Darcie, T.E., Palais, J.C., Kaminow, I P. Optical Communication The Electrical Engineering Handbook Ed. Richard C. Dorf Boca raton crc Press llc. 2000
Darcie, T.E., Palais, J.C., Kaminow, I.P. “Optical Communication” The Electrical Engineering Handbook Ed. Richard C. Dorf Boca Raton: CRC Press LLC, 2000
71 Optical Communication 71.1 Lightwave Technology for Video Transmission Video Formats and applic Intensity Modulation. Noise Limitations· Linearity Requirements· Laser Linearity. Clipping. External Modulation. Miscellaneous Impairments· Summary 71.2 Long Distance Fiber Optic Communications TE. Darcie Fiber· Modulator· Light Source Source ATe)T Bell laboratories Coupler. Isolator. Connectors and Splices. Optical Amplifier. Repeater. Photodetector Receiver. Other Joseph C. Palais Components. System Considerations. Error Rates andSignal-to- Arizona State University Noise Ratio. System Design 71.3 Photonic Networks Ivan p kamino Data Links. Token Ring: FDDI, FFOL Active Star Networks: ATeTBell laboratories Ethernet, Datakit".New Approaches to Optical Networks 71.1 Lightwave Technology for Video Transmission T.E. Darcie Lightwave technology has revolutionized the transmission of analog and, in particular, video information Because the light output intensity from a semiconductor laser is linearly proportional to the injected current, and the current generated in a photodetector is linearly proportional to the incident optical intensity, analog information is transmitted as modulation of the optical intensity. The lightwave system is analogous to a linear electrical link, where current or voltage translates linearly into optical intensity. High-speed semiconductor lasers and photodetectors enable intensity-modulation bandwidths greater than 10 GHz. Hence, a wide variety radio frequency(RF)and microwave applications have been developed [ Darcie, 1990] Converting microwaves into intensity-modulated (IM) light allows the use of optical fiber for transmission in place of bulky inflexible coaxial cable or microwave waveguide. Since the fiber attenuation is 0. 20.4 dB/km, ompared with several decibels per meter for waveguide, entirely new applications and architectures are possible. In addition, the signal is confined tightly to the core of single-mode fiber, where it is immune to electromagnetic interference, cross talk, or spectral regulatory control. To achieve these advantages, several limitations must be overcome. The conversion of current to light intensity must be linear. Several nonlinear mechanisms must be avoided by proper laser design or by the use of various linearization techniques. Also, because the photon energy is much larger than in microwave systems, the signal fidelity is limited by quantum or shot noise This section describes the basic technology for the transmission of various video formats. We begin by describing the most common video formats and defining transmission requirements for each. Sources of noise, including shot noise, relative intensity noise(RIN), and receiver noise are then quantified Limitations impose by source nonlinearity, for both direct modulation of the laser bias current and external modulation using an interferometric LiNbO, modulator, are compared. Finally, several other impairments caused by fiber non- nearity or fiber dispersion are discussed c 2000 by CRC Press LLC
© 2000 by CRC Press LLC 71 Optical Communication 71.1 Lightwave Technology for Video Transmission Video Formats and Applications • Intensity Modulation • Noise Limitations • Linearity Requirements • Laser Linearity • Clipping • External Modulation • Miscellaneous Impairments • Summary 71.2 Long Distance Fiber Optic Communications Fiber • Modulator • Light Source • Source Coupler • Isolator • Connectors and Splices • Optical Amplifier • Repeater • Photodetector • Receiver • Other Components • System Considerations • Error Rates andSignal-toNoise Ratio • System Design 71.3 Photonic Networks Data Links • Token Ring: FDDI, FFOL • Active Star Networks: Ethernet, Datakit“ • New Approaches to Optical Networks 71.1 Lightwave Technology for Video Transmission T. E. Darcie Lightwave technology has revolutionized the transmission of analog and, in particular, video information. Because the light output intensity from a semiconductor laser is linearly proportional to the injected current, and the current generated in a photodetector is linearly proportional to the incident optical intensity, analog information is transmitted as modulation of the optical intensity. The lightwave system is analogous to a linear electrical link, where current or voltage translates linearly into optical intensity. High-speed semiconductor lasers and photodetectors enable intensity-modulation bandwidths greater than 10 GHz. Hence, a wide variety of radio frequency (RF) and microwave applications have been developed [Darcie, 1990]. Converting microwaves into intensity-modulated (IM) light allows the use of optical fiber for transmission in place of bulky inflexible coaxial cable or microwave waveguide. Since the fiber attenuation is 0.2–0.4 dB/km, compared with several decibels per meter for waveguide, entirely new applications and architectures are possible. In addition, the signal is confined tightly to the core of single-mode fiber, where it is immune to electromagnetic interference, cross talk, or spectral regulatory control. To achieve these advantages, several limitations must be overcome. The conversion of current to light intensity must be linear. Several nonlinear mechanisms must be avoided by proper laser design or by the use of various linearization techniques. Also, because the photon energy is much larger than in microwave systems, the signal fidelity is limited by quantum or shot noise. This section describes the basic technology for the transmission of various video formats. We begin by describing the most common video formats and defining transmission requirements for each. Sources of noise, including shot noise,relative intensity noise (RIN), and receiver noise are then quantified. Limitations imposed by source nonlinearity, for both direct modulation of the laser bias current and external modulation using an interferometric LiNbO3 modulator, are compared. Finally, several other impairments caused by fiber nonlinearity or fiber dispersion are discussed. T.E. Darcie AT&T Bell Laboratories Joseph C. Palais Arizona State University Ivan P. Kaminow AT&TBell Laboratories
Video Formats and applications Each video format represents a compromise between transmission bandwidth and robustness or immunity impairment. With the exception of emerging digital formats, each is also an entrenched standard that often reflects the inefficiencies of outdated techno FM Video Frequency-modulated (FM) video has served for decades as the basis for satellite video transmission [Pratt and Bostian, 1986], where high signal-to-noise ratios(SNRs)are difficult to achieve. Video information with a bandwidth of B,=4.2 MHz is used to FM modulate an RF carrier. The resulting channel bandwidth B is given by B~△fPp+2fm (71.1) where Afp, is the frequency deviation(22.5 MHz) and m is the audio subcarrier frequency(6.8 MHz). As a result of this bandwidth expansion to typically 36 MHz, a high SNR can be obtained for the baseband video bandwidth B, even if the received carrier-to-noise ratio(CNR) over the fm bandwidth B is small. The SnR is given by SNR=CNR+ 10 log/3B(]pe +w+ pe (71.2) where W is a weighting factor(13. 