Reduction of dispersion-induced distortion in SCM transmissionsystems by using predistortion-linearized MQW-EA modulators

The application of low-chirp MQW (multiquantum well)-EA (electro-absorption) modulators to subcarrier multiplexing (SCM) optical transmission systems is studied. The authors show that a third-order predistortion circuit is feasible for compensating the nonlinearity of this type of modulator. The degree of frequency chirping per optical intensity modulation depth of the modulator is theoretically determined to be about 1.4 MHz. A 42-channel AM-SCM signal transmitter with the predistortion circuit realizes both composite second-order distortion (CSO) and composite triple beat distortion (CTB) values under -57 dBc after 200-km-long transmission. The authors confirm dispersion-induced distortion of the MQW-EA modulator is as small as that of the LiNbO₃ modulator. Carrier-to-noise ratios (CNRs) of 45.2 dB for channel 1 and 43.8 dB for channel 42 were measured after 100 km transmission. It is found that the deterioration of a CNR is caused by the cascaded erbium doped fiber amplifiers (EDFAs), Rayleigh backscattered power and the optical phase noise. Theoretical CNRs show that the output power of the modulator should be higher to improve CNR     

入射偏振态偏离TE模对光纤AM CATV外调制发射机性能影响的研究

从理论上分析了入射到调制器的偏振态偏离ＴＥ模时对外调制光发射机组合二次失真（ＣＳＯ）和组合三次差拍（ＣＴＢ）的影响。得出在ＴＥ，ＴＭ模插入损耗相同时，传输５９个ＰＡＬ－Ｄ频道，在保证ＣＳＯ＝－７０ｄＢｃ时，偏离ＴＥ模的角度应小于６°。单频传输的实验结果和理论分析一致     

Calculating the CSO/CTB spectrum of CATV amplifiers and opticalreceivers

The discrete CSO/CTB intermodulation distortion has a big influence on the CATV net quality. This article describes a new method to calculate the composite second order (CSO) and composite triple beat (CTB) intermodulation distortion spectrum for the full frequency range of a coaxial cable CATV amplifier for any input frequency plan. The calculation takes into account the output level ripple and preemphasis, using frequency dependent intermodulation coefficients. These coefficients have been established through CSO/CTB measurements on multichannel test systems for standard frequency plans at 5-8 intermodulation frequencies or on distortion arrays for the two (three) oscillator measuring test system. The defined intermodulation coefficients are unrelated to the frequency plan and the input/output level ripple. It is also possible to calculate the CSO/CTB spectrum caused by analogue channels within digital QAM channels. The results equally apply to optical CATV receivers. Simulation results have been verified using extensive data from multichannel measurement systems     

The impact of CSO/CTB distortion on BER characteristics by hybridmultichannel analog/QAM transmission systems

The FEC limit (uncorrectable errors) with CSO/CTB (composite second order/composite triple beats) distortion presence in a channel is not for modulated lasers only, but for coaxial networks with amplifiers too. The system quality reserve depends not only on CNR, but is more limited on CSO/CTB. It is important, for the definition of the system quality reserve, to calculate the CSO/CTB spectrum caused by analogue channels within analogue and digital QAM channels and also to calculate the changing of CNR from the intermodulation interaction between analog and digital channels in a CATV network     

Radio-on-hybrid WDM transport systems based on mutually injection-locked Fabry–Perot laser diodes

A radio-on-hybrid wavelength-division-multiplexing (WDM) transport system employing mutually injection-locked Fabry–Perot laser diodes (F–P LDs) is proposed and demonstrated. System performances evaluated by carrier to noise ratio (CNR), composite second order (CSO), composite triple beat (CTB), bit error rate (BER), and third-order intermodulation distortion to carrier ratio (IMD₃/C) for simultaneous transmission of CATV/LAN/ITS are improved. Our proposed systems are relatively simple and potentially low cost due to the use of mutually injection-locked F–P LDs as broadband light source (BLS).     

