Crosstalk penalty in a two-channel ASK heterodyne detection system with non-negligible laser line width

Crosstalk penalties as a function of channel spacing were measured in a two-channel ASK heterodyne system using 1·55?m DFB lasers. For an IF linewidth/bit rate ratio of 0.5, the channel spacing in the IF domain must be greater than 13 times the bit rate for negligible crosstalk penalty.     

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     

Effect of laser linewidth on crosstalk penalty in two-channel ASKheterodyne detection system

The effect of laser linewidth on the crosstalk penalty in a two-channel ASK heterodyne system is calculated through a simple frequency domain analysis. The results show that the crosstalk penalty increases steeply when the linewidth at the IF stage exceeds one-tenth of the bit rate. The results of calculation show very good agreement with experiment by Park et al     

Impact of laser linewidth on optical channel spacing requirements for multichannel FSK and ASK systems

The sensitivity penalty is evaluated for amplitude-shift-keyed and frequency-shift-keyed multichannel coherent systems that use lasers with linewidths which are a significant fraction of the bit rate. The study was conducted for both ASK and FSK systems using a single-filter receiver with nonsynchronous detection. For FSK systems, both NRZ (nonreturn-to-zero) and alternate mark inversion (AMI) signal formats were studied. The optical channel spacing is strongly determined by the laser linewidth. For example, with the FSK-NRZ data rate of 150 Mb/s, the optical channel spacing which gives 1-dB crosstalk penalty is 4 GHz when the intermediate frequency linewidth is 50 MHz (laser linewidth is 25 MHz), as opposed to 1.8 GHz when the linewidth is negligible.<>     

ASK multiport optical homodyne receivers

Several types of ASK multiport homodyne receivers are investigated, and the impact of the phase noise and of the shot noise on these receivers is analyzed. The simplest structure is the conventional multiport receiver with a matched filter in each branch. This structure can tolerateDeltavT[deltavis the laser finewidth andTis the bit duration) of several percent with a small power penalty (3.6 percent for 1-dB penalty and 5.2 percent for 2-dB penalty). Optimization of branch filters of conventional multiport receivers does not help when the linewidth (and the penalty) is small but does improve the receiver performance for larger linewidths. The most important point of the paper is the novel wide-band filter-rectifier-narrow-band filter (WIRNA) structure, proposed and investigated here for the first time for optical communication systems. It is shown that the optimized WIRNA homodyne receivers are extremely robust with respect to the phase noise: the WIRNA tolerable value ofDeltavTis 3.6 percent for 1-dB penalty and more than 50 percent for 2-dB penalty. Thus, the WIRNA structure opens, for the first time, the possibility of constructing homodyne receivers operating at several hundred megabits per second with conventional DFB lasers without complicated external cavities. Under no-phase-noise conditions, all the multiport receivers investigated here have the same performance, which is identical to that of heterodyne ASK receivers. In addition, the optimized WIRNA receivers can tolerate tapproximately) the same laser linewidth as the heterodyne ASK receivers. Thus, the main difference between the WIRNA multiport homodyne and heterodyne receivers is that the former shifts the processing to a lower frequency range, in return for a more complicated implementation. This difference makes the WIRNA multiport homodyne receivers particularly attractive at high (say, several gigabit per second) bit rates.     

Performance of coherent ASK lightwave systems with finiteintermediate frequency

The impact of finite intermediate frequency (IF) on the performance of heterodyne ASK lightwave systems is examined and quantified in the presence of laser phase noise and shot noise. For negligible linewidths, it is shown that certain finite choices of IF (R _b,3R_b/2,2R_b,5R_b/2, etc.) lead to the same ideal bit-error-rate (BER) performance as infinite choices of IF. Results indicate that for negligible linewidths the worst case sensitivity penalty is 0.9 dB for proper heterodyne detection and occurs when f_IF=1.25 R_b. For nonnegligible linewidths (e.g., when ΔνT⩾0.04) the sensitivity penalty is always less than 0.9 dB for finite choices of IF. The analysis presented does lead to a closed-form signal-to-noise ratio (SNR) expression at the decision gate of the receiver which can readily be used for BER and sensitivity penalty computations. The SNR expression provided includes all the key system parameters of interest such as system bit rate (R_b), the peak IF SNR (ξ), laser linewidth (Δν), and the IF filter expansion factor (α). The findings of this work suggest that the number of channels in a multichannel heterodyne ASK lightwave system can be increased substantially by properly choosing a small value for the IF at the expense of a small penalty <1 dB. On the negative side, IF frequency stabilization becomes a more critical requirement in multichannel systems employing small values of IF     

The effect of crosstalk and phase noise in multichannel coherentoptical ASK systems

The authors extend the analysis of G.J. Foschini et al. (1988) to a two-channel system and to a wider class of IF filters. The authors carry out a comprehensive study of the sensitivity of an optical receiver in a two-channel ASK system, taking into account phase noise and crosstalk, including crosstalk arising from phase noise in the second channel. They compare the results with previous work in the limit of no phase noise and show how the agreement is good. They compute the penalty due to crosstalk for different linewidths and filter shapes. It is shown that the minimum channel spacing is a few bit rates more than the IF filter bandwidth, and is therefore increased significantly by significant phase noise. The detailed IF filter shape appears to have only a weak effect     