8 dB)that accounts for the way the eye responds to noise in the video bandwidth, and PE is a pre-emphasis factor(0-5 dB) that is gained by emphasizing the high-frequency video components to improve the performance of the FM modulator. High-quality video(snr=55 dB)requires a CNR of only 16 dB. This is achieved easily in a lightwave transmission system. Applications for lightwave FM video transmission include links to satellite transmission facilities, transport of video between cable television company head-ends(super-trunking), and perhaps delivery of video to subscribers over large fiber distribution networks [ Way et al, 1988; Olshansky et al., 1988] AM-VSBⅤideo The video format of choice both for broadcast and cable television distribution is Am-vSB. each consists of an RF carrier that is amplitude modulated(AM) by video information. Single-sideband (VSB )filtering is used to minimize the bandwidth of the modulated spectrum. The resultant RF spectrum is dominated by the remaining RF carrier, which is reduced by typically 5.6 dB by the AM, and contains relativel low-level signal information, including audio and color subcarriers. An AM-VSB channel requires a bandwidth of only 6 MHz, but CNRs must be at least 50 dB For cable distribution, many channels are frequency-division multiplexed( FDM), separated nominally by 6 MHz (8 MHz in Europe), over the bandwidth supported by the coaxial cable. A typical 60-channel cable system operates between 55.25 and 439.25 MHz Given the large dynamic range required to transmit both the remaining RF carrier and the low-level sidebands, transmission of this multichannel spectrum is a challenge for lightwave technology The need for such systems in cable television distribution systems has motivated the development of suitable high-performance lasers. Before the availability of lightwave AM-VSB systems, cable systems used long(up to 20 km)trunks of coaxial cable with dozens of cascaded electronic amplifiers to overcome cable loss.Accumu lations of distortion and noise, as well as inherent reliability problems with long cascades, were serious limi Fiber AM-VSB trunk systems can replace the long coaxial trunks so that head-end quality video can be delivered deep within the distribution network [Chiddix et al., 1990]. Inexpensive coaxial cable extends from he optical receivers at the ends of the fiber trunks to each home. Architectures in which the number of electronic amplifiers between each receiver and any home is approximately three or fewer offer a good compromise between cost and performance. The short spans of coaxial cable support bandwidths approaching 1 GHz, two e 2000 by CRC Press LLC
© 2000 by CRC Press LLC Video Formats and Applications Each video format represents a compromise between transmission bandwidth and robustness or immunity to impairment. With the exception of emerging digital formats, each is also an entrenched standard that often reflects the inefficiencies of outdated technology. FM Video Frequency-modulated (FM) video has served for decades as the basis for satellite video transmission [Pratt and Bostian, 1986], where high signal-to-noise ratios (SNRs) are difficult to achieve. Video information with a bandwidth of Bv = 4.2 MHz is used to FM modulate an RF carrier. The resulting channel bandwidth B is given by B ; Dfpp + 2fm (71.1) where Dfpp is the frequency deviation (22.5 MHz) and fm is the audio subcarrier frequency (6.8 MHz). As a result of this bandwidth expansion to typically 36 MHz, a high SNR can be obtained for the baseband video bandwidth Bv even if the received carrier-to-noise ratio (CNR) over the FM bandwidth B is small. The SNR is given by (71.2) where W is a weighting factor (13.8 dB) that accounts for the way the eye responds to noise in the video bandwidth, and PE is a pre-emphasis factor (0–5 dB) that is gained by emphasizing the high-frequency video components to improve the performance of the FM modulator. High-quality video (SNR = 55 dB) requires a CNR of only 16 dB. This is achieved easily in a lightwave transmission system. Applications for lightwave FM video transmission include links to satellite transmission facilities, transport of video between cable television company head-ends (super-trunking), and perhaps delivery of video to subscribers over large fiber distribution networks [Way et al., 1988; Olshansky et al., 1988]. AM-VSB Video The video format of choice, both for broadcast and cable television distribution, is AM-VSB. Each channel consists of an RF carrier that is amplitude modulated (AM) by video information. Single-sideband vestigial (VSB) filtering is used to minimize the bandwidth of the modulated spectrum. The resultant RF spectrum is dominated by the remaining RF carrier, which is reduced by typically 5.6 dB by the AM, and contains relatively low-level signal information, including audio and color subcarriers. An AM-VSB channel requires a bandwidth of only 6 MHz, but CNRs must be at least 50 dB. For cable distribution, many channels are frequency-division multiplexed (FDM), separated nominally by 6 MHz (8 MHz in Europe), over the bandwidth supported by the coaxial cable. A typical 60-channel cable system operates between 55.25 and 439.25 MHz. Given the large dynamic range required to transmit both the remaining RF carrier and the low-level sidebands, transmission of this multichannel spectrum is a challenge for lightwave technology. The need for such systems in cable television distribution systems has motivated the development of suitable high-performance lasers. Before the availability of lightwave AM-VSB systems, cable systems used long (up to 20 km) trunks of coaxial cable with dozens of cascaded electronic amplifiers to overcome cable loss. Accumulations of distortion and noise, as well as inherent reliability problems with long cascades, were serious limitations. Fiber AM-VSB trunk systems can replace the long coaxial trunks so that head-end quality video can be delivered deep within the distribution network [Chiddix et al., 1990]. Inexpensive coaxial cable extends from the optical receivers at the ends of the fiber trunks to each home. Architectures in which the number of electronic amplifiers between each receiver and any home is approximately three or fewer offer a good compromise between cost and performance. The short spans of coaxial cable support bandwidths approaching 1 GHz, two SNR CNR W PE = + Ê Ë Á ˆ ¯ ˜ È Î Í Í ˘ ˚ ˙ ˙ 10 + + 3 2 log B B f v B pp v D
or three times the bandwidth of the outdated long coaxial cable trunks. With fewer active components, reliability is improved. The cost of the lightwave components can be small compared to the overall system cost. These ompelling technical and economic advantages resulted in the immediate demand for lightwave AM-VSB Compressed Digital Video [Netravali and Haskel, 1988]. For years digital"NTSC-like"video required s ital video(CDV)technology The next generation of video formats will be the product of compressed dig Mbps. CDV technology can reduce the required bit rate to less than 5 Mbps. This compression requires complex digital signal processing and large-scale circuit integration, but advances in chip and microprocessor design have made inexpensive implementation of the compression algorithms feasible. Various levels of compression complexity can be used, depending on the ultimate bit rate and quality required. Each degree of complexity removes different types of redundancy from the video image. The image is broken nto blocks of pixels, typically 8X 8. By comparing different blocks and transmitting only the differences (DPCM), factors of 2 reduction in bit rate can be obtained. No degradation of quality need result. Much of the information within each block is imperceptible to the viewer. Vector quantization(vQ)or discrete-cosine transform(DCT)techniques can be used to eliminate bits corresponding to these imperceptible details. This intraframe coding can result in a factor of 20 reduction in the bit rate, although the evaluation of image quality becomes subjective. Finally, stationary images or moving objects need not require constant retransmission of every detail. Motion compression techniques have been developed to eliminate