Effect of phase noise on optical minimum-shift keying transmission systems

Phase-modulated (PM) signals have recently been used in long-haul lightwave communication systems to reach record distances. Added directly to the signal phase, linear and nonlinear phase noise cause performance degradation in PM systems. Minimum-shift keying (MSK), as a special format of PM signals, presents some different features of phase noise tolerance. This paper investigates the effect of phase noise on the performance degradation in the 10 Gb/s optical MSK systems and analyzes the contribution of phase noise to the bit error rate (BER), compared with the conventional PM format, 50% duty cycle optical return-to-zero differential phase-shift keying (RZ-DPSK) signals, in several typical local fiber dispersion transmission systems. The effect of intensity noise on the performance degradation in the corresponding transmission systems is also investigated.     

Multi-amplitude minimum shift keying modulation format for optical communications

We present the multi-amplitude level minimum shift keying (M-ary MSK) modulation scheme for long haul optically amplified transmission systems. New configurations of optical M-ary MSK transmitters using two cascaded electro-optic phase modulators are proposed and its detailed operation principles are expressed. The optical receiver for optical multi-amplitude MSK modulation format requires both amplitude direct-detection and differential phase balanced-detection. Numerical results on spectral characteristics, dispersion tolerance and the relationship between amplitude and phase with launched average power for long-haul transmission of multi-level MSK modulation format are presented. The dispersion tolerance shows that the 2-amplitude MSK at 80 Gb/s is well compared with 40 Gb/s binary MSK modulation format.     

Tolerance of laser frequency offset in optical minimum-shift keying transmission systems

A mismatch between laser frequency and delay interferometer (DI) phase is found to be the most critical impairment for the receiver performance in a practical phase-modulated (PM) system. This paper investigates the receiver performance degradation caused by frequency offset between optical signal and DI in a 10 Gb/s minimum-shift keying (MSK) system, and compares it with the conventional PM formats, optical differential-phase-shift keying (DPSK) and differential-quadrature-phase-shift keying (DQPSK), which are nonreturn-to-zero (NRZ) and 50% duty cycle return-to-zero (RZ). Results show that the MSK system is about double times and six times more robust to frequency offset than the DPSK and DQPSK systems operating at the same bit rate, respectively.     

基于串行最小频移键控的8进制调制的研究

提出了一种新型的基于串行结构的最小频移键控调制技术,产生了相位连续,频谱宽度窄,每码元时间有两次π/2相移,类似于传统最小频移键控调制信号,给出了该调制方案的理论推导,并将其应用于8进制的多维多阶调制系统中。理论和仿真分析了不同的8进制调制方案在频谱特性,残留色散容限,自相位调制容限的不同,还分析了其受到系统滤波带宽的影响。最后仿真实现了通过350km的色散完全补偿的光纤120Gb/s的数据传输。     

Linear and nonlinear distortion effects in direct detection 40 Gb/s MSK modulation formats multi-span optically amplified transmission

The generation of minimum shift keying (MSK) requires a linear variation of the phase, hence a constant frequency of the optical carrier. However, the generation of the optical phase may be preferred by driving an optical modulator using sinusoidal signal for practical implementation. Thus a nonlinear variation of the carrier phase, hence some distortion effects are produced. In this paper, we investigate the use of linear and nonlinear phase shaping filtering and their impacts on MSK modulated optical signals transmission over optically amplified long haul communications system. The evolution of the phasor of the in-phase and quadrature components is illustrated for lightwave-modulated signal transmission. The distinct features of three different MSK modulation formats: linear MSK, weakly nonlinear MSK and strongly nonlinear MSK and their transmission are simulated. Transmission performance obtained indicates the resilience of the MSK signals in transmission over multi-optically amplified multi-spans.     

