Effects of Upstream Incoherent Crosstalk Caused by ASE Noise from Tx‐Disabled ONUs in XG‐PONs and TWDM‐PONs

A large incoherent crosstalk (IC) caused by amplified spontaneous emission (ASE) noise power from Tx‐disabled optical network units and a differential path loss has been shown to degrade upstream transmission performance in time‐division multiplexing passive optical networks. This paper considers the IC‐induced power penalty of an upstream signal both in an XG‐PON and in a TWDM‐PON. We investigate the degradation of the extinction ratio and relative intensity noise through a simulation and experiments. For the XG‐PON case, we observe a 9.6 dB difference in the level of ASE noise power from Tx‐disabled ONUs (hereafter known simply as ASE noise) between our result and the ITU‐T XG‐PON PMD recommendation. We propose an optical filtering method to mitigate an IC‐induced power penalty. In the TWDM‐PON case, the IC‐induced power penalty is naturally negligible because the ASE noise is filtered by a wavelength multiplexer at the optical line terminal. The results provide design guidelines for the level of ASE noise in both XG‐PONs and TWDM‐PONs.     

Degradation of Bi-Directional Single Fiber Transmission in WDM-PON Due to Beat Noise

This paper presents a general formulation for the impact of back-reflection in wavelength division multiplexing-passive optical networks (WDM-PON) access networks. The analysis is applied to various wavelength-independent optical network unit (ONU) configurations such as amplified spontaneous emission (ASE)-locked Fabry-Perot (FP) lasers, reflective semiconductor optical amplifiers or injection locked FP using the downstream signal. The power penalty due to beat noise and its dependence on relative intensity noise, transmitter linewidth, and receiver bandwidth is investigated. The optimal gain at the ONU that minimizes the effect of beat noise is also found. The results show that the power penalty decreases as the linewidth of the the optical line terminal (OLT) and ONU light source increase.     

Suppression of common-mode beat noise from optical amplifiers using a balanced receiver

When a coherent (heterodyne or homodyne), single-mixer type optical receiver is combined with an optical preamplifier or in-line amplifier(s), the S/N is deteriorated severely due to the common-mode (signal-ASE and ASE-ASE) beat noise. When a balanced-type coherent receiver is used, the common-mode beat noise can be suppressed and the ideal, so-called LO-ASE beat-noise limited, S/N ratio is attainable. The authors present a clear experimental demonstration of the suppression of common-mode beat noise by a balanced receiver. The minimum required LO power is determined by the thermal noise or the common-mode rejection ratio (CMRR) of the balanced receiver.<>     

WDM transmission system performance: influence of non-Gaussiandetected ASE noise and periodic DEMUX characteristic

This paper presents the first unified wavelength division multiplexing (WDM) transmission model for systems incorporating cascaded optical amplifiers and a realistic demultiplexing (DEMUX) characteristic with periodic transfer function. The bit error ratio (BER) is evaluated accounting in rigorous form for the influence of non-Gaussian detected amplified spontaneous emission (ASE) noise, noise correlation between stochastic noise samples in the receiver, the bandwidth of the electrical receiver noise filter, the gain tilt and effective noise figure of optical amplifiers (with as well as without optical ASE noise filtering), channel crosstalk, signal extinction ratio and a one-or two-stage DEMUX implementation. The model is compared to the Gaussian receiver model in realistic design cases providing important information as to the validity of the Gaussian model. Practical design results indicate the link budget dependence on the DEMUX design and the ASE noise filtering     

掺铒光纤放大器噪声特性的全面分析

本文对掺铒光纤放大器（ＥＤＦＡ）的噪声特性进行了全面的分析。把光信号和ＥＤＦＡ中放大自发辐射（ＡＳＥ）产生的量子噪声，以及信号－ＡＳＥ拍频噪声和ＡＳＥ－ＡＳＥ拍频噪声一并加以考虑。通过数值模拟，给出了不同信号功率和泵浦功率下的ＥＤＦＡ噪声系数。这些结果对优化ＥＤＦＡ的工作参量及其工程制作具有一定的指导作用。     

Q-factor estimation and impact of spontaneous-spontaneous beat noise on the performance of optically preamplified systems with arbitrary optical filtering