these interframe redundancies. Combinations of these techniques have resulted in coders that convert NTSC-like video(100 Mbps uncom pressedinto a few megabits per second and HDTV images (1 Gbps uncompressed) into less than 20 Mbp CDV can be transmitted using time-division multi plexing(TDM) and digital lightwave systems or by using channel to modulate an RF carrier and transmitting analog lightwave systems. There are numerous COMPRESSION applications for both alternatives. TDM systems for CDV 10 are no different from any other digital transmission sys- tem and will not be discussed further Using RF techniques offers an additional level of RF ompression, wherein advanced multilevel modulation d formats are used to maximize the number of bits per hertz 4 QAM uses 8 ampli- tude and 8 phase levels and requires only 1 Hz for 5 bits NTER-3-DIGITAL of information. As the number of levels, hence the num- ber of bits per hertz, increases, the CNR of the channel o must increase to maintain error-free transmission. A 64 QAM channel requires a CNR of approximately 30 dB A synopsis of the bandwidth and CNR requirem 2-LEVEL 64-QAM for FM, AM-VSB, and CDv is shown in Fig. 71.1. AM VSB requires high CNR but low bandwidth. FM is the opposite. Digital video can occupy a wide area, depending FIGURE 71.1 versu on the degree of digital and RF compression. The com bination of CDV and QAM offers the possibility of (CNR) required for AM-VSB, FM, and digital video ueezing a high-quality video channel into 1 MHz of reduce the bit rate required for nTsc-like video from bandwidth, with a required CNR of 30 dB. This drastic 100 Mbps to less than 5 Mbps. Bandwidth efficient rF improvement over AM-VSB or FM could have tremen- techniques like QAM minimize the bandwidth require dous impact on future video transmission systems for each bit rate but require greater CNRs. e 2000 by CRC Press LLC
© 2000 by CRC Press LLC or three times the bandwidth of the outdated long coaxial cable trunks.With fewer active components,reliability is improved. The cost of the lightwave components can be small compared to the overall system cost. These compelling technical and economic advantages resulted in the immediate demand for lightwave AM-VSB systems. Compressed Digital Video The next generation of video formats will be the product of compressed digital video (CDV) technology [Netravali and Haskel, 1988]. For years digital “NTSC-like” video required a bit rate of approximately 100 Mbps. CDV technology can reduce the required bit rate to less than 5 Mbps. This compression requires complex digital signal processing and large-scale circuit integration, but advances in chip and microprocessor design have made inexpensive implementation of the compression algorithms feasible. Various levels of compression complexity can be used, depending on the ultimate bit rate and quality required. Each degree of complexity removes different types of redundancy from the video image. The image is broken into blocks of pixels, typically 8 ¥ 8. By comparing different blocks and transmitting only the differences (DPCM), factors of 2 reduction in bit rate can be obtained. No degradation of quality need result. Much of the information within each block is imperceptible to the viewer. Vector quantization (VQ) or discrete-cosine transform (DCT) techniques can be used to eliminate bits corresponding to these imperceptible details. This intraframe coding can result in a factor of 20 reduction in the bit rate, although the evaluation of image quality becomes subjective. Finally, stationary images or moving objects need not require constant retransmission of every detail. Motion compression techniques have been developed to eliminate these interframe redundancies. Combinations of these techniques have resulted in coders that convert NTSC-like video (100 Mbps uncompressed) into a few megabits per second and HDTV images (1 Gbps uncompressed) into less than 20 Mbps. CDV can be transmitted using time-division multiplexing (TDM) and digital lightwave systems or by using each channel to modulate an RF carrier and transmitting using analog lightwave systems. There are numerous applications for both alternatives. TDM systems for CDV are no different from any other digital transmission system and will not be discussed further. Using RF techniques offers an additional level of RF compression, wherein advanced multilevel modulation formats are used to maximize the number of bits per hertz of bandwidth [Feher, 1987]. Quadrature-amplitude modulation (QAM) is one example of multilevel digitalto-RF conversion. For example, 64-QAM uses 8 amplitude and 8 phase levels and requires only 1 Hz for 5 bits of information. As the number of levels, hence the number of bits per hertz, increases, the CNR of the channel must increase to maintain error-free transmission. A 64- QAM channel requires a CNR of approximately 30 dB. A synopsis of the bandwidth and CNR requirements for FM, AM-VSB, and CDV is shown in Fig. 71.1. AMVSB requires high CNR but low bandwidth. FM is the opposite. Digital video can occupy a wide area, depending on the degree of digital and RF compression. The combination of CDV and QAM offers the possibility of squeezing a high-quality video channel into 1 MHz of bandwidth, with a required CNR of 30 dB. This drastic improvement over AM-VSB or FM could have tremendous impact on future video transmission systems. FIGURE 71.1 Bandwidth versus carrier-to-noise ratio (CNR) required for AM-VSB, FM, and digital video. Increasingly complex digital compression techniques reduce the bit rate required for NTSC-like video from 100 Mbps to less than 5 Mbps. Bandwidth efficient RF techniques like QAM minimize the bandwidth required for each bit rate but require greater CNRs
Intensity Modulation As mentioned in the introduction, the light output from the laser should be linearly proportional to the injected current. The laser is prebiased to an average output intensity Lo. Many video channels are combined electron- ically, and the total RF signal is added directly to the laser current. The optical modulation depth(m)is defined the ratio of the peak modulation Lo for one channel, divided by Lo For 60-channel AM-VSB systems, m is typically near The laser(optical carrier) is modulated by the sum of the video channels that are combined to form the total RF signal spectrum. The resultant optical spectrum contains sidebands from the IM superimposed on unintentional frequency modulation, or chirp, that generally accompanies IM. This complex optical spectrum must by understood if certain subtle impairments are to be avoided a photodetector converts the incident optical power into current. Broadband In GaAs photodetectors with responsivities(Ro)of nearly 1.0 A/W and bandwidths greater than 10 GHz are available. The detector generates a dc current corresponding to the average received optical power L, and the complete RF modulation spectrum that was applied at the transmitter. An ac-coupled electronic preamplifier is used to remove the dc component and boost the signal to usable levels. Noise limitations The definition of Cnr deserves clarification. Depending on the video format and RF modulation technique, the rF power spectrum of the modulated RF carrier varies widely. For AM-VSB video the remaining carrier is the dominant feature in the spectrum. It is thereby convenient to define the Cnr as the ratio of the power remaining in the carrier to the integrated noise power in a 4-MHz bandwidth centered on the carrier frequency. then necessary to define the CNR as the ratio of the integrated signal power within the channel bandwib For FM or digitally modulated carriers, the original carrier is not generally visible in the RF spectrum. the integrated noise power. Shot noise Shot noise is a consequence of the statistical nature of the photodetection process. It results in a noise power spectral density, or electrical noise power per unit bandwidth(dBm/Hz) that is proportional to the received photocurrent I (Ro Lr). The total shot noise power in a bandwidth B is given by N=2el B where e is the electronic charge. With small m, the detected signal current is a small fraction of the total received current. The root mean square(rms)signal power for one channel is P,=1(ml)2 (714) The total shot noise power then limits the CNr (P /N )to a level referred to as the quantum limit Received powers near 1 mW are required if CNRs greater than 50 dB are to be achieved for 40-to 80-channel AM-VSB Receiver noise Receiver noise is generated by the electronic amplifier used to boost the detected photocurrent to usable levels. The easiest receiver to build consists of a pin photodiode connected directly to a low-noise 50-to 75-Q2 amplific as shown in Fig. 71.2(a). The effective input current noise density,(n), for this simple receiver is given by (71.5) RL e 2000 by CRC Press LLC
