, amplify the digital photocurrent, and reshape the signal to produce an undistorted output electrical signal. Analog receivers detect the input optical signal and amplify the generated photocurrent.
For most digital applications the designs of the digital fiber optic receivers are similar. For low-data- rate applications, PIN diodes and high impedance amplifiers are generally used. Receiver sensitivities are maximized by using large load resistors in the photodiode circuit. For moderate-data-rate applications, PIN diodes and either high impedance amplifiers with smaller load resistances or transimpedance amplifiers are used. For high-data-rate applications, PINs or APDs are used with transimpedance amplifiers. APDs are rarely used in low- or moderate-data-rate applications unless receivers with extremely low sensitivities are required.
For each digital application, the receiver will generally contain a low-pass filter. The pass-band of the filter depends on the data rate of the application. The filter is used to smooth the amplified signal to
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remove some of the high frequency noise before the signal is further processed. The digital receiver generally contains a comparator, which reshapes the amplified electrical signal to remove any distortions introduced in the transmission process. In some cases the receiver may also contain clock recovery circuitry, which retimes the output electrical signal as well.
Q27. For what types of applications are APDs generally used? Q28. Why is a low-pass filter generally part of a digital fiber optic receiver?
Analog receivers are similar in design to digital receivers with the exception that digital signal restoring circuitry is not used. The preamplifier and postamplifiers are designed to be more linear than those used in digital receivers in some cases.
For low-frequency applications, PIN diodes and high impedance amplifiers are generally used. For moderate-frequency applications, PIN diodes and either high impedance amplifiers or transimpedance amplifiers are used. For high-frequency applications, PINs or APDs are used with transimpedance amplifiers. As in digital applications, APDs are rarely used in low- or moderate-frequency applications unless receivers with extremely low sensitivities are required.
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 8.
A FIBER OPTIC RECEIVER is an electro-optic device that accepts optical signals from an optical fiber and converts them into electrical signals. A typical fiber optic receiver consists of an optical detector, a low-noise amplifier, and other circuitry used to produce the output electrical signal.
RECEIVER SPECTRAL RESPONSE, SENSITIVITY, FREQUENCY RESPONSE, and DYNAMIC RANGE are key receiver performance parameters that can affect overall system operation.
RECEIVER SENSITIVITY is the minimum amount of optical power required to achieve a specific receiver performance. For digital transmission at a given data rate and coding, this performance is described by a maximum bit-error rate (BER). In analog systems, for a given modulation and bandwidth, it is described by a minimum signal-to-noise ratio (SNR).
DYNAMIC RANGE refers to the range of optical power levels over which the receiver operates within the specified values. It usually is described by the ratio of the maximum input power to the sensitivity.
A TRANSDUCER is a device that converts input energy of one form into output energy of another.
An OPTICAL DETECTOR is a transducer that converts an optical signal into an electrical signal. It does this by generating an electrical current proportional to the intensity of incident optical radiation.
The semiconductor POSITIVE-INTRINSIC-NEGATIVE (PIN) PHOTODIODE and AVALANCHE PHOTODIODE (APD) are the principal optical detectors used in fiber optic systems.
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A PHOTOCURRENT is generated when photons are absorbed by a photodiode.
RESPONSIVITY is the ratio of the optical detector's output photocurrent in amperes to the incident optical power in watts.
DARK CURRENT, or reverse-leakage current, is the current that continues to flow in the photodetector when there is no incident light.
The RESPONSE TIME of a photodiode and its output circuitry depends primarily on the thickness of the detector active area and the detector RC time constant.
The TRANSIT TIME is the time it takes electrons to travel out of the detector active area.
The RC TIME CONSTANT is defined by the capacitance (C) of the photodiode and the resistance (R) of the load. The RC time constant is given by tRC = RC.
A HIGH-SPEED RESPONSE requires short transit times and low capacitance. However, any change in photodiode parameters to optimize the transit time and capacitance can also affect quantum efficiency, dark current, and coupling efficiency.
Detector LINEARITY means that the output electrical current (photocurrent) of the photodiode is linearly proportional to the input optical power.
An AVALANCHE PHOTODIODE (APD) is a photodiode that internally amplifies the photocurrent by an avalanche process.
In APDs, a large REVERSE-BIAS VOLTAGE, typically over 100 volts, is applied across the active region.
AVALANCHEMULTIPLICATION occurs when accelerated electrons collide with other electrons in the semiconductor material, causing a fraction of them to become part of the photocurrent.
TRADE-OFFS are made in APD design to optimize responsivity and gain, dark current, response time, and linearity.
The RESPONSE TIME of APDs accounts for the avalanche build-up time in addition to transit time and RC time constant.
The PREAMPLIFIER is defined as the first stage of amplification following the optical detector.
The POSTAMPLIFIER is defined as the remaining stages of amplification required to raise the detectors electrical signal to a level suitable for further signal processing.
RECEIVER SENSITIVITY, BANDWIDTH, and DYNAMIC RANGE are key operational parameters used to define receiver performance.
NOISE is the main factor that determines receiver sensitivity.
RECEIVER NOISE includes thermal noise, dark current noise, and quantum noise.
THERMAL NOISE is the noise resulting from the random motion of electrons in a conducting medium.
SHOT NOISE is noise caused by current fluctuations due to the discrete nature of charge carriers.
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DARK CURRENT NOISE results from dark current that continues to flow in the photodiode when there is no incident light.
QUANTUM NOISE results from the random generation of electrons by the incident optical radiation.
The HIGH-IMPEDANCE AMPLIFIER and the TRANSIMPEDANCE AMPLIFIER are the two basic types of amplifiers used in fiber optic receivers.
The HIGH-IMPEDANCE PREAMPLIFIER provides a high sensitivity, but limits receiver bandwidth and dynamic range.
The TRANSIMPEDANCE PREAMPLIFIER provides improvements in bandwidth and dynamic range with some degradation in sensitivity from an increase in noise.
PIN PHOTODIODES are used as the detector in most applications.
AVALANCHE PHOTODIODES are only used in high-speed applications and applications where detectors with extremely low sensitivities are required.
ANSWERS TO QUESTIONS Q1. THROUGH Q28. A1. An electro-optic device that accepts optical signals from an optical fiber and converts them into
electrical signals.
A2. Amplifier. A3. Receiver sensitivity.
A4. The range of optical power levels over which the receiver operates within the specified values. It usually is described by the ratio of the maximum input power to the sensitivity.
A5. It is a transducer that converts an optical signal into an electrical signal. It does this by generating an electrical current proportional to the intensity of incident optical radiation.
A6. High.
A7. The semiconductor positive-intrinsic-negative (PIN) photodiode and avalanche photodiode (APD).
A8. Silicon, gallium arsenide, germanium, and indium phosphide.
A9. The current produced when photons are incident on the detector active area.
A10. The ratio of the optical detector's output photocurrent in amperes to the incident optical power in watts.
A11. Reverse-biased.
A12. The leakage current that continues to flow through a photodetector when there is no incident light.
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A13. Increase.
A14. Small.
A15. Fast transit time.
A16. Because fiber optic communications systems operate at low optical power levels. A17. The electrons initially generated by the incident photons accelerate as they move through the
APD active region. As these electrons collide with electrons in the semiconductor material, they cause a fraction of them to become part of the photocurrent.
A18. By increasing the reverse-bias voltage.
A19. The preamplifier. A20. Receiver sensitivity, bandwidth, and dynamic range.
A21. Thermal noise, dark current noise, and quantum noise.
