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.     

OSNR Monitoring Method for OOK and DPSK Based on Optical Delay Interferometer

We describe a simple method to measure the optical signal-to-noise ratio (OSNR) of on-off-keying (OOK) and differential phase-shift-keying (DPSK) signals by using an optical delay interferometer (ODI) having a sinusoidal and tunable passband. This OSNR monitoring method is independent of chromatic dispersion, polarization-mode dispersion, and noise polarization. We show experimentally that accurate OSNR measurements are made for a 10-Gb/s OOK signal by using a 1-bit ODI and a 40-Gb/s DPSK signal by using a partial-bit ODI with the OSNR ranging from 5 to 25 dB.     

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.      

Monitoring optical network performance degradation due to amplifier noise

The relationship between optical performance as monitored within a network and the end terminal optical signal-to-noise ratio (OSNR) and bit error ratio (BER) is measured for the case of performance degradation due to amplified spontaneous emission noise. Measurements on 10-Gb/s signals reveal that performance monitoring sensitivity to OSNR levels of 26 dB is sufficient for identifying degradations that impact the end terminal BER.     

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.     

OSNR Monitoring Technique for DPSK/DQPSK Signals Based on Self-Heterodyne Detection

We describe a new method for monitoring the “in-band” optical signal-to-noise ratio (OSNR) of differential phase-shift-keying (DPSK) and differential quadrature phase-shift-keying (DQPSK) signals. This technique estimates the OSNR by analyzing the radio frequency spectrum obtained by the self-heterodyne detection. The results show that the performance of the proposed technique is not sensitive to the effects of chromatic dispersion and polarization-mode dispersion. In addition, this technique can be used for monitoring the polarization-scrambled signals. For a demonstration, we measure the OSNR of the polarization-scrambled 10-Gb/s DPSK and 20-Gb/s DQPSK signals in a 640-km-long transmission link.      

Experimental study on optically band-limited 40-Gb/s RZ signals with optically time-division demultiplexing receiver

The impact of optical filtering on 40-Gb/s return-to-zero (RZ) signals was experimentally investigated with an optically time-division multiplexing (OTDM) receiver. Through the evaluation of the signal performance by changing the interpulse-phase conditions of the symmetrically band-limited 40-Gb/s signals, we have confirmed that similar performance was obtained regardless of the interpulse-phase condition, owing to the pulse-reshaping capability of an OTDM receiver. A performance comparison was also conducted between symmetrically and asymmetrically filtered 40-Gb/s RZ signals. It was found that the symmetrically filtered signal was more tolerant for the dispersion-compensation error, while the asymmetrically filtered signal was more tolerant for fiber nonlinearity with optical filters that have a 3-dB bandwidth of 45 GHz.     

Superposition of DQPSK over inverse-RZ for 3-bit/symbol modulation-demodulation

We successfully demonstrated overwriting of differential quadrature phase-shift keying (DQPSK) on inverse return-to-zero (RZ) pulses for simple 3-bit/symbol operation at a 10-Gb/s symbol rate (30-Gb/s bit rate). We adopted cross-gain modulation (XGM) in a semiconductor optical amplifier (SOA) for inverse-RZ generation, which allows both low and high levels of RZ optical signal to have a finite pulse energy in a bit time slot. We verified a wide tolerance of 20% of the bit-slot for time slot alignment between amplitude-shift keying and differential phase-shift keying modulation in the proposed scheme. We also demonstrated wide dynamic range characteristics at the extinction ratio for both 2- and 3-bit/symbol operation, compared to the conventional scheme. The proposed scheme allows a cross-modulation penalty, due to the intensity to phase modulation, of less than 1.5 dB in 2-bit/symbol and less than 5 dB in 3-bit/symbol operation.     

Experimental Investigation of Delay-Tap Sampling Technique for Online Monitoring of RZ-DQPSK Signals

We experimentally demonstrate optical performance monitoring of 20-Gb/s return-to-zero differential quadrature phase-shift keying signals using an asynchronous delay-tap sampling technique. This method allows online monitoring of accumulated chromatic dispersion in the range from $-$600 to $+$600 ps/nm. We also show evaluation of optical signal-to-noise ratio at a level of 6.7 dB and monitor operation at input power of $-$ 16 dBm.      

All-Optical RZ-DPSK WDM to RZ-DQPSK Phase Multiplexing Using Four-Wave Mixing in Highly Nonlinear Fiber

We propose and experimentally demonstrate an all-optical phase multiplexing scheme using four-wave mixing in a highly nonlinear fiber. Two 10-Gb/s pi/2-shifted return-to-zero (RZ) differential phase-shift-keying (DPSK) wavelength-division-multiplexing (WDM) signals are successfully phase-multiplexed into a 20-Gb/s RZ differential quadrature phase-shift-keying signal with a negative 1.6-dB power penalty. With more input DPSK WDM signals, the proposed scheme can be applied to obtain a multilevel phase-shift-keying signal with increased capacity and enhanced spectral efficiency.     

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。     

40-Gb/s NRZ wavelength conversion with 3R regeneration using an EA modulator and SOA polarization-discriminating delay interferometer

We propose and demonstrate, for the first time, to our knowledge, a 40-Gb/s nonreturn-to-zero wavelength converter with 3R regeneration based on an electroabsorption modulator and semiconductor optical amplifier polarization-discriminating delay interferometer. The results show that the root mean square timing jitter and optical signal-to-noise ratio (OSNR) of the regenerated signal are kept to 0.68 ps and higher than 40 dB/0.1 nm, respectively. In addition, the measured receiver sensitivity (BER=10/sup -9/) is improved from -19 to -25.2 dBm at the input OSNR of 26 dB/0.1 nm.     

