Impact of nonlinear phase noise on direct-detection DQPSK WDM systems

The performance degradation of differential quadrature phase-shift keying (DQPSK) wavelength-division-multiplexed (WDM) systems due to self-phase modulation (SPM)- and cross-phase modulation (XPM)-induced nonlinear phase noise is evaluated in this letter. The XPM-induced nonlinear phase noise is approximated as Gaussian distribution and summed together with the SPM-induced nonlinear phase noise. We demonstrate that 10-Gb/s systems, whose walkoff length is larger than 40-Gb/s systems', are more sensitive to XPM-induced nonlinear phase noise than 40-Gb/s systems. Furthermore, DQPSK WDM systems show lower tolerance to both SPM- and XPM-induced nonlinear phase noise than differential phase-shift keying WDM systems.     

Duty-ratio control of nonlinear phase noise in dispersion-managed WDM transmissions using RZ-DPSK modulation at 10 Gb/s

The authors compare analytical and numerical estimates, showing that the nonlinear phase noise of short optical pulses associated with the coupling between amplified spontaneous emission noise and fiber nonlinearity may be controlled by adjusting the duty cycle of the return-to-zero (RZ) signal modulation format. The impact of this effect in the optimization of the performance of 10-Gb/s dispersion-managed wavelength division multiplexed (WDM) systems using RZ-differential phase-shift keying (DPSK) modulation is discussed. By extensive numerical simulations, it is shown that the transmission quality of ultradense WDM systems using the RZ-DPSK modulation format may be significantly enhanced by optimizing the duty cycle of the RZ pulses.     

Amplitude Regeneration of RZ-DPSK Signals in Single-Pump Fiber-Optic Parametric Amplifiers

The input power tolerance of a single-pump fiber-optic parametric amplifier (FOPA) is experimentally shown to be enhanced for return-to-zero differential phase-shift keying (RZ-DPSK) modulation compared to RZ on–off keying modulation at 40 Gb/s. The improved nonlinear tolerance is exploited to demonstrate amplitude regeneration of a distorted RZ-DPSK signal in a gain-saturated FOPA. An optical signal-to-noise ratio penalty of 3.5 dB after amplitude distortion is shown to be reduced to 0.2 dB after the FOPA, thus clearly demonstrating the regenerative nature of saturated FOPAs for RZ-DPSK modulation.      

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.      

WDM PON using 10-Gb/s DPSK downstream and re-modulated 10-Gb/s OOK upstream based on SOA

A signal remodulation scheme of 10-Gb/s differential phase-shift keying(DPSK) downstream and 10-Gb/s on-off keying(OOK) upstream using a semiconductor optical amplifier(SOA) and a Mach-Zehnder intensity modulator(MZ-IM) at the optical networking unit(ONU) side for wavelength division multiplexed passive optical network(WDM PON) is proposed.Simulation results indicate that error-free operation can be achieved in a 20-km transmission,and the receiver sensitivity of return-to-zero differential phase-shift keying(RZ-DPSK) is higher than nonreturn-to-zero differential phase-shift keying(NRZ-DPSK) in the proposed scheme.     

WDM transmission at 6-Tbit/s capacity over transatlantic distance, using 42.7-Gb/s differential phase-shift keying without pulse carver

We report the transmission of a record 6 Tbit/s capacity over 6120 km distance, involving channels modulated at 42.7-Gb/s bit-rate with differential phase-shift keying (DPSK). The performance is found similar to DPSK with subsequent pulse carving, namely RZ-DPSK.     

Accurate computations of the nonlinear phase noise and BER in DPSK modulation-balanced direct detection high-speed optical fiber communication systems

This article analyzes the nonlinear phase noise caused by an amplified spontaneous-emission of erbium-doped optical fiber amplifier and self-phase modulation (SPM) in a differential phase-shift keying modulation- balanced direct detection optical fiber communication system. Using numerical methods, the characteristic function of the differential nonlinear phase noise cos(Dj){\cos (\Delta \varphi)} in the received electrical current of signals and the accurate computation formula of the BER are achieved when SPM is dominant comparing to cross phase modulation and four wave mixing. The result is verified by a simulation of a 40-Gbit/s transmission system.     

