Signal distortion and noise in AM-SCM transmission systemsemploying the feedforward linearized MQW-EA external modulator

In subcarrier multiplexing (SCM) optical video distribution systems, the nonlinear signal distortion generated by the combined action of laser chirp and fiber dispersion limits the transmission distance. This paper for the first time shows that low-chirp MQW (multiquantum well)-EA (electroabsorption) external modulators are applicable to AM-SCM transmission systems. The feedforward linearization technique is used to compensate the signal distortion due to the nonlinearity in the modulator's L-V (light power versus voltage) characteristic. The effectiveness of this type of modulator for suppressing dispersion-induced distortion is clarified experimentally. A feedforward linearized transmitter with composite second order (CSO) intermodulation distortion <-62 dBc, composite triple beat (CTB) intermodulation distortion <-59 dBc and a carrier-to-noise ratio (CNR) >50.5 dB is successfully constructed. It is shown that the transmitter can achieve a CSO <-59 dBc, a CTB <-57 dBc and a CNR >49.5 dB for a 32 channel AM signal and 10 km transmission. Furthermore, estimations of the signal distortion cancellation and the noise characteristic achieved with feedforward circuit modeling are shown. The results obtained here emphasize that MQW-EA external modulators are applicable to AM-SCM transmission systems     

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     

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.     

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     

中间链路光学相位共轭补偿相干光正交频分复用系统的光纤非线性损伤

由于相干光正交频分复用(CO-OFDM)系统具有很高的峰均功率比(PAPR)以及非常近的子载波间隔,使得光纤非线性损伤成为系统的决定因素。提出中间位置光学相位共轭(OPC)补偿算法补偿CO-OFDM的Kerr损伤,由于OPC两端链路对称,可以最大限度地保证满足补偿条件,具有很好的非线性补偿效果。而且无链路色散补偿和有链路色散补偿系统均适用。该算法能使单信道40 Gb/s CO-OFDM的最大Q因子提高3 dB,非线性阈值提高4 dB;波分复用(WDM)系统的最大Q因子能提高1.1 dB,非线性阈值提高1 dB。     

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。     

Generation of optical phase-conjugate waves and compensation forpulse shape distortion in a single-mode fiber

The generation of optical phase-conjugate waves and the application of optical phase conjugation (OPC) to optical communication systems is described. The method of pulse shape distortion compensation by OPC is outlined including distortion due to both fiber dispersion and the optical Kerr effect. The generation of a forward-going phase-conjugate wave in a third-order nonlinear medium is discussed and that by a nondegenerate forward four-wave mixing in a zero-dispersion single-mode fiber (SMF) is investigated. Suppressing the stimulated Brillouin scattering (SBS) of a pump wave in the fiber prevents saturation of the generation efficiency of the phase-conjugate wave even when the pump power exceeds the SBS threshold. In transmission experiments through a 200-km standard SMF with a 16-Gb/s intensity-modulated signal and a 5-Gb/s continuous-phase FSK (CPFSK) modulated signal, it is shown the applicability of OPC is modulation independent and that OPC effectively compensates for both chromatic dispersion and the optical Kerr effect     

Fundamental nonlinear distortions in analog links with fiberamplifiers

The author analyzes the second-order distortion in a modulated laser-based AM subcarrier multiplexed (AM-SCM) video transport system using fiber amplifiers for power boosting. All three of the involved system elements: the laser, the fiber amplifier, and the transmission fiber, contribute to the system distortion. The laser distortion originates from the internal photon-electron interaction, whereas the fiber amplifier and dispersive distortion are directly related to the intrinsic frequency chirping of the modulated laser. From the distortion magnitude and phase analysis, the author finds that under most situations the distortions have an electronic counterpart and can be electronically compensated. In addition, the system distortion can be minimized with a combination of critically chosen elements that have minimum or counteracting distortion mechanisms. Optically amplified AM-SCM systems can thus be engineered to perform well within the stringent distortion requirements for both 1.3- and 1.55-μm wavelength ranges     

CSO/CTB performances improvement in a bi-directional DWDM CATV system

We proposed and demonstrated a four-wavelength bi-directional dense-wavelength-division-multiplexing (DWDM) CATV system that uses chirped fiber gratings (CFGs) as the dispersion compensation devices to reduce the fiber dispersion and cross-phase modulation (XPM)-induced crosstalk simultaneously. Our proposed system not only reduces the required number of fibers, but also offers the advantages of capacity doubling. Excellent performances of carrier-to-noise ratio (CNR) /spl ges/ 50 dB, composite second order (CSO) /spl ges/ 72 dB and composite triple beat (CTB) /spl ges/ 69 dB were obtained over a 50-km single-mode fiber (SMF) transport.     

Nonlinear distortions and compensations of DFB laser diode inAM-VSB lightwave CATV applications

The nonlinear distortions of a directly modulated laser diode applied to AM-VSB lightwave CATV systems are studied from the viewpoint of rate equations. A simple method for calculating composite second-order distortion (CSO) is proposed, which is called the quasidynamic approach. The contribution to nonlinearity from fiber dispersion and laser chirping is also discussed. The dependences on the variations of parameters of rate equations are investigated to determine the dominant factor for CSO degradation. This approach explains quite well the experimental results and suggests a method for constructing a predistortion circuit with at least 10 dB improvement     

Nonlinear noise amplification in optical transmission systems with optical phase conjugation

Taking into account the influence of group-velocity dispersion (GVD) and the nonlinear Kerr effect, the nonlinear amplification of the amplified spontaneous emission (ASE) noise in fiber transmission systems using optical phase conjugation (OPC) is studied. Under a path-averaged power approximation for long-haul transmission systems, an equivalent system is developed to evaluate ASE noise amplification and accumulation in OPC systems. Combining the theoretical analysis and numerical simulations, the noise suppression effect in OPC systems is demonstrated and discussed. By using the numerical calculation method, the power variation along the system is involved in the evaluation of noise amplification. It is shown that the power variation through the system results in an imperfect compensation of the modulation instability (MI) effect, which furthermore causes the degradation of the noise suppression performance in OPC systems with anomalous dispersion.     

