A novel chromatic dispersion monitoring method for 400 Gbit/s 256 QAM fiber-optic system based on asynchronous amplitude sampling

A novel chromatic dispersion (CD) monitoring technique based on asynchronous amplitude sampling (AAS) is proposed for a higher modulation format and higher rate system. The dispersion and other impairment factors can be separated with the definition of monitoring parameter M. A 400 Gbit/s 256 quadrature amplitude modulation (QAM) system is built using Optisystem13.0 beta software. Simulations of CD monitoring technique for different bandwidths of sampling Gaussian filter, optical signal to noise ratios (OSNRs) and duty cycles are investigated, and the tolerance is also discussed. Simulation results show that the method can be less affected by noise, and a higher accuracy of 600 ps/(nm.km) can be achieved. The technique supports a wide range of data traffic and enhances operation flexibility of optical networks.     

基于异步幅度抽样的高速光信号色散监测方法

针对目前异步幅度统计直方图（AAH）对色散监测范围小、灵敏度低的缺点,提出一种基于异步幅度抽样（AAS）的色散监测方法,定义无量纲色散监测参数G,将色散与其他损伤因子分离开。使用 OptiSystem9.0 软件搭建了200Gbps DP-16QAM 监测系统,在不同信噪比和占空比下进行色散监测仿真实验,探讨了信噪比和占空比对该方法的监测精度的影响,并对色散监测误差进行了分析。仿真结果表明基于该参数的色散监测方法受噪声因子影响小、精确度高,能实现在0~600 ps/nm 全范围内对色散的有效监测,为在更高阶调制信号的光纤通信系统中进行色散监测管理提供一定的参考。     

An chromatic dispersion monitoring method based on spectral-shift of SOA

A remnant dispersion monitoring method based on spectral-shift of SOA in high speed optical communication system with CSRZ format and single channel speed of 40 Gbit/s is proposed. The system performance can be optimized by careful choosing bandwidth and center wavelength of the optical fiber grating filter. The dynamical monitoring range is ±60 ps/nm and the monitoring precision is about 5 ps/nm. This method can be suitable for the application in dynamical dispersion compensation of high speed optical communication systems.     

Optimum filter bandwidths for optically preamplified NRZ receivers

We present a comprehensive treatment of optically preamplified direct detection receivers for non-return-to-zero (NRZ) and return-to-zero (RZ) on/off keying modulation, taking into account the influence of different (N)RZ optical pulse shapes, specified at the receiver input, and filter transfer functions; optical Fabry-Perot filters (FPFs) and Bragg gratings as well as electrical fifth-order Bessel and first-order RC low-pass filters are considered. We determine optimum optical and electrical filter bandwidths and analyze the impact of bandwidth deviations on receiver sensitivity. Optimum receiver performance relies on a balance between noise and intersymbol interference (ISI) for NRZ transmission, while for RZ reception detection noise has to be traded against filter-induced signal energy rejection. Both for NRZ and 33% duty cycle RZ, optical filter bandwidths of around twice the data rate are found to be optimum. Receivers using RZ coding are shown to closely approach the quantum limit, and thus to outperform NRZ-based systems by several decibels. We further analyze the impact of important degrading effects on receiver sensitivity and optimum receiver bandwidths, including receiver noise, finite extinction ratio, chirp, and optical carrier frequency (or optical filter center frequency) fluctuations     

Improved all optical chromatic dispersion monitoring method for WDM systems

In this paper we propose an improved all optical chromatic dispersion (CD) monitoring method applicable to wavelength division multiplexing (WDM) systems with intensity or phase modulation format. This method can operate over a much larger bandwidth and CD monitoring range and is insensitive to optical signal-to-noise ratio (OSNR) and polarization mode dispersion (PMD). These improvements are achieved by utilizing a wavelength tunable laser as the probe to maintain the appropriate phase-matching condition and a loop-back controlled tunable CD compensator to discriminate CD distortions from PMD and OSNR influences. Numerical simulations show that by this method the operational bandwidth and CD monitoring range can be expanded to cover C + L bands and ±1600 ps/nm, respectively, for different modulation formats and the sensitivity is higher than the other methods proposed before.     