Simple explicit expressions to estimate the Q-factor and the sensitivity of optically preamplified receivers with arbitrary optical filtering, which only require the eye-diagram analysis and the knowledge of the optical and electrical filters transfer functions, are proposed. The physical insight and fast computation time are its main advantages. The noise-equivalent bandwidths associated with the nonuniformity of the amplified spontaneous emission (ASE) noise spectrum at the photodetector input are fully characterized and taken into account in the expressions derivation. By using the noise-equivalent bandwidths, a simple way of designing the optical receiver filters to bound the impact of the ASE-ASE beat noise on the receiver performance is provided. Results show that this impact can be neglected for extinction ratio below 25 dB, as long as the optical-filter -3 dB bandwidth does not exceed 8/spl times/ the bit rate. Numerical results reveal that our Q-factor expression provides acceptable estimates. Only for systems where the impact of the ASE-ASE beat noise is significant (high extinction ratio) or in case of low Q (below 4) and high intersymbol interference (ISI), less accurate estimates have been found. In case of high ISI and low Q, the accuracy is improved by taking the probability of occurrence of the nearest rails to the decision threshold in the eye-diagram into account.     

Noise filtering with the nonlinear optical loop mirror

The operation of a nonlinear optical loop mirror as a passive and signal polarization independent noise filter, is investigated experimentally. Spontaneous emission not within the signal spectrum is rejected leaving a receiver noise limited solely by signal-spontaneous beat noise. The receiver power penalty caused by spontaneous emission (@BER=10^-9 (PRBS)) was improved from 3 dB to 0.4 dB by propagating a signal through the loop. A comparison between this nonlinear filtering: technique and a variable band-pass filter is also made     

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     

Metrics for dispersion ripple in optical systems

An analysis of the relationship between group delay ripple and eye closure penalty is used to develop two metrics that parameterise signal degradation due to optical filter group delay ripple. These metrics, which are based upon the standard deviation of the group delay ripple, correlate very well with the signal eye closure penalty. The metrics provide an upper and lower limit to the eye closure penalty applicable over a wide range of group delay ripple amplitudes and pitches as well as bit rates.     

Optical amplifier noise figure reduction for opticalsingle-sideband signals

This paper analyzes the amplification of optical single-sideband signals, highlighting the significance of the optical signal bandwidth in determining signal-amplified spontaneous emission (ASE) beat noise. It predicts that an electrical noise figure of one may be realized for single-sideband signals using appropriate optical filtering. It verifies noise figure reduction experimentally, demonstrating an electrical noise figure of 1.1× the spontaneous emission factor of the amplifier, which is 2.6 dB below the limit for conventional intensity modulated signals     

Optical filter impact on eye opening in vestigial sideband systems

An analytical tool is presented in order to analyze and elucidate the impact of the optical filter position and steepness on eye opening in an optical vestigial sideband system with direct detection. It is shown that bit sequences containing solitarities (i.e., bit sequences containing single bits surrounded by several bits of opposite kind) are the major bit sequences to cause eye closure. In comparison with double-sideband modulation format, an eye opening penalty of 3-6 dB can be achieved with vestigial sideband format. The analytical tool helps to understand the interrelation between the bit sequence properties and the optical filter parameters.     

Effects of In-Band Crosstalk in Wavelength-Locked Fabry–PÉrot Laser-Diode-Based WDM PONs

We investigated the impact of in-band crosstalk on the system's performance in wavelength-locked Fabry-Perot laser-diode (FP-LD)-based wavelength-division-multiplexed passive optical networks (WDM PONs). As expected, wavelength-locked FP-LD, which was injected by using an incoherent broadband light source, was more tolerant to in-band crosstalk than distributed-feedback (DFB) laser diode. A 1-dB power penalty in the wavelength-locked FP-LD-based PON system was observed when the crosstalk-to-signal ratio was ~-9 dB. We also compared the measured power penalties with the calculated power penalties. It has been shown that the in-band crosstalk-induced power penalty in wavelength-locked FP-LD-based WDM PON could be estimated properly by taking into account both effects of power addition and signal-crosstalk beat noise.     

SOA功率均衡技术的误码率性能分析

对光交叉连接 (OXC)节点中采用半导体光放大器 (SOA)功率均衡技术的方案性能进行了理论分析。首先模拟计算了链路中引入SOA后的功率代价 ;然后逐一讨论了发射机消光比 ,信号码率 ,SOA的噪声系数对系统性能的影响 ;最后计算了OXC节点级联链路的误码率性能。从而为OXC节点和网络设计提供了理论分析和指导依据。     

Reducing ASE Effect in Coherent Detection by Employing Double-Pass Fiber Preamplifier and Time-Domain Filter

We report on a novel fiber based coherent detection system employing a double-pass fiber preamplifier, a spectral bandpass filter, and a time-domain filter. The time-domain filter, a synchronous time gate, reduces the in-band amplified spontaneous emission (ASE) beat noise, which cannot be achieved by the spectral bandpass filter alone. The double-pass fiber amplifier further reduces the out-band ASE by about 20 dB with a fiber Bragg grating (FBG) at the end of the fiber amplifier. In preliminary experiments with a 100 GHz bandpass filter, no degradation is observed from the optically preamplified coherent detection compared to pure coherent detection. With a 10 ns pulse width, 500 kHz repetition rate, and 10 pW average input power, about 2 dB and 1 dB signal-to-noise ratio (SNR) improvements are achieved when 5% and 50% time gating duty cycles are used, respectively.     