M-ary FSK Performance for Coherent Optical Communication Systems Using Semiconductor Lasers

This paper describes the design and performance of anMary frequency shift keyed (FSK) signaling and demodulation scheme for an optical communication system using semiconductor lasers and heterodyne detection. Frequency or phase noise in semiconductor lasers causes spectral spreading, producing a nonzero linewidth laser signal. This degrades communication performance when compared to a system using an ideal laser with zero linewidth. We present estimates of the bit error rate (BER) performance ofM-ary frequency shift keying (FSK) with noncoherent demodulation in the presence of white Gaussian frequency noise and additive channel noise. This is typical of an optical system using semiconductor lasers and heterodyne detection. Estimates use the union-Chernoff bound with a simplified channel model to predict the effects of frequency noise. Two effects of frequency noise are identified: signal attenuation or suppression, and crosstalk. These cause an offset in the BER curve from the BER in the absence of frequency noise, and an error rate floor, respectively. The error rate floor is lower than previously predicted. When performance is not crosstalk limited,M-ary FSK is found to perform better than binary FSK with the same system bandwidth constraints, as would be predicted if ideal lasers are used. Theoretical results are compared with Monte Carlo simulations of the system.     

560 Mbit/s optical PSK heterodyne detection using carrier recovery

An optical PSK heterodyne synchronous detection experiment was performed at a bit rate of 560 Mbit/s using carrier recovery in the IF stage. A receiver sensitivity of -51.6 dBm was achieved, and the power penalty due to the phase noise of the laser diodes was suppressed to less than that of DPSK.<>     

Accurate performance evaluation of weakly coherent optical systems

An accurate performance evaluation approach which uses a closed-form exact analytical expression of the phase noise moments is presented. This enables one to derive a high-order Gaussian quadrature rule for the integrations needed to take into account the phase noise in the computation of error probability. A systematic comparison with results obtained through a Monte Carlo simulation shows that the approach is more accurate than previous methods. The analysis is performed on ASK and FSK heterodyne receivers with integrate-and-dump filtering, envelope detection, and optimized postdetection low-pass filtering. The feasibility of ASK and FSK heterodyne systems at bit rates comparable to the spectral line bandwidth of the laser sources is confirmed. The theory applied seems to be adequate to attack other problems, such as the evaluation of the effects of crosstalk between the FSK filters or among frequency division multiplexed channels     

Analysis of the effect of thermal chirp on interferometric homodyne and heterodyne crosstalk in optical communication systems employing directly Modulated DFB lasers

A closed-form expression is derived for the probability density function (pdf) of the beat noise created by homodyne and heterodyne interferometric crosstalk in optical communication systems employing directly modulated distributed feedback lasers at bit rates between 155 Mb/s and 2.5 Gb/s. Thermal chirp is shown to be the predominant chirp mechanism affecting homodyne-crosstalk-induced penalty at bit rates up to 2.5 Gb/s. Theoretical calculations of the crosstalk-induced power penalty are verified experimentally.     

Dynamic range of an optical amplifier cascade in an ASK system withsignificant phase noise

The authors present detailed results concerning the dynamic range of an amplitude-shift keyed (ASK) heterodyne optical communication system where the transmitted signal is impaired by significant amounts of laser phase noise. Theoretical results are evaluated for a system with up to ten in-line optical amplifiers and a significant influence of the laser phase noise is found for the maximum allowable transmission span and the dynamic range. With ten amplifiers, a linewidth of 11% of the bit rate causes the span to be reduced by 12.2 dB and the dynamic range to be reduced by 5.7 dB. The results have been confirmed using two amplifiers in a field trial experiment at a bit rate of 636 Mb/s with an IF linewidth of 70 MHz     

Multichannel coherent optical communications systems

Balanced coherent receivers perform substantially better than single-detector receivers in multichannel optical fiber FDM communications systems since the balanced approach eliminates the direct-detection and signal-cross-signal interference. The permissible channel spacingDdepends on the intermediate frequency fIF, on the bit rate Rb, and on the modulation/demodulation format. In particular,Dincreases by 2 Hz for every 1-Hz increase of the fIF. The signal-to-interference ratio SIR, as defined in the text, provides a simple measure of the amount of the interference generated by undesired channels. The criterion SIR = 30 dB is selected in this paper and leads to the following minimum channel spacings: for heterodyne systems,3.8R_{b}for FSK,9.5R_{b}for ASK, and12.4R_{b}for PSK; for homodyne systems,7.5R_{b}for ASK and10.5R_{b}for PSK. Simultaneous transmission of several channels generates an excess shot noise studied here for the first time. If the local oscillator power is 40 dB above the received signal power and 2000 channels are transmitted without optical prefiltering, the excess shot noise power penalty is less than 1 dB.     

