EDFA-based DWDM lightwave transmission systems with end-to-end power and SNR equalization

An approximate analysis is presented which can be used to predict the performance of power and signal-to-noise ratio (SNR) equalization schemes when applied to dense wavelength-division multiplexing (DWDM) lightwave systems employing erbium-doped fiber amplifier (EDFA) cascades. Expressions are provided which relate the maximum number of amplifiers, EDFA gain imbalance, bit rate (R/sub b/), transmitter power, receiver dynamic range and number of channels. The relative advantages of these two equalization strategies are quantified by comparing the maximum number of amplifiers allowed by each scheme. It is shown that, while SNR equalization represents, on balance, the more desirable equalization strategy for future EDFA-based DWDM lightwave transmission systems, under certain conditions power equalization may be a better choice. When employing an APD receiver, for instance, power equalization can support 1.9 times more amplifiers than SNR equalization. However, when employing the more conventional preamplified PIN/FET receiver, SNR equalization can support 1.7 times more amplifiers than power equalization.     

Photoconductor receiver sensitivity

The various contributions to photoconductor (PC's) noise are calculated and are used to determine the sensitivity of digital photoconductor receivers for use in lightwave communication systems. We find that Johnson noise is the most significant source of noise current up to bit rates as high as 4 Gbit/s, above which FET channel noise becomes dominant. In comparing the results obtained for ideal photoconductive receivers with receivers employing p-i-n photodetectors, we find that the sensitivities of both circuits are comparable, provided that low-capacitance p-i-n receivers are employed. In contrast, we find that avalanche photodiode receivers have higher sensitivities than either photoconductor or p-i-n receivers over the entire bit-rate range considered. It is concluded that equalization necessary for photoconductor receiver operation at high bit rates due to a limited gain-bandwidth product significantly degrades the sensitivity of the receiver.     

The sensitivity of photoconductor receivers for long-wavelength optical communications

We calculate the sensitivity of In0.53Ga0.47As photoconductor receivers for use in moderate to high bit-rate lightwave transmission applications. It is found that the noise of photoconductive receivers is dominated at all bit ratesB < 4Gbit/s by Johnson noise in the conductive channel. Nevertheless, the total noise current decreases approximately linearly with photoconductive gain, and therefore the sensitivity of photoconductive receivers can be comparable to high-sensitivity p-i-n photodiode receivers. The sensitivity of practical photoconductive receivers compares most favorably with p-i-n receivers in the bit-rate range of 500-2 Gbit/s. However, receivers employing high-speed In0.53Ga0.47As/InP avalanche photodiodes are expected to be more sensitive than photoconductive receivers over the entire bit-rate range considered. In this analysis, we consider the effects of slow photoconductor response on receiver sensitivity, and find that the limited gain-bandwidth product of practical photoconductors increases the complexity of the receiver circuit by necessitating equalization, resulting in a decrease in receiver sensitivity and dynamic range.     

Gain-bandwidth-limited response in long-wavelength avalanche photodiodes

The gain-bandwidth(GB)-limited response of In0.53Ga0.47As/ InP heterostructure avalanche photodiodes (APD's) and related devices used in long-wavelength digital optical receivers is calculated. We find that these diodes, as currently designed, are useful at bit ratesB lsim 2Gbit/s when employed in conjunction with high-sensitivity optical receivers. Response at higher bit rates may be obtained depending on the details of device design. On the other hand, use of poor-quality receivers that require moderate-to-high values of optimum gain can significantly degrade the performance of heterostructure APD's at high bit rates due to GB limitations. We also show that APD receiver bandwidth can be expressed in terms of the sensitivity obtained using the receiver in conjunction with a p-i-n photodiode. It is found that the response speed of optimized receivers is lowest for an APD effective ionization rate ratio ofk = 0.5.     

Analysis of sensitivity degradation caused by the flicker noise ofGaAs-MESFETs in fiber-optic receivers

The total input noise current and sensitivity of the fiber-optic receiver was calculated. The flicker noise source was included by adopting a pertinent flicker noise model. Power penalties caused by the flicker noise were calculated for various fiber-optic receivers using the calculated noise current. It has been found that the flicker noise affects the sensitivity over the whole range of the bit rates, and that the total input capacitance is an important parameter affecting the power penalty which is serious in the case of a high-impedance-type p-i-n FET receiver. The optimum feedback resistance for practical p-i-n FET receiver design is also suggested     

Multigigabit-per-second avalanche photodiode lightwave receivers

High-speed avalanche photodiodes and high-sensitivity receivers are vital components for future multigigabit-per-second lightwave transmission systems. We review theoretical and experimental performance of high-speed III-V avalanche photodiodes, and also that of multigigabit-per-second lightwave receivers using FET and bi-polar amplifiers. Particular attention is given to APD gain-bandwidth product, and to its effect on high-speed receiver sensitivity. Comparisons between measured receiver sensitivities and calculated performance are presented for bit rates up to 8 Gbit/s.     

