Influence of laser phase on 400-Mbit/s coherent optical DPSK system

The influence of laser phase noise on a 400-Mb/s optical DPSK (differential phase-shift keying) system is experimentally investigated with linewidths ranging from 1.2 MHz to 8 MHz. This range corresponds to linewidth to bit rate ratios ϵ of 0.33-2%. The system performance with these nonzero linewidths is evaluated against a negligible linewidth performance baseline. The sensitivity degradation at a bit error rate of 10^-9 increases from 1.8 to 7 dB as ϵ is increased from 0.33-1%. When ϵ is increased beyond 1%, bit error rate floors higher than 10^-9 develop. These findings agree well with the existing theories and allow the generalization of these results to other bit rates, as well as establishing practical criteria for lasers to be used in DPSK systems     

Dynamic range and switching speed limitations of an N×Noptical packet switch based on low-gain semiconductor optical amplifiers

Two important system performance limitations-dynamic range and switching speed-of an integrated packet switch fabric based on low-gain semiconductor optical amplifiers (SOA's) have been examined by using cascaded blocks of an SOA model, which includes transient effect, nonlinear pulse distortion effect, and amplified spontaneous emission (ASE) noise. Low-gain SOA's were used to minimize ASE noise considering that no optical filters can be integrated in an SOA-based switch fabric. The system performance with and without a narrowband optical filter at the receiver were both studied. By assuming fixed-wavelength transmitters and no optical filter can be used at the receiving end owing to the unpredictability of arriving packet wavelengths, our simulation results indicate that the dynamic ranges of 4×4 and 8×8 SOA-based packet switches at 2.5 Gb/s can only be about 3.2 and 0.8 dB, respectively. However, at 155 Mb/s, even without a receiving-end optical filter, the dynamic range of each switch size can be increased by more than 17 dB as compared to the cases of 2.5 Gb/s. Note that the dynamic ranges were estimated under the conditions of a bit error rate (BER) ⩽10^-9 and a pulse distortion ratio ⩽30%. We have also shown that, when an optical filter with a 1 nm bandwidth was used at the receiving end to simulate (1) a circuit-switched condition where the center wavelength of the filter can be adjusted according to the established circuit, or (2) a packet-switched condition where each receiver has a wavelength demultiplexer and a detector array, the dynamic range of 4×4 and 8×8 switches can be increased to 16.3 and 14 dB, respectively, at 2.5 Gb/s     

噪声对相干光通信锁相环性能影响的量化分析

以PSK(相移键控)外差光通信系统作为研究对象,对相位噪声和散弹噪声对锁相环的性能影响进行了深入的量化分析,得出了锁相环的最优带宽与激光器线宽的关系以及误码率和比特率对激光器线宽的要求,可以作为光锁相环设计的理论指导。     

Theory for optical heterodyne narrow-deviation FSK receivers withdelay demodulation

A theoretical model is presented that includes the effects of laser phase noise, receiver noise, imperfect modulation, IF bandwidth, and postdetection filtering. Detailed numerical results for 140-Mb/s and 400-Mb/s systems are presented, showing excellent agreement with independent published experimental results and strongly supporting the theoretical analysis. It is found that an IF linewidth of less than 0.25% of bit rate is required to avoid degrading the receiver sensitivity by more than 1 dB in a system with a strong local oscillator and modulation index of 0.7. A larger modulation index allows a larger linewidth to be accommodated. If the demodulation is not optimal, a narrower linewidth is necessary     

Experimental linewidth-insensitive coherent analog optical link

We constructed an experimental linewidth-insensitive coherent analog optical link. The transmitter utilizes an external electro-optic amplitude modulator and a semiconductor laser. The receiver consists of a heterodyne front-end, a wideband filter, square law detector and narrowband lowpass filter. We performed experimental measurements and theoretical analyses of the spurious-free dynamic range (SFDR), link gain and noise figure for both the coherent AM and the direct detection links; we investigated the dependencies of the foregoing parameters on the received optical signal power, laser linewidth, IF bandwidth, and the laser relative intensity noise (RIN). By selecting a wide enough bandpass filter, we made the coherent AM link insensitive to laser linewidth. The coherent AM link exhibits a higher SFDR than the corresponding direct detection link when the received optical signal power is less than 85 μW. The noise figure for the coherent link is greater than that for the direct detection link under all conditions investigated. For received optical signal powers greater than 4 μW, the link gain for the direct detection link is greater than that for the coherent AM link. The following are the link parameters that have been achieved for the coherent AM link investigated: SFDR=88 dB·Hz^2/3, link gain=-25 dB and noise figure=78 dB; this performance has been obtained with a received optical signal power of 85 μW, and a local oscillator power at the photodetector of 228 μW. The link performance can be further improved by auxiliary subsystems such as a balanced receiver and impedance matched transmitter and receiver ends; and/or by using better optical and electrical devices like higher power lasers, linearized optical modulators, low-noise and high gain RF amplifiers, and optical amplifiers,     

Impact of laser intensity noise on ASK two-port optical homodyne receivers

The letter describes initial experimental results obtained with a multiport optical homodyne receiver employing a DFB laser. The receiver performance is found to be limited by the intensity noise of the local oscillator rather than by the phase noise, even when the product of the IF linewidth and the bit duration is as large as 0.56. A relative intensity noise level of at least ? 140dB/Hz will be required for a satisfactory receiver performance with ? 15dBm local oscillator power.     

