Performance analysis of differential-phase-shift-keying optical receivers in the presence of in-band crosstalk noise

In-band crosstalk can pose important limitations in an all-optical wavelength-division-multiplexed (WDM) network. Recent studies have demonstrated that differential phase shift keying (DPSK), can tolerate higher in-band crosstalk-noise levels compared to amplitude shift keying (ASK). In this paper, the performance of a DPSK receiver, limited by in-band crosstalk noise, is studied theoretically. The model takes into account both the in-band crosstalk noise as well as the amplified-spontaneous-emission (ASE) noise of the optical amplifiers. The model is based on the evaluation of the moment-generating function (MGF) of the decision variable through which, the error probability (EP) can be calculated by applying the saddle point approximation. This provides a rigorous model for the evaluation of the EP of a DPSK receiver in the presence of ASE and in-band crosstalk noises. In the absence of the ASE noise, a closed-form formula for the EP is also given that is useful for estimating the error floor set by the in-band crosstalk noise.     

Multicanonical Monte Carlo Modeling of Wavelength Division Multiplexed Differential Phase Shift Keying Systems

Differential phase shift keying (DPSK) modulation is being considered as a possible candidate for future optical wavelength division multiplexed (WDM) transmission systems. In a single channel link, the balanced interferometric DPSK receiver exhibits increased tolerance against amplified spontaneous emitting (ASE) noise and fiber nonlinear effects. In this paper, a model is presented that can be used to estimate the performance of a multichannel DPSK system taking into account the influence of interchannel phenomena, namely cross-phase modulation (XPM) and four wave mixing (FWM), in the phase noise statistics. The model is based on an approximate solution of the fiber propagation equation and the multicanonical Monte Carlo (MCMC) method. It provides an efficient tool that can be used to investigate the influence of many link design parameters such as channel spacing, launch power, and fiber dispersion. The model is illustrated in the comparison of the performance of multichannel DPSK to on-off keying (OOK) systems. It is verified that, even in the presence of interchannel effects, DPSK modulation greatly enhances the system performance compared to OOK.     

Performance analysis of an optical serial-to-parallel converterwith erbium-doped fiber amplifier

In this paper, we develop a performance analysis of an surface-emitting second harmonic generation (SESHG) optical serial-to-parallel converter using an erbium-doped fiber amplifier (EDFA) as a preamplifier. The analysis is complicated by the fourth-order nonlinearity that acts on the signal plus amplified spontaneous emission (ASE) noise to create many beat noises at the binary decision device. However, we demonstrate that Gaussian approximation for the beat noise statistics is reasonable. We calculate the BER of the system as a function of the SHG nonlinear cross section (A^NL), EDFA gain, the bandwidth of the optical filter that band-limits the ASE noise, and the timing pulse-to-data pulse power ratio. We find that for reasonable values of these and other parameters, the EDFA/SESHG serial-to-parallel converter combination should be able to operate at or below a BER of 10^-12. We find that small increments (0-2 dB) in the signal power that is input to the EDFA are enough to compensate the effects of ASE noise for most of the parameter variations we consider. From this point of view, the ASE noise has little effect on system performance. However, when the input power is fixed, we show evidence in terms of BER that the ASE noise plays a significant role, particularly in the high A^NL, high gain case. Also in this case, we show that the optimal timing pulse-to-data pulse power ratio is somewhat different from the value that is optimal for the system without an EDFA     

Parametric-gain approach to the analysis of single-channel DPSK/DQPSK systems with nonlinear phase noise

This paper presents a novel method based on a parametric gain (PG) approach to study the impact of nonlinear phase noise in single-channel dispersion-managed differentially phase-modulated systems. This paper first shows through Monte Carlo simulations that the received amplified spontaneous emission (ASE) noise statistics, before photodetection, can be reasonably assumed to be Gaussian, provided a sufficiently large chromatic dispersion is present in the transmission fiber. This paper then evaluates in a closed form the ASE power spectral density by linearizing the interaction between a signal and a noise in the limit of a distributed system. Even if the received ASE is nonstationary in time due to pulse shape and modulation, this paper shows that it can be approximated by an equivalent stationary process, as if the signal were continuous wave (CW). This paper then applies the CW-equivalent ASE model to bit-error-rate evaluation by using an extension of a known Karhunen-Loe/spl acute/ve method for quadratic detectors in colored Gaussian noise. Such a method avoids calculation of the nonlinear phase statistics and accounts for intersymbol interference due to a nonlinear waveform distortion and optical and electrical postdetection filtering. This paper compares binary and quaternary schemes with both nonreturn- and return-to-zero (RZ) pulses for various values of nonlinear phases and bit rates. The results confirm that PG deeply affects the system performance, especially with RZ pulses and with quaternary schemes. This paper also compares ON-OFF keying (OOK) differential phase-shifted keying (DPSK) systems, showing that the initial 3-dB advantage of DPSK is lost for increasing nonlinear phases because DPSK is less robust to PG than OOK.     

