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 =