Analysis methods for optical heterodyne DPSK receivers corrupted bylaser phase noise

Two methods are presented for analyzing the effects of phase noise on the performance of an optical heterodyne binary differential-(DPSK) system. The first method utilizes a perturbation solution for filtered phase noise. By comparing the results of this analysis with simulated results, it is shown that the perturbation solution is accurate for laser linewidths up to at least 10% of the bit rate. Using this analysis, the accuracy of the widely used approximation, whereby the effects of filtering on the magnitude of the phase-noise corrupted signal are neglected, is verified. The author's second method is based on moments of random variables. As the level of phase noise in a practical DPSK system must be small, an improved formulation for the moments of the filtered phase noise is derived. It removes the major cause of this numerical instability. A maximum-entropy probability density function estimation technique is applied to the problem of analyzing the performance of a DPSK receiver. By comparing results with those obtained using the perturbation analysis, it is found that the moment-based method is effectively limited to relatively large error probabilities     

Low detection penalty in an optical DPSK heterodyne system using asaturated optical amplifier as phase modulator

A model established from the rate equations of a near traveling-wave optical amplifier (NTWOA) used as a phase modulator shows that the modulation rate is limited by the carrier lifetime to 500 Mb/s and that the worst-case detection penalty is about 0.7 dB. It is then experimentally demonstrated that close control of the injected light polarization can reduce the detection penalty in a DPSK (differential phase-shift keying) heterodyne system, using an NTWOA phase modulator instead of an LiNbO₃ modulator, to 0.6 dB. This device and a balance receiver have allowed the implementation of 282-Mb/s DPSK link operating at 1.522 μm over 242 km of fiber with a 3-dB margin     

1.4 Gbit/s optical DPSK heterodyne transmission system experiment

The authors report the performance of a 1.4 Gbit/s differential phase shift keying (DPSK) coherent fibre system experiment using a balanced receiver. They show that the detection sensitivity as a function of local oscillator power follows the same functional dependence as the theoretical prediction     

Analysis and design of a DPSK optical heterodyne receiver in the presence of laser phase noise and frequency detuning

This paper is concerned with the analysis and design of an improved differential heterodyne optical receiver for differentially-encoded binary PSK (briefly, DPSK) signals in a coherent lightwave communication system. In the first part of the paper, we discuss the relevant design criteria to be employed when dealing with asynchronous heterodyne receivers for ASK, FSK or DPSK optical signals. In particular, we introduce a convenient definition of the signal-to-noise ratio at the data detector input to be assumed as the system performance measure when the non-negligible linewidth of the transmit/receive laser sources are to be taken into account. Following this design approach, we show that by properly modifying the traditional delay-and-multiply DPSK receiver, i. e. by allowing the delay to be a fraction of the symbol interval, we can considerably reduce the performance degradation caused by laser phase noise. We show thus that the superior power-efficiency of DPSK can be traded in favour of a decreased sensitivity to phase noise through a proper choice of the differential detector delay. In this respect, our results reveal that DPSK may still be competitive with other modulation formats even with non-negligible linewidth sources. In the last part of the paper, the behaviour of the optimized DPSK, ASK and large-deviation FSK data demodulators in the presence of a quasistationary frequency detuning of the local laser is also discussed under the same set of conditions as in the previous analysis. The results can be employed to derive accurate design requirements for the AFC loop of the receiver.     

光外差系统噪声等效功率的一种测量方法

本文介绍了利用工作于Bragg衍射的Ge晶体声光调制器作为外差系统中的频移器来测量外差系统的噪声等效功率(NEP)的方法。该方法具有测试系统简单、信号光功率、偏频可调的优点。实验测试与理论分析吻合。     

BER for optical heterodyne DPSK receivers using delay demodulationand integration detection