Simulation for all-optical format conversion from NRZ-DPSK to RZ-DPSK

We propose and simulate a simple scheme of all-optical format conversion from non-retum-to-zero differential phase-shift keying (NRZ-DPSK) to return-tozero differential phase-shift keying (RZ-DPSK) by using phase modulators and detuning filters.The operation principle is theoretically analyzed and simulated by exploiting spectra,temporal waveform and eye diagram with commerical optical design software VPI Transmission Maker 8.5.The use of electrical clock recovery and linear phase modulation in the conversion scheme may be potiental in practise use.     

Self-Pumping Wavelength Conversion for DPSK Signals and DQPSK Generation Through Four-Wave Mixing in Highly Nonlinear Optical Fiber

A novel scheme is proposed to achieve self-pumping wavelength conversion for two differential phase-shift keying (DPSK) signals at different wavelengths through four-wave mixing (FWM) effect in a highly nonlinear optical fiber. By changing the phase modulation depths to pi/2 for both of the DPSK signals, the two signals can be multiplexed to generate a differential quadrature phase-shift keying signal. The simulations and experimental results demonstrate the feasibility of phase manipulations for phase-shift keying signals through the FWM process     

10Gb/s和40Gb/s RZ-DPSK电预失真系统中非线性效应的研究

通过仿真研究了10和40Gb/s RZ-DPSK电预失真(EPD)系统中的自相位调制(SPM)和交叉相位调制(XPM)等非线性效应。EPD系统中的非线性效应比光色散补偿(ODC)中的大,但在不同比特率下非线性效应不同。对800km标准单模光纤(SSMF)传输的仿真的结果表明:单信道传输时受到SPM影响,比特率为10Gb/s的EPD系统的非线性阈值比ODC系统的小6dBm以上,而当比特率为40Gb/s时的EPD系统非线性阈值只比ODC系统小2dBm。波分复用(WDM)系统中受到SPM和XPM的影响,比特率为10Gb/s的EPD系统的非线性阈值比ODC系统的小6dBm,而比特率为40Gb/s时的EPD系统非线性阈值比ODC系统的小2dBm。研究结果表明,当比特率升高时,EPD系统的非线性效应减弱。     

Nonlinear signal–noise interactions in dispersion-managed links with various modulation formats

Purpose of this paper is to highlight the principles of the nonlinear signal–noise interaction (NSNI) in dispersion-managed long-haul optical links and provide a quantitative understanding of the system parameters for which NSNI sets the nonlinear performance of the most popular intensity and phase modulation formats, namely on–off keying, differential binary and quadrature phase-shift keying and coherent quadrature phase-shift keying.     

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.     

Signal Design and Detection in Presence of Nonlinear Phase Noise

In optical fiber transmission systems using inline amplifiers, the interaction of a signal and amplifier noise through the Kerr effect leads to nonlinear phase noise that can impair the detection of phase-modulated signals. We present analytical expressions for the maximum-likelihood (ML) decision boundaries and symbol-error rate (SER) for phase-shift keying and differential phase-shift keying systems with coherent and differentially coherent detection, respectively. The ML decision boundaries are in the form thetas(r) = c₂r² + c₁r + c₀, where thetas and r are the phase and the amplitude of the received signal, respectively. Using the expressions for the SER, we show that the impact of phase error from carrier synchronization is small, particularly for transoceanic links. For modulation formats such as 16-quadrature amplitude modulation, we propose various transmitter and receiver phase rotation strategies such that the ML detection is well approximated by using straight-line decision boundaries. The problem of signal constellation design for optimal SER performance is also studied for a system with four signal points.     