Phase diversity techniques for coherent optical receivers

In the present state of the art, coherent optical receivers most often operate in the heterodyne mode. Here a photodiode-amplifier combination having bandwidth greater than twice the bit rate (B) is needed: indeed bandwidths considerably greater than2Bare preferably employed to ease design of the bandpass filter needed for noise limitation, and to avoid demodulator penalties in some modulation schemes. For the high bit rate systems now coming into service (560 Mbit/s-2.4 Gbit/s), the optical receiver design requirements become more stringent for coherent heterodyne operation. The various modes of "zero IF" operation, however, require only baseband receiver module bandwidth. The options available are either homodyne (phase locked) operation, or phase diversity (multiport) techniques. In this paper, we compare these options, and show that phase diversity techniques are capable of good performance for high bit rate coherent receivers. In phase diversity operation, not only is phase locking avoided, but also the necessary frequency locking does not have high stability requirements. Furthermore, there are advantages in operating with a small frequency offset from zero (of the order of 1 percent of the bit rate). An experimental receiver has been operated at 320 and 680 Mbit/s, demodulating both amplitude shift keying (ASK) and differential phase shift keying (DPSK). Operation with FSK is also possible. Sensitivities so far achieved of -47.5 dBm (320-Mbit/s ASK) and -42 dBm (680- Mbit/s ASK) with limited local oscillator power are capable of substantial improvement when higher power local oscillators and lower noise receive modules become available. Demodulation of DPSK at 320 Mbit/s has also been achieved and shows a measured receiver sensitivity improvement of over 4 dB over ASK at the same bit rate and local oscillator power. These practical results show clearly that phase diversity is a very realistic option for high bit rate systems.     

ASK and FSK coherent lightwave systems: a simplified approximateanalysis

A simplified approximate analysis of amplitude shift keying (ASK) and frequency shift keying (FSK) coherent optical communication systems is presented. The analysis accounts for the phase noise of the transmitter and the local oscillator lasers and for the additive Gaussian noise stemming from the shot noise and thermal noise. The analysis yields a closed-form estimate of the bit error rate (BER) and allows an immediate physical insight and appreciation of the impact of the IF filter bandwidth, laser linewidth, and bit rate on the system performance. The theory also yields reasonably accurate estimates of the optimum IF bandwidths and of the sensitivity penalty stemming from the phase noise     

Crosstalk degradation caused by optical amplification in amultichannel FSK heterodyne system

The crosstalk degradation caused by an optical amplifier in a four-channel FSK (frequency-shift-keyed) heterodyne communication system is measured. A bit error rate (BER) floor of 3×10^-4 is observed when the channels are spaced by 200 MHz, FSK modulation at 45 Mb/s, and when the optical input signal is large enough such that the gain is compressed by 2 dB relative to its small-signal value. The receiver is substantially improved by reducing the optical power amplifier input. However, the sensitivity increases only to a maximum value beyond which it degrades as the optical power of the demodulated channel becomes small relative to the noise of the optical amplifier. The combined effect of the crosstalk and the amplifier noise yields an optimum sensitivity of 250 photons/b for BER=10^-9. This result is 5 dB poorer than the sensitivity obtained in the absence of an optical amplifier     

Performance analysis of optical heterodyne PSK receivers in thepresence of phase noise and adjacent channel interference

A phase-shift-keying (PSK) optical heterodyne receiver using synchronous detection by means of a Costas phase-locked loop (PLL) is investigated. Taking into account the laser phase noise and adjacent channel interference (ACI), an expression of the phase error variance is derived and error probability calculation is performed. Plots of the error probability versus the number of photons per bit are presented as a function of the optical domain channel spacing (D) and for several linewidth-to-bit-rate ratios (δf/R_b ). Relative sensitivity penalties, based on the performance with and without ACI, are evaluated for several combinations of D and δf/R_b. It is shown that, if lasers with larger linewidths are used, the frequency separation between optical carriers has to be increased in order to allow the same relative sensitivity penalty     

Influence of Crosstalk on Signal Power Based on Arrayed-waveguide Grating as N × N Optical Router

The fact that the signal results in signal-crosstalk is confirmed for arrayed-waveguide grating as N × N optical router,and the relation between the crosstalk and power penalty is obtained.The method reveals the random distributions of optical path phase errors in two multiplexers with channel numbers of 10 and 160. It is shown that the crosstalk must be less than -28 dB for a power penalty below 1 dB at a bit error rate of 1×10 -9 .It is found that when N =100,crosstalk power value is -20 dB with compensation power of 2-3 dB,so the compensation power is not ignored.     

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。     

Heterodyne ASK multiport optical receivers using postdetectionfiltering

A theoretical model for heterodyne amplitude-shift-keying (ASK) multiport optical receivers is presented. It applies when the transmitter and local oscillator have significant linewidths. The modeled receiver uses square law detection and postdetection filtering. The model allows for the essentially non-Gaussian character of the probability density function of the noise process at the input of the threshold comparator. Numerical computations are made for the {2×2} multiport receiver. They show that such a receiver can handle laser linewidths on the order of the bit rate with 1.5-dB loss as compared to the ideal heterodyne receiver with zero linewidth