Resonant p-i-n-FET receivers for lightwave subcarrier systems

A theoretical and experimental analysis of narrowband resonant direct-detection p-i-n-FET receivers for subcarrier multiple-access networks is described. It is shown how a small inductance can be used to optimize the coupling between the p-i-n and FET, over a range of microwave subcarrier frequencies, minimizing the frequency-dependent thermal noise and leaving shot-noise as the ultimate limitation. Shot-noise then establishes a fixed ratio of the total usable bandwidth to the minimum received power per channel, which for the binary FSK system considered is 6.1 GHz/μmW. A resonant p-i-n-FET receiver, designed to provide maximum sensitivity between 2.5 and 5.0 GHz, has been constructed. The measured signal-to-noise ratio is in excellent agreement with that predicted by the noise analysis     

孔径接收下各向异性海洋湍流UWOC系统误码分析

为了研究孔径接收对各向异性海洋湍流条件下水下无线光通信(UWOC)系统误比特率的影响, 系统采用高斯光束传输, 接收端通过孔径接收, 在脉冲位置调制方式下通过各向异性海洋湍流信道。引入各向异性海洋湍流结构常数, 通过对闪烁的形成原理和各向异性海洋湍流条件下闪烁系数的分析, 数值模拟得到了在不同接收孔径和各向异性因子下, 海洋湍流参量、传输距离、雪崩光电二极管(APD)平均增益和调制阶数对系统误比特率的影响。结果表明, 相同各向异性因子和海洋湍流参量下, 大孔径接收能有效提升系统误比特率性能; 相同孔径直径和海洋湍流参量下, 各向异性因子越大, 系统通信性能越好; 均方温度耗散率、温度和盐度对海洋功率谱变化贡献的比值较小, 湍流动能耗散率、动力粘度较大以及传输距离越短, 系统误码性能越好; APD增益为100或150时, 系统通信性能最佳; 调制阶数M=8时, 系统通信性能最佳, M>64时, 系统误比特率变化程度几乎饱和。该研究为UWOC系统平台搭建和性能估计提供了参考。     

Application of preemphasis to achieve flat output OSNR intime-varying channels in cascaded EDFAs without equalization

In this paper, we present a new method for optical signal-to-noise ratio (OSNR) equalization of wavelength division multiplexed (WDM) channels at the end of a cascade of several erbium-doped fiber amplifiers (EDFAs) by use of preemphasis, as well as the proper choice of EDFA design parameters. Identical OSNR at the end of the cascade ensures better signal detection and quality of service. The dynamics of the equalizing method have been demonstrated by simulation for single- and double-stage amplifier designs using a numerical model incorporating time variation effects in EDFA. Calculations are based on the solution of a transcendental equation describing the dynamics of the reservoir, i.e., the total number of excited ions, for each EDFA. Traffic on eight WDM channels is modeled as statistically independent ON-OFF time-slotted sources. In addition, we investigate the effect of gain clamping of the first amplifier in the cascade-by implementing a ring laser and propagating the lasing power through the cascade-on the statistics of OSNR variation. We show that it is possible to achieve dynamic OSNR equalization for a WDM system by the use of preemphasis and an appropriate choice of EDFA parameters, without resorting to optical equalization filters. Most previous equalization methods are static with flat gain for a given inversion level in the amplifier. Changes in the input power (due to network reconfiguration or packetized traffic) will lead to a varying inversion level and hence non optimal equalization     

Bit error probability reduction in direct detection opticalreceivers using RZ coding