Performance evaluation of 100 and 200-Gb/s WDM PM-QPSK transmission systems: tolerance analysis to the optical link impairments according to the optical carrier shape

Polarization-multiplexing quadrature phase shift keying (PM-QPSK) has become a key ingredient in the design of 100 and 200-Gb/s dense wavelength-division multiplexed (DWDM) networks. The performance of the system using these formats varies according to the shape of the pulses employed by the optical carrier: non-return to zero (NRZ), return to zero (RZ) or carrier-suppressed return to zero (CSRZ). In this paper, we analyse the tolerance of PM-QPSK to linear and nonlinear optical impairments, including amplified spontaneous emission (ASE) noise, crosstalk, distortion by optical filtering, chromatic dispersion (CD), polarization mode dispersion (PMD) and fibre Kerr nonlinearities. The simulation results show that RZ formats with a low-duty cycle have a higher tolerance to CD, PMD and intrachannel nonlinearities due to the reduction of pulse-to-pulse interaction. Conversely, the PM–NRZ-QPSK format has a higher spectral efficiency (SE) than other options and shows better tolerance to filtering distortion and CD in filterless networks.     

Data-aided channel estimation and frequency domain equalization of minimum-shift keying in optical transmission systems

We demonstrate a digital optical communication system based on minimum shift keying （MSK） signal transmission with coherent detection. 5-Gb/s MSK signal can transmit over a 160-km standard single mode fiber （SSMF） without phase compensation. At the receiver, we use data-aided channel estimation and frequency domain equalization （FDE） techniques in the digital signal processing （DSP） algorithm, then analyze its performance characteristics compared with quadrature phase shift keying （QPSK） format. The simulation results show that the MSK format will be a potential candidate for next-generation access network.     

High speed optical transmission utilizing constant envelope modulation based on frequency shift keying and minimum-shift keying

A novel 40-Gb/s constant envelope optical frequency shift keying （FSK） transmitter and the transmission characteristics are investigated both by simulation and experiment. Meanwhile, to increase the spectrum efficiency of FSK, we propose a novel optical minimum-shift keying （MSK） scheme and analyze its per-formance compared with other MSK schemes and other traditional modulation formats. Simulation and experimental results show that the novel FSK scheme could be a potential candidate for the future high speed transmission and label switching systems. And the novel MSK scheme deserves future deep research for its potential excellent performance.     

A detailed analysis of cross-phase modulation effects on OOK and dpsk optical WDM transmission systems in the presence of GVD,SPM, and ASE noise

A performance analysis is carried out to evaluate the effect of cross-phase modulation (XPM) on a dispersion-managed 20 Gb/s optical wavelength-division multiplexing (WDM) transmission system using either the on-off keying (OOK) or the different-phase-shifting keying (DPSK) modulation, in the presence of the group-velocity dispersion (GVD), self-phase modulation (SPM), and amplified spontaneous emission (ASE). It is found that to achieve a bit error rate (BER) of 10⁻⁹ at a distance of 160 km, a 1.0 dB XPM power penalty is incurred for input channel power of 3 dBm in the OOK transmission and 7 dBm in the DPSK transmission. The power penalty increases with input channel powers and is inversely proportional and exhibits oscillations with respect to the channel separation. The oscillation is evenly spaced for the DPSK but not for the OOK and suggests the presence of optimum separation values. The XPM penalty decreases when a high dispersion fiber is used and increases linearly with increasing dispersion slope. Small residual dispersion can reduce the penalty of nonlinear effects.     

Adding channels with PSBT format at 40 Gbit/s in an existing 10 Gbit/s optical network