© 2000 by CRC Press LLC Intensity Modulation As mentioned in the introduction, the light output from the laser should be linearly proportional to the injected current. The laser is prebiased to an average output intensity L0. Many video channels are combined electronically, and the total RF signal is added directly to the laser current. The optical modulation depth (m) is defined as the ratio of the peak modulation L0 for one channel, divided by L0. For 60-channel AM-VSB systems, m is typically near 4%. The laser (optical carrier) is modulated by the sum of the video channels that are combined to form the total RF signal spectrum. The resultant optical spectrum contains sidebands from the IM superimposed on unintentional frequency modulation, or chirp, that generally accompanies IM. This complex optical spectrum must by understood if certain subtle impairments are to be avoided. A photodetector converts the incident optical power into current. Broadband InGaAs photodetectors with responsivities (R0) of nearly 1.0 A/W and bandwidths greater than 10 GHz are available. The detector generates a dc current corresponding to the average received optical power Lr and the complete RF modulation spectrum that was applied at the transmitter. An ac-coupled electronic preamplifier is used to remove the dc component and boost the signal to usable levels. Noise Limitations The definition of CNR deserves clarification. Depending on the video format and RF modulation technique, the RF power spectrum of the modulated RF carrier varies widely. For AM-VSB video the remaining carrier is the dominant feature in the spectrum. It is thereby convenient to define the CNR as the ratio of the power remaining in the carrier to the integrated noise power in a 4-MHz bandwidth centered on the carrier frequency. For FM or digitally modulated carriers, the original carrier is not generally visible in the RF spectrum. It is then necessary to define the CNR as the ratio of the integrated signal power within the channel bandwidth to the integrated noise power. Shot Noise Shot noise is a consequence of the statistical nature of the photodetection process. It results in a noise power spectral density, or electrical noise power per unit bandwidth (dBm/Hz) that is proportional to the received photocurrent Ir (= R0Lr ). The total shot noise power in a bandwidth B is given by Ns = 2eIr B (71.3) where e is the electronic charge. With small m, the detected signal current is a small fraction of the total received current. The root mean square (rms) signal power for one channel is (71.4) The total shot noise power then limits the CNR (Ps /Ns ) to a level referred to as the quantum limit. Received powers near 1 mW are required if CNRs greater than 50 dB are to be achieved for 40- to 80-channel AM-VSB systems. Receiver Noise Receiver noise is generated by the electronic amplifier used to boost the detected photocurrent to usable levels. The easiest receiver to build consists of a pin photodiode connected directly to a low-noise 50- to 75-W amplifier, as shown in Fig. 71.2(a). The effective input current noise density, (n), for this simple receiver is given by (71.5) P mI s r = 1 2 2 ( ) n kTF RL 2 4 =
where k is the Boltzmann constant, T is the absolute temperature, F (a)I PINA-LNA is the noise figure of the amplifier, and R, is the input impedance. For a 50- input impedance and F=2, n=20 pA/VHz A variety of more complicated receiver designs can reduce the noise current appreciably [Kasper, 1988]. The example shown in Fig. 71. 2(b)uses a high-speed FET. Ri can be increased to maximize the voltage developed by the signal current at the FET input. Input capacitance becomes a limitation by shunting high-frequency com ponents of signal current High-frequency signals are then reduced (b)[PIN-FET] with respect to the noise generated in the FET, resulting in poor high rformance. Various impedance matching technique have been proposed to maximize the CNR for specific frequency Relative Intensity Noise GATE Relative intensity noise(RIN) can originate from the laser or from reflections and Rayleigh backscatter in the fiber. In the laser, RIN FIGURE 71.2 Receivers for broadband caused by spontaneous emission in the active layer. Spontaneous to a low-noise amplifier(a)is simple, but ion drives random fluctuations in the number of photons he laser which appear as a random modulation of the output inten- using designs like the pin FET(b)C,is the ity, with frequency components extending to tens of gigahertz. The undesirable input capacitance noise power spectral density from RiN is 1? RIN, where RIN expressed in decibels per hertz. RIN is also caused by component reflections and double-Rayleigh backscatter in the fiber, by a process called multipath interference. Twice-reflected signals arriving at the detector can interfere coherently with the unre- flected signal. Depending on the modulated optical spectrum of the laser, this interference results in noise that can be significant[Darcie et al, 1991] The CNR, including all noise sources discussed, is given by 21 CNR (71.6) 2BIn2 +2el +RIN All sources of intensity noise are combined into RIN. Increasing m improves the CNR but increases the impairment caused by nonlinearity, as discussed in the following subsection. The optimum operating value for m is then a balance between noise and distortion Figure 71.3 shows the noise contributions from shot noise, receiver noise, and RIN For FM or digital systems, the low CNR values required allow operation with small received optical powers. Receiver noise is then generally the limiting factor. Much larger received powers are required if AM-VSB noise requirements are to be met Although detecting more optical power helps to overcome shot and receiver noise, the ratio of signal to RIN remains constant. RIN can be dominant in high-CNR systems, when the received power is large. AM-VSB systems require special care to minimize all sources of rin. The dominant noise source is then shot noise, with receiver noise and RIN combining to limit CNRs to within a few decibels of the quantum limit. Linearity Requirements Source linearity limits the depth of modulation that can be applied. Linearity, in this case, refers to the linearity of the current-to-light-intensity (I-L) conversion in the laser or voltage-to-light(V-L)transmission for an external modulator. Numerous nonlinear mechanisms must be considered for direct modulation and no existing external modulator has a linear transfer function A Taylor-series expansion of the I-Lor V-L characteristic, centered at the bias point, results in linear, quadratic, cubic, and higher-order terms. The linear term describes the efficiency with which the applied signal is converted e 2000 by CRC Press LLC