基于仿真软件的DPSK光纤传输性能研究

介绍了DPSK码型的产生原理,通过仿真软件模拟级联调制器产生各种DPSK光调制码型,给出了频谱,并基于仿真软件建立了DPSK光纤传输模型,重点考虑了光放大器的自发辐射(ASE)噪声对DPSK系统的性能影响.实际仿真结果表明了ASE噪声是限制级联DPSK传输系统传输距离的最主要因素.     

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。     

基于光纤四波混频DPSK系统相位噪声提取检测和抑制

差分相移键控(DPSK）在长距离光纤通信系统中有高接收机灵敏度、高频谱效率以及抗非线性效应方面的优势,最近受到广泛关注。在DPSK通信系统中,线性相位噪声和非线性相位噪声是影响系统性能的主要因素。研究了如何利用光纤四波混频（FWM）效应来监测和抑制相位噪声,降低相位噪声对DPSK通信系统的影响。结果表明利用光纤FWM效应产物可以检测相位噪声值,并且基于光纤饱和FWM效应全光限幅器可以有效地降低非线性相位噪声,提高了DPSK系统性能。该结果对研究高速全光通信有重要意义。     

Suppression of beat noise from optical amplifiers using coherentreceivers

Coherent optical fiber communications have been studied intensively because of their high receiver sensitivity and high-frequency selectivity. With the advent of an erbium-doped fiber amplifier (EDFA), however, the first advantage seems to have become less attractive. Nevertheless, the combination of the EDFA and coherent techniques offers a number of attractive features. In this paper, we show both theoretically and experimentally that the excess beat noises (common-mode and image-band beat noises) from optical amplifiers can be suppressed by using coherent receivers such as a balanced receiver and a double-stage phase-diversity (DSPD) receiver. The noise figure (NF) of the excess-noise-suppressed coherent receivers with an optical preamplifier is shown to be 0 dB. Bit-error-rate (BER) formulas are also discussed     

掺铒光纤放大器的非线性色散效应

给出一种模型来考虑非线性色散对光通信系统的影响。数值结果表明 ,考虑非线性色散时 ,光脉冲的传播速度比不考虑非线性色散时的传播速度要快 ,而脉冲展宽是一样的     

Modulation and demodulation techniques in optical heterodyne PSKtransmission systems

Modulation and demodulation techniques are described for an optical PSK heterodyne transmission system operating at 560 Mb/s and 1.2 Gb/s. Performance limitations affecting the receiver sensitivity in a 1.2-Gb/s DPSK system, such as laser phase noise, phase modulation depth, IF center frequency deviation, and local laser power, are studied. High receiver sensitivities for PSK systems were achieved. The applicability of the Mach-Zehnder modulator as a phase modulator for 1.2-Gb/s DPSK is also demonstrated. A 1.2-Gb/s DPSK transmission of over 100 km, using polarization diversity with novel polarization-insensitive automatic frequency control in an attempt to overcome signal fading caused by polarization fluctuation in the transmitting fiber, is also described. A receiver sensitivity of less than -42.8 dBm and varying within 1.4 dB for all states of polarization was achieved. A multichannel high-definition TV (HDTV) transmission experiment using a DPSK polarization-diversity tunable receiver is described     