A22. Noise.
A23. Increased.
A24. Dark current and quantum noises.
A25. The high-impedance amplifier and the transimpedance amplifier. A26. Transimpedance.
A27. For high-data-rate applications and for low- or moderate-data-rate applications where receivers with extremely low sensitivities are required.
A28. To smooth the amplified signal to remove some of the high frequency noise before the signal is further processed.
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Upon completion of this chapter, you should be able to do the following:
1. Describe a basic point-to-point fiber optic data link.
2. Explain the difference between digital and analog fiber optic communications systems.
3. Discuss the most common types of line coding used in digital fiber optic communications including non-return-to-zero (NRZ), return-to-zero (RZ), and biphase (or Manchester).
4. Describe the main type of analog modulation.
5. State several precautions that need to be emphasized when installing fiber optic links on board ships.
Most of the discussion on fiber optic data links provided earlier in this training manual refers to simple point-to-point links. A point-to-point fiber optic data link consists of an optical transmitter, optical fiber, and an optical receiver. In addition, any splices or connectors used to join individual optical fiber sections to each other and to the transmitter and the receiver are included. Figure 8-1 provides a schematic diagram of a point-to-point fiber optic data link.
A common fiber optic application is the full duplex link. This link consists of two simple point-to- point links. The links transmit in opposite directions between the equipments. This application may be configured using only one fiber. If configured with one fiber, fiber optic splitters are used at each end to couple the transmit signal onto the fiber and receive signal to the detector.
All fiber optic systems are simply sets of point-to-point fiber optic links. Different system topologies arise from the different ways that point-to-point fiber optic links can be connected between equipments. The term topology, as used here, refers to the configuration of various equipments and the fiber optic components interconnecting them. This equipment may be computers, workstations, consoles, or other equipments. Point-to-point links are connected to produce systems with linear bus, ring, star, or tree topologies. Point-to-point fiber optic links are the basic building block of all fiber optic systems.
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A linear bus topology consists of a single transmission line that is shared by a number of equipments (see figure 8-2). Generally the transmission line in a fiber optic linear bus consists of two optical lines, one for each direction of communication. Optical taps (optical splitters) are used by each equipment to connect to each line. For each line, the optical tap couples signals from the line to the equipment receiver and from the equipment transmitter onto the line. The connection between any two equipments is a simple point-to-point link that contains the optical tap for each equipment.
A ring topology consists of equipments attached to one another in a closed loop or ring (see figure 8-3). The connection between each equipment is a simple point-to-point link. In some systems each equipment may have an associated optical switch. In normal operation, the switch routes signals from the fiber connected to the previous equipment to the receiver. It also routes signals from the transmitter to the fiber connected to the next equipment. In bypass operation, the switch routes signals from the fiber connected to the previous equipment to the fiber connected to the next equipment. In each case, the connection between adjacent equipments on the ring is a simple point-to-point link through fiber, connectors, and switches.
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In the star topology, each equipment is connected to a common center hub (see figure 8-4). The center hub can be a passive fiber optic star coupler or an active equipment. If the center hub is a passive star coupler, each equipment transmitter is connected to an input port of the coupler and an output port of the coupler is connected to each equipment receiver. The connection between any two equipments is a simple point-to-point link through the star coupler. If the center hub is an active equipment, the connection between any two equipments consists of two point-to-point links. Each connection consists of one link from the first equipment to the center hub and a second link from the center hub to the second equipment.
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A tree topology consists of a transmission line that branches, or splits (see figure 8-5). A tree topology may have many different branching points. At each branching point either a passive fiber optic splitter or an active branching device is used. In many cases both passive couplers and active branching devices are used within a particular system. Regardless of the branching method, each connection within the tree is a simple point-to-point link through splitters or multiple point-to-point links through active branching devices.
Q1. List four system topologies that can be constructed using point-to-point fiber optic links. Q2. Which topology (linear bus, ring, star, or tree) consists of equipments attached to one another in
a closed loop?
Q3. Which topology (bus, ring, star, or tree) has a center hub interconnecting the equipments?
While there are several ways to classify fiber optic links, this chapter classifies links according to the modulation type: either digital or analog.Modulation is the process of varying one or more characteristics of an optical signal to encode and convey information. Generally, the intensity of the optical signal is modulated in fiber optic communications systems. Digital modulation implies that the optical signal consists of discrete levels. Analog modulation implies that the intensity of the optical signal
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is proportional to a continuously varying electrical input. Most fiber optic systems are digital because digital transmission systems generally provide superior performance over analog transmission systems.
Q4. Define modulation. DIGITAL TRANSMISSION
A digital signal is a discontinuous signal that changes from one state to another in discrete steps. A popular form of digital modulation is binary, or two level, digital modulation. In binary modulation the optical signal is switched from a low-power level (usually off) to a high-power level. There are a number of modulation techniques used in digital systems, but these will not be discussed here. For more information on digital modulation techniques, refer to the references listed in appendix 2.
Line coding is the process of arranging symbols that represent binary data in a particular pattern for transmission. The most common types of line coding used in fiber optic communications include non- return-to-zero (NRZ), return-to-zero (RZ), and biphase, or Manchester. Figure 8-6 illustrates NRZ, RZ, and biphase (Manchester) encoding.
NRZ code represents binary 1s and 0s by two different light levels that are constant during a bit duration. The presence of a high-light level in the bit duration represents a binary 1, while a low-light level represents a binary 0. NRZ codes make the most efficient use of system bandwidth. However, loss of timing may result if long strings of 1s and 0s are present causing a lack of level transitions.
RZ coding uses only half the bit duration for data transmission. In RZ encoding, a half period optical pulse present in the first half of the bit duration represents a binary 1. While an optical pulse is present in the first half of the bit duration, the light level returns to zero during the second half. A binary 0 is represented by the absence of an optical pulse during the entire bit duration. Because RZ coding uses only half the bit duration for data transmission, it requires twice the bandwidth of NRZ coding. Loss of timing can occur if long strings of 0s are present.
Biphase, or Manchester, encoding incorporates a transition into each bit period to maintain timing information. In Manchester encoding, a high-to-low light level transition occurring in the middle of the bit duration represents a binary 1. A low-to-high light level transition occurring in the middle of the bit duration represents a binary 0.
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For further information on digital encoding schemes and modulation techniques, refer to the reference material listed in appendix 2.
Digital transmission offers an advantage with regard to the acceptable signal-to-noise ratio (SNR) at the optical receiver. Digital communications systems can tolerate large amounts of signal loss and dispersion without impairing the ability of the receiver to distinguish a binary 1 from a binary 0. Digital signalling also reduces the effects that optical source nonlinearities and temperature have on system performance. Source nonlinearities and temperature variations can severely affect analog transmission. Digital transmission provides superior performance in most complex systems (such as LANs) and long- haul communications systems. In short-haul systems, the cost and complexity of analog-to-digital and digital-to-analog conversion equipment, in some cases, outweigh the benefits of digital transmission.
Q5. What is a digital signal? Q6. In NRZ code, does the presence of a high-light level in the bit duration represent a binary 1 or a
binary 0?
Q7. How can the loss of timing occur in NRZ line coding? Q8. How is a binary 1 encoded in RZ line coding? Q9. In Manchester encoding, does a low-to-high light level transition occurring in the middle of the
bit duration represent a binary 1 or a binary 0?
An analog signal is a continuous signal whose amplitude, phase, or some other property varies in a direct proportion to the instantaneous value of a physical variable. An example of an analog signal is the output power of an optical source whose intensity is a function of a continuous electrical input signal.