© 2000 by CRC Press LLC where k is the Boltzmann constant, T is the absolute temperature, F is the noise figure of the amplifier, and RL is the input impedance. For a 50-W input impedance and F = 2, n = 20 pA/ . A variety of more complicated receiver designs can reduce the noise current appreciably [Kasper, 1988]. The example shown in Fig. 71.2(b) uses a high-speed FET. RL can be increased to maximize the voltage developed by the signal current at the FET input. Input capacitance becomes a limitation by shunting high-frequency components of signal current. High-frequency signals are then reduced with respect to the noise generated in the FET,resulting in poor highfrequency performance. Various impedance matching techniques have been proposed to maximize the CNR for specific frequency ranges. Relative Intensity Noise Relative intensity noise (RIN) can originate from the laser or from reflections and Rayleigh backscatter in the fiber. In the laser, RIN is caused by spontaneous emission in the active layer. Spontaneous emission drives random fluctuations in the number of photons in the laser which appear as a random modulation of the output intensity, with frequency components extending to tens of gigahertz. The noise power spectral density from RIN is I 2 r RIN, where RIN is expressed in decibels per hertz. RIN is also caused by component reflections and double-Rayleigh backscatter in the fiber, by a process called multipath interference. Twice-reflected signals arriving at the detector can interfere coherently with the unre- flected signal. Depending on the modulated optical spectrum of the laser, this interference results in noise that can be significant [Darcie et al., 1991]. The CNR, including all noise sources discussed, is given by (71.6) All sources of intensity noise are combined into RIN. Increasing m improves the CNR but increases the impairment caused by nonlinearity, as discussed in the following subsection. The optimum operating value for m is then a balance between noise and distortion. Figure 71.3 shows the noise contributions from shot noise,receiver noise, and RIN. For FM or digital systems, the low CNR values required allow operation with small received optical powers. Receiver noise is then generally the limiting factor. Much larger received powers are required if AM-VSB noise requirements are to be met. Although detecting more optical power helps to overcome shot and receiver noise, the ratio of signal to RIN remains constant. RIN can be dominant in high-CNR systems, when the received power is large. AM-VSB systems require special care to minimize all sources of RIN. The dominant noise source is then shot noise, with receiver noise and RIN combining to limit CNRs to within a few decibels of the quantum limit. Linearity Requirements Source linearity limits the depth of modulation that can be applied. Linearity, in this case, refers to the linearity of the current-to-light-intensity (I-L) conversion in the laser or voltage-to-light (V-L) transmission for an external modulator. Numerous nonlinear mechanisms must be considered for direct modulation, and no existing external modulator has a linear transfer function. A Taylor-series expansion of the I-L or V-L characteristic, centered at the bias point,results in linear, quadratic, cubic, and higher-order terms. The linear term describes the efficiency with which the applied signal is converted FIGURE 71.2 Receivers for broadband analog lightwave systems. Coupling a pin to a low-noise amplifier (a) is simple, but improved performance can be obtained using designs like the pin FET (b). Ct is the undesirable input capacitance. Hz CNR RIN] = + + m I B n eI I r r r 2 2 2 2 2 [ 2
TOTAL FIGURE 71.3 Current noise densities from receivers, RiN, and not noise as a function of total received photocurrent. Receiver o oise is dominant in FM or some digital systems where the total received power is small. The solid line for receiver noise represents IN(-150 dB/Hz) the noise current for a typical 50-52 low-noise amplifier. More sophisticated receiver designs could reduce the noise to the levels wn approximately by the dotted lines. RIN and shot noise more important in AM-VSB systems 8 ll mmmmmmmn 0.8 0.4 H0bttio 250 FIGURE 71.4 Second-order(a)and third-order(b)distortion products for 42-channel AM-VSB system. The maximum number of second-order products occurs at the lowest frequency channel, where 30 products contribute to the CSO. The maximum number of third-order products occurs near the center channel, where 530 products contribute to the CtB to linear intensity modulation. The quadratic term results in second-order distortion, the cubic produces third order distortion and so on. Requirements on linearity can be derived by considering the number and spectral distribution of the distortion products generated by the nonlinear mixing between carriers in the multichannel signal. Second order nonlinearity results in sum and difference +f) mixing products for every combination of the two channels. This results in as many as 50 second-order products within a single channel, in a 60-channel AM VSB system with the standard U.S. frequency plan. Similarly, for third-order distortion, products result from mixing among all combinations of three channels. However, since the number of combinations of three channels much larger than for two, up to 1130 third-order products can interfere with one channel. The cable industry defines the composite second-order( CSo)distortion as the ratio of the carrier to the largest group of second order products within each channel. For third-order distortion, the composite triple beat( CTB)is the ratio of the carrier to the total accumulation of third-order distortion at the carrier frequency in each channel. The actual impairment from these distortion products depends on the spectrum of each RF channel and on the exact frequency plan used. A typical 42-channel AM-VSB frequency plan, with carrier frequencies shown as the vertical bars on Fig 71.4, results in the distributions of second- and third-order products shown in Fig. 71.4(a)and(b), respectively. Since the remaining carrier is the dominant feature in the spectrum of each annel, the dominated by the video requires that the CSO is -60 dBc(dB relative to the carrier), each sum or difference product must be less than-73 dBc. Likewise, for the CTB to be less than 60 dB, each product must be less than approximately -90dB e 2000 by CRC Press LLC
© 2000 by CRC Press LLC to linear intensity modulation. The quadratic term results in second-order distortion, the cubic produces thirdorder distortion, and so on. Requirements on linearity can be derived by considering the number and spectral distribution of the distortion products generated by the nonlinear mixing between carriers in the multichannel signal. Secondorder nonlinearity results in sum and difference (fi ± fj) mixing products for every combination of the two channels. This results in as many as 50 second-order products within a single channel, in a 60-channel AMVSB system with the standard U.S. frequency plan. Similarly, for third-order distortion, products result from mixing among all combinations of three channels.However, since the number of combinations of three channels is much larger than for two, up to 1130 third-order products can interfere with one channel. The cable industry defines the composite second-order (CSO) distortion as the ratio of the carrier to the largest group of secondorder products within each channel. For third-order distortion, the composite triple beat (CTB) is the ratio of the carrier to the total accumulation of third-order distortion at the carrier frequency in each channel. The actual impairment from these distortion products depends on the spectrum of each RF channel and on the exact frequency plan used. A typical 42-channel AM-VSB frequency plan, with carrier