Multichannel AM video transmission using a high-power Nd:YAG laserand linearized external modulator

A high-power Nd:YAG laser with a linearized external modulator-based system is demonstrated for a transmission of 50 channels of vestigal sideband amplitude modulated (VSM AM) video. The 1320-nm diode-pumped Nd:YAG laser is an attractive source for AM video transmission due to the high output power (~200 mW) and low relative intensity noise (<-170 dB/Hz). However, the required external modulator generates third-order distortion products, which limit the performance of such a system. A predistortion linearization technique is described. It reduces the modulator generated third-order distortion products by 16.4 dB, allowing a modulation index of 3.37% per channel. When combined with the Nd:YAG laser, this yields a 50-channel video transmission system with a 12 dB optical power budget for video carrier-to-noise ratios of 52 dB and composite distortion products (CSO and CTB) of less than -60 dBc. The demonstrated power budget is sufficient to allow four-way power splitting, with the remaining margin comparable to the test results obtained with DFB (distributed-feedback) laser-based AM transmission systems     

Second-order distortion of amplified intensity-modulated signalswith chirping in erbium-doped fiber

Second-order distortion produced in an erbium-doped fiber amplifier (EDFA) is investigated theoretically and experimentally. Rate equations for intermodulation products are led by the perturbation method. From numerical calculation, it is shown that optimum combinations of input power to EDFA and signal wavelength exist that give a distortion-free condition, Improvement of the composite second-order (CSO) distortion, which results from the cancellation of the laser distortion and the gain tilt induced distortion, was confirmed for VHF multichannel transmission. Furthermore, CSO distortion-free transmission in cascaded EDFA's has been demonstrated for a 42-channel AM-SCM signal by experimentally determining the optimum operating condition of the EDFA such that each amplifier generates no additive distortion     

Predistortion of electroabsorption modulators for analog CATVsystems at 1.55 μm

A predistortion circuit with adjustable amounts of third- and fifth-order predistortion is studied both theoretically and experimentally. The circuit is used to cancel the third- and fifth-order intermodulation distortion of a 1.55-μm integrated electroabsorption modulator/DFB laser. The CSO obtained is -61 dBc and the CTB is reduced 22.6 dB to -65 dBc. This performance is maintained after fiber amplification and propagation through 13 km of nondispersion shifted fiber due to the modulator's low chirp. Dithering of the DFB laser's injection current is employed to increase the stimulated Brillouin scattering (SBS) threshold to +13.4 dBm     

Polarization dependent distortion in AM-SCM video transmissionsystems

The second-order intermodulation distortion product, namely composite second-order (CSO) distortion, of an AM-SCM video transmission system can be caused by the optical isolators, cascaded fiber amplifiers, or the fiber transmission cable employed. Moreover, the absolute distortion value depends on the polarization angle of light input to the optical isolators, cascaded fiber amplifiers, and fiber transmission line. The degraded distortion values are measured and their mechanisms are clarified     

The impact of CSO/CTB distortion on BER characteristics by hybridmultichannel analog/QAM transmission systems

This article describes the measurements of composite second order (CSO), composite triple beats (CTB) frequency spectrum and time-varying distortion in the basic bandwidth (BB) (0-4 MHz) of the 64-QAM digital channels by multichannel AM/64-QAM optical and coaxial transmission systems. For any regular standard frequency plans (Europe, USA, NCTA, etc.) there is, for the I and Q components in the BB of the QAM channels, the harmonic series of the CSO/CTB with the first harmonic f ₁. It has been shown that for all the same laser-optics or coaxial transmission systems the CSO/CTB distortions spectrum form in the QAM BB 60-80 μs duration bursts of a series pulse with a pulse period T=1/f₁ and a pulse duration τ≈0.4/f_m, where f_m is the CSO/CTB harmonic with maximum amplitude in the QAM BB. The bursts result in the essential unevenness of error bits in time domain distribution. These bursts can deteriorate essential error correction by the interleaver-deinterleaver and Reed-Solomon (RS) forward error correction (FEC) code of the transmission systems also with a relative low bit error rate (BER) of about 10^-5 before FEC. There is, white Gaussian noise like, intermodulation distortion from interaction of digital-, digital-analog channels too, that have added white Gaussian noise     

Compensation for fiber-induced composite second-order distortion inexternally modulated lightwave AM-SCM systems using optical-phaseconjugation

Compensation for composite second-order (CSO) distortion due to the interplay of fiber chromatic dispersion and self-phase modulation in externally modulated amplitude modulated subcarrier multiplexing (AM-SCM) systems employing optical-phase conjugation (OPC) is investigated. The simulation results show that significant reductions (>20 dB) of fiber-induced CSO may be achieved using OPC, which results in an increase of the transmission distance in AM-SCM video distribution systems operating near 1550 nm. In addition, a comparison with another compensation technique, the phase modulation technique, is established, showing that OPC is much less sensitive to variations in the fiber length. However it is also calculated that for practical implementation of the OPC-based compensation technique, 25 to 30 dB reduction in the amplified spontaneous emission (ASE) associated with the OPC will be required     