Most analog fiber optic communications systems intensity modulate the optical source. In intensity modulation, the intensity of the optical source's output signal is directly modulated by the incoming electrical analog baseband signal. A baseband signal is a signal that is in its original form and has not been changed by a modulation technique.
In some cases, the optical source may be directly modulated by a incoming electrical signal that is not a baseband signal. In these cases the original electrical signal generally modulates an electrical subcarrier frequency. The most common form of analog subcarrier modulation in fiber optic systems is frequency modulation (FM). The optical source is intensity modulated by the electrical subcarrier.
While most fiber optic systems employ digital modulation techniques, there are certain applications where analog modulation techniques are preferred. The transmission of video using analog techniques is very popular, especially for shorter distances, where costs can be minimized and complex multiplexing and timing equipment is unnecessary. The transmission of analog voice signals may also be attractive in small, short-haul systems. In addition, fiber optic sensor systems may incorporate analog transmission. Requirements that analog transmission places on applications include high signal-to-noise ratio and high source linearity. While analog transmission can be attractive for short-haul or medium-haul systems, it is unattractive for long-haul systems where digital techniques provide better performance.
Q10. What is an analog signal? Q11. What type of modulation do most analog fiber optic communications systems use?
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Q12. Why has the transmission of video using analog techniques been very popular, especially for shorter distances?
Fiber optic systems can be simple point-to-point data links or can involve more complex topologies. However, it is generally necessary only to refer to point-to-point data links when discussing the process of link design. Fiber optic systems that incorporate complex architectures can be simplified into a collection of point-to-point data links before beginning the design process.
Fiber optic system design is a complicated process that involves link definition and analysis. The design process begins by providing a complete description of the communication requirements. This information is used to develop the link architecture and define the communications links. System designers must decide on the operational wavelength and types of components to use in the system. These decisions affect numerous system and link design parameters, such as launched power, connection losses, bandwidth, cost, and reliability.
Once a system design has been formulated, each link is analyzed to determine its viability. Link analysis involves calculating each link's power budget and risetime budget. Calculating a power budget involves identifying all of the sources of loss in the fiber optic link. These losses and an additional safety margin are then compared to the difference between the transmitter output power and the receiver sensitivity. The difference between the transmitter output power and the receiver sensitivity is referred to as the available power. If the sources of loss plus the safety margin are less than the available power in the link, the design is viable.
Calculating the risetime budget involves calculating the risetimes of the link transmitter and the optical fiber. The composite optical transmitter/fiber risetime is referred to as the fiber exit risetime. If the fiber exit risetime is less than the maximum input risetime specified for the link receiver, then the link design is viable.
If a proposed link design is not viable, the system designer will reevaluate various decisions made earlier in the system design. These reevaluations may include using a different transmitter or receiver or may involve redesigning the physical configuration of the link. Because there are many variables involved in link design and analysis, it may take several iterations before the variables are combined in a manner that ensures link operation. For more information of fiber optic system design, refer to the Navy Fiber Optic System Design Standard.
Q13. Why is it generally only necessary to refer to point-to-point data links when discussing the process of fiber optic system design?
Q14. List five system design parameters considered when system designers choose the system operational wavelength and link components.
Q15. What two analyses are performed to determine if a link design is viable?
The Navy has a standard to provide detailed information and guidance to personnel concerned with the installation of fiber optic cable plants on naval surface ships and submarines. The fiber optic cable
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plant consists of all the fiber optic cables and the fiber optic interconnection equipment within the ship, including connectors, splices, and interconnection boxes. The fiber optic cable plant installation standard consists of a basic standard and six numbered parts dealing with the following:
There are other standards that discuss fiber optic system installation. Many of these standards incorporate procedures for repair, maintenance, and testing. The techniques developed for installing fiber optic hardware are not much different than for installing hardware for copper-based systems. However, the primary precautions that need to be emphasized when installing fiber optic systems on board ships are as follows:
• Optical fibers or cables should never be bent at a radius of curvature less than a certain value, called theminimum bend radius. Bending an optical fiber or cable at a radius smaller than the minimum bend radius causes additional fiber loss.
• Fiber optic cables should never be pulled tight or fastened over or through sharp corners or cutting edges. Extremely sharp bends increase the fiber loss and may lead to fiber breakage.
• Fiber optic connectors should always be cleaned before mating. Dirt in a fiber optic connection will significantly increase the connection loss and may damage the connector.
• Precautions must be taken so the cable does not become kinked or crushed during installation of the hardware. Extremely sharp kinks or bends increase the fiber loss and may lead to fiber breakage.
• Only trained, authorized personnel should be allowed to install or repair fiber optic systems.
Q16. Optical fibers or cables should never be bent at a radius of curvature smaller than a certain value. Identify this radius of curvature.
Q17. List five precautions to take when installing fiber optic systems on board naval ships.
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Now that you have completed this chapter, let's review some of the new terms, concepts, and ideas that you have learned. Understanding the basics of fiber optic system classification, design, and installation is recommended before you begin studying specific fiber optic system applications.
A basic POINT-TO-POINT fiber optic data link consists of an optical transmitter, optical fiber, and an optical receiver. In addition, any splices or connectors used to join individual optical fiber sections to each other and to the transmitter and the receiver are included.
The term TOPOLOGY refers to the configuration of various equipments and the fiber optic components interconnecting them.
A LINEAR BUS TOPOLOGY consists of a single transmission line that is shared by a number of equipments.
A RING TOPOLOGY consists of equipments attached to one another in a closed loop or ring.
In the STAR TOPOLOGY, each equipment is connected to a common center hub. The center hub can be a passive fiber optic star coupler or an active equipment.
A TREE TOPOLOGY consists of a transmission line that branches, or splits.
FIBER OPTIC LINKS are classified according to the modulation type: either digital or analog.
DIGITAL MODULATION implies that the optical signal consists of discrete levels.
ANALOGMODULATION implies that the intensity of the optical signal is proportional to a continuously varying electrical input.
MODULATION is the process of varying one or more characteristics of an optical signal to encode and convey information.
A DIGITAL SIGNAL is a discontinuous signal that changes from one state to another in discrete steps.
BINARY, or two level, digital modulation is a popular form of digital modulation.
LINE CODING is the process of arranging symbols that represent binary data in a particular pattern for transmission. The most common types of line coding used in fiber optic communications include non- return-to-zero (NRZ), return-to-zero (RZ), and biphase, or Manchester.
DIGITAL TRANSMISSION offers an advantage with regard to the acceptable SNR at the optical receiver.
An ANALOG SIGNAL is a continuous signal that varies in a direct proportion to the instantaneous value of a physical variable.
Most ANALOG FIBER OPTIC COMMUNICATIONS SYSTEMS intensity modulate the optical source.
In INTENSITY MODULATION, the intensity of the optical source's output signal is directly modulated by the incoming electrical analog baseband signal.
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A BASEBAND SIGNAL is a signal that is in its original form and has not been changed by a modulation technique.
FIBER OPTIC SYSTEMS that have complex architectures can be simplified into a collection of point-to-point data links.
LINK ANALYSIS involves calculating each link's power budget and risetime budget.
Calculating a POWER BUDGET involves identifying all of the sources of loss in the fiber optic link. These losses and an additional safety margin are then compared to the difference between the transmitter output power and the receiver sensitivity.
Calculating the RISETIME BUDGET involves calculating the risetimes of the link transmitter and the optical fiber.