Until recently, the wavelength-division-multiplexed (WDM) transmission system has reached record capacities and distances due to innovations such as FEC (Forward Error Correction), distributed Raman amplification, new transmission fiber and advanced optical format. Optical-communication systems exclusively employed conventional On-Off Keying signals in either Non-Return-To-Zero (NRZ) or Return-To-Zero (RZ) format. Recently a number of advanced modulation formats have attracted attention. Some of these formats carry information through On-Off-Keying but also modulate the optical phase in order to enhance the robustness of signal to chromatic dispersion, optical filtering and non-linearities. Through extensive sets of simulation results, we showed that it is possible to replace a channel with higher bit-rate on existing DPSK or OOK at 10Gbit/s transmission link. Duobinary formats are ideal candidates to do it and are known for their low spectral range and high tolerance to residual chromatic dispersion. These particularities make them very attractive for both high bit rates and high distance-transmissions. Today, Phase Shaped Binary Transmission (PSBT) is considered as being the promising format for the deployment of 40Gbit/s technology on existing links at 10Gbit/s WDM long haul transmissions.     

Effect of carrier reshaping and narrow MUX-DEMUX filtering in 0.8 bit/s/Hz WDM RZ-DPSK transmission

Effect of the carrier shape in the ultra high dense wavelength division multiplexing (WDM) return to zero differential phase shift keying (RZ-DPSK) transmission has been examined through numerical optimization of the pulse form, duty cycle and narrow multiplex/de-multiplex (MUX/DEMUX) filtering parameters.     

Comparison of polarization-mode dispersion tolerances in polarization-multiplexing systems with different modulation formats

The polarization-mode dispersion (PMD) tolerance of 10 Gb/s polarization-multiplexing (PM) system is investigated. Using the importance sampling (IS) method, the outage probabilities of the PM systems with three modulation formats, including on-off keying (OOK), differential phase-shift keying (DPSK) and differential quadrature phase-shift keying (DQPSK), are quantified. When the amplified spontaneous emission (ASE) noise is assumed to be dominant, we evaluate the optical power penalties caused by the PMD effect at bit error rate (BER) of 10⁻¹². The performance of compensated PM systems with variable optical delay line is also described. The simulation results indicate that the OOK signal with higher duty cycle (DC) performs better in the PM systems with PMD compensation. It is found that the higher-order PMD impairs seriously the performance of the PM system, and phase-keying formats are more sensitive to the PMD than the OOK.     

Absolute polar duty cycle division multiplexing over wavelength division multiplexing system

The performance of absolute polar duty cycle division multiplexing (AP-DCDM) over wavelength division multiplexing (WDM) system is presented based on the simulation results. The AP-DCDM signal has narrower bandwidth than conventional time division multiplexing (TDM) signal, which makes its implementation in WDM system advantageous. In this paper, characteristics of AP-DCDM and TDM signals in WDM system are compared at the speed of 40 Gbit/s per channel, for the minimum allowed channel spacing and the chromatic dispersion tolerance. The results clearly show that AP-DCDM performs significantly better than TDM. By using AP-DCDM, 1.28 Tbit/s (32 × 40 Gbit/s) was successfully transmitted over 320 km standard single mode fiber. Spectral efficiency of 0.64 b/s/Hz was achieved by using 10 Gbit/s transmitters and receivers without polarization multiplexing.     

Phase encoded spread in time signals for suppression of propagation non-linearities in 40 Gb/s CDM/WDM communications systems using optical phase conjugation for dispersion compensation

The non-linear degradation along a RZ-WDM 40 Gb/s transmission link is shown to be reduced when a spectral direct sequence bipolar phase shift keying (DS/BPSK) encoding format is used. Standard single mode fibre transmission links are studied which use optical phase conjugation for dispersion compensation in the encoded system. For a WDM system with 100 GHz channel spacing, an improvement of 6 dB is found in the link output power margin at a distance of 300 km after five amplification stages.     

Characterization of wavelength interleaving in radio-over-fiber systems employing WDM/SCM

A radio-over-fiber (RoF) distribution system incorporating both sub-carrier multiplexing and wavelength division multiplexing (WDM) technologies is presented. This signal is directly modulated onto three high-speed lasers. Bragg filters are employed at the receiver base station in order to both demultiplex the required optical channel, and ensure that the detected signal is single side band (in order to overcome dispersion limitations of the link). System spectral efficiency is optimised by wavelength interleaving. The channel spacing between the WDM channels is varied and the system performance for different values of channel spacing and spectral efficiencies is investigated. The results show that wavelength interleaving is a reliable technique that could be used to increase the spectral efficiency of RoF systems.     