变占空比脉冲测量非线性光环镜阈值特性的实验研究

光阈值器件是多用户光码分多址(OCDMA)系统接收机中抑制基底噪声和互相关旁瓣峰的重要部件。非线性光环镜(NOLM)是实现光阈值的一种有效方法。对基于高非线性光纤(HNLF)NOLM的阈值特性进行了实验研究及数据分析,提出利用改变抽运脉冲占空比分析阈值特性的方法,实验发现抽运脉冲占空比D为1/25时出现阈值点,阈值功率为3.75W。讨论了抽运光与信号光的波长间隔对NOLM输出信号的影响,得出阈值点处的最优波长间隔为5.8nm,脉冲压缩量为114.47ps。实验搭建了接收机阈值系统,眼图分析表明该系统能较好地抑制噪声。     

Optical Performance Monitoring of Phase-Modulated Signals Using Asynchronous Amplitude Histogram Analysis

As optical networks continue to grow towards high capacity and high flexibility, new transmission technologies are being introduced. In order to maintain the quality of signal and control over network in the transparent domains, optical performance monitoring (OPM) systems are becoming a necessity. Phase modulation formats emerge as the solution of choice in transparent domains because of their sensitivity, spectral efficiency, and resilience to optical impairments. In this paper, we demonstrate a flexible OPM method for phase-modulated signals using asynchronous amplitude histogram analysis. We show numerically and experimentally the monitoring of optical signal-to-noise ratio (OSNR), chromatic dispersion (CD), and polarization-mode dispersion (PMD) for differential phase-shift keying (DPSK) and differential quadrature phase-shift keying (DQPSK) signals. The OSNR can be measured within range of 20-35 dB and accumulated chromatic dispersion between 600 and 600 ps/nm. The asynchronous amplitude histogram monitoring method is proved to be a precise and versatile monitoring tool for high-capacity optical networks.     

Simple and highly sensitive method for wavelength measurement of low-power time-multiplexed signals using optical amplifiers

We describe a simple method for the wavelength measurement of optical signals that is easily capable of measuring a 1-nW average power optical signal with a wavelength resolution of 0.1 pm/Hz/sup 1/2/ while maintaining a large measurement range in excess of 12 nm. The system uses an erbium-doped fiber amplifier to increase the signal level before being measured with a wide-band edge filter. This technique is well suited to the measurement of low duty cycle time-multiplexed signals such as those in multiplexed fiber sensor systems. We show that the measurement of the amplified signal is improved despite the broadband nature of the amplified spontaneous emission noise. We show for the first time that the addition of an amplifier can increase the detection capabilities of the edge filter method beyond the shot noise limit of an unamplified measurement.     

高非线性光纤中并行交叉相位调制的偏振膜色散监测方法

利用信号光和插入的连续泵浦光之间产生的交叉相位调制（XPM）效应,提出了一种基于并联的XPM效应来监测光相位调制信号的一阶偏振模色散（PMD）的新技术。泵浦光的光谱会随着信号光中PMD和色散（CD）的变化而发生变化,所以导致泵浦光的光功率发生变化,在并联的一个支路中抑制PMD的影响,利用并联的两路同一波段泵浦光功率的差值来进行监测。仿真结果显示,新的技术可以实现对40 Gb/s非归零差分四相移相键控（NRZ-DQPSK）光信号从0~20 ps的监测。在20 ps的监测范围内,新技术的动态范围大于3 dB,可以用来进行准确的监测。对信号速率、色散、泵浦光功率和滤波器带宽对新技术的影响做了详细的研究。     

All-Optical Chromatic Dispersion Monitoring for Phase-Modulated Signals Utilizing Cross-Phase Modulation in a Highly Nonlinear Fiber

We propose and experimentally demonstrate an all-optical chromatic dispersion (CD) monitoring technique for phase-modulated signals utilizing the cross-phase-modulation effect between the input signal and the inserted continuous-wave probe. The probe's optical spectrum changes with the accumulated CD on the input signal, indicating that the optical power variations can be measured for monitoring. The experimental results show that this technique can monitor up to 120 ps/nm of CD for a 40-Gb/s return-to-zero differential phase-shift keying (RZ-DPSK) transmission system, with the maximum measured optical power increment of 16.5 dB. The applicability of this monitoring technique to higher bit-rate phase-modulated signals, such as 80-Gb/s RZ differential quadrature phase-shift keying and 80-Gb/s polarization-multiplexed RZ-DPSK, is also investigated via simulation.      