The FIBER OPTIC CABLE PLANT consists of all the fiber optic cables and the fiber optic interconnection equipment within the ship, including connectors, splices, and interconnection boxes.
OPTICAL FIBERS or CABLES should never be bent at a radius of curvature less than a certain value, called the minimum bend radius.
FIBER OPTIC CONNECTORS should always be cleaned before mating.
ANSWERS TO QUESTIONS Q1. THROUGH Q17. A1. Linear bus, ring, star, and tree topologies.
A2. Ring.
A3. Star.
A4. The process of varying one or more characteristics of an optical signal to encode and convey information.
A5. A discontinuous signal that changes from one state to another in discrete steps. A6. Binary 1.
A7. If long strings of 1s or 0s are present causing a lack of level transitions. A8. A half-period optical pulse present in the first half of the bit duration. A9. Binary 0.
A10. A continuous signal that varies in a direct proportion to the instantaneous value of a physical variable.
A11. Intensity modulation.
A12. Because cost can be minimized and complex multiplexing and timing equipment is unnecessary.
A13. Because fiber optic systems that incorporate complex architectures can be simplified into a collection of point-to-point data links before beginning the design process.
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A14. Launch power, connection losses, bandwidth, cost, and reliability.
A15. Power budget and risetime budget.
A16. Minimum bend radius.
A17.
a. Never bend an optical fiber or cable at a radius of curvature less than the minimum bend radius.
b. Never pull fiber optic cables tight or fasten them over or through sharp corners or cutting edges.
c. Always clean fiber optic connectors before mating. d. Do not kink or crush fiber optic cable during installation of the hardware. e. Allow only trained, authorized personnel to install or repair fiber optic systems.
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NOTE: The following references were current at the time this TRAMAN was published, but you should be sure you have the current edition.
An Introduction to Fiber Optics: Handbook, HB 5444, Rev. A., AMP Incorporated, Harrisburg, Penn., 1982.
Brown, G. D., J. P. Ingold, D. R. Knudsen, and S. E. Spense, "A Ship-Wide System Engineering Approach for Fiber Optics for Surface Combatants," Naval Engineers Journal, Vol. 102, No. 3, The American Society of Naval Engineers, Alexandria, Va., May 1990.
Costa, B., ed., "Historical Remarks," Optical Fiber Communications, Technical staff of CSELT, McGraw-Hill, New York, N.Y., 1980.
Darden, P. C., "Introduction," Fiber Optic Communications: From the Learning Tree, Course 440, Integrated Computer Systems, Los Angeles, Calif., 1985.
Glossary, Fiber Optics, MIL-STD-2196, Naval Sea Systems Command, Washington, D.C., 1989.
IEEE Standard Definitions of Terms Relating to Fiber Optics, ANSI/IEEE Std 812-1984, The Institute of Electrical and Electronics Engineers, Inc., New York, N.Y., 1984.
Keiser, G., "Overview of Optical Fiber Communications," T. M. Slaughter, and M. Eichberg, eds, Optical Fiber Communications, McGraw-Hill, New York, N.Y., 1983.
Palladino, J. R., "Historical Perspective," Fiber Optics: Communicating by Light, Bellcore, Piscataway, N.J., 1990.
Senior, J. M., "Introduction," P. J. Dean, ed., Optical Fiber Communications: Principles and Practice, Prentice-Hall, Englelwood Cliffs, N.J., 1985.
Costa, B., ed. "The Optical Fibre," Optical Fiber Communications, Technical staff of CSELT, McGraw-Hill, New York, N.Y., 1980.
Darden, P. C., "Fiber Technology," Fiber Optic Communications: From the Learning Tree, Course 440, Integrated Computer Systems, Los Angeles, Calif., 1985.
DiVita, P., and V. Rossi, "Light Propagation Theory in Optical Fibers," Optical Fiber Communications, Technical staff of CSELT, McGraw-Hill, New York, N.Y., 1980.
Glossary, Fiber Optics, MIL-STD-2196, Naval Sea Systems Command, Washington, D.C., 1989.
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Hopiavuori, E. M., and J. A. Jay, "Dispersion-Shifted Fiber Hits Its Stride," Photonics Spectra, Sep 1990, pp. 153-158.
IEEE Standard Definitions of Terms Relating to Fiber Optics, ANSI/IEEE Std 812-1984, The Institute of Electrical and Electronics Engineers, Inc., New York, N.Y., 1984.
Keiser, G., "Optical Fibers: Structures and Waveguide Fundamentals," T. M. Slaughter, and M. Eichberg, eds., Optical Fiber Communications, McGraw-Hill, New York, N.Y., 1983.
"Light and Optical Fibers," An Introduction to Fiber Optics: Handbook, HB 5444, Rev. A, AMP Inc., Harrisburg, Penn., 1982.
Naval Electricity and Electronics Training Series, Introduction to Wave Propagation, Transmission Lines, and Antennas, Module 10, Naval Education and Training Professional Development and Technology Center, Pensacola, Fla., 1983.
Palladino, J. R., "Transmission Principles," Fiber Optics: Communicating by Light, Bellcore, Piscataway, N.J., 1990.
Quinn, D. E., "Optical Fibers," Frederick C. Allard, ed., Fiber Optics Handbook for Engineers and Scientists, McGraw-Hill, New York, N.Y., 1990.
Senior, J. M., "Optical Fiber Waveguides and Transmission Characteristics of Optical Fibers," P. J. Dean, ed., Optical Fiber Communications: Principles and Practice, Prentice-Hall, Englewood Cliffs, N.J., 1985.
Senior, J. M., "Transmission Characteristics of Optical Fibers," P. J. Dean, ed., Optical Fiber Communications: Principles and Practice, Prentice-Hall, Englewood Cliffs, N.J., 1985.
Solitro, M., "Basic Fiber-Optic Concepts," Communications Technology, Sep 1990, p. 32.
Balchin, G. A., G. D. Brown, J. P. Ingold, and S. E. Spense, "The Development of Steady State Modal Conditions in Short Length Concatenated Systems," Fiber Optics Reliability: Benign and Adverse Environment II, SPIE Vol. 992, International Society for Optical Engineers, Bellingham, Wash., 1988.
Brown, G. D., J. P. Ingold, D. R. Knudsen, and S. E. Spense, "A Ship-Wide System Engineering Approach for Fiber Optics for Surface Combatants," Naval Engineers Journal, Vol. 102, No. 3, The American Society of Naval Engineers, Alexandria, Va., May 1990.
Cocito, G., G. Roba, and P. Vergnano, "Material and Technologies for Optical Fibers," Optical Fiber Communications, Technical staff of CSELT, McGraw-Hill, New York, N.Y., 1980.
Costa, B., ed., "The Optical Fibre," Optical Fiber Communications, Technical staff of CSELT, McGraw-Hill, New York, N.Y., 1980.
Darden, P. C., "Fiber Technology," Fiber Optic Communications: From the Learning Tree, Course 440, Integrated Computer Systems, Los Angeles, Calif., 1985.
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DiVita, P., and V. Rossi, "Light Propagation Theory in Optical Fibres," Optical Fiber Communications, Technical staff of CSELT, McGraw-Hill, New York, N.Y., 1980.
Fiber Optic Cable Plant Installation Standard Methods for Naval Ships (Cables), DOD-STD- XXXX-1 (Navy), Naval Sea Systems Command, Washington, D.C., draft 1990.
Fluevog, J. B., K. Kathiresan, and L. R. Sherrets, "Fiberoptic Cables for Shipboard Applications," Fiberoptic Product News, July 1991, pp. 15-20.