frequencies shown as the vertical bars on Fig. 71.4, results in the distributions of second- and third-order products shown in Fig. 71.4(a) and (b), respectively. Since the remaining carrier is the dominant feature in the spectrum of each channel, the distortion products are dominated by the mixing between these carriers. Because high-quality video requires that the CSO is –60 dBc (dB relative to the carrier), each sum or difference product must be less than –73 dBc. Likewise, for the CTB to be less than 60 dB, each product must be less than approximately –90dB. FIGURE 71.3 Current noise densities from receivers, RIN, and shot noise as a function of total received photocurrent. Receiver noise is dominant in FM or some digital systems where the total received power is small. The solid line for receiver noise represents the noise current for a typical 50-W low-noise amplifier. More sophisticated receiver designs could reduce the noise to the levels shown approximately by the dotted lines. RIN and shot noise are more important in AM-VSB systems. FIGURE 71.4 Second-order (a) and third-order (b) distortion products for 42-channel AM-VSB system. The maximum number of second-order products occurs at the lowest frequency channel, where 30 products contribute to the CSO. The maximum number of third-order products occurs near the center channel, where 530 products contribute to the CTB
FM EXTERNAL FIGURE715 Resonance distortion for directly modulated laser 9-60 with resonance frequency of 7 GHz. Both the second-harmonic 2 and two-tone third-order 2fi f; distortion peak near half the res- m=004 onance frequency and are small at low frequency. Also shown is the _10 same third-order distortion for an external modulator biased at the FREQUENCY(GHz) FM or CDV systems have much less restrictive linearity requirements, because of the reduced sensitivity to impairment. Distortion products must be counted, as with the AM-VSB example described previously, but each product is no longer dominated by the remaining carrier. Because the carrier is suppressed entirely by the modulation, each product is distributed over more than the bandwidth of each channel. The impairment resulting from the superposition of many uncorrelated distortion products resembles gous to the CSO and CtB can be defined for these systems Laser linearity Several factors limit the light-versus-current(L-D linearity of directly modulated lasers. Early work on laser dynamics led to a complete understanding of resonance-enhanced distortion(RD). Rd arises from the same carrier-Photon interaction within the laser that is responsible for the relaxation-oscillation resonance The second-harmonic distortion(2ff) and two-tone third-order distortion(2f: -fi) for a typical 1.3-um wavelength directly modulated semiconductor laser are shown in Fig. 71.5 [ Darcie et al, 1986. Both distortions are small at low frequencies but rise to maxima at half the relaxation resonance frequency. AM-VSB systems are feasible only within the low-frequency window. FM or uncompressed digital systems require enough band- one-octave frequency band(eg-,2-4 GHz), such that all second-order products are out ofbant ating withina e. width per channel that multichannel systems must operate in the region of large RD. Fortunately, the CNR require ments allow for the increased distortion. The large second-order RD can be avoided entirely by op within the frequency range between 50 and 500 MHz, nonlinear gain and loss, intervalence-band absorption, and, more importantly, spatial-hole burning(SHB)and carrier leakage can all be significant. Carrier leakage prevents all of the current injected in the laser bond wire from entering the active layer. This leakage must be ations SHB results from the nonuniform distribution of optical power along the length of the laser In DFB lasers, because of the grating feedback, the longitudinal distribution of optical power can be highly nonuniform. This results in distortion [Takemoto et al., 1990] that can add to or cancel other distortion, making it, in some cases, a desirable effect Even if all nonlinear processes were eliminated, the allowable modulation would be limited by the fact that the minimum output power is zero. Typical operating conditions with, for example, 60 channels, each with an average modulation depth(m)near 4%, result in a peak modulation of 240%. Although improbable, modu- lations of more than 100%result in clipping The effects of clipping were first approximated by Saleh [1989], who calculated the modulation level at which the total power contained in all orders of distortion became appreciable. Even for perfectly linear lasers, the modulation depth is bounded to values beyond which all orders of distortion increase rapidly. Assuming that half the total power in all orders of distortion generated by clipping is distributed evenly over each of N channels, clipping results in a carrier-to-interference ratio(CIR) given by CIR (71.7) e 2000 by CRC Press LLC
© 2000 by CRC Press LLC FM or CDV systems have much less restrictive linearity requirements, because of the reduced sensitivity to impairment. Distortion products must be counted, as with the AM-VSB example described previously, but each product is no longer dominated by the remaining carrier. Because the carrier is suppressed entirely by the modulation, each product is distributed over more than the bandwidth of each channel. The impairment resulting from the superposition of many uncorrelated distortion products resembles noise. Quantities analogous to the CSO and CTB can be defined for these systems. Laser Linearity Several factors limit the light-versus-current (L-I) linearity of directly modulated lasers. Early work on laser dynamics led to a complete understanding of resonance-enhanced distortion (RD). RD arises from the same carrier-photon interaction within the laser that is responsible for the relaxation-oscillation resonance. The second-harmonic distortion (2fi) and two-tone third-order distortion (2f i – fj) for a typical 1.3-mm wavelength directly modulated semiconductor laser are shown in Fig. 71.5 [Darcie et al., 1986]. Both distortions are small at low frequencies but rise to maxima at half the relaxation resonance frequency. AM-VSB systems are feasible only within the low-frequency window. FM or uncompressed digital systems require enough bandwidth per channel that multichannel systems must operate in the region of large RD. Fortunately, the CNR requirements allow for the increased distortion. The large second-order RD can be avoided entirely by operating within a one-octave frequency band (e.g., 2–4 GHz), such that all second-order products are out of band. Within the frequency range between 50 and 500 MHz, nonlinear gain and loss, intervalence-band absorption, and, more importantly, spatial-hole burning (SHB) and carrier leakage can all be significant. Carrier leakage prevents all of the current injected in the laser bond wire from entering the active layer. This leakage must be reduced to immeasurable levels for AM-VSB applications. SHB results from the nonuniform distribution of optical power along the length of the laser. In DFB lasers, because of the grating feedback, the longitudinal distribution of optical power can be highly nonuniform. This results in distortion [Takemoto et al., 1990] that can add to or cancel other distortion, making it, in some cases, a desirable effect. Clipping Even if all nonlinear processes were eliminated, the allowable modulation would be limited by the fact that the minimum output power is zero. Typical operating conditions with, for example, 60 channels, each with an average modulation depth (m) near 4%, result in a peak modulation of 240%. Although improbable, modulations of more than 100% result in clipping. The effects of clipping were first approximated by Saleh [1989], who calculated the modulation level at which the total power contained in all orders of distortion became appreciable. Even for perfectly linear lasers, the modulation depth is bounded to values beyond which all orders of distortion increase rapidly. Assuming that half the total power in all orders of distortion generated by clipping is distributed evenly over each of N channels, clipping results in a carrier-to-interference ratio (CIR) given by (71.7) FIGURE 71.5 Resonance distortion for directly modulated laser with resonance frequency of 7 GHz. Both the second-harmonic 2fi and two-tone third-order 2fi ± fj distortion peak near half the resonance frequency and are small at low frequency. Also shown is the same third-order distortion for an external modulator biased at the point of zero second-order distortion. CIR = + 2 1 6 2 3 1 2 2 p m m ( ) / m e