Enhanced Optical Generation of 42.7 Gbps Phase Shaped Binary Transmission

A novel method for optical phase shaped binary transmission (PSBT) generation has been recently reported and experimentally evaluated. An amelioration of the optical signal‐to‐noise ratio (OSNR) sensitivity of the optical PSBT modulation format is proposed with the application of the concepts of enhanced electrical PSBT signal generation. Enhanced optical PSBT is proposed here as a modulation format producing a 0.7 dB gain in OSNR sensitivity compared to OPSBT, while maintaining a good robustness to group velocity dispersion: 100 ps/nm for enhanced optical PSBT and 120 ps/nm for optical PSBT, compared to 50 ps/nm for standard NRZ signals.     

Chromatic Dispersion Monitoring for High-Speed WDM Systems Using Two-Photon Absorption in a Semiconductor Microcavity

This paper presents a theoretical and experimental investigation into the use of a two-photon absorption (TPA) photodetector for use in chromatic dispersion (CD) monitoring in high-speed, wavelength division multiplexing network. In order to overcome the inefficiency associated with the nonlinear optical-to-electrical TPA process, a microcavity structure is employed. An interesting feature of such a solution is the fact that the microcavity enhances only a narrow wavelength range determined by device design and angle at which the signal enters the device. Thus, a single device can be used to monitor a number of different wavelength channels without the need for additional external filters. When using a nonlinear photodetector, the photocurrent generated for Gaussian pulses is inversely related to the pulse width. However, when using a microcavity structure, the cavity bandwidth also needs to be considered, as does the shape of the optical pulses incident on the device. Simulation results are presented for a variety of cavity bandwidths, pulse shapes and durations, and spacing between adjacent wavelength channels. These results are verified experimental using a microcavity with a bandwidth of 260 GHz (2.1 nm) at normal incident angle, with the incident signal comprising of two wavelength channels separated by 1.25 THz (10 nm), each operating at an aggregate data rate of 160 Gb/s. The results demonstrate the applicability of the presented technique to monitor accumulated dispersion fluctuations in a range of 3 ps/nm for 160 Gb/s return-to-zero data channel.      

NRZ-DPSK and RZ-DPSK Signals Signed Chromatic Dispersion Monitoring Using Asynchronous Delay-Tap Sampling

In this paper, we demonstrated a signed chromatic dispersion (CD) monitoring method of 10 GHz nonreturn-to-zero differential phase-shift keying (NRZ-DPSK) and return-to-zero differential phase-shift keying (RZ-DPSK) signals by using asynchronous delay-tap sampling and an imperfect tuned delay interferometer. This method could monitor not only the value but also the polarity of residual CD. The demodulated signals show amplitude shoulders on the rising edge or the trailing edge with CD accumulation. Delay-tap sampling scatter plots could reflect this signal distortion by a unique characteristic and realize the signed CD monitoring. The monitoring range can be up to plusmn400 and plusmn720 ps/nm for NRZ-DPSK and RZ-DPSK signals, respectively. Simulation and experimental results are also proposed.     

Power penalty assessment in optically preamplified receivers witharbitrary optical filtering and signal-dependent noise dominance

The assessment of the power penalty of optically preamplified receivers with signal-dependent noise (SDN) dominance is often accomplished by neglecting the influence of the optical filtering of the amplified spontaneous emission (ASE) noise on the signal-ASE beat noise. In this paper, it is shown that the optical filtering of the ASE noise can have a strong impact on the signal-ASE beat noise and remarkably affect the power penalty, even for optical filter bandwidths five times wider than the signal bandwidth. A simple expression to analytically evaluate the power penalty due to optical filtering, which describes reasonably well the influence of the optical filter on the signal-ASE beat noise variance, is proposed. The accuracy of the new expression is investigated, in the case of assessment of the optical filter detuning impact on receiver performance and the case of optical filter bandwidth optimization, and its predictions are satisfactory in comparison with rigorous estimates. Two new expressions of power penalty due to extinction ratio and to eye closure are also presented. It is shown that the power penalty due to eye closure depends on the extinction ratio and vice versa. Our results show also that the power penalty due to eye closure is remarkably dependent on the eye closure asymmetry     

A novel chromatic dispersion monitoring technique for 16/64-QAM system based on asynchronous amplitude histogram

A novel chromatic dispersion (CD) monitoring technique based on asynchronous amplitude histogram (AAH) for higher order modulation formats is proposed in this paper. Without demodulating the signal, in the monitoring scheme, the received signal is sampled asynchronously, and thus clock information and high-speed sampling units are unnecessary, resulting in low cost and high reliability. Simulations of CD monitoring technique for non-return-to-zero/return-to-zero (NRZ/RZ) 16- and 64-quadrature amplitude modulation (QAM) systems with different optical signal-to-noise ratios (OSNRs) and duty cycles are investigated, and the tolerance of the scheme is also discussed. Simulation results show that the presented CD monitoring technique with high sensitivity can be applied to monitor the residual CD of a transmission link in the next-generation optical networks.     