波分复用系统的光信噪比与代价测试方法探讨

本文通过分析光放大器噪声的成分,指出了波分系统中在OSNR表象一致的情况下光放大噪声与ASE噪声的差异,以及这种差异在检测系统传输性能时所造成的影响,并设计了相关的代价测试实验系统。通过对实验数据的分析,指出了进行与系统OSNR代价相关的性能测试时如何获得真实系统参数的办法。     

Impact of crosstalk in an arrayed-waveguide multiplexer onN×N optical interconnection

The impact of crosstalk in an arrayed-waveguide N×N wavelength multiplexer is investigated precisely in relation to its application to wavelength-routing N×N all optical networks. In such systems multiple crosstalk light which has the same wavelength as the signal results in signal-crosstalk beat noise. We confirm that the noise is Gaussian and obtain the relation between crosstalk and power penalty. It is shown that the crosstalk must be less than -38 dB for a 16×16 system to keep the power penalty below 1 dB at a bit error rate of 10^-9     

Direct detection of optical digital transmission based onpolarization shift keying modulation

Receiver structures for the direct detection of binary and multilevel digital optical modulation schemes employing the modulation of the state of polarization of light, or polarization shift keying (POLSK), are proposed and accurately analyzed, in the presence of optical amplifier ASE noise and electrical receiver noise. A comprehensive set of results shows that the performance of the binary system is approximately 3 dB better than IM/DD (on peak optical power), and stays so for a wide range of optical filter bandwidths. As a somewhat unexpected result, the multilevel schemes show a lower sensitivity to the use of a wide optical filter than the binary one. As a consequence, transmitting 3 b/symbol on a cubic constellation on the Poincare sphere brings about virtually no penalty, whereas transmitting 2 b/symbol has a better performance than binary transmission, for even relatively low values of the optical filter bandwidth. A clear explanation of this phenomenon has been found. These results suggest that narrow-bandwidth and therefore low-dispersion, multilevel, POLSK transmission could be performed with no penalty. Together with the low excitation of nonlinear effects in the fiber that polarization modulation seems to ensure, these results make POLSK schemes look promising for very-long-haul transmission     

Influence of extinction ratio on performance of optical receivers incorporating laser preamplifiers

Reports the performance degradation of optical receivers incorporating semiconductor laser amplifiers (SLAs) caused by the nonzero extinction ratio of the input optical signal. The resulting sensitivity penalty dependencies on bit rate, SLA gain and facet reflectivity are also investigated. The results clearly show that the new receiver can be more affected by a finite extinction ratio compared to a conventional pin or APD receiver. Using a bandpass optical filter to reduce amplifier spontaneous emission noise will increase the extinction ratio penalty.<>     

Power penalty due to optical reflections in erbium-doped fiberpreamplifier

The effect of optical reflections in erbium-doped fiber amplifiers (EDFAs) is investigated primarily as they are used for preamplifying the received signal in a digital transmission system. Both an increase in amplified spontaneous emission (ASE) due to its multiple reflection and an increase in relative noise intensity (RIN) due to interferometric phase-to-intensity-noise conversion are considered as sources of degradation of the receiver sensitivity. The power penalty is calculated and confirmed experimentally     

Effect of optical amplifier noise on laser linewidth requirementsin long haul optical fiber communication systems with Costas PLLreceivers

The impact of optical amplifier noise is analyzed in investigating the performance of optical long-haul PSK homodyne communication systems with Costas phase locked loop (PLL) receivers. The laser linewidth requirement for an optically amplified system becomes relaxed in comparison with the system with no optical amplifier, owing to the fact that the effect of incomplete phase tracking becomes less important as a larger signal power is demanded to maintain a fixed bit-error rate. Also, it is found that the power splitting ratio regarding the power distributions for the I-arm and the Q-arm of a Costas loop can vary in a wide range without having much influence on the performance of an optically amplified system. As a matter of fact, the power penalty induced by incomplete phase tracking for a system with a large number of cascaded optical amplifiers is mainly due to the finite phase error and not due to the power splitting ratio, and this may fail a previously-reported method for finding the required laser linewidth by assigning a certain amount of power penalty that is due to the power splitting ratio