where the rms modulation index u is N/2 External Modulation Laser-diode-pumped YAG lasers with low RiN and output powers greater than 200 mW have been developed cently. Combined with linearized external LiNbO, modulators, these lasers have become high-performance competitors to directly modulated lasers. YAG lasers with external modulation offer a considerable increase in launched power, and the low riN of the YAG laser translates into a slight CNR improvement. The most challenging technical hurdle is to develop a linear low-loss optical intensity modulator Low-loss LiNbO, Mach-Zehnder modulators are available with insertion losses less than 3 dB, modulation bandwidths greater than a few gigahertz, and switching voltages near 5 V. The output intensity of these modulators is a sinusoidal function of the bias voltage. By prebiasing to 50% transmission, modulation applied to the mach-Zehnder results in the most linear intensity modulation. This bias point, which corresponds the point of inflection in the sinusoidal transfer function, produces zero second-order distortion. Unfortunately, laser, at low frequencies. This comparison is shown on Fig.71.5. For igih-etregzing the third-order nonlinearity is essential for AM-VSB applications Various linearization techniques have been explored. The two most popular approaches are feedforward and predistortion. Feedforward requires that a portion of the modulated output signal be detected and compared riginal applied voltage signal to provide an error signal. This error signal is then used to modulate a second laser, which is combined with the first laser such that the total instantaneous intensity of the two lasers is a replica of the applied voltage. In principle, this technique is capable of linearizing any order of distortion and correcting RIN from the laser Predistortion requires less circuit complexity than feedforward. A carefully designed nonlinear circuit is placed before the nonlinear modulator, such that the combined transfer function of the predistorter-modulator is linear. Various nonlinear electronic devices or circuits can act as second-or third-order predistorters. Difficulties include matching the frequency dependence of the predistorter with that of the modulator, hence achieving good linearity over a wide frequency range. Numerous circuit designs can provide reductions in third-order distortion by 15 dB Miscellaneous Impairment Laser chirp can cause problems with direct laser modulation Chirp is modulation of the laser frequenc by modulation of the refractive index of the laser cavity in response to current modulation. The interaction of chirp and chromatic dispersion in the fiber can cause unacceptable CSO levels for AM-VSB systems as short a few kilometers. Dispersion converts the FM into IM, which mixes with the signal IM to produce second order distortion Phillips et al, 1991]. These systems must operate at wavelengths corresponding to low fiber dispersion, or corrective measures must be taken Chirp also causes problems with any optical component that has a transmission that is a function of optical frequency. This can occur if two optical reflections conspire to form a weak interferometer or in an erbium- doped fiber amplifier(EDFA)that has a frequency-dependent gain [Kuo and Bergmann, 1991]. Once again, the chirp is converted to IM, which mixes with the signal IM to form second-order distortion Although externally modulated systems are immune to chirp-related problems, fiber nonlinearity, in the rm of stimulated Brillouin scattering(SBS), places a limit on the launched power. SBS, in which light is scattered from acoustic phonons in the fiber, causes a rapid decrease in CNR for launched powers greater than approximately 10 mw [Mao et al, 1991]. Since the SBS process requires high optical powers within a narrow systems broadens the optical spectrum so that SBS is unimportan eternally modulated systems. Chirp in DFB optical spectral width(20 MHz), it is a problem only in low-chirp e 2000 by CRC Press LLC
© 2000 by CRC Press LLC where the rms modulation index m is m = m (71.8) External Modulation Laser-diode-pumped YAG lasers with low RIN and output powers greater than 200 mW have been developed recently. Combined with linearized external LiNbO3 modulators, these lasers have become high-performance competitors to directly modulated lasers. YAG lasers with external modulation offer a considerable increase in launched power, and the low RIN of the YAG laser translates into a slight CNR improvement. The most challenging technical hurdle is to develop a linear low-loss optical intensity modulator. Low-loss LiNbO3 Mach–Zehnder modulators are available with insertion losses less than 3 dB, modulation bandwidths greater than a few gigahertz, and switching voltages near 5 V. The output intensity of these modulators is a sinusoidal function of the bias voltage. By prebiasing to 50% transmission, modulation applied to the Mach–Zehnder results in the most linear intensity modulation. This bias point, which corresponds to the point of inflection in the sinusoidal transfer function, produces zero second-order distortion. Unfortunately, the corresponding third-order distortion is approximately 30 dB worse than a typical directly modulated DFB laser, at low frequencies. This comparison is shown on Fig. 71.5. For high-frequency applications where RD is important, external modulators can offer improved linearity.A means of linearizing the third-order nonlinearity is essential for AM-VSB applications. Various linearization techniques have been explored. The two most popular approaches are feedforward and predistortion. Feedforward requires that a portion of the modulated output signal be detected and compared to the original applied voltage signal to provide an error signal. This error signal is then used to modulate a second laser, which is combined with the first laser such that the total instantaneous intensity of the two lasers is a replica of the applied voltage. In principle, this technique is capable of linearizing any order of distortion and correcting RIN from the laser. Predistortion requires less circuit complexity than feedforward. A carefully designed nonlinear circuit is placed before the nonlinear modulator, such that the combined transfer function of the predistorter-modulator is linear. Various nonlinear electronic devices or circuits can act as second- or third-order predistorters. Difficulties include matching the frequency dependence of the predistorter with that of the modulator, hence achieving good linearity over a wide frequency range. Numerous circuit designs can provide reductions in third-order distortion by 15 dB. Miscellaneous Impairments Laser chirp can cause problems with direct laser modulation. Chirp is modulation of the laser frequency caused by modulation of the refractive index of the laser cavity in response to current modulation. The interaction of chirp and chromatic dispersion in the