We analyze the bit error probability reduction for direct detection ON-OFF keying optical receivers using return-to-zero (RZ) coding instead of the nonreturn-to-zero (NRZ) format. For the same average optical power, RZ is shown to outperform NRZ, even when employing the same receiver bandwidth. Results are given for receivers whose noise variance is i) dominated by a signal-independent term (e.g., simple pin diode receivers), ii) dominated by a signal-dependent term (e.g., optically preamplified receivers), and iii) made up of two equally important contributions [e,g,, avalanche photodiode (APD) receivers]. Based on semianalytic simulations including intersymbol interference, we show that the achievable RZ sensitivity gain is typically less for dominating signal-independent noise than for dominating signal-dependent noise, where it amounts to about 3 dB. We also quantitatively discuss the influence of the optical pulse shape on the achievable RZ coding gain, and show that finite extinction ratios can significantly reduce that gain, especially when the RZ signals are produced by direct-modulation methods     

Optical preamplifier receiver for spectrum-sliced WDM

Spectrum-slicing provides a low-cost alternative to the use of multiple coherent lasers for wavelength division multiplexing (WDM) applications by utilizing spectral slices of a single broadband noise source for creating the multichannel system. In this paper we analyze the performance of both p-i-n and optical preamplifier receivers for spectrum-sliced WDM using actual noise distributions, and the results are compared with those using the Gaussian approximation. This extends prior results of Marcuse for the detection of deterministic signals in the presence of optical amplifier and receiver noise. Although the methodology is similar, the results are considerably different when the signal is itself noise-like. For the case of noise-like signals, it is shown that when an optical preamplifier receiver is used, there exists an optimum filter bandwidth which minimizes the detection sensitivity for a given error probability. Moreover the evaluated detection sensitivity, in photons/bit, represents an order of magnitude (>10 dB) improvement over conventional detection techniques that employ p-i-n receivers. The Gaussian approximation is shown to be overly conservative when dealing with small ratios of the receiver optical to electrical bandwidth, for both p-i-n and preamplifier receivers     

Effect of chirping-induced waveform distortion on the performanceof direct detection receivers using traveling-wave semiconductor opticalpreamplifiers

The influence of chirping-induced waveform distortion on the performance of multigigabit-per-second traveling-wave semiconductor optical amplifier (TWSOA)/p-i-n direct detection receivers is evaluated. The results are based on a novel method of evaluating the probability of error in the presence of the signal-spontaneous and spontaneous-spontaneous beat noise components. Laser chirping causes the dependence of the receiver sensitivity on the fiber dispersion coefficient×length product DL to be different for TWSOA/p-i-n and avalanche photodiode (APD) receivers. Compared to the APD receiver, the sensitivity of the TWSOA/p-i-n receiver degrades less quickly. So for cases of practical interest, the TWSOA/p-i-n receiver is more tolerant of chirping-induced waveform distortion     

Low-noise receivers for fiber-optic microwave signal transmission

The problem of low-noise reception of high-frequency narrowband modulated optical radiation is discussed. Relatively simple p-i-n diode/FET based optical receiver structures which achieve good narrowband optical sensitivity are proposed. These receivers achieve good gain and noise performance due to a lossless parallel resonance circuit which shunts the input to the detector and FET. Design characteristics are presented for S-band receivers constructed using low-capacitance photodetectors, and GaAs MESFETs. Experimental realizations of these receiver designs, which achieve conversion efficiencies on the order of 250 V/W and optical noise equivalent power of less than 10 pW/√Hz for 0.8-μm radiation, are reported. Scaling rules which should be of use in optimizing the performance of more advanced designs are stated and other types of receiver structures which can be expected to yield high narrowband optical sensitivity are discussed     

Multiple Access Channel Optimal Spectrum Balancing for Upstream DSL Transmission

Upstream DSL transmission suffers from in-domain crosstalk as well as out-of-domain or alien crosstalk. Here, the use of multi-user receiver signal coordination e.g. generalized decision feedback equalization, can lead to spectacular performance gains. This paper presents a transmission scheme, referred to as MAC-OSB, which focuses on the weighted rate sum capacity by joint receiver signal coordination and transmit spectrum coordination. The proposed scheme incorporates per-user total power constraints, spectral mask constraints and discrete bit or power loading constraints. Furthermore a low-complexity scheme, referred to as MAC-ISB, is presented which performs similar to MAC-OSB. Simulations show large performance gains over existing methods especially for scenarios with significant alien crosstalk     

Performance analysis of digital lightwave systems using efficientcomputer simulation techniques