Performance characteristics and applications of hybrid multichannelANI-VSB/M-QAM video lightwave transmission systems

The performance characteristics and applications of hybrid multichannel amplitude modulation vestigal sideband (AM-VSB)/M-quadrature amplitude modulation (QAM) video lightwave transmission systems operating at either 1310 or 1550 nm are reviewed. These systems can transport up to 80 AM-VSB video channels and more than 30 64/256-QAM digital video channels over a standard single-mode fiber (SMF) using a single laser transmitter. Five main transmission impairment mechanisms for these systems are reviewed as follows: (a) clipping-induced impulse noise, (b) bursty nonlinear distortions, (c) multiple optical reflections, (d) stimulated Brillouin scattering, and (e) self-phase modulation. For AM-QAM video lightwave trunking applications, the in-line erbium-doped fiber amplifier (EDFA) selection is discussed using a frequency-domain simulation model. Such lightwave trunking systems can provide an AM carrier-to-noise ratio (CNR) greater than 50 dB with composite second order (CSO) and composite-triple-beat (CTB) distortions less than -65 dBc, and nearly error-free transmission (BER⩽10^-9) for the 64-QAM channels with signal-to-noise ratio (SNR) of 30-dB or better. Comparison between 64-QAM and 256-QAM video channel transmission and the effect of the QAM channels on the AM-VSB channels are also presented. The implications of these results and others in hybrid multichannel AM-QAM video lightwave trunking systems are discussed     

Numerical simulation of clipping induced distortion in externallymodulated lightwave AM-SCM systems

For the first time to our knowledge the results of numerical simulation, and experimental verification, of clipping induced distortion in externally modulated lightwave AM-SCM CATV systems are presented. Comparison of these results with clipping induced distortion in directly modulated systems show a remarkable outcome: under the same modulation conditions the composite triple beat (CTB) distortion component in externally modulated systems is 6 dB higher than in directly modulated systems     

High Efficiency Automatic-Power-Controlled and Gain-Clamped EDFA for Broadband Passive Optical Networking Systems

The configuration of a simple improved high efficiency automatic-power-controlled and gain-clamped EDFA (APC-GC-EDFA) for broadband passive optical networking systems (BPON) is presented here. In order to compensate the phase and amplitude variation due to the different distance between the optical line terminal (OLT) and optical network units (ONU), the APC-GC-EDFA need to be employed. A single 980 nm laser module is employed as the primary pump. To extend the bandwidth, all C-band ASE is recycled as the secondary pump to enhance the gain efficiency. An electrical feedback circuit is used as a multi-wavelength channel transmitter monitor for the automatic power control to improve the gain-flattened flatness for stable amplification. The experimental results prove that the EDFA system can provide flatter clamped gain in both C-band and L-band configurations. The gain flatness wavelength ranging from 1530 to 1610 nm is within 32.83 ± 0.64 dB, i.e. below 1.95 %. The gains are clamped at 33.85 ± 0.65 dB for the input signal power of −40 dBm to −10 dBm. The range of noise figure is between 6.37 and 6.56, which is slightly lower compared to that of unclamped amplifiers. This will be very useful for measuring the gain flatness of APC-GC-EDFA. Finally, we have also demonstrated the records of the overall simultaneous dynamics measurements for the new system stabilization. The carrier to noise ratio (CNR) is 49.5 to 50.8 dBc which is above the National Television System Committee (NTSC) standard of 43 dBc, and both composite second order (CSO) 69.2 to 71.5 dBc and composite triple beat (CTB) of 69.8 to 72.2 dBc are above 53 dBc. The recorded corresponding rise-time of 1.087 ms indicates that the system does not exhibit any overshoot of gain or ASE variation due to the signal at the beginning of the pulse.     