fiber can cause unacceptable CSO levels for AM-VSB systems as short as a few kilometers. Dispersion converts the FM into IM, which mixes with the signal IM to produce secondorder distortion [Phillips et al., 1991]. These systems must operate at wavelengths corresponding to low fiber dispersion, or corrective measures must be taken. Chirp also causes problems with any optical component that has a transmission that is a function of optical frequency. This can occur if two optical reflections conspire to form a weak interferometer or in an erbiumdoped fiber amplifier (EDFA) that has a frequency-dependent gain [Kuo and Bergmann, 1991]. Once again, the chirp is converted to IM, which mixes with the signal IM to form second-order distortion. Although externally modulated systems are immune to chirp-related problems, fiber nonlinearity, in the form of stimulated Brillouin scattering (SBS), places a limit on the launched power. SBS, in which light is scattered from acoustic phonons in the fiber, causes a rapid decrease in CNR for launched powers greater than approximately 10 mW [Mao et al., 1991]. Since the SBS process requires high optical powers within a narrow optical spectral width (20 MHz), it is a problem only in low-chirp externally modulated systems. Chirp in DFB systems broadens the optical spectrum so that SBS is unimportant. N 2
Summary A wide range of applications for transmission of video signals over optical fiber has been made possible by refinements in lightwave technology. Numerous technology options are available for each application, each with advantages or disadvantages that must be considered in context with specific system requirements. Evo- lution of these video systems continues to be driven by development of new and improved photonic devices Defining Terms Chirp: Modulation of the optical frequency that occurs when a laser is intensity modulated Composite second order(CSO): Ratio of the power in the second-order distortion products to power in the cable television channel Composite triple beat(CTB): Same as CSo but for third-order distortion Direct modulation: Modulation of the optical intensity output from a semiconductor diode laser by direct adulation of the bias current Erbium-doped fiber amplifier: Fiber doped with erbium that provides optical gain at wavelengths near 1.55 um when pumped optically at 0.98 or 1.48 um External modulation: Modulation of the optical intensity using an optical intensity modulator to modulate a constant power(cw) laser Fiber dispersion: Characteristic of optical fiber by which the propagation velocity depends on the optical Fiber nonlinearity: Properties of optical fibers by which the propagation velocity, or other characteristic, Lightwave technology: Technology based on the use of optical signals and optical fiber for the transmission f information Linear: Said of any device for which the output is linearly proportional to the input. Noise figure: Ratio of the output signal-to-noise ratio(SNR) to the input SNR in an amplifier. Rayleigh backscatter: Optical power that is scattered in the backwards direction by microscopic inhomoge neities in the composition of optical fibers. Relative intensity noise: Noise resulting from undesirable fluctuations of the optical power detected in an optical communication system. Shot noise: Noise generated by the statistical nature of current flowing through a semiconductor p-n junction or photodetector. Related Topics 42.1 Lightwave Waveguides.69.1 Modulation and Demodulation.73.6 Data Compression References T.E. Darcie, Subcarrier multiplexing for lightwave networks and video distribution systems, IEEE J. Selected Areas in Communications, vol 8, P. 1240, 1990 T Pratt and C W. Bostian, Satellite Communications, New York: Wiley, 1986. W. Way, C. Zah. C. Caneau, S. Menmocal, E. Favire, F. Shokoochi, N. Cheung, and T.P. Lee, Multichannel FM video transmission using traveling wave amplifiers for subscriber distribution, Electron. Lett., vol. 24, P. R Olshansky, V. Lanzisera, and P. Hill, Design and performance of wideband subcarrier multiplexed lightwave ystems, in Proc. ECOC 88, Brighton, U.K., Sept. 1988, Pp. 143-1 J.A. Chiddix, H. Laor, D M. Pangrac, L.D. williamson, and R W. Wolfe, AM video on fiber in CATV systems, need and implementation, IEEE J. Selected Areas in Communications, vol 8, P. 1229, 1990 A N. Netravali and B.G. Haskel, Digital Pictures, New York: Plenum Press, 1988. K. Feher, Ed, Advanced Digital Communications, Englewood Cliffs, N J. Prentice-Hall, 198 e 2000 by CRC Press LLC
© 2000 by CRC Press LLC Summary A wide range of applications for transmission of video signals over optical fiber has been made possible by refinements in lightwave technology. Numerous technology options are available for each application, each with advantages or disadvantages that must be considered in context with specific system requirements. Evolution of these video systems continues to be driven by development of new and improved photonic devices. Defining Terms Chirp: Modulation of the optical frequency that occurs when a laser is intensity modulated. Composite second order (CSO): Ratio of the power in the second-order distortion products to power in the carrier in a cable television channel. Composite triple beat (CTB): Same as CSO but for third-order distortion. Direct modulation: Modulation of the optical intensity output from a semiconductor diode laser by direct modulation of the bias current. Erbium-doped fiber amplifier: Fiber doped with erbium that provides optical gain at wavelengths near 1.55 mm when pumped optically at 0.98 or 1.48 mm. External modulation: Modulation of the optical intensity using an optical intensity modulator to modulate a constant power (cw) laser. Fiber dispersion: Characteristic of optical fiber by which the propagation velocity depends on the optical wavelength. Fiber nonlinearity: Properties of optical fibers by which the propagation velocity, or other characteristic, depends on the optical intensity. Lightwave technology: Technology based on the use of optical signals and optical fiber for the transmission of information. Linear: Said of any device for which the output is linearly proportional to the input. Noise figure: Ratio of the output signal-to-noise ratio (SNR) to the input SNR in an amplifier. Rayleigh backscatter: Optical power that is scattered in the backwards direction by microscopic inhomogeneities in the composition of optical fibers. Relative intensity noise: Noise resulting from undesirable fluctuations of the optical power detected in an optical communication system. Shot noise: Noise generated by the statistical nature of current flowing through a semiconductor p-n junction or photodetector. Related Topics 42.1 Lightwave Waveguides • 69.1 Modulation and Demodulation • 73.6 Data Compression References T.E. Darcie, “Subcarrier multiplexing for lightwave networks and video distribution systems,” IEEE J. Selected Areas in Communications, vol. 8, p. 1240, 1990. T. Pratt and C.W. Bostian, Satellite Communications, New York: Wiley, 1986. W. Way, C. Zah. C. Caneau, S. Menmocal, F. Favire, F. Shokoochi, N. Cheung, and T.P. Lee, “Multichannel FM video transmission using traveling wave amplifiers for subscriber distribution,” Electron. Lett., vol. 24, p. 1370, 1988. R. Olshansky, V. Lanzisera, and P. Hill, “Design and performance of wideband subcarrier multiplexed lightwave systems,” in Proc. ECOC ’88, Brighton, U.K., Sept. 1988, pp. 143–146. J.A. Chiddix, H. Laor, D.M. Pangrac, L.D. Williamson, and R.W. Wolfe, “AM video on fiber in CATV systems, need and implementation,” IEEE J. Selected Areas in Communications, vol. 8, p. 1229, 1990. A.N. Netravali and B.G. Haskel, Digital Pictures, New York: Plenum Press, 1988. K. Feher, Ed., Advanced Digital Communications, Englewood Cliffs, N.J.: Prentice-Hall, 1987