Optical coherent broad-band transmission for long-haul anddistribution systems using subcarrier multiplexing

Signal multiplexing techniques for coherent optical transmission are compared, and appropriate application for a coherent subcarrier multiplexing (SCM) system is discussed. Optical frequency modulation (FM) using direct modulation of a distributed-feedback laser diode (DFB-LD) and a heterodyne detection is shown to be feasible. A transmission system using a local laser in the transmitter is unaffected by polarization and is cost effective. Phase noise can be suppressed by a phase-noise-canceling circuit (PNC) in a heterodyne receiver. This circuit can also effectively compensate for the frequency of instability of light sources. A theoretical simulation of a coherent SCM system showed that 100 channels of 30-MHz FM signal or 15 channels of 155-Mb/s signal can be distributed to 10000 subscribers using single-stage or double-stage optical amplifiers     

A 10 Gb/s optical code division multiplexing using 8-chip opticalbipolar code and coherent detection

bOptical code division multiplexing (OCDM) using optical bipolar code and coherent detection is a new multiplexing method for future fiber-optic communication networks. Some optical bipolar codes, which are described by the combination of phase of optical chip pulses, with a repetition frequency 10 GHz are generated experimentally. Temporal matched filtering for the 10 Gb/s OCDM is demonstrated using optical encoder and decoder, and their fundamental properties are investigated. Effect of interference code is shown experimentally and discussed. The novel coherent detection system with autocorrelation sidelobe suppression using balanced detector and local light source is proposed. The pseudocoherent detection is demonstrated experimentally by using a clock pulse. Advantageous properties of the coherent detection are shown. The fundamental functions to realize the gigabit rate OCDM system is revealed     

Ultimate performance of heterodyne/coherent optical fiber communications

Research and development of heterodyne/homodyne and coherent optical fiber communications have expanded rapidly because of the expectation for repeater separation elongation and superwide-band frequency-division multiplexing. In this paper, recent progress in heterodyne/coherent optical fiber communications is reviewed with emphasis on theoretical and experimental studies pursuing its ultimate performance. The topics discussed are: the recent and future trend in the system design, the transmitter power limit due to fiber nonlinearity and the resulting repeater separation limit, the laser noise problem, the fiber polarization control and polarization diversity, the possibility of narrow frequency-separation frequency-division multiplexing, and finally, a comparison of heterodyne/coherent and photon-counting communications.     

基于频域均衡的Tb/s级相干光单载波系统

对相干光单载波频域均衡系统(CO-SCFDE)、相干光单载波频分复用系统(CO-SCFDM)以及相干光正交频分复用系统(CO-OFDM)的理论和实验进行了对比研究,结果表明基于频域均衡的相干光单载波系统既保留了正交频分复用方式计算复杂度较低、频谱效率高和抗色散性能好的优点,又有效地减小了光纤非线性带来的传输损伤,是高速长距离光纤传输的一种有潜力的技术方案。     

Synchronized Mode-Locked Semiconductor Lasers and Applications in Coherent Communications

The general features and characteristics of two different external-cavity mode-locked semiconductor lasers (MSLs) were studied. Coherent optical frequency combs and short pulses from the MSLs were used to develop a synchronized MSL system. Exploiting the synchronized MSLs as a phase-locked transmitter and a local oscillator, various heterodyne and homodyne coherent detection techniques were experimentally demonstrated. The experimental results have shown that the phase-coherent optical frequency combs and short pulses from the synchronized MSLs are very promising optical sources for many future coherent photonic systems, especially for spectrally phase-encoded optical code-division multiple access (SPE-OCDMA).     

基于相干光正交频分复用的WDM传输系统及其性能分析

相干光正交频分复用由于其良好的传输性能成为近年来光传输领域的研究热点,波分复用技术可以在光纤中通过增加并行波长的数量来提高系统的容量,将CO-OFDM和WDM技术结合,可以构造出高速率、大容量、低成本的光传输网络。文章首先对基于CO-OFDM的WDM传输系统的理论模型和基本原理进行了研究,然后对基于CO-OFDM的100Gb/s×32-信道WDM传输系统进行了仿真分析。并研究了该系统的传输性能。结果表明：在没有任何光纤的色散及非线性补偿的情况下,当信号速率为3.2 Tb/s时,系统的Q因子高于16.0 dB,在标准单模光纤中的传输距离可达1500km。     

Phase Noise Effects on High Spectral Efficiency Coherent Optical OFDM Transmission