Simultaneous 4 × 10 Gb/s NRZ-to-RZ Modulation Format Conversion in Nonlinear Optical Loop Mirror With a Photonic Crystal Fiber

Orthogonal cross-phase-modulation (XPM) scheme in a nonlinear optical loop mirror (NOLM) is proposed to simultaneously convert four synchronized 10-Gb/s nonreturn-to-zero signals into return-to-zero (RZ) format. Modulation format conversion is achieved by using a synchronized optical control pulse train as the pulse carver in a NOLM. The control pulse train and the targeted signals are orthogonal in their states of polarization. This orthogonal nonlinear interaction substantially suppresses the undesirable four-wave mixing (FWM) induced crosstalk in the multichannel operation. Experimental demonstration of the proposed scheme shows that the suppression of the generated FWM idler is >26 dB. Error-free operation is achieved for the four converted 10-Gb/s RZ signals in a single NOLM by sharing the nonlinear XPM effect induced by the control pulse train.     

Optical performance monitoring using data stream intensity autocorrelation

We demonstrate optical performance monitoring (OPM) using intensity autocorrelations of 10-Gb/s return-to-zero (RZ) data. Data stream intensity autocorrelations are used for the quantitative determination of the optical signal-to-noise ratio (OSNR) and accumulated dispersion values up to half the Talbot dispersion. Autocorrelations are performed using sensitive two-photon absorption in photon counting silicon avalanche photodiodes. Simulations and experimental results demonstrate the retrieval of optical signal-to-noise ratio values in the presence of large accumulated dispersion.     

Payload-envelope detection and label-detection integrated photonic circuit for asynchronous variable-length optical-packet switching with 40-gb/s RZ payloads and 10-gb/s NRZ labels

A photonic integrated circuit that performs 40-Gb/s payload-envelope detection (PED) and 10-Gb/s label detection for asynchronous variable-length optical-packet switching is demonstrated. The circuit consists of an InP photonic integrated device combined with electronic GaAs and InP devices on a carrier. Asynchronous variable-length optical packets with 40-Gb/s return-to-zero (RZ) payloads and 10-Gb/s non-RZ (NRZ) labels are processed by the circuit. The circuit outputs a PED electrical signal that represents the temporal location of the payload and a 10-Gb/s electrical signal representing the optical label. The optical label is detected error free. The PED signal has a rise/fall time of 3-ns and 150-ps jitter. The PED signal was also used to erase and rewrite the optical labels error free.     

Wavelength-Shift-Free 3R Regenerator for 40-Gb/s RZ System by Optical Parametric Amplification in Fiber

We demonstrate wavelength-shift-free 3R-regenerator for 40-Gb/s optical return-to-zero (RZ) system by optical parametric amplification (OPA) with a clock-modulated pump in highly nonlinear fiber. The power penalty is improved by 2.6 dB, and the signal power is amplified by 7 dB     

A novel scheme for generating optical dark return-to-zero pulses and its application in a label switching optical network

We experimentally demonstrate a novel and simple method to generate a dark return-to-zero (RZ) pulse with tunable pulsewidth and extinction ratio by using a dual-arm LiNbO /sub 3/ intensity modulator. Our experimental results show that this dark RZ pulse signal can be used in a 10-Gb/s optical packet switching system as an optical label. In addition, we demonstrate that this dark RZ label can be easily erased using the gain saturation effect in a semiconductor optical amplifier.     

Maximum-likelihood optimal-sampling-phase estimation for ultra-high-sensitivity optically preamplified duobinary receivers

This paper describes the demonstration of a novel method for detecting optical duobinary signals. Oversampling in combination with maximum-likelihood estimation-based data processing of the optical-to-electronic converted signal results in a record sensitivity for an optically preamplified receiver. The concept is investigated at 10 Gb/s and allows data recovery at bit-error rates of 1/spl times/10/sup -9/ and 1/spl times/10/sup -3/ with a receiver sensitivity of better than -39.5 dBm and an optical signal-to-noise ratio (OSNR) of only 7.8 dB, respectively.     

Low insertion loss and low dispersion penalty InGaAsP quantum-well high-speed electroabsorption modulator for 40-Gb/s very-short-reach, long-reach, and long-haul applications

We present a metal-organic-chemical-vapor-deposition-grown low-optical-insertion-loss InGaAsP/InP multiple-quantum-well electroabsorption modulator (EAM), suitable for both nonreturn-to-zero (NRZ) and return-to-zero (RZ) applications. The EAM exhibits a dynamic (RF) extinction ratio of 11.5 dB at 1550 nm for 3 Vp-p drive under 40-Gb/s modulation. The optical insertion loss of the modulator in the on-state is -5.2 dB at 1550 nm. In addition, the EAM also exhibits a 3-dB small-signal response (S21) of greater than 38 GHz, allowing it to be used in both 40-Gb/s NRZ and 10-Gb/s RZ applications. The dispersion penalty at 40 Gb/s is measured to be 1.2 dB over /spl plusmn/40 ps/nm of chromatic dispersion. Finally, we demonstrate 40-Gb/s transmission performance over 85 km and 700 km.     

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.