Galliano, G., and F. Tosco, "Optical Fibre Cables," Optical Fiber Communications, Technical staff of CSELT, McGraw-Hill, New York, N.Y., 1980.
Glossary, Fiber Optics, MIL-STD-2196, Naval Sea Systems Command, Washington, D.C., 1989.
Hartman, J.,"Outdoor Fiber Installation: Part 1," Communications Technology, Oct 1990, pp. 88- 92.
Hopiavuori, E. M., and J. A. Jay, "Dispersion-Shifted Fiber Hits Its Stride," Photonics Spectra, Sep 1990, pp. 153-158.
IEEE Standard Definitions of Terms Relating to Fiber Optics, ANSI/IEEE Std 812-1984, The Institute of Electrical and Electronics Engineers, Inc., New York, N.Y., 1984.
Keiser, G., "Optical Fiber Fabrication and Cabling," T. M. Slaughter, and M. Eichberg, eds., Optical Fiber Communications, McGraw-Hill, New York, N.Y., 1983.
Keiser, G., "Optical Fibers: Structures and Waveguide Fundamentals," T. M. Slaughter, and M. Eichberg, eds., Optical Fiber Communications, McGraw-Hill, New York, N.Y., 1983.
Keiser, G., "Signal Degradation in Optical Fibers," T. M. Slaughter, and M. Eichberg, eds., Optical Fiber Communications, McGraw-Hill, New York, N.Y., 1983.
"Optical Fiber Choices," Multimode Fiber Optic Link Transmission Design, FO-2.2, Subcommittee on Local Area Networks, Telecommunications Systems Bulletin, May 1992.
"Optical Fibers," An Introduction to Fiber Optics: Handbook, HB 5444, Rev. A, AMP Inc., Harrisburg, Penn., 1982.
Palladino, J. R., "Transmission Principles and Properties of Fiber and Cable," Fiber Optics: Communicating by Light, (updated), Bellcore, Piscataway, N.J., 1990.
Pearson, E. R., "Comparing Fiber Types," Lightwave, Nov 1991, pp. 29-31.
Quinn, D. E., "Optical Fibers," Frederick C. Allard, ed., Fiber Optics Handbook for Engineers and Scientists, McGraw-Hill, New York, N.Y., 1990.
Ramsay, M. M., "Fiber Optic Cables," Frederick C. Allard, ed., Fiber Optics Handbook for Engineers and Scientists, McGraw-Hill, New York, N.Y., 1990.
Schnecker, J., "Optical Fiber Termination and End Preparation," Fiberoptic Product News, Sep 1990, pp. 29-31.
Senior, J. M., "Optical Fibers, Cables, and Connections," P. J. Dean, ed., Optical Fiber Communications: Principles and Practice, Prentice-Hall, Englewood Cliffs, N.J., 1985.
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Senior, J. M., "Transmission Characteristics of Optical Fibers, P. J. Dean, ed., Optical Fiber Communications: Principles and Practice, Prentice-Hall, Englewood Cliffs, N.J., 1985.
Surat, C., "Fiber Cable: Market Trends and Technology," Photonics Spectra, Dec 1990, pp. 95- 98.
Anderson T. W., "A Method of Measuring Splice Loss with an Injection/Detector Test Set," Lightwave, Dec 1990, pp. 35-36.
Barnoski, M. K., "Fiber Couplers," H. Kressel, ed., Topics in Applied Physics: Semiconductor Devices for Optical Communication, 2d updated ed, Springer-Verlag, New York, N.Y., 1982.
Darden, P. C., "Fiber Technology," Fiber Optic Communications: From the Learning Tree, Course 440, Integrated Computer Systems, Los Angeles, Calif., 1985.
De Jong, M., "Connector Designs Simplify Field Installations," Laser Focus World, Aug 1990, pp. 165-167.
Esposto, F., and E. Vezzoni, "Connecting and Splicing Techniques," Optical Fiber Communications, Technical staff of CSELT, McGraw-Hill, New York, N.Y., 1980.
Fahey, M., "Cleave Findings Could Cut Costs in Loop," Lightwave, June 1991, p. 1.
DOD-STD-XXXX-5 (Navy), Naval Sea Systems Command, Washington, D.C., draft 1990. Glossary, Fiber Optics, MIL-STD-2196, Naval Sea Systems Command, Washington, D.C., 1989.
IEEE Standard Definitions of Terms Relating to Fiber Optics, ANSI/IEEE Std 812-1984, The Institute of Electrical and Electronics Engineers, Inc., New York, N.Y., 1984.
"Interconnections," An Introduction to Fiber Optics: Handbook, HB 5444, Rev. A, AMP Inc., Harrisburg, Penn., 1982.
Keiser, G., "Power Launching and Coupling," T. M. Slaughter, and M. Eichberg, eds., Optical Fiber Communications, McGraw-Hill, New York, N.Y., 1983.
Midkiff, J. A., "Fusion vs Mechanical Splices: A Question of Cost and Application," Lightwave, Aug 1990, pp. 20-23.
Morra, P., and E. Vezzoni, "Fiber-Optic Splices, Connectors, and Couplers," Frederick C. Allard, ed., Fiber Optics Handbook for Engineers and Scientists, Mcgraw-Hill, New York, N.Y., 1990.
Murphy, K. W., "Building Blocks Offer Coupler Flexibility," Photonics Spectra, Aug 1990, pp. 133-138.
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Palladino, J. R., "Properties of Fiber and Cable," Fiber Optics: Communicating by Light, Bellcore, Piscataway, N.J., 1990.
"Passive Optical Devices," Multimode Fiber Optic Link Transmission Design, FO-2.2, Subcommittee on Local Area Networks, Telecommunications Systems Bulletin, May 1992.
Schnecker, J., "Optical Fiber Termination and End Preparation," Fiberoptic Product News, Sep 1990, pp. 29-31.
Senior, J. M., "Optical Fiber, Cables, and Connections," P. J. Dean, ed., Optical Fiber Communications: Principles and Practice, Prentice-Hall, Englewood Cliffs, N.J., 1985.
Vitale, A., "Fiber Alignment: Stability, Precision," Photonics Spectra, Jan 1991, pp. 125-128.
Anderson T. W., "A Method of Measuring Splice Loss with an Injection/Detector Test Set," Lightwave, Dec 1990, pp. 35-36.
Balchin, G. A., G. D. Brown, J. P. Ingold, and S. E. Spense, "The Development of Steady State Modal Conditions in Short Length Concatenated Systems," Fiber Optics Reliability: Benign and Adverse Environment II, SPIE Vol. 992, International Society for Optical Engineers, Bellingham, Wash., 1988.
Costa, B., and B. Sordo, eds., "Fibre Characterization," Optical Fiber Communications, Technical staff of CSELT, McGraw-Hill, New York, N.Y., 1980.
Diamond, J., "Buying an OTDR," Fiberoptic Product News, Oct 1991, pp. 25-26.
Fahey, M., "Cleave Findings Could Cut Costs in the Loop," Lightwave, June 1991, p. 1.
Fiber Optic Cable Plant Installation Standard Methods for Naval Ships (Tests), DOD-STD- XXXX-6 (Navy), Naval Sea Systems Command, Washington, D.C., draft Sep 1990.
FOTP-34: Interconnection Device Insertion Loss Test, EIA-455-34, Electronics Industries Association, Washington, D.C., May 1985.