There are three major advantages for coherent optical orthogonal frequency-division multiplexing (CO-OFDM) transmission using digital signal processing. First, coherent detection is realized by digital phase estimation without the need for optical phase-locked loop. Second, OFDM modulation and demodulation are realized by the well-established computation-efficient fast Fourier transform (FFT) and inverse FFT. Third, adaptive data rates can be supported as different quadrature amplitude modulation (QAM) constellations are software-defined, without any hardware change in transmitter and receiver. However, it is well-known that coherent detection, OFDM, and QAM are all susceptible to phase noise. In this paper, theoretical, numerical, and experimental investigations are carried out for phase noise effects on high spectral efficiency CO-OFDM transmission. A transmission model in the presence of phase noise is presented. By using simulation, the bit error rate floors from finite laser linewidth are presented for CO-OFDM systems with high-order QAM constellations. In the experiments, the phase noise effects from both laser linewidth and nonlinear fiber transmission are investigated. The fiber nonlinearity mitigation based on receiver digital signal processing is also discussed.     

Recent advances in coherent optical fiber communication systems

Research and development of coherent optical fiber communications have been accelerated mainly because of the possibility of receiver sensitivity improvement reaching 20 dB, and partly because of the possibility of frequency-division multiplexing (FDM) with very fine frequency separation. In this paper, recent advances in the research on coherent optical fiber communication systems are reviewed, with emphasis on those reported in the past two years. The bit-error rate measurements so far reported are classified and investigated in four categories: PCM-ASK, PCM-FSK, PCM-PSK, and PCM-DPSK. The states-of-the-art of polarization-state stabilization techniques is also discussed.     

Self-heterodyne multiterminal system concepts for frequencydivision multiplexed fiber-optic communication

System structures are described that use a centralized optical source for supplying all optical power needs-signal generation and coherent detection. In the configurations described the following benefits occur. Functional systems can be implemented with already demonstrated, and in many cases readily available components. There is no need for local oscillator optical sources and there is no need to stabilize the local oscillators with respect to a remote transmitter frequency to achieve coherent detection. Increases selectivity against other signal frequencies and increased signal-to-noise result from the use of coherent detection. When configured to equalize reference and signal time delay paths, both slow and fast laser source phase and frequency jitter are compensated for. One may therefore efficiently implemented multiple-access frequency division multiplexed networks in which the individual communication bandwidths are small compared to the linewidth of the laser power source     

The effect of four-wave mixing in fibers on opticalfrequency-division multiplexed systems

The limiting effects of four-wave mixing on optical frequency-division multiplexing (OFDM) systems are described. The optical nonlinearity in a single-mode fiber imposes a fundamental limitation on the capacity of optical frequency-division multiplexed systems. In particular, four-wave mixing (FWM) crosstalk may severely degrade the system performance when the fiber input powers are large and/or the channel spacing is too small. Theoretical and experimental results of the effects of FWM in OFDM systems are presented. The theoretical results demonstrate the dependence of FWM on various system parameters. An analysis of FWM in both undirectional and bidirectional transmission systems is included. The receiver sensitivity degradation from FWM crosstalk is measured in a 16-channel coherent system     

Coherent Optical 25.8-Gb/s OFDM Transmission Over 4160-km SSMF

We discuss coherent optical orthogonal frequency division multiplexing (CO-OFDM) as a suitable modulation technique for long-haul transmission systems. Several design and implementation aspects of a CO-OFDM system are reviewed, but we especially focus on phase noise compensation. As conventional CO-OFDM transmission systems are very sensitive to laser phase noise a novel method to compensate for phase noise is introduced. With the help of this phase noise compensation method we show continuously detectable OFDM transmission at 25.8 Gb/s data rate (20 Gb/s after coding) over 4160-km SSMF without dispersion compensation.     

新型的光纤无源器件——光纤法布里-珀罗调谐滤波器

本文介绍了国外可调谐光纤法布里-珀罗(FFP)滤波器不同类型的结构、制作、主要参数及其用途。每一种滤波器都可覆盖一个独立的由谐振腔长度所决定的自由光谱区。滤波器采用压电元件调谐,在信号处理、传感器、波分复用(WDM)系统、相干光通信系统、特别是近几年才发展起来的光频复用(OFDM)系统中将得到广泛的应用。     