光发射机中MZ型波导调制器产生CTB的分析

AM－CATV外调制光发射机中的MZ型波导调制器，其转移特性呈正弦曲线。当多路信号同时调制时，将产生组合二次失真（CSO）和组合三次差拍（CTB）。本文对CTB进行了理论分析，并对我国450、550MHz二种CATV系统调制时的CTB进行了计算。这对非线性补偿电路的设计具有一定的指导价值。     

K频段自适应前馈行波管放大器设计

空间通信系统高保真转发对大功率行波管放大器的线性化度提出了更高的要求。针对高频段和大功率应用,基于预失真和前馈线性化技术,设计了一种K频段50 W自适应前馈线性化行波管放大器,介绍了其基本原理和参数,研究了前馈双检波电路结构及并行变步长的组合自适应控制算法。在K频段400 MHz带宽内,实现了输入功率20 dB大动态调整情况下失真信号自适应对消抑制,在相对于单载波饱和输出功率回退3 dB点,三阶交调小于-35 dBc,验证了前馈线性化技术在线性度改善方面的巨大优势。     

Performance comparison between DCF and RDF dispersion compensation in fiber optical CATV systems

An architecture of an externally modulated AM-VSB CATV 77-channel erbium-doped fiber amplifier (EDFA)-repeated system which use the combination of a single-mode fiber (SMF) and reverse dispersion fiber (RDF) as a dispersion compensation device is proposed and demonstrated. Compared to the conventional externally modulated fiber optical CATV systems with or without a dispersion compensation fiber (DCF), excellent performance of composite second order (CSO) /spl ges/ 78 dB accompanied by satisfied carrier-to-noise ratio (CNR) /spl ges/ 50 dB and composite triple beat (CTB) /spl ges/ 65 dB was obtained in the proposed system.     

Multichannel 64-QAM transmission in AM/digital and all-digitalhybrid-fiber-coax based video dial-tone systems

A hybrid-fiber-coax (HFC) system is operated with a combination of 63 AM and 21 64-QAM subcarriers as well as with the 21 64-QAM subcarriers alone. The 64-QAM subcarriers are 6-MHz wide, decorrelated, and of the type intended for compressed digital video. The AM signal levels (over both the optical and coaxial links) are set to produce standard carrier-to-noise ratios (CNRs), composite second order distortion (CSO), and composite triple beat (CTB). The optical modulation depth (OMD) is independently varied and transmission tests are carried out over the optical link alone, as well as over the entire system (including two coaxial amplifiers, or actives). The relative degradation of the system due to the optical link versus the actives is investigated. For an OMD of 24% or less and with the raw bit-error ratio (BER) set to 10^-5 (as anticipated for use with forward error correction (FEC) systems), the performance is limited by the actives. There is an approximate 3.5 dB penalty in the necessary CNR for 64-QAM reception, due to the presence of the AM signals. On the other hand, when the system is operated with all-digital subcarriers, the optical modulation depth may be increased from 24% to 37% with no penalty in the required digital CNR     

AlGaAs/GaAs HBT linearity characteristics

Communication systems require linear power amplifiers with high efficiency and very low intermodulation distortion. AlGaAs/GaAs heterojunction bipolar transistors (HBT's) were found to have very low intermodulation distortion in power operation. Two-tone tests were carried out on both common-emitter (CE) and common-base (CB) power HBT's. At 7 GHz, the CE HBT showed -20 dBc IM₃ (third-order intermodulation ratio) and 12% power added efficiency (PAE) per tone at the 1 dB gain compression point; IM₃ dropped to -30 dBc at 1.5 dB output power backoff. The CE HBT has lower intermodulation distortion than CB HBT. Load pull data were collected to aid the understanding of the intermodulation. Parameters of the Gummel-Poon model (as used in SPICE) were derived for HBT's based on dc data and small-signal S parameters at various bias points. The accuracy and validity of the model were confirmed by comparison to experimental two-tone results. SPICE predicts that the emitter and base resistances linearize the HBT and reduce the third-order intermodulation distortion. The excellent third-order intermodulation performance of the CE HBT makes it a very attractive choice for linear power amplifiers     

Erbium-doped fiber amplifier for video distribution networks

The possibility of an application of Er-doped fiber amplifiers to AM-FDM (frequency division multiplexing) or FM-FDM video distribution networks is discussed. The measured noise and modulation distortion properties of ER-doped fiber amplifiers are good enough to meet even the severe quality standards set for trunk lines. A carrier-to-noise ratio (CNR) of 57 dB for an AM-FDM transmission was measured. A second-order harmonic distortion (HD2) of less than -56 dB was measured by the monotone method (modulation depth m=50%), and a cross modulation distortion (XM) of less than -63 dB was measured by the two-tone method (m=25%×channel). On the basis of these values, composite second-order distortion (CSO) and XM in a 40-channel transmission were estimated as less than -57 dB and -73 dB, respectively. Significant reduction of noise and modulation distortion is made possible by optimizing the length of Er-doped fiber amplifiers and using input- and output-port isolators