基于异步延迟采样的光通信性能监测方法

基于异步延迟采样和人工神经网络统计学习提出了一种光通信性能监测方法。通过对高速光信号进行异步延迟采样,获得信号二维幅度直方图,然后提取其中特征参数并对人工神经网络进行训练,最后以人工神经网络的预测输出实现对光信号损伤的监测。构建10 Gb/s非归零码开关键控,40 Gb/s光学双二进制码和归零码差分移相键控光通信仿真系统,并对光信噪比、色散和偏振模色散损伤进行监测。仿真结果表明,所提方法对被监测光信号的速率、码型调制格式透明,可同时准确监测多种并存的传输损伤,损伤参数监测误差小于5%。该方法具有电域处理带宽要求低、采样机制简单的特点,适用于分布式在线光性能监测。     

Novel optical orthogonally modulation scheme for superimposing DPSK signals on dark RZ signals

A configuration of transmitter for superimposing DPSK signals on dark RZ signals was proposed. The transmitter comprises two optical phase modulators and a delayed line Mach-Zehnder interferometer. A new orthogonally modulation format with tunable duty cycle and extinction ratio in the optical domain was generated and discussed. The results show that the proposed modulation format offers significant advantages in offering high spectral efficiency values, high extinction ratio, and requiring electronics with reduced bandwidth.     

Performance comparison between digital back propagation (DBP) and pilot-aided method for fiber nonlinearity compensation in different fiber links

We numerically investigate and compare the performance of fiber nonlinearity compensation using digital back propagation (DBP) method and pilot-aided method in coherent optical transmission systems using different fiber links. Simulations for wavelength division multiplexed (WDM) 112 Gb/s polarization division multiplexed quadrature phase shift keying (PDM-QPSK) systems with dispersion unmanaged (no DM) and dispersion managed (DM) fiber links are implemented. System Q-factor and maximum transmission distance at bit error rate (BER) of 3.8 ÿ 10^?3 are calculated for performance comparison. The results show that, for system with no DM fiber link, DBP method outperforms pilot-aided method, because DBP method has better performance for intra-channel fiber nonlinearity compensation. However, for system with DM fiber link where inter-channel fiber nonlinearity plays an important role, pilot-aided method performs better than DBP method, because of its ability for inter-channel fiber nonlinearity compensation.     

Polarization mode dispersion in WDM systems

The coupled nonlinear Schrodinger equations for two wavelength optical pulses in a birefringence fiber are given. The model for treating polarization mode dispersion in two channel (WDM) system is established. Based on this model, optical signal propagation behaviors in two channel WDM with polarization mode dispersion are numerically simulated. The influences of polarization mode dispersion on two channel WDM system are also analyzed.     

Design of adaptive dispersion compensation and demodulation system for 40 Gbps RZ-DQPSK optical fiber communication receivers

An adaptive terminal dispersion compensation and demodulation scheme based on dual-feedback strategy for 40 Gbps return-to-zero differential quaternary phase shift keying (RZ-DQPSK) fiber communication receivers is proposed to achieve high reliability receiving performances. In the receiving system introduced in this paper we utilize both the balanced receiver feedback and framer error rate to achieve accurate RZ-DQPSK optical signal demodulation and adaptive dispersion compensation. Compared to traditional RZ-DQPSK receivers, the proposed design combines the chromatic dispersion (CD) compensation and Mach-Zehnder interferometer (MZI) adjusting and automatically estimates the dispersion compensation parameters without human intervention. And another improvement is introducing the framer error rate as feedback quantity to ensure the accuracy of CD and MZI adjusting. Finally, we present the performance of the designed receiver by showing the simulations and experiment results in actual telecommunication engineering projects.