Analytically-based methods for evaluating the performance of digital lightwave systems in terms of bit error rates (BERs) are extremely difficult to develop without making restrictive assumptions. A Monte Carlo simulation approach can offer an attractive alternative. However, for typical optical systems, this approach would require an excessive amount of computer time. Importance sampling (IS) is a variance reduction method which can substantially increase the computational efficiency of Monte Carlo simulations. This paper presents an IS method to efficiently evaluate the BERs of direct-detection optical systems employing avalanche photodiode (APD) receivers. Specifically, using a heuristic argument based on large deviations theory, a large class ℱ of exponentially twisted sampling distributions for the APD-based receiver is developed. It is then demonstrated that when used as a sampling distribution, the “optimized” exponentially twisted distribution of large deviations theory is the most efficient distribution among the sampling distributions in ℱ. Further, it is demonstrated that such a distribution would estimate the performance of optical systems with a high degree of accuracy to warrant its possible use as a powerful and flexible tool in computer-aided design, analysis and modeling of fiber-optic transmission systems     

High-sensitivity direct-detection receiver with a 1.5 ?m optical preamplifier

A 1.5 ?n optical preamplifier was used to improve the sensitivity of a PINFET receiver by 12dB. The resonant optical amplifier provided 17dB of fibre-to-fibre gain including coupling losses in an isolator and a narrowband optical filter. The achieved sensitivity, -45.6 dBm at 500 Mbit/s, or 420 photons/bit, is the best reported for any direct-detection receiver and a factor of two better than previous results using optical amplifiers. However, the results are virtually identical to the best APD receiver results.     

Evaluation of Decision Feedback Equalization and Viterbi Algorithm Detection for Voiceband Data Transmission--Part I

The application of QAM receivers employing passband decision feedback equalization or Viterbi algorithm detection is considered for high-speed data transmission in voiceband telephone channels. Analytical results for the Viterbi receiver indicate that it can permit digital data transmission with symbol rates exceeding the nominal bandwidth capabilities of the channel. However, it is also shown that the minimum distance of partial response-type impulse responses can substantially decrease due to small demodulation phase errors. Thus, decision feedback receivers may offer more robust performance on channels with phase jitter.     

Comparison of available detectors for digital optical fiber systems for the 1.2-1.55 µm wavelength range

Receiver sensitivity is estimated at 1.3 and 1.5 μm for commercial Ge APD's for bit rates between 8 and 1200 MBd, for a variety of APD diameters and operating temperatures. Although both holes and electrons are injected into the depletion region at these wavelengths, the measured photocurrent excess multiplication noise is found empirically to be well described by the simple expression for unilateral carrier injection into the depletion region, while the measured noise on the bulk leakage current can be characterized by the photocurrent parameters for wavelengthssim1.8 mum. Measurements at BTRL on a 140 Mbit/s system receiver using an Optitron GA-1 Ge APD at temperatures in the range20-60degC agree within 1 dB with the theoretical model using these data. The performance of Ge APD-based receivers is strongly influenced by noise on the leakage current and is therefore susceptible to temperature fluctuations. The ionization rates for holes and electrons are comparable in Ge, resulting in a high excess noise factor and a strong dependence of multiplication factor on bias voltage. Thus, APD's for long-wavelength digital optical receivers operating below ∼1 GBd require a bulk leakage current densityll 10^{-4}A/cm²and markedly different ionization rates for holes and electrons, in order to match the otherwise superior performance of the present-day high-impedance p-i-n/FET hybrid.     

A performance comparison of two p-i-n FET receiver circuit architectures

Two conceptually different p-i-n FET receiver circuit architectures are evaluated using a SPICE circuit simulation. The popular p-i-n FET transimpedance amplifier is compared to a new architecture that uses distributed gain and dual feedback. To highlight the importance of circuit architecture to receiver performance, identical device parameters are used in each circuit model. Frequency, phase, and pulse responses are computed and presented in graphical form. Results demonstrate that the popular receiver is adversely sensitive to FET transconductance variations and distorts the pulse reponse, whereas the distributed gain and dual feedback design is substantially independent of transistor parameters and free of pulse distortion.     

A performance comparison of two p-i-n FET receiver circuit architectures

Two conceptually different p-i-n FET receiver circuit architectures are evaluated using a SPICE circuit simulation. The popular p-i-n FET transimpedance amplifier is compared to a new architecture that uses distributed gain and dual feedback. To highlight the importance of circuit architecture to receiver performance, identical device parameters are used in each circuit model. Frequency, phase, and pulse responses are computed and presented in graphical form. Results demonstrate that the popular receiver is adversely sensitive to FET transconductance variations and distorts the pulse reponse, whereas the distributed gain and dual feedback design is substantially independent of transistor parameters and free of pulse distortion.