FOTP-43: Output Near-Field Radiation Pattern Measurement, EIA-455-43, Electronics Industries Association, Washington, D.C., Dec 1984.
FOTP-46: Spectral Attenuation Measurement for Long Length, Graded-Index Fibers, EIA-455- 46, Electronics Industries Association, Washington, D.C., May 1983.
FOTP-47: Output Far-Field Radiation Pattern Measurement, EIA/TIA-455-47A, Electronics Industries Association, Washington, D.C., May 1989.
FOTP-50: Light Launch Conditions for Long-Length Graded-Index Optical Fiber Spectral Attenuation Measurements, EIA-455-50A, Electronics Industries Association, Washington, D.C., Nov 1987.
FOTP-57: Optical Fiber End Preparation and Examination, EIA/TIA-455-57A, Electronics Industries Association, Washington, D.C., Apr 1990.
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FOTP-58: Core Diameter Measurement of Graded-Index Optical Fibers, EIA/TIA-455-58A, Electronics Industries Association, Washington, D.C., Oct 1990.
FOTP-59: Measurement of Fiber Point Defects Using an OTDR, EIA/TIA-455-59, Electronics Industries Association, Washington, D.C., Oct 1989.
FOTP-60: Measurement of Fiber or Cable Length Using an OTDR, EIA/TIA-455-60, Electronics Industries Association, Washington, D.C., Oct 1989.
FOTP-61: Measurement of Fiber or Cable Attenuation Using an OTDR, EIA/TIA-455-61, Electronics Industries Association, Washington, D.C., Oct 1989.
EIA/TIA-455-77, Electronics Industries Association, Washington, D.C., draft.
FOTP-78: Spectral-Attenuation Cutback Measurement for Single-Mode Optical Fibers, EIA/TIA-455-78A, Electronics Industry Association, Washington, D.C., Apr 1990.
FOTP-80: Cutback Wavelength of Uncabled Single-Mode Fiber by Transmitted Power, EIA/TIA-455-80, Electronics Industry Association, Washington, D.C., July 1988.
FOTP-167: Mode Field Diameter Measurement--Variable Aperture Method in the Far Field, EIA/TIA-455-167, Electronics Industries Association, Washington, D.C., July 1987.
FOTP-170: Cable Cutoff Wavelength of Single-Mode Fiber by Transmitted Power, EIA/TIA- 455-170, Electronics Industries Association, Washington, D.C., Oct 1989.
FOTP-176: Measurement Method for Optical Fiber Geometry by Automated Gray-Scale Analysis, EIA/TIA 455-176, Electronics Industries Association, Oct 1989.
FOTP-177: Numerical Aperture Measurement of Graded-Index Optical Fibers, EIA/TIA-455- 177, Electronics Industries Association, Washington, D.C., Oct 1989.
Glossary, Fiber Optics, MIL-STD-2196, Naval Sea Systems Command, Washington, D. C., 1989.
IEEE Standard Definitions of Terms Relating to Fiber Optics, ANSI/IEEE Std 812-1984, The Institute of Electrical and Electronics Engineers, Inc., New York, N.Y., 1984.
Kapron, F. P., "Fiber Optic Test Methods," Frederick C. Allard, ed., Fiber Optics Handbook for Engineers and Scientists, McGraw-Hill, New York, N.Y., 1990.
Keiser, G., "Measurements," T. M. Slaughter and M. Eichberg, eds., Optical Fiber Communications, McGraw Hill, New York, N.Y., 1983.
Optical Power Loss Measurements of Installed Multimode Fiber Cable Plant, EIA/TIA-526-14, Electronics Industry Association, Washington, D.C., Nov 1990.
Schnecker, J., "Optical Fiber Termination and End Preparation," Fiberoptic Product News, Sep 1990, pp. 29-31.
AII-7
Senior, J. M., "Optical Fiber Measurements," P. J. Dean, ed., Optical Fiber Communications: Principles and Practice, Prentice-Hall, Englewood Cliffs, N.J., 1985.
Darden, P. C., "Photoemitters: Solid State Laser and Light-Emitting Diodes and Their Application to Fiber Optic Data Transmission," Fiber Optic Communications: From the Learning Tree, Course 440, Integrated Computer Systems, Los Angeles, Calif., 1985.
DiDomenico, M., and P. W. Shumate, "Lightwave Transmitters," H. Kressel, ed., Topics in Applied Physics: Semiconductor Devices for Optical Communication, 2d updated ed, Springer-Verlag, New York, N.Y., 1982.
Ettenberg, M., H. Kressel, J. P. Wittke, and I. Ladany, "Laser Diodes and LEDs for Fiber Optical Communication," H. Kressel, ed., Topics in Applied Physics: Semiconductor Devices for Optical Communication, 2d updated ed, Springer-Verlag, New York, N.Y., 1982.
"Fiber Optic Transmitter/Source Choices," Multimode Fiber Optic Link Transmission Design, FO-2.2, Subcommittee on Local Area Networks, Telecommunications Systems Bulletin, May 1992.
Glossary, Fiber Optics, MIL-STD-2196, Naval Sea Systems Command, Washington, D.C., 1989.
Higgins, T. V., ed., "Fiberoptic Sources and Detectors Come of Age," Laser Focus World, Nov 1990, pp. 173-188.
IEEE Standard Definitions of Terms Relating to Fiber Optics, ANSI/IEEE Std 812-1984, The Institute of Electrical and Electronics Engineers, Inc., New York, N.Y., 1984.
Jungbluth, E. D., ed., "Transmitters and Receivers Must Deliver High Performance," Laser Focus World, July 1991, pp. 167-183.
Keiser, G., "Optical Sources," T. M. Slaughter and M. Eichberg, eds., Optical Fiber Communications, McGraw-Hill, New York, N.Y., 1983.
Keiser, G., Appendix E: "Topics from Semiconductor Physics," T. M. Slaughter, and M. Eichberg, eds., Optical Fiber Communications, McGraw-Hill, New York, N.Y., 1983.
Lai Yu, P. K., and K. Li, "Optical Sources for Fibers," Frederick C. Allard, ed., Fiber Optics Handbook for Engineers and Scientists, McGraw-Hill, New York, N.Y., 1990.
Millman, J., "Part 1: Semiconductor Device Characteristics," J. V. Brown, M. Eichberg, and P. D. Nalle, eds., Microelectronics: Digital and Analog Circuits and Systems, McGraw-Hill, New York, N.Y., 1979.
Randone, G., "Electro-Optical Sources," Optical Fiber Communications, Technical staff of CSELT, McGraw-Hill, New York, N.Y., 1980.
Senior, J. M., "Optical Sources 1: The Laser" P. J. Dean, ed., Optical Fiber Communications: Principles and Practice, Prentice-Hall, Englewood Cliffs, N.J., 1985.
AII-8
Senior, J. M., "Optical Sources 2: The Light Emitting Diode," P. J. Dean, ed., Optical Fiber Communications: Principles and Practice, Prentice-Hall, Englewood Cliffs, N.J., 1985.
"Sources and Transmitters," An Introduction to Fiber Optics: Handbook, HB 5444, Rev. A, AMP Inc., Harrisburg, Penn., 1982.
Carni, P., "Photodetectors," Optical Fiber Communications, Technical staff of CSELT, McGraw- Hill, New York, N.Y., 1980.
Darden, P. C., "Noise and Bit Error Rate," Fiber Optic Communications: From the Learning Tree, Course 440, Integrated Computer Systems, Los Angeles, Calif., 1985.