Nonlinear Phase Noise in Optical-Fiber-Communication Systems

Gordon and Mollenauer, in their famous paper published in 1990, laid out how the interplay between the nonlinear Kerr effect in optical fibers and the amplified spontaneous-emission (ASE) noise from the optical-amplifiers results in enhanced levels of noise and degrades the performance of modulation schemes that encode information in, particularly, the phase of the optical carrier. This phenomenon has been termed as nonlinear phase noise in the literature. In this paper, we first present a comparative and critical review of previous techniques that have been proposed for the analysis of nonlinear phase noise by forming a classification framework that reveals some key underlying features. We then present a unifying theory and a comprehensive methodology and computational techniques for the analysis and characterization of nonlinear phase noise and its impact on system performance by building on and extending previous work that we identify as most favorable and systematic. In our treatment, we consider a multichannel multispan optically amplified dense wavelength-division multiplexed system and develop general techniques for the analysis of the intricate interplay among Kerr nonlinearity, chromatic dispersion, and ASE noise, and for computing the bit-error-ratio performance of differential phase-shift-keying (DPSK) systems. By means of the extensive results we present, we demonstrate and argue that correlated noise behavior plays a most significant role in understanding nonlinear phase noise and its impact on DPSK system performance.     

Performance of open-loop all-optical chaotic communication systems under strong injection condition

A numerical investigation of the performance of an open-loop optical chaotic communication system for the isochronous synchronization solution has been carried out, under strong optical injection conditions achieved using antireflective coating at the input facet of the receiver laser in combination with an optical erbium-doped fiber amplifier (EDFA). Different message encoding techniques have been considered and tested at multigigabit rates and for different levels of optical injection to the receiver. The effects induced by the amplified spontaneous emission (ASE) noise of the EDFA to the performance of the chaotic communication system have also been studied. The performance of all the examined encryption methods for the 1 Gb/s bit-rate messages was quite satisfactory and was characterized by Q-factor values that exceeded 10, after synchronizing in the strong injection regime. For higher message bit rates, the Q-factor values for all methods decrease considerably due to the residual frequency components of the chaotic carrier that are now more significant in the message spectral region. The effect of the amplifier's ASE noise to the system's performance was deteriorated as long as the EDFA chaotic input was kept in relatively high power levels.     

Probability of error for optical heterodyne DPSK system with quantum phase noise

An expression is derived for the probability of error in an optical heterodyne DPSK system, subject to shot noise and quantum phase noise of the optical sources. Results are given for the maximum linewidth of the transmit and local optical sources to satisfy a given degradation in optical receiver sensitivity.     

Single and Dual-Level Minimum Shift Keying Optical Transmission Systems

We present optical transmission systems employing minimum shift keying modulation formats of single and dual-amplitude level under linear, weakly nonlinear, strongly nonlinear variation of the lightwave carrier within a bit-period depending on whether the phase variation within a symbol period is linear or nonlinear. These formats are externally modulated, incoherently and differentially detected based on the Mach-Zehnder delay interferometric optical balanced receiver. Transmission performance of these optical transmission systems is evaluated in terms of receiver sensitivity, amplification stimulated emission noise loading, dispersion tolerances. These performance characteristics are compared with return-to-zero (RZ) differential phase shift keying (DPSK) and carrier-suppressed RZ on-off keying modulation formats. Accurate bit-error ratios are obtained and confirmed by different statistical techniques: Monte Carlo, single-Gaussian or multiple Gaussian distributions and generalized Pareto distribution statistical methods, especially when the eye diagrams are distorted. Among the three minimum shift keying (MSK) types, the weakly nonlinear optical MSK is found to be the most promising because of its robust transmission performance and more importantly, its reduced-complexity of the electrical driving signals for transmitter in modulating the lightwave carrier as compared to the linear MSK counterpart. Transmission performance of dual-level MSK optical transmission systems depends on the intensity-splitting ratio of the two levels. The performance of three ratios: 0.7/0.3,0.8/0.2, and 0.9/0.1 are demonstrated. The spectral attributes of 80 Gb/s dual-level MSK optical signals for these three ratios are similar to each other and compatible with that of 40 Gb/s optical MSK, but narrower than that of 40 Gb/s optical nonreturn-to-zero DPSK, hence high spectral efficiency of the dual-level MSK.     

