Multichannel coherent optical communications systems

Balanced coherent receivers perform substantially better than single-detector receivers in multichannel optical fiber FDM communications systems since the balanced approach eliminates the direct-detection and signal-cross-signal interference. The permissible channel spacingDdepends on the intermediate frequency fIF, on the bit rate Rb, and on the modulation/demodulation format. In particular,Dincreases by 2 Hz for every 1-Hz increase of the fIF. The signal-to-interference ratio SIR, as defined in the text, provides a simple measure of the amount of the interference generated by undesired channels. The criterion SIR = 30 dB is selected in this paper and leads to the following minimum channel spacings: for heterodyne systems,3.8R_{b}for FSK,9.5R_{b}for ASK, and12.4R_{b}for PSK; for homodyne systems,7.5R_{b}for ASK and10.5R_{b}for PSK. Simultaneous transmission of several channels generates an excess shot noise studied here for the first time. If the local oscillator power is 40 dB above the received signal power and 2000 channels are transmitted without optical prefiltering, the excess shot noise power penalty is less than 1 dB.     

Multichannel CPFSK coherent optical communications systems

A detailed theoretical analysis of multichannel coherent CPFSK communications systems is presented. The analysis accounts for the crosstalk between adjacent channels, the intersymbol interference and correlation between noise samples stemming from the limited IF bandwidth the non-Gaussian statistics of the noise at the decision gate, and the impact of the laser phase noise. It is found that the IF bandwidth needed to avoid intersymbol interference is 2.2 bit rates for a modulation index m=1; it is larger for other modulation index values. For m=1, receiver sensitivity is within 1 dB of the shot noise limit, and the electrical domain channel spacing can be as small as 2.05 bit rates with 1-dB sensitivity penalty. The foregoing conclusions are valid for a negligibly narrow linewidth; the degradation due to phase noise is shown to be modest as long as the linewidth does not exceed 1% of the bit rate if m=1. Larger linewidth can be tolerated if the modulation index is larger than unity     

双模SU(1,1)相干态场与四能级原子相互作用的量子动力学

利用全量子理论导出了双模SU(1,1)相干态场与四能级原子相互作用系统的态函数,研究了双模SU(1,1)相干态场与四能级原子相互作用中光场的互关联函数、Cauchy-Schwartz不等式及二阶相干性质.讨论了光场与原于间耦合系数和初始光场强度对光场量子动力学特性的影响.数值计算结果表明:光场光子的统汁性质依赖于系统的初始状态,光场两模之间总是呈现止相关,而且是非经典相关,光场第一模光子总是呈现反聚束效应,第二模光子交替呈现聚束和反聚束效应.     

Crosstalk interference in FSK coherent optical systems

In this work, the impairment due to crosstalk interference in a FSK coherent optical system is investigated. The crosstalk can arise from both co-channel interference due to the dual-filter receiver structure, and inter-channel interference due to the frequency division multiplexing technique in multichannel systems. Here, we perform the analysis of a multichannel system, taking into account simultaneously the two contributions to interference. An approximation of the crosstalk contributions seems necessary in order to carry out the analysis. The joint moment characterization of the filtered phase noise and of the interference is suitable to get accurate results     

Accurate performance evaluation of weakly coherent optical systems

An accurate performance evaluation approach which uses a closed-form exact analytical expression of the phase noise moments is presented. This enables one to derive a high-order Gaussian quadrature rule for the integrations needed to take into account the phase noise in the computation of error probability. A systematic comparison with results obtained through a Monte Carlo simulation shows that the approach is more accurate than previous methods. The analysis is performed on ASK and FSK heterodyne receivers with integrate-and-dump filtering, envelope detection, and optimized postdetection low-pass filtering. The feasibility of ASK and FSK heterodyne systems at bit rates comparable to the spectral line bandwidth of the laser sources is confirmed. The theory applied seems to be adequate to attack other problems, such as the evaluation of the effects of crosstalk between the FSK filters or among frequency division multiplexed channels     

相干光纤通信系统对激光器线宽的要求

通过仿真相干光接收机载波相位估计性能,研究了采用不同调制格式时对所使用激光器线宽的要求,分析了16-QAM采用Star和Square两种不同星座图时线宽要求差异的原因.仿真结果表明:对于既定的比特速率,采用高阶调制格式的系统降低了线路符号速率,但要求激光器线宽更窄;同一调制格式对激光器线宽的要求与所采用的星座图有关.     

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.     

Long-haul coherent optical fiber communication systems usingoptical amplifiers

Studies on long-haul coherent optical fiber communication systems with in-line optical amplifier repeaters are made theoretically and experimentally. By theoretical calculation it was found that coherent systems can achieve wider dynamic range for an amplifier input power as compared with the intensity-modulation direct-detection (IM-DD) systems. The feasibility of such systems using traveling-wave semiconductor laser amplifiers (TWSLAs) and erbium-doped fiber amplifiers (EDFAs) was investigated, and 546 km, 140 Mb/s CPFSK transmission using TWSLAs and 1028-km, 560-Mb/s CPFSK transmission using EDFAs were successfully demonstrated     

ASK coherent optical communications systems: simplified analysis

Presents a simple approximate analysis of ASK coherent optical communications systems and obtain, for the first time, a closed-form estimate of the bit error rate. The inaccuracy introduced by the approximation analysis is about 0.4 dB. The main application of the presented theory is expected to be in the analysis of practical imperfections (such as a finite IF filter bandwidth and a nonzero laser linewidth) on the system performance.<>     

Operations achievable with coherent optical information processing systems

Coherent optical information processing systems are well known to be capable of performing a certain limited class of operations, such as Fourier transformation, Fresnel transformation, and linear, space-invariant filtering. Recently, advances have been made in broadening the class of achievable operations to include nonlinear operations and space-variant linear filtering. In this paper we review the operations commonly achieved in the past, and discuss newer techniques which are capable of significantly extending the class of achievable operations.     

