An optical FSK heterodyne dual filter detection system for takingadvantage of DFB LD applications

An optimum system configuration for an optical frequency-shift keying (FSK) heterodyne dual-filter detection system with distributed feedback laser diodes (DFB LDs) is investigated, taking into consideration LD phase noise influence. Experimental and theoretical examination show that an IF filter bandwidth greater than 10 times the beat spectral linewidth is necessary to avoid LD phase noise influence. A 301-km long-span transmission experiment has been successfully carried out with an optimum configuration for 34 Mb/s. High receiver sensitivity, -61.8 dBm with more than 10 dB improvement over the direct detection system, has also been achieved. Experimental results at higher bit rates of 140, 200, and 280 Mb/s indicate that a modulation index greater than two is desirable to avoid cross talk between mark and space signals. With sufficient frequency deviations, high receiver sensitivities of -54.7 dBm (140 Mb/s) and -52.5 dBm (200 Mb/s) have been achieved. These represent 9.6- and 9-dB sensitivity improvement, respectively, over direct detection systems. A guide for FSK dual-filter detection system design is derived from the experimental and theoretical results. Potential application regions for a dual-filter detection system with DFB LDs are determined     

Theoretical analysis of heterodyne optical receivers for transmission systems using (semiconductor) lasers with nonnegligible linewidth

We present a general theoretical model of receivers for coherent optical communication systems where transmitters and local oscillators having nonzero linewidth are used. Key issues in the model are the concept of single realization measurements of a stochastic intermediate frequency, and development of the probability density function for this stochastic process. Analytical results are derived for heterodyne ASK and dual filter FSK receivers and include the shot-noise limit, the asymptotic error-probability limits in ASK and FSK receivers, the influence of the IF on receiver noise, and the effective local oscillator strength. Detailed numerical results for typical p-i-n-FET wide-band receivers illustrate the influence on receiver sensitivity of IF filter bandwidth and relative threshold setting in ASK systems and of modulation index and IF filter bandwidth in FSK systems. A receiver sensitivity penalty for nonzero linewidth is found to be, for IF linewidths of 0.1 to 0.3 of the bit-rate, 3 to 9 dB in optimum ASK receivers, and 2 to 8 dB in optimum FSK receivers. Thus DFB lasers of linewidth 5 to 20 MHz could be used without external cavities in simple systems with near-ideal performance, which could find application wherever the great multiplexing advantage of coherent systems is a prime advantage. We present some guidelines for system design based on the results of this work.     

Optimum system design for CPFSK heterodyne delay demodulationsystem with DFB LDs

An optimum system configuration for an optical continuous-phase frequency-shift-keying (CPFSK) heterodyne delay demodulation system with distributed feedback laser diodes (DFB LDs) is discussed. The optimum modulation index was determined by evaluating the LD phase noise effect and the IF noise effect. The IF noise effect was investigated in detail, considering the noise conversion effect through delay demodulation. In the case of 10-MHz IF beat spectral width, the modulation index m =1.5 is optimum for a 1.2-Gb/s system. With this optimum modulation index, a 204-km long-span transmission experiment, with -41.5 dBm receiver sensitivity, has been successfully performed. The feasibility of using stand-alone DFB LDs for a high-sensitivity CPFSK delay demodulation system has been confirmed through this experiment     

Long-span optical FSK heterodyne single-filter detection transmission experiment using a phase-tunable DFB laser diode

High receiver sensitivity (?51.9 dBm) and long span (243 km) transmission expriments have been achieved with a 140 Mbit/s optical FSK heterodyne single-filter detection system, using a phase-tunable DFB laser diode as a transmitter. This has enabled direct FSK modulation without waveform distortion. Also, a 280 Mbit/s 204 km transmission experiment has been carried out successfully.     

The effect of error control coding in multichannel FSK coherentlightwave communication systems influenced by laser phase noise

It has been shown that coherent optical fiber receivers with a two-filter structure (TFS) consisting of a wide-band IF filter and a narrow-band postdetection filter are less susceptible to the influence of phase noise. However, the expanded IF bandwidth required to achieve optimum sensitivity performance is large, particularly in multichannel FSK systems. Forward error control coding can relax the laser linewidth requirement and improve receiver sensitivity. In this paper a multichannel asynchronous FSK scheme equipped with (31, k) Reed-Solomon codes is used to verify the coding benefit. A systematic error probability analysis is developed and a stable and accurate performance evaluation procedure is provided. The sensitivity penalties due to the combined phase noise and interchannel crosstalk for both coded and uncoded systems are calculated for comparison. The results show that the performance reduction due to phase noise can be largely alleviated by choosing a proper code rate and an optimum value of the expanded IF bandwidth     