Coherent optical tapping using semiconductor optical amplifier

Semiconductor optical amplifiers are attractive not only as optical repeaters but also as functional devices, since carrier density modulation in amplifiers causes a nonlinear phenomenon. Utilizing the effect of the carrier density modulation on the semiconductor optical amplifier junction voltage, a coherent optical tapping is proposed for signal monitoring or control signal extraction. A 155 Mb/s FSK (frequency shift keying) signal tapping was realized with a simple configuration using heterodyne single-filter detection with -24.4 dBm sensitivity. Many applications for this coherent optical tapping are discussed, and basic characteristics for frequency-selective tapping from FDM (frequency division multiplexing) signals and optical amplifier gain control are examined     

Channel allocation and crosstalk penalty in coherent opticalfrequency division multiplexing systems

Theoretically studies the crosstalk penalty of coherent optical frequency division multiplexing systems by varying the intermediate frequency and the channel spacing. Results, applicable to ASK, PSK and CPFSK modulation formats, provide a quantitative criterion for designing a practical multichannel coherent lightwave system     

Compensation of IQ mismatch in optical PDM–OFDM coherent receivers

The performance enhancements based on Gram–Schmidt orthogonalization procedure (GSOP) for compensating IQ mismatch in coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems are investigated. We analytically explore IQ mismatch in optical OFDM systems and investigate the impacts of phase and amplitude IQ mismatch on required optical signal-to-noise ratio (OSNR) for the different values of data mapping and polarization multiplexing. The impacts of analog-to-digital converter (ADC) resolution and the number of samples in GSOP are also evaluated. The results show that the GSOP operation efficiently compensate IQ mismatch induced performance degradations regardless of the amount of IQ phase mismatch, density of data mapping, and polarization multiplexing.     

Theoretical analysis of heterodyne optical receivers for transmission systems using (semiconductor) lasers with nonnegligible linewidth

We present a general theoretical model of receivers for coherent optical communication systems where transmitters and local oscillators having nonzero linewidth are used. Key issues in the model are the concept of single realization measurements of a stochastic intermediate frequency, and development of the probability density function for this stochastic process. Analytical results are derived for heterodyne ASK and dual filter FSK receivers and include the shot-noise limit, the asymptotic error-probability limits in ASK and FSK receivers, the influence of the IF on receiver noise, and the effective local oscillator strength. Detailed numerical results for typical p-i-n-FET wide-band receivers illustrate the influence on receiver sensitivity of IF filter bandwidth and relative threshold setting in ASK systems and of modulation index and IF filter bandwidth in FSK systems. A receiver sensitivity penalty for nonzero linewidth is found to be, for IF linewidths of 0.1 to 0.3 of the bit-rate, 3 to 9 dB in optimum ASK receivers, and 2 to 8 dB in optimum FSK receivers. Thus DFB lasers of linewidth 5 to 20 MHz could be used without external cavities in simple systems with near-ideal performance, which could find application wherever the great multiplexing advantage of coherent systems is a prime advantage. We present some guidelines for system design based on the results of this work.     

Linear Fiber Impairments Mitigation of 40-Gbit/s Polarization-Multiplexed QPSK by Digital Processing in a Coherent Receiver

To provide higher capacity networks, 40-Gb/s transmission systems are under active development and their cost is on the way to be competitive with the one of 410 Gb/s. However, their lower tolerance to linear and nonlinear fiber impairments remains a major drawback for field deployment. To address the issue of linear impairments, coherent detection of multilevel formats with polarization division multiplexing appears as a promising solution by reducing the symbol rate to 10 Gbaud. Indeed, such coherent based systems have already demonstrated an improved tolerance to optical noise and an interesting capability to compensate for large amount of chromatic dispersion. In this paper, the tolerances to narrow optical filtering, chromatic dispersion, and polarization mode dispersion are investigated with coherent detection of 10-Gbaud quadrature phase shift keying (QPSK) with and without polarization division multiplexing. Moreover, the efficient mitigation of these linear impairments by digital processing in a coherent receiver is demonstrated in an ultralong haul transmission (4080 km) of 40-Gb/s QPSK polarization multiplexed data.     

Optical frequency combs from semiconductor lasers and applications in ultrawideband signal processing and communications

Modelocked semiconductor lasers are used to generate a set of phase-locked optical frequencies on a periodic grid. The periodic and phase coherent nature of the optical frequency combs makes it possible for the realization of high-performance optical and RF arbitrary-waveform synthesis. In addition, the resulting optical frequency components can be used for communication applications relying on direct detection, dense wavelength division multiplexing (WDM), coherent-detection WDM, optical time-division multiplexing, and optical code division multiple access. This paper highlights the recent results in the use of optical frequency combs generated from semiconductors for ultrawideband signal processing and communication applications.