Darden, P. C., "Photodetectors: Solid State Silicon and Gallium Arsenide Photodtectors and Their Applications," Fiber Optic Communications: From the Learning Tree, Course 440, Integrated Computer Systems, Los Angeles, Calif., 1985.
"Detectors and Receivers," An Introduction to Fiber Optics: Handbook, HB 5444, Rev. A, AMP Inc., Harrisburg, Penn., 1982.
"Fiber Optic Receiver/Detector Choices," Multimode Fiber Optic Link Transmission Design, FO- 2.2, Subcommittee on Local Area Networks, Telecommunications Systems Bulletin, May 1992.
Glossary, Fiber Optics, MIL-STD-2196, Naval Sea Systems Command, Washington, D.C., 1989.
Hartman, A. R., D. P. Schinke, and R. G. Smith, "Photodetectors," H. Kressel, ed., Topics in Applied Physics: Semiconductor Devices for Optical Communication, 2d updated ed., Springer-Verlag, New York, N.Y., 1982.
Higgins, T. V., ed., "Fiberoptic Sources and Detectors Come of Age," Laser Focus World, Nov 1990, pp. 173-188.
IEEE Standard Definitions of Terms Relating to Fiber Optics, ANSI/IEEE Std 812-1984, The Institute of Electrical and Electronics Engineers, Inc., New York, N.Y., 1984.
Jungbluth, E. D., ed., "Transmitters and Receivers Must Deliver High Performance," Laser Focus World, July 1991, pp. 167-183.
Keiser, G., "Optical Receiver Operation," T. M. Slaughter, and M. Eichberg, eds., Optical Fiber Communications, McGraw-Hill, New York, N.Y., 1983.
Keiser, G., "Photodetectors," T. M. Slaughter, and M. Eichberg, eds., Optical Fiber Communications, McGraw-Hill, New York, N.Y., 1983.
Lai Yu, P. K., and K. Li, "Optical Detectors for Fibers," Frederick C. Allard, ed., Fiber Optics Handbook for Engineers and Scientists, McGraw-Hill, New York, N.Y., 1990.
Personick, S. D., and R. G. Smith, "Receiver Design for Optical Fiber Communication Systems," H. Kressel, ed., Topics in Applied Physics: Semiconductor Devices for Optical Communication, 2d updated ed, Springer-Verlag, New York, N.Y., 1982.
AII-9
Senior, J. M., "Optical Detectors," P. J. Dean, ed., Optical Fiber Communications: Principles and Practice, Prentice-Hall, Englewood Cliffs, N.J., 1985.
Senior, J. M., "Receiver Noise Considerations," P. J. Dean, ed., Optical Fiber Communications: Principles and Practice, Prentice-Hall, Englewood Cliffs, N.J., 1985.
"Communication and Data Transmission," An Introduction to Fiber Optics: Handbook, HB 5444, Rev. A, AMP Inc., Harrisburg, Penn., 1982.
Daly, J. C., and A. Fascia, "Modulation," Frederick C. Allard, ed., Fiber Optics Handbook for Engineers and Scientists, McGraw-Hill, New York, N.Y., 1990.
Fiber Optic Cable Plant Installation Standard Methods for Naval Ships, DOD-STD-XXXX-1 through -6 (Navy), Naval Sea Systems Command, Washington, D.C., draft Sep 1990.
Fiber Optic System Design, DOD-STD-YYYY (Navy), Naval Sea Systems Command, Washington, D.C., draft.
Glossary, Fiber Optics, MIL-STD-2196, Naval Sea Systems Command, Washington, D.C., 1989.
Hartman, J.,"Outdoor Fiber Installation: Part 1," Communications Technology, Oct 1990, pp. 88- 92.
IEEE Standard Definitions of Terms Relating to Fiber Optics, ANSI/IEEE Std 812-1984, The Institute of Electrical and Electronics Engineers, Inc., New York, N.Y., 1984.
Keiser, G., "Transmission Link Analyses," T. M. Slaughter, and M. Eichberg, eds., Optical Fiber Communications, McGraw-Hill, New York, N.Y., 1983.
Luvison, A., "Topics in Optical Fibre Communication Theory," Optical Fiber Communications, Technical staff of CSELT, McGraw-Hill, New York, N.Y., 1980.
Moncalvo, A., and L. Sacchi, "System Considerations," Optical Fiber Communications, Technical staff of CSELT, McGraw-Hill, New York, N.Y., 1980.
Multimode Fiber Optic Link Transmission Design, FO-2.2, Subcommittee on Local Area Networks, Telecommunications Systems Bulletin, May 1992.
Naval Electricity and Electronics Training Series, Modulation Principles, Module 12, Naval Education and Training Professional Development and Technology Center, Pensacola, Fla., 1983.
Pitt, G. D., "Optical Fiber Sensors," Frederick C. Allard, ed., Fiber Optics Handbook for Engineers and Scientists, McGraw-Hill, New York, N.Y., 1990.
Senior, J. M., "Optical Fiber Systems," P. J. Dean, ed., Optical Fiber Communications: Principles and Practice, Prentice-Hall, Englewood Cliffs, N.J., 1985.
Sunak, H., "Fiber-Optic System Design," Frederick C. Allard, ed., Fiber Optics Handbook for Engineers and Scientists, McGraw-Hill, New York, N.Y., 1990.
AII-10
The Fiber Optic LAN Handbook, (Expanded 3d edition for 1991), Engineering Staff of Codenoll Technology Corp., Codenoll Technology Corporation, Yonkers, N.Y., 1990.
INDEX-1
INDEX-2
INDEX-3
INDEX-4
INDEX-5
INDEX-6
Information: The text pages that you are to study are provided at the beginning of the assignment questions.
1
Textbook assignment: Chapter 1, “Background on Fiber Optics,” pages 1-1 through 1-7. Chapter 2, “Fiber Optic Concepts,” pages 2-1 through 2-35.
1-1. Fiber optics uses what medium to send information?
1. Electrons 2. Phonons 3. Link 11 4. Light
1-2. What are the three parts of a fiber optic data link?
1. Transmitter, optical fiber, receiver 2. Transmitter, optical fiber, optical
connectors 3. Optical fiber, optical connectors,
receiver 4. Optical fiber, optical connectors,
1-3. The fiber optic transmitter has which of the following functions?
1. Amplifies the optical signal 2. Converts the electrical input signal to
an optical signal 3. Converts the input optical signal to
an electrical signal 4. Amplifies the output electrical signal
1-4. Fiber optic systems use what two types of optical sources?
1. LEDs and APDs 2. PIN diodes and LEDs 3. LEDs and laser diodes 4. Laser diodes and APDs
1-5. The optical source performs which of the following functions?
1. Amplifies the optical signal 2. Amplifies the electrical signal 3. Launches the optical signal into the
fiber 4. Converts the optical signal to an
1-6. What fiber mechanisms weaken and distort the optical signal launched into the fiber?
1. Scattering, absorption, and dispersion 2. Scattering, radiation, and absorption 3. Dispersion, radiation, and absorption 4. Scattering, reflection, and refraction
1-7. The fiber optic receiver performs which of the following functions?
1. Amplifies the optical signal 2. Amplifies the electrical signal 3. Converts the electrical signal back
into an optical signal 4. Converts the optical signal back into
1-8. What are the two parts of a fiber optic receiver?
1. Fiber and case 2. Fiber and optical detector 3. Optical detector and PIN diode 4. Optical detector and signal-
2
1-9. Noise has what effect, if any, on the quality of a signal?