Previous work on the bit error rate (BER) performance of differential phase-shift keying (DPSK) including the effects of additive white Gaussian noise (AWGN) and phase noise has concentrated on the delay demodulator with narrow-band intermediate frequency (IF) bandpass filtering (BPF) and sampling detection. No similar analysis has yet been performed for the delay demodulator with wideband IF bandpass filtering and integration detection. Phase noise is an important consideration in coherent optical communication systems and the most widely accepted model is a Brownian motion process. A closed-form BER expression along with detailed Monte Carlo simulation results are presented for the DPSK delay demodulator with wideband IF bandpass filtering and integration detection filtering including phase noise effects using the Brownian motion model. Analytic expressions are also obtained for the moments of the phase noise component of the decision variable. Using these moments, estimates of the phase noise BER floor are produced. It is found that this receiver has noise performance comparable to receivers with narrowband IF bandpass filtering and sampling detectors for signal-to-noise ratio (SNR) and phase noise in the range of practical interest, but with potentially less degradation due to intersymbol interference (ISI)     

Influence of the nonlinear phase and ASE noise on DPSK balanced optical receiver in optical fiber communication system

The analytical expression of bit error probability in a balanced differential phase-shift keying(DPSK) optical receiver considering nonlinear phase noise and EDFA ASE noise is given,which is very useful to estimate the performance of DPSK balanced and unbalanced receiver in optical communication system.Through analysis,if only nonlinear phase noise is considered,both the balance and unbalanced receivers have the same performances.But if adding the ASE noise of EDFA,the balanced receiver is better.     

Error probability of 2DPSK with phase noise

A simple tight upper bound on the BEP of 2DPSK over the AWGN channel with phase noise in the received signal is obtained. The phase is modeled as a Gaussian random process which is slowly varying compared to the bit rate so that a piecewise-constant approximation can be made. The bound is verified by computer simulations, and it provides good estimates of the error probability. It shows that for high SNR the error probability decreases as the reciprocal of the square-root of the SNR. The results are applicable in particular to heterodyne optical communications     

400 Mbit/s optical heterodyne DPSK system experiment with inline1.5 μm amplifiers

Reports a 400 Mbit/s differential phase shift keying (DPSK) heterodyne optical transmission system experiment using two inline packaged resonant type amplifiers. The optical amplifier gave a 14.5 dB net gain from fibre to fibre at λ=1.488 μm with no significant system penalty     

An improved optical heterodyne DPSK receiver to combat laser phasenoise

Presents a method to improve the performance of a standard delay-and-multiply type optical heterodyne binary-DPSK receiver. This is achieved by using separate “weighting” bandpass filters to shape the phase noise corrupted direct and delayed signals before combining and decoding as for the standard receiver. The optimization of a filter parameter allows this “weighted” receiver to exhibit a lower phase noise sensitivity than the standard receiver. The most significant achievement of the weighted receiver is the ability to tradeoff excess signal power to obtain low bit error rates when large amounts of phase noise are present     

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.     

Optical DPSK heterodyne detection experiments using DBR laser diodes with external optical feedback

High receiver sensitivity, with 7 dB improvement over a direct detection system, has been achieved in a 400 Mbit/s optical DPSK heterodyne detection experiment using DBR laser diodes with external optical feedback. The influence of the laser output phase noise was evaluated experimentally in good agreement with theory.     

Calculation of bit error rate in DPSK system with interfering echoes from optical amplifier chain

The bit error rate degradation due to echoes caused by cascaded nearly travelling wave amplifiers in an optical DPSK-heterodyne transmission system is calculated. Using these results, an estimate is derived for the maximal allowable facet reflectivity in dependence of the number of in-line optical amplifiers.<>     

A postdetection strategy for the reduction of error floors ofoptical DPSK receivers impaired by laser phase noise