RZ-DPSK传输系统相位噪声的比较分析

建立了单信道40Gbit／s归零-差分相移键控码（RZ-DPSK）传输系统模型，详细地分析并比较了线性相位噪声和非线性相位噪声对系统性能的影响。结果表明，由放大器自发辐射（ASE）噪声带来的线性相位噪声对传输系统的性能影响最大，ASE与自相位调制（SPM）相互作用引起的Gordon-Mollenauer（G-M）相移次之，带内四波混频（IFWM）的影响最小。在最优输入功率的条件下，由ASE带来的线性相位噪声引起的系统差分相位Q值代价大于204dB，由G-M相移引起的系统差分相位Q值代价小于7．5dB。仿真结果表明，对于采用掺铒光纤放大器（EDFA）级联放大的单信道40Gbit／s RZ-DPSK传输系统，ASE噪声是限制系统无中继传输距离的最主要因素。     

XPM Statistics in 100% Precompensated WDM Transmission for OOK and DPSK Formats

We numerically study cross-phase modulation (XPM) statistics in 100% precompensated wavelength-division-multiplexing transmission with on–off keying (OOK) and differential phase-shift keying (DPSK) formats. XPM and its randomness are determined by nonlinear diffusion bandwidth and specific modulation format in the target channel. XPM degradation in DPSK channels shows much less stochastic than that in OOK channels. The small duty cycle also helps to reduce XPM randomness.      

Comparison of nonlinear phase noise and intrachannel four-wave mixing for RZ-DPSK signals in dispersive transmission systems

Self-phase-modulation-induced nonlinear phase noise is reduced with the increase of fiber dispersion but intrachannel four-wave mixing (IFWM) is increased with dispersion. Both degrading differential phase-shift keying signals, the standard deviation of nonlinear phase-noise-induced differential phase is about three times that from IFWM even in highly dispersive transmission systems.     

Transmission Performance Comparison of Direction Detection‐Based 100‐Gb/s Modulation Formats for Metro Area Optical Networks

Transmission performances of direct detection‐based 100‐Gb/s modulation formats are investigated and compared for metro area optical networks. The effects of optical signal‐to‐noise ratio sensitivity, chromatic dispersion, cross‐channel nonlinearity, and transmission distance on the performance of differential 8‐ary phase‐shift keying (D8PSK), differential phase‐shift keying plus three‐level amplitude‐shift keying (DPSK+3ASK), and dual‐carrier differential quaternary phase‐shift keying (DC‐DQPSK) are evaluated. The performance of coherent dual‐polarization quadrature phase‐shift keying (DP‐QPSK) with block phase estimation and coherent DP‐QPSK with digital differential detection are also presented for reference. According to our analysis, all three direct detection modulation formats could transmit a 100‐Gb/s signal over several hundred kilometers of a single‐mode fiber link. The results also show that DC‐DQPSK outperforms D8PSK and DPSK+3ASK, and the performance of DC‐DQPSK is comparable to that of coherent DP‐QPSK with digital differential detection. The maximum transmission distance of DC‐DQPSK is over 1,000 km, which is enough distance for metro applications.     

Compensation improvement of DPSK signal with nonlinear phase noise

When nonlinear phase noise is compensated by the received intensity, simple formulas are derived for the error probability of differential phase-shift keying signals. Simulation is conducted to verify the error probability. The tolerance of nonlinear phase noise is doubled by the compensator, allowing doubling of the transmission distance if nonlinear phase noise is the dominant impairment.     

Modulation and coding for mobile radio communications: channelswith correlated Rice fading and Doppler frequency shift

We discuss the relative performance of coherent phase-shift keying (PSK), differential PSK (DPSK), and double differential PSK (DDPSK) modulation schemes over a mobile radio channel in which transmission is affected by additive noise, a constant carrier phase offset, a constant Doppler frequency shift, and correlated Rice fading. We first compare the performance of these schemes to assess the amount of degradation caused on each one of them by fading. Among our findings, we observe that DDPSK turns out to be less sensitive to the effects of correlated fading than the other two schemes and that in these conditions interleaving may not be beneficial. We then consider the introduction of trellis-coded modulation (TCM). The system we advocate as offering the best tradeoff between performance and complexity with the channel model assumed here includes double-differential encoding, differential detection, an open-loop Doppler phase tracking circuit, and differential decoding in addition to a simple TCM scheme