Detailed Theoretical and Experimental Characterization of 10 Gb/s Clock Recovery Using a Q-Switched Self-Pulsating Laser

A detailed characterization of the clock recovery properties of a self-pulsating, three-section distributed feedback laser is presented by directly comparing simulation and experimental results for the dependence of the RMS timing jitter of the recovered clock signal on important properties of the input signal. These properties include the duty cycle, peak power, extinction ratio, state-of-polarization, optical signal-to-noise ratio (OSNR), and waveform distortion due to residual group velocity dispersion and polarization mode dispersion. The permissible range for each of these is identified in terms of the RMS timing jitter of the recovered clock signal being less than 2 ps. In particular, the self-pulsating laser is effective for input signals degraded by amplified spontaneous emission noise as it provides this level of jitter performance for input OSNRs larger than 8.8 dB (0.1 nm noise bandwidth).     

一种信号检测预处理的改进多尺度形态学滤波方法

通信信道冲击噪声干扰和宽带接收机非线性影响会导致接收的信号噪声基底不平坦,若不考虑噪声基底起伏而直接进行检测信号,可能会导致漏检。为提高信号检测精度,需估计信号噪声基底。为解决该问题,提出了一种信号检测预处理的改进多尺度形态学滤波方法。该方法采用多尺度开-闭组合运算对信号频谱进行形态学滤波,利用不同尺度的结构元素对不同带宽的信号进行局部滤波,并利用滤波后凸包的存在性作为滤波终止条件。仿真结果表明,该方法可以有效将不同带宽的信号滤除,有效提取噪声基底噪声,同时可消除频谱中低电平毛刺对噪声估计的影响。     

Statistical Analysis of Optical Signal-to-Noise Ratio Monitoring Using Delay-Tap Sampling

We derive analytical expressions for the optical signal-to-noise ratio (OSNR) monitoring using delay-tap sampling for intensity-modulated direct-detection (IM/DD) systems. We demonstrate through our theoretical analysis and experimental results that modeling of noise correlation due to low-pass filtering between delay-tap samples enables accurate monitoring of OSNR, without recalibration for each system setup. Effects of chromatic dispersion (CD) and polarization-mode dispersion on the proposed technique are investigated through numerical simulations. The monitoring error remains within $pm$ 1 dB for CD up to 400 ps/nm and differential group delay up to 45 ps.      

Sensitivity enhancement of optical receivers by impulsive coding

A theory for the signal-to-noise ratio (SNR) of optical direct-detection receivers employing return-to-zero (RZ) coding (and possibly optical preamplification) is developed. The results are valid for both signal-independent noise limited and signal-dependent noise limited receivers, as well as for arbitrary optical pulse shapes and receiver filter characteristics. Even if the same receiver bandwidth is used, RZ coding is seen to perform better than nonreturn-to-zero (NRZ) coding. Asymptotic expressions for the SNR in case of very high and very low receiver bandwidths show that the full sensitivity enhancement potential of RZ coding is exhausted at fairly moderate duty cycles. A realistic example taking into account intersymbol interference (ISI) shows that a receiver sensitivity gain (compared to NRZ coding) of, e.g., 3.2 dB can be obtained in a signal-independent noise limited receiver with a bandwidth of 80% of the data rate, using a duty cycle of three     

Enhancing the dynamic range and DGD monitoring windows in DOP-based DGD monitors using symmetric and asymmetric partial optical filtering

We propose and demonstrate a new type of degree-of-polarization (DOP)-based differential-group-delay (DGD) monitor using an optical filter such that the DGD monitoring range and DOP dynamic range are dramatically increased. We apply this technique to varying pulsewidth return-to-zero (RZ), carrier-suppressed RZ (CSRZ), and alternate-chirped RZ (ACRZ) signals and show that by optimally setting the position, bandwidth, and shape of a filter, we can double the DGD monitoring range compared to traditional DOP-based DGD monitors. Using our technique, the DGD monitoring ranges for 10, 20, and 40 Gb/s /spl sim/12.5-ps pulsewidth RZ signals are increased by 32, 33, and 12 ps, respectively. We also show that a narrow-band optical filter, offset from the center of the optical spectrum by the bit-rate frequency, can double the dynamic range of DOP-based DGD monitors for non-RZ (NRZ) signals.