1. Increases 2. Reduces 3. None; noise has no effect
1-10. What are the two types of optical
detectors?
1. LEDs and APDs 2. PIN diodes and APDs 3. APDs and laser diodes 4. Laser diodes and PIN diodes
1-11. Besides the transmitter, the optical fiber, and the receiver, which of the following components may be found in a fiber optic link?
1. Splices only 2. Couplers only 3. Connectors only 4. Splices, couplers, and connectors
1-12. Extremely high losses occurred in early fibers because of which of the following conditions?
1. Cracks in the fibers 2. Holes in the fiber sides 3. Core areas too small in the fibers 4. Impurities in the fiber material
1-13. What was the first light source developed that could be easily coupled into a fiber?
1. LED 2. Lamp 3. YAG laser 4. PIN diode
1-14. Compared to multimode fibers, single mode fibers tend to have lower loss and produce less signal distortion.
1. True 2. False
1-15. What multimode fiber properties help reduce connection losses?
1. Larger core size and lower NA 2. Larger core size and higher NA 3. Smaller core size and lower NA 4. Smaller core size and higher NA
1-16. Fiber optics is being used in which of the following types of applications?
1. Long-haul communications 2. Subscriber-loop applications 3. Military applications 4. All of the above
1-17. Of the following advantages, which one does NOT apply to fiber optics?
1. Established standards 2. Increased bandwidth 3. Improved environmental 4. Improved signal security
1-18. Of the following factors, which ones are advantages of fiber optic systems?
1. Electrical isolation and low
bandwidth 2. Immunity to noise and low
bandwidth 3. Electrical isolation and immunity to
noise 4. Signal security and high price
1-19. To describe the nature of light, which of the following ways can be used?
1. Electromagnetic wave only 2. Particles of energy only 3. Electromagnetic wave and particles
of energy 4. Element
1-20. Light exhibits what kind of wave motion?
1. Longitudinal 2. Transverse 3. Turbulent 4. Aperiodic
3
1-21. Which of the following factors is a description of transverse wave motion?
1. The wave motion is not predictable 2. The wave magnitude varies parallel
to the direction of propagation 3. The wave magnitude varies
1-22. What does a transparent substance do to
light rays that fall on it?
1. Absorbs them 2. Reflects them 3. Refracts them 4. Transmits them
1-23. What does a translucent substance do to light rays that fall on it?
1. Reflects and absorbs them 2. Refracts and absorbs them 3. Transmits and diffuses them 4. Transmits and reflects them
1-24. Which of the following substances is a good example of a translucent substance?
1. Cardboard 2. Clear glass 3. Frosted glass 4. Aluminum foil
1-25. Which of the following substances is a good example of a transparent substance?
1. Cardboard 2. Clear glass 3. Frosted glass 4. Aluminum foil
1-26. What does an opaque substance do when light rays fall on it?
1. Refracts them 2. Reflects or absorbs them 3. Transmits them only 4. Transmits and diffuses them
1-27. Which of the following substances is an example of an opaque substance?
1. Cardboard 2. Clear glass 3. Oiled paper 4. Frosted glass
1-28. Which of the following objects is NOT an example of a luminous source?
1. Sun 2. Gas flame 3. Mirror 4. Flashlight
1-29. What are light waves called that strike a surface but are neither transmitted nor absorbed?
1. Diffused 2. Refracted 3. Reflected 4. Diffracted
1-30. What is the name of the law that states "The angle of incidence is equal to the angle of reflection"?
1. Snell's Law 2. Murphy's Law 3. Law of Entropy 4. Law of Reflection
1-31. A light wave is incident on a surface. The reflected power is the greatest in which of the following incidences?
1. 30º 2. 45º 3. Perpendicular 4. Almost parallel
4
1-32. A light wave passes from one medium into another medium with a different velocity. As the wave enters the second medium, the change of direction is known by which of the following terms?
1. Reflection 2. Refraction 3. Absorption 4. Diffusion
1-33. If a light wave passes from a less dense medium to a more dense medium, how does the angle of refraction compare to the angle of incidence?
1. Greater than the angle of incidence
only 2. Equal to the angle of incidence only 3. Greater than or equal to the angle of
incidence 4. Less than the angle of incidence
1-34. What is another word for diffused?
1. Absorbed 2. Refracted 3. Scattered 4. Attenuated
1-35. When light falls on a piece of black paper, what happens to most of the light?
1. It is absorbed 2. It is reflected 3. It is scattered 4. It is refracted
1-36. Light is transmitted along an optical fiber by what two methods?
1. Ray theory and mode theory 2. Ray theory and photon theory 3. Ray theory and quantum theory 4. Mode theory and photon theory
1-37. How does the speed of light in the fiber compare to the speed of light in the air?
1. It is slower in the fiber 2. It is faster in the fiber 3. It is the same in both the fiber and the
1-38. The relationship between the incident rays and the refracted rays at a boundary between mediums with different indexes of refraction describes what law?
1. Bragg's Law 2. Snell's Law 3. Murphy's Law 4. Law of Reflection
1-39. Total internal reflection occurs at which of the following angles?
1. Obtuse angle 2. Fresnel angle 3. Right angle 4. Critical angle of incidence
1-40. What are the three basic parts of an optical fiber?
1. Core, cladding, and coating 2. Inside, middle, and outside 3. Fiber, kevlar, and jacket 4. Hole, shell, and coating
1-41. The cladding performs all except which of the following functions?
1. Reduces the loss of light from the
core 2. Reduces the scattering loss at the
surface of the core 3. Protects the fiber core from
absorbing surface contaminants 4. Reduces mechanical strength
5
1-42. What is the definition of a bound ray?
1. A ray that cannot move 2. A ray that travels in the air 3. A ray that is refracted out of the fiber 4. A ray that propagates through the
1-43. The fiber NA relates to which of the following characteristics?
1. Physical size of the fiber 2. Tensile strength of the fiber 3. Maximum angle within the fiber
acceptance cone 4. Speed of light within the fiber
1-44. A skew ray is which of the following types of rays?
1. An unbound ray 2. A meridional ray 3. An unbalanced ray 4. A ray that propagates without passing
1-45. Electromagnetic wave behavior is used to describe the propagation of light along the fiber in what theory?
1. Mode theory 2. Particle theory 3. Darwin's theory 4. Rayleigh's theory
1-46. High-order modes cross the axis of the fiber at steeper angles than low-order modes.
1. True 2. False
1-47. Compared to a low-order mode, the electrical and magnetic fields of a high- order mode are distributed more toward the center of a fiber.
1. True 2. False
1-48. What are the two basic types of fibers?
1. Small and large 2. Glass and plastic 3. Opaque and diffuse 4. Single mode and multimode
1-49. Compared to multimode fibers, single mode fibers have a larger core size.
1. True 2. False
1-50. The lowest signal loss and the highest bandwidth are characteristic of which of the following types of fibers?
1. Air core 2. Multimode 3. Single mode 4. Plastic core
1-51. Compared to single mode fibers, multimode fibers have which of the following advantages?
1. Ease of making connections only 2. Ease of launching light into them
only 3. Ease of both making connections and
launching light into them 4. Lower dispersion
1-52. System performance is affected most by which of the following fiber properties?
1. NA and delta 2. Core diameter and NA 3. Attenuation and dispersion 4. Core diameter and cladding
1-53. The loss of optical power as light travels along a fiber is called
1. attenuation 2. scattering 3. absorption 4. dispersion
6
1-54. Attenuation is specified in what units?
1. dB 2. dB/km 3. µm 4. µm/km