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     

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     

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     

Evaluation of 4-Gbit/s optical fiber transmission distance with direct and external modulation

Transmission characteristics for a recently modulated measured distributed-feedbacked (DFB) laser and an externally modulated DFB laser using a Ti:LiNbO/sub 3/, Mach-Zehnder modulator at 4 Gb/s are discussed. The transmission characteristics are estimated by an advanced eye-pattern analysis method. The maximum measured fiber dispersion with a directly modulated laser is 100 to 140 ps/nm when the chirp power penalty is 1 dB. However, for external modulation, there is no power penalty after transmission over a 2220-ps/nm dispersive fiber. This confirms that external modulation has superior transmission characteristics. The modulation scheme for 4-Gb/s systems in terms of these results is discussed.<>     

30-gb/s signal transmission over 40-km directly modulated DFB-laser-based single-mode-fiber links without optical amplification and dispersion compensation

Based on a recently proposed novel optical-signal-modulation technique of adaptively modulated optical orthogonal frequency-division multiplexing (AMOOFDM), numerical simulations of the transmission performance of AMOOFDM signals are undertaken in directly modulated DFB laser (DML)-based single-mode-fiber (SMF) links without optical amplification and dispersion compensation. It is shown that a 30-Gb/s transmission over a 40-km SMF with a loss margin of greater than 4.5 dB is feasible in the aforementioned simple configuration using intensity modulation and direct detection (IMDD). In addition, the DFB-laser frequency chirp and the transmission-link loss are identified to be the key factors limiting the maximum achievable transmission performance of the technique. The first factor is dominant for transmission distances of < 80 km and the second one for transmission distances of > 80 km. It is also observed that fibers of different types demonstrate similar transmission performances, on which fiber nonlinear effects are negligible.     

通过恒包络调制提高相干光OFDM系统的光纤非线性容限

相干光正交频分复用(CO-OFDM)对光纤链路中 的色度色散(CD)和偏振模色散(PMD)具有较强的容忍性,但 是OFDM信号高峰均功率比(PAPR)的特点使其对光纤非线性效应 非常敏感,严重影响了系统传输性能。 本文提出了基于恒包络(CE)调制的方法使得系统中光信号PAPR降低为0dB,从而提高了CO-OFDM系统的非 线性传输性能。仿真结果表明,子载波采用16QAM调制的40Gbit/s单信道CE调制CO -OFDM系统,在经800km无色散补偿、欠色散补偿和周期全色散补偿 标准单模光纤(SSMF)链 路传输后,虽然较传统CO-OFDM存在约1.8dB的代价,但是系统最大 发射光功率分别提高 了6.2、9.3dB。并且,将本文方案应 用 到CO-OFDM和10Gbit/s NRZ-OOK混合传输WDM系统中,信道最大发 射光功率仍获得了5.2dB的提高。因此,本文提出的CE调制方法能有 效地提高CO-OFDM系统在不同传输环境中的光纤非线性容限。     

Transmission Performance Improvement in Microwave/Millimeter-Wave Optical System Using Chirped Fiber Grating

A chirped fiber grating (CFG) written by the moving fiber/phase mask scanning beam technique with a uniform mask is used to demonstrate the dispersion compensation in the externally modulated microwave/millimeter-wave over fiber system. It is found that the use of the CFG is an efficient way to compensate for the severe RF power degradation induced by the fiber chromatic dispersion, which results in a significant improvement in the transmission performance. The signal to noise ratio is improved from 4.8 dB to 24.4 dB when 10.58 GHz RF signal carrying 5 Mbit/s QPSK format data is transmitted over 35 km single mode fiber.     

SCM/WDM系统中抑制载波的光学单边带调制技术

为了有效抑制SCM/WDM光纤通信系统中的群速度色散和非线性效应,利用SCM/WDM光纤通信系统中信号经过两次调制这一特性,提出了一种在SCM/WDM系统中容易实现的抑制载波的光学单边带调制技术。抑制载波的光学单边带调制信号具有很大的调制深度,即通过抑制载波可以有效的增大了调制深度,进而可以减小调制器的外加电压升高而激发的一系列有害的非线性效应。实测的调制信号的频谱只剩下了半个边频,有效地减小了光信号的频带宽度,从而可以有效地减小群速度色散对信号的劣化。接收机灵敏度的实测图表明,利用文中所述的抑制载波的光学单边带调制技术可以有效地提高接收机的灵敏度达3-5dB。     

Optical phase conjugation for ultra long-haul phase-shift-keyed transmission

Nonlinear phase noise (Gordon-Mollenauer phase noise) can limit the transmission distance for phase-shift-keyed modulation formats. In this paper, the compensation of nonlinear phase noise by a midlink optical phase conjugation (OPC) is studied. A proof-of-principle experiment is presented showing an over 4-dB improvement in Q factor when OPC is employed in a differential phase-shift-keying (DPSK) system. Also, an ultra long-haul OPC-based differential quadrature phase-shift-keying (DQPSK) transmission experiment is studied to show the impact of self-phase modulation (SPM)-induced impairments, including nonlinear phase noise, in a transmission line. OPC results in a 44% increase in transmission distance when compared to a "conventional" transmission system using dispersion compensating fiber (DCF) for chromatic dispersion compensation.