A postdetection strategy for the performance enhancement of optical binary differential phase shift keying (DPSK) receivers impaired by phase noise is proposed and investigated. First, the bandwidth of integrate-and-dump (I&D) filters is increased so that multiple samples per symbol duration of the received signal are obtained. Previously, a scheme, hereafter referred to as fixed-window (FW) postdetection processing, was introduced. In this approach, the samples separated in time by a symbol time at the output of I&D filters are correlated. The decision circuitry, subsequently, provides a sum of the resulting signal over a symbol time to yield a decision variable. In the proposed scheme here, hereafter referred to as symmetrical expanding window (S-EW) postdetection processing scheme, samples that are centered about the signal transition point in a two-symbol signaling interval are correlated. Numerical results indicate that the S-EW scheme achieves an error floor that is significantly smaller than that of the its FW counterpart     

Effect of semiconductor laser phase noise on BER performance in an optical DPSK heterodyne-type experiment

We report the first experimental investigation of the effect of laser-diode phase noise on the error-rate performance in a PSK heterodyne differential detection fibre communication experiment. A newly designed AlGaAs laser emitter with adjustable linewidth is used as both the signal carrier and the local oscillator. Error rate is measured for 25 Mbit/s and 50 Mbit/s signals at an intermediate frequency of 250 MHz, and a BER (bit error rate) of 10?10 has been achieved.     

A dual-detector optical heterodyne receiver for local oscillator noise suppression

The performance of a dual-detector optical heterodyne receiver was analyzed and compared with the performance of a conventional single-detector heterodyne receiver. The dual-detector receiver is found to offer two main advantages over the single-detector receiver-1) increased performance in the presence of local oscillator intensity fluctuations that might severely degrade single-detector receiver performance, and 2) decreased local oscillator power requirements. These two advantages are particularly important in a communication system which uses semiconductor laser diodes as local oscillators. Such lasers suffer from intrinsic wide-band intensity fluctuations and can also impose strict power constraints on receiver design. Based on the analysis, suggestions for the optimal design of a dual-detector heterodyne receiver are made. Also, several experiments were performed to demonstrate the improved performance of the dual-detector receiver-both for unguided- and guided-wave receivers.     

Analysis of the phase noise in saturated SOAs for DPSK applications

A theoretical model is used to analyze the impact of phase noise on the performance of semiconductor optical amplifiers (SOAs) in the saturation regime for differential phase-shift keying (DPSK) applications. It is found that the variance of the differential phase error scales as T/sup 2///spl tau//sub c//sup 2/ for /spl tau//sub c//spl Gt/T, where T is the bit period and /spl tau//sub c/ is the carrier lifetime of the SOA. This suggests that the adverse effect of saturation-induced phase noise can be significantly reduced by increasing the bit rate or the carrier lifetime.     

Quantum noise and excess noise in optical homodyne and heterodyne receivers

A parallel development of the semiclassical and quantum statistics of multispatiotemporal mode direct, homodyne, and heterodyne detection using an ideal (except for its subunity quantum efficiency) photon detector is presented. Particular emphasis is placed on the latter two coherent detection Configurations. The primary intent is to delineate the semiclassical theory's regime of validity and to show, within this regime of validity, how the quantum theory's signal quantum noise, local oscillator quantum noise, the quantum noise incurred because of subunity detector quantum efficiency, plus (for heterodyning only) image band quantum noise produce the quantitative equivalent of the semiclassical theory's local oscillator shot noise. The effects of classical fluctuations on the local oscillator, and the recently suggested dual-detector arrangement for suppressing these fluctuations, are treated. It is Shown that previous studies of this arrangement have neglected a potentially significant noise contribution.     

基于延时自零差光相干接收机的激光器相位噪声测试系统

在相干光通信系统中,激光器相位噪声是影响接收机灵敏度的重要因素。针对相干光通信中的激光光源相位噪声测试提出并研究了表征激光器相位噪声的3个关键指标,分别是电场的功率谱密度、相位误差方差和FM噪声谱线,建立了基于延时自零差相干接收技术的窄线宽激光器相位噪声测试系统,实现了系统仿真,并对一窄线宽激光器进行了相位噪声测试,相比传统的自外差线宽测量技术,此方法在满足测试分辨率要求的同时能够更全面表征激光器相位噪声特性。