Effects of nonlinear dispersion in EDFA's on optical communicationsystems

The signal-induced change of the refractive index in an erbium-doped fiber amplifier (EDFA) causes a phase modulation imposed on a signal when passing the EDFA. In this paper, we apply our extended EDFA model on an optical communication system. The model includes this phase modulation, by including the nonlinear dispersion in an EDFA, and the spontaneous emission noise. The influence of these effects on an optical communication system is examined by means of Q-factor and eye diagram. We assume an intensity modulated-direct detection (IM-DD) system operating at 193 THz (1552.5 nm) with a bit rate of 10 Gb/s in the anomalous dispersion regime and a total fiber length of 500 km. The fibers are assumed to be dispersion shifted ones, EDFAs are used to compensate for the fiber loss. By numerical simulation we obtain results for the influence of the phase modulation (nonlinear dispersion) due to the signal induced change of the refractive index in an EDFA and the spontaneous emission noise at different input peak powers. Neglecting the signal-induced change of the refractive index strongly underestimates the Q-factor in the anomalous dispersion regime. Therefore it should be included for reliable system simulations. This can be done with the numerical model presented here     

Polarization independent coherent optical receiver

This paper describes an optical heterodyne receiver for DPSK signals which can receive an optical signal having an arbitrary polarization state. This is achieved by splitting the received signal between two orthogonal polarization axes and processing the resulting two signals as in a conventional DPSK heterodyne receiver. The sum of the two demodulated signals provides a baseband signal independent of the polarization state of the received optical signal. When the receiver noise is dominated by the shot noise of the photodetectors, the receiver provides a BER of 10^-9for an average number of 22 photon/bit. In comparison, a conventional optical heterodyne receiver requires under the same noise condition 20 photon/bit to achieve the same BER for a received optical signal polarized along the polarization axis of the local optical signal.     

Performance analysis of an asymptotically quantum-limited opticalDPSK receiver

An optical, direct-detection differential phase-shift keying (DPSK) receiver whose error probability is quantum-limited as the transmitting laser linewidth vanishes is analyzed. The receiver design is based on a binary equiprobable hypothesis test with doubly stochastic point process observations, the conditional random rates of which depend on the transmitting laser phase noise, which is modeled as a Brownian motion. The receiver structure consists of a simple delay-and-sum optical preprocessor followed by a photoelectric converter and an integrate-and-dump circuit. Upper and lower bounds on the receiver bit error rate are derived by developing bounds on the conditional rates of the point process, and it is shown that the error probability bounds converge to the true value as the transmitting laser linewidth decreases. Bounds on the power penalty are computed for parameters corresponding to existing semiconductor injection lasers, and are seen to be less than the limiting power penalty for the balanced DPSK receiver     

基于EDFA的分布式光纤喇曼温度解调技术

为了消除掺铒光纤放大器的放大自发辐射噪声对分布式光纤温度传感器测温精度的影响,采用精确的修正公式进行温度解调是一种有效手段。通过分析掺铒光纤放大器的放大自发辐射噪声对系统产生的影响,并将计算的中间结果代入理论公式获得了修正后的温度解调公式。实验中分别采用理论公式和修正公式对温度进行分布式测量并获得了实验数据。结果表明,修正后的温度解调公式有效补偿了掺铒光纤放大器噪声引起的误差,并显著提高了系统的信噪比和测温精度;修正公式可将系统的整体测温精度提高到1℃～2℃;此外,实验中观察到多模光纤中喇曼背向自发散射同样会出现放大现象,这与单模光纤中的情形类似,且放大喇曼自发散射依然具有理想的温度效应。     

Phase noise cancellation in a carrier recovered optical heterodynereceiver

The performance and application of an optical heterodyne receiver which uses a carrier recovery demodulator are described. Phase sensitive demodulators used in coherent optical transmission are compared, and the suppression of both phase noise and frequency instability of light sources by a carrier recovery (CR-) demodulator is described. A carrier recovered PSK (CR-PSK) demodulator and a phase noise canceling circuit (PNC) for a coherent SCM receiver are introduced as examples of CR-demodulators. The relationship between laser diode spectral linewidths and the delay time difference between the two paths in the CR-PSK demodulator necessary to keep the system performance within a certain power penalty is then derived. In a preliminary experiment using 560-Mb/s CR-PSK transmission, a receiver sensitivity of -51.6 dBm was obtained, and a laser phase noise suppression of about ¹/₂ that of DPSK was confirmed. The results suggest the possibility of constructing a heterodyne receiver which has no AFC-loop. Applications of a CR-demodulator to an optical frequency division multiplexing (OFDM) system and to a multivalue modulation scheme are discussed