Coding of optical on-off keying signals impaired by phase noise

The benefits of coding for an optical communication system that employs binary on-off keying and heterodyne detection are quantified. The system is impaired by laser phase noise as well as by additive white Gaussian noise (AWGN). A receiver structure especially designed to mitigate the effects of phase noise in the presence of AWGN is assumed. This special receiver structure requires a wider-band front-end IF filter than would be required for a phase-noise-free signal. The results, computed for several different coding schemes, indicate that the benefits of coding are large and the costs are small. For a linewidth-to-bit-rate ratio (βT) of 0.64 (for example, 45 Mb/s and 29 MHz linewidth), a half-rate binary code that can correct 3 bit errors provides a 50% reduction in the required IF filter bandwidth (and, therefore, the required IF) and about 5 dB of reduction in required laser power. The benefits of coding are greatest under high-βT conditions, corresponding to low bit rates where coders and decoders are most practical to implement     

Balanced phase-locked loops for optical homodyne receivers: Performance analysis, design considerations, and laser linewidth requirements

Balanced phase-locked loops for optical homodyne receivers are investigated. When a balanced loop is employed in a communications system, a part of the transmitter power must be used for unmodulated residual carrier transmission. This leads to a power penalty. In addition, the performance of the balanced loops is affected by the laser phase noise, by the shot noise, and by the crosstalk between the data-detection- and phase-lock-branches of the receiver. The impact of these interferences is minimized if the loop bandwidthBis optimized. The value of Boptand the corresponding optimum loop performance are evaluated in this paper. Further, the maximum permissible laser linewidthdeltanuis evaluated and found to be5.9 times 10^{-6}times Rb, where Rb(bit/s) is the system bit rate. This number corresponds toBER = 10^{-10}and power penalty of 1 dB (0.5 dB due to residual carrier transmission, and 0.5 dB due to imperfect carrier phase recovery). For comparison, decision-driven phase-locked loops require onlydeltanu = 3.1 times 10^{-4}. R_{b}. Thus, balanced loops impose more stringent requirements on the laser linewidth than decision-driven loops, but have the advantage of simpler implementation. An important additional advantage of balanced loops is their capability to suppress the excess intensity noise of semiconductor lasers.     

Performance of homodyne detection of binary PSK optical signals

This study evaluates the performance of an optical receiver for binary phase shift keyed (BPSK) signals in the presence of noise originating from the photodetectors and the phase fluctuations of the optical sources. Analysis of the homodyne detection process shows that the performance is degraded by two effects: One due to the phase error fluctuations of the recovered carrier and the other due to reduction of the energy per bit available for data recovery. The resulting power penalty can be minimized by dividing in an optimal way the received optical signal between the carrier recovery and the data recovery circuits of the receiver. The minimum penalty thus obtained depends on the 3-dB linewidth and on the transmission rate. For example, a penalty of 0.5 dB, relative to the quantum limit of 9 photon bit needed to achieve a BER of 10^-9, imposes a minimum transmission rate of about 180 Gbit/s when the optical source has a 3-dB linewidth of 20 MHz.     

Wide-linewidth phase diversity homodyne receivers

The use of phase diversity homodyne receivers, which have excellent performance even when the laser linewidth is of the same order of magnitude as the bit rate, to construct coherent systems with semiconductor lasers and moderate bandwidth receivers is considered. Theoretical, experimental, and computer simulation results of a study of a linewidth homodyne phase-diversity receiver is presented. A 150-Mb/s system with an IF linewidth of more than 50% of the bit rate is investigated in depth and is experimentally shown to operate within 1.8 dB from its theoretical limit     

Multichannel system design using optical preamplifiers andaccounting for the effects of phase noise, amplifier noise, and receivernoise

A rigorous and accurate model for multichannel direct detection systems where optical preamplification is used is presented. The model accounts for the influence of an optical bandlimiting filter as well as of a polarization filter. Effects of laser phase noise, of spontaneous emission noise from the amplifier, and of shot noise and thermal noise in the receiver front end are accounted for. The predicted phase noise penalty for a single channel configuration is found. For multichannel system design, it is found that a system with many optical channels and no phase noise should have a channel separation of 9.4 bit rates to have less than 1-dB worst-case crosstalk penalty. For a linewidth of 27% of the bit rate in each channel the required spacing increases to 25 bit rates or more     

Properties of fiber Raman amplifiers and their applicability todigital optical communication systems