Homodyne optical coherent systems based on polarization modulation

The first homodyne receiver for polarization modulated optical signals is proposed and analyzed. It is based on the detection at the receiver of the four quadratures that completely characterize the received optical field and on the estimation of the Stokes parameters starting from them. This allows homodyne detection to be carried out without phase locking the local oscillator to the received signal. An accurate performance evaluation is carried out both at the quantum limit and in the presence of phase noise for a binary system based on an antipodal decision in the Stokes space. The system structure is described. The quantum limit performance is evaluated, and the phase-noise induced penalty is evaluated using an accurate analytical model     

Moment characterization of phase noise in coherent optical systems

The statistical characterization of the phase noise introduced by a semiconductor laser in a coherent optical transmission system is a key problem in the system performance evaluation. The authors consider the moment characterization, of the complex random process. Starting from the implicit representation of the probability density function through the Fokker-Planck equation, the authors obtain closed form analytical expressions for the moments of the filtered phase noise both in stationary and nonstationary conditions. Then the use of the moments for the computation of probability densities through orthogonal polynomial series expansion and maximum entropy approach is considered in application examples     

CO-OFDM系统中一种改进的同步算法方案

对相干光正交频分复用(CO-OFDM)系统时序和频率 同步算法进行了改进,消除了Park的方法中由于时间标尺旁瓣引起 的定时模糊性,利用单个训练符号实现了和Schmidl方法一样宽的频偏估计范围,且保持 同等的估计精 度。在20Gb/s的CO-OFDM系统上对改进算法进行仿真测试的结果表 明,通过色散补偿的辅助,改进的同步 算法方案实现了精确的定时估计,和节省训练符号且宽范围的频差估计。此外,色散补偿的 辅助提升了子载波恢复的性能,使得误比特率(BER)为10－3时系统的光信噪比(OSNR)要求降低了3dB。     

Receiver analysis for synchronous coherent optical fiber transmission systems

A receiver sensitivity expression applicable for both PSK homodyne and heterodyne optical fiber transmission systems is derived taking account of polarization misalignment, reduced modulation depth, preamplifier thermal noise, power coupling ratio of the fiber coupler, local oscillator excess intensity noise, and reference phase errors. From a comparison of recent studies on system performance degradation due to laser phase noise a generalized expression relating beat linewidth to phase error variance for pilot carrier and Costas phase-locked-loop receivers is defined.     

Experimental 1.5 μm coherent optical fibre transmission systems

The authors consider the design requirements for 1.52 μm coherent optical fibre transmission systems. Some of the essential requirements are: narrow linewidth sources, polarisation stable transmission media, low insertion loss optical mixers and frequency tracking or phase locked local oscillators. Having been successful in reducing the linewidth of semiconductor lasers to less than 30 kHz, by using injection locking techniques, and in fabricating optical mixers with insertion losses of the order of dB, they have been able to experiment with coherent optical fibre transmission systems to assess their practicability. In this paper it will be shown that coherent systems can be operated over long lengths of conventional low loss monomode fibre without any polarisation penalty and give far better performance than direct detection systems.     

Weakly coherent optical systems using lasers with significant phasenoise

Current theories for coherent receiver performance as affected by phase noise are compared, the main prediction of these theories are highlighted, and the reasons why many coherent system experiments have failed to approach the level of performance which is theoretically possible are considered. Different theoretical predictions are compared under idealized receiver conditions concerning the appropriate IF bandwidth, the penalty in receiver sensitivity resulting from laser phase noise, and other system properties. In general, good agreement is found. It is shown how these results differ for realistic receiver parameters, and optimized front-end design using tuning of parasitics is suggested     

Analysis of optical amplifier noise in coherent opticalcommunication systems with optical image rejection receivers

A detailed theoretical analysis of optical amplifier noise in coherent optical communication systems with heterodyne receivers is presented. The analysis quantifies in particular how optical image rejection receiver configurations reduce the influence of optical amplifier noise on system performance. Two types of optical image rejection receivers are investigated: a novel, all-optical configuration and the conventional, microwave-based configuration. The analysis shows that local oscillator-spontaneous emission beat noise (LO-SP), signal-spontaneous emission beat noise (S-SP), and spontaneous-spontaneous beat noise (SP-SP) can all be reduced by 3 dB, thereby doubling the dynamic range of the optical amplifier. A 2.5-dB improvement in dynamic range has been demonstrated experimentally with the all-optical image rejection configuration. The implications of the increased dynamic range thus obtained are also discussed from a systems point of view     

Random access optical heterodyne receiver for coherent FDM broadcasting systems

A random access optical heterodyne receiver for a coherent FDM broadcasting system has been developed, using a wavelength tunable local DBR LD, polarisation diversity technique and a random access automatic frequency controller. In a 10-channel FDM experimental system, 114 GHz capture range, 80 GHz lock range and arbitrary channel selection within less than 1 ms were realised.<>     

Dichroism effect on polarization-modulated optical systems usingStokes parameters coherent detection

Antipodal Stokes parameters shift keying (ASPSK) performance is analyzed in the presence of dichroism. Two different approximate analytical approaches are used, and the obtained results are in good agreement. In the presence of dichroism not only does a sensitivity penalty arise, but also the system performance is not independent of the received state of polarization. However, these effect are not very relevant even in situations in which a large amount of dichroism is present, such as when inline amplifiers are present, so that the system shows a good tolerance with respect to dichroism. The results confirm the feasibility of ASPSK for point-to-point and multipoint systems even in conjunction with emerging optical technologies such as optical amplifiers and nonlinear couplers