Noncoherent detection of coherent lightwave signals corrupted byphase noise

Waves are treated that modulate by either on-off keying (OOK) or binary frequency-shift keying (FSK) and are further impaired by additive Gaussian noise. Heterodyne detection of such a waveform produces an electronic bandpass signal, which, to ease demodulation in the presence of phase noise, is noncoherently demodulated to extract the baseband pulse stream. The treatment goes beyond previous bit error rate (BER) analyses of optical heterodyne receivers for OOK and FSK. First, there is full adherence to the standard (Brownian motion) model of phase noise. Also, the receiver structure is formulated in such a way that the probability density function of the receiver output samples can be accurately determined. This permits calculations of the additive noise and phase noise tolerable when achieving bit error rates as small as 10 ^-9. Finally, the study is comprehensive regarding the range of parameters explored. Filtering at an intermediate frequency (IF) alone, as well as IF filtering plus postdetection low-pass filtering, is considered. When the receiver parameters decision threshold (for OOK) and IF filter bandwidth are optimized, large amounts of phase noise can be accommodated with only minor increases in required signal-to-noise ratio. This is especially important when the bit rate is moderate compared to the laser linewidth     

Adaptive quantised feedback equalisation for FSK heterodynetransmission at 150 Mbit/s and 1 Gbit/s

An adaptive quantised feedback equaliser has been used in an FSK heterodyne receiver to overcome the nonuniform frequency modulation response of a DFB laser transmitter in FSK transmission experiments at 150 Mbit/s and 1 Gbit/s with fibre spans of 121 km and 136 km, respectively     

A cold-start 1.7 Gb/s FSK heterodyne detection system usingsingle-electrode DFB lasers with no equalization

A cold-start, 1.7-Gb/s frequency-shift keying (FSK) heterodyne single-filter detection system using ordinary single-electrode distributed feedback (DFB) lasers is discussed. These lasers were frequency-locked to an atomic krypton line at 1.5224 μm independently. Thus, this system can maintain a constant IF without an IF locking circuit and consequently does not require any IF acquisition circuitry for cold-start operation. The measured receiver sensitivity of this system was -41 db (1 mW) at an error rate of 10^-9     

Novel optical FSK heterodyne single filter detection system using a directly modulated DFB-laser diode

A novel optical FSK heterodyne single filter detection system, which can use direct modulation and can tolerate the relatively large spectral width of a laser diode, is proposed. The receiver sensitivity improvement, as much as 8 dB over direct detection using a Ge APD, has been achieved after 105 km transmission at 100 Mbit/s.     

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     

Over 300 km transmission experiment on an optical FSK heterodyne dual filter detection system

An optical FSK heterodyne dual filter detection system has been developed for the first time at 34 Mbit/s bit rate with a high receiver sensitivity of -61.4 dBm. A 66.9 dB large span loss margin has been achieved with high transmitted power and high receiver sensitivity. As a result, a 301 km transmission experiment has been performed.     

Distance limits of 10 to 20 Gbit/s FSK transmission on standardfibre

A simple analysis is performed on an FSK transmission system using a high FM efficiency, high bandwidth DFB laser as a simple DFB transmitter and an FM/AM converter as an optical signal processor. It shows that optimum optical processing pushes the transmission limits to 150 km at 10 Gbit/s and to 40 km at 20 Gbit/s     

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     

1 Gbit/s bipolar optical FSK transmission experiment over 121 km offibre

Bipolar direct modulation has been used to overcome the nonuniform low-frequency modulation response of a DFB laser transmitter in a 1 Gbit/s optical FSK transmission system. The heterodyne receiver sensitivity (nP¯) was -39 dBm for a 2²³-1 bit pseudorandom pattern, with no degradation in receiver sensitivity after transmission through 121 km of fibre     

Evaluation of optimal bandwidth in optical FSK system influenced bylaser phase noise

In this paper, an optical frequency shift keying (FSK) system with TFS (two-filter structure), also known as dual-filter FSK, is observed in the presence of laser phase noise. Sensitivity of the receiver is shown as a function of intermediate frequency (IF) bandwidth and a bandwidth of the low-pass postdetection filter. It is shown that there is an optimal pair of bandwidths which minimizes required sensitivity; a few figures for different amounts of phase noise are given. The results in this paper are based on truncated Taylor expansion method and a method based on complex analysis is used to evaluate the integrals involved     

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.     

Optical FSK heterodyne detection using image rejection mixer

A new optical FSK heterodyne detection scheme is proposed and demonstrated. This scheme uses an optical image rejection mixer. The intermediate frequency bandwidth is the same as ASK heterodyne detection while equalling dual filter FSK detection receiver sensitivity.<>     

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     

4-Gb/s heterodyne transmission experiments using ASK, FSK and DPSKmodulation

An experimental heterodyne lightwave transmission system operating at 4 Gb/s is described. The optical sources were 1.5-μm-wavelength DFB (distributed feedback) and DBR (distributed Bragg reflector) semiconductor lasers. ASK, FSK, and DPSK modulation formats were investigated; baseline receiver sensitivities of 175, 191, and 209 photons/bit, respectively, were achieved. Transmission experiments through up to 160 km of conventional single-mode fiber and up to 175 km of dispersion-shifted fiber are also reported