Bit error rate performances of fiber Raman amplifiers both as booster amplifiers and as detection preamplifiers are studied in intensity modulation/direct detection optical communication systems. It is theoretically shown that, in the booster amplifier application, receiver sensitivity degradation due to amplification can be made less than 0.2 dB for signal-to-noise power ratio larger than 20 dB, and thus an allowable transmission line loss can be increased approximately by the value of Raman gain. For detection preamplifier use, receiver sensitivities in the Raman preamplifier system are numerically calculated in terms of minimum detectable signal power at 100-Mbit/s and 2-Gbit/s bit rates. In both bit rates, it is shown that, for a Raman gain greater than 20 dB, minimum detectable signal power can be improved by more than 15 dB over the conventional detection level without Raman amplification. Preliminary experiments are carried out using a 1.32-μm Nd:YAG laser and a 1.4-μm laser diode as pump and signal light sources, respectively. The experimental results are in good agreement with theoretical estimations     

Theory of direct-detection lightwave receivers using opticalamplifiers

A simple theory is presented for analyzing the sensitivity and bit-error rate (BER) performance of direct-detection lightwave receivers using optical amplifiers. The analysis provides closed-form expressions for the BER and receiver sensitivity, and includes the impact of phase noise. Furthermore, the proposed theory predicts the optimum filter bandwidth values required for linewidths of practical interest (up to 100% of the bit rate). For the special case of zero linewidth, the average signal energy predicted by this work is 42.3 photons/b at BER=10 ^-9. This result is within 0.47 dB of the sensitivity predicted by more accurate techniques     

Linewidth-insensitive coherent AM optical links: design,performance, and potential applications

The use of coherent detection in analog optical links offers several advantages over direct detection: improved receiver sensitivity, inherent frequency translation, and the ability to utilize angle modulation and separate wavelength division multiplexed (WDM) signals. In this paper, we investigate an externally modulated coherent AM optical link. We study the dynamic range of the coherent AM link, considering receiver noise, laser phase noise, laser relative intensity noise (RIN), and system nonlinearities. With proper selection of the receiver's IF bandwidth, the coherent AM link can be made insensitive to the laser linewidth. For optical powers less than 5 mW, RIN of less than -160 dB/Hz reduces the spurious-free dynamic range (SFDR) by less than 3 db with the use of a balanced receiver. The external modulator nonlinearity is the dominant nonideal effect; it reduces the SFDR by 5-19 dB from the theoretical limit for 100% modulation index. We compare the performance of the coherent AM link with that of a conventional direct detection link for two applications: point-to-point links and distribution networks. When the received optical power is less than 1 mW, the coherent link can provide higher SFDR than the direct detection link. Thus, coherent links are well-suited for long distance point-to-point links and FM video distribution systems     

Modulation performance of a semiconductor laser coupled to anexternal high-Q resonator

The dynamic response of a semiconductor laser coupled to an external resonator is studied using the single-mode rate equations modified to account for the dispersive feedback. Both the frequency and the damping rate of relaxation oscillations are affected by the feedback. The frequency chirp that invariably accompanies amplitude modulation is significantly reduced. The feedback also reduces the phase noise and the linewidth. To investigate the usefulness of external-resonator lasers in high-speed optical communication systems, the rate equation have been solved numerically to obtain the emitted chirped pulse; the pulse is propagated through the fiber, detected, and filtered at the receiver. The simulated-eye diagrams show that such lasers can be operated at high bit rates with negligible dispersion penalty owing to their reduced frequency chip     

激光器线宽对-OTDR系统性能影响的研究

为了对相位敏感光时域反射系统进行优化设计,采用相位敏感光时域反射原理搭建了一套多点入侵检测系统,通过数值模拟和实验分析了激光器线宽对系统性能的影响,由理论分析发现,随着激光器线宽的增大,系统探测到的后向相干瑞利散射的波纹逐渐平滑,系统的信噪比和定位精度也随之降低,直至不能定位和识别干扰信息;并利用7km的光纤进行了相关实验。结果表明,当激光器线宽由5kHz增加到200kHz时,系统探测到的后向瑞利散射信号出现了较大的平滑,信噪比则由12dB减小到3dB,这与理论分析的规律一致。     

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     

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     

Optical demultiplexing at 6 Gb/s using a semiconductor laser amplifier as an optical gate

A semiconductor laser amplifier (SLA) has been employed successfully for optical demultiplexing in two-channel optical time division multiplexed system experiments at 6 and 2 Gb/s. Demultiplexing of 6-Gb/s (2-Gb/s) signals was demonstrated with a power penalty of 1.6 dB (3.0 dB) at bit error rates of 10/sup -9/. It is also shown that a reduction of the generated amplified spontaneous emission can be obtained by optical gating/demultiplexing for systems incorporating inline amplifiers. A 0.5-dB improvement in sensitivity was achieved as a result of using an SLA for demultiplexing from 2.0 to 1.0 Gb/s in a system with one inline Er/sup 3+/-doped fiber amplifier.<>     

Optical homodyne frequency domain reflectometer using an externalcavity semiconductor laser

An external cavity traveling-wave semiconductor ring laser with narrow linewidth is used as a light source for research in frequency domain reflectometry. The optical frequency of the laser is linearly chirped by an intra-cavity phase modulator. The time-delayed reflection signal is mixed with a reference signal to produce a microwave frequency that indicates the position of the reflection. For optical fiber measurement, a spatial resolution of 30 m and a one-way dynamic range of 28 dB for Rayleigh backscattered light have been achieved