Novel FSK heterodyne single-filter detection system using no IFfrequency lock

A frequency-shift keyed (FSK) modulation/single-filter detection system which does not require an intermediate-frequency (IF) locking loop is reported. This system utilizes a laser frequency-locked to an atomic line (krypton 2p₈-3d₁" transition at 1.5224 μm) as both transmitter and local oscillator. As a result, communication can be initiated without any manual adjustment to match the wavelengths of transmitter and local oscillator lasers. No significant degradation in the receiver sensitivity was observed due to the slight frequency dither used to lock the laser frequency to an atomic transition line     

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.     

High-performance, two-channel optical FSK heterodyne system with polarisation diversity receiver

A two-channel FSK heterodyne system with 54 dB loss span and microprocessor-controlled channel selection is presented. The sensitivity of the polarisation-diversity receiver is investigated for both fixed and changing signal polarisations.<>     

First sea trial of FSK heterodyne optical transmission system usingpolarisation diversity

Operations of an FSK heterodyne optical transmission system on the cable ship during the cable-laying, holding, and recovering process in a 6000 m-deep sea trial is demonstrated. It is shown that polarisation diversity is a powerful technique for application to optical fibre coherent systems in submarine cable systems     

Polarisation- and pattern-independent optical FSK heterodyne system using data-induced polarisation switching

Data-induced polarisation switching is a simple yet powerful technique to solve the polarisation problem in optical FSK heterodyne systems. The authors achieve a polarisation- and pattern-independent sensitivity of -53.5 dBm for a 140 Mbit/s FSK receiver. The response time to polarisation changes is estimated as 100 ns.<>     

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     

Polarisation-insensitive optical heterodyne receiver for coherent FSK communications

An optical FSK heterodyne communications receiver is proposed using a polarisation diversity technique which has a reduced level of complexity. The proposed receiver is constructed and tested at 100kbit/s and shown to have a maximum fluctuation in sensitivity of 17% over conditions of originally complete polarisation fading.     

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     

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     

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.     

1.244-Gb/s 32-channel transmission using a shelf-mountedcontinuous-phase FSK optical heterodyne system

A total capacity of 40 Gb/s is achieved using a shelf-mounted continuous-phase frequency-shift-keying (CPFSK) optical heterodyne frequency-division-multiplexing (FDM) transmission system with 32 optical channels and a bit-rate of 1.244-Gb/s per channel. For achieving a stable bit-error-rate (BER) characteristics with high-sensitivity, narrow-linewidth laser diodes, a channel-spacing stabilization circuit, and an optical tuner are developed. The obtained sensitivity at a BER of 10^-9 for fiber transmission over 121 km ranges from -45.1 to -44.2 dBm, which is 9.8-10.7 dB lower than the shot-noise-limited sensitivity. The crosstalk penalty is suppressed to within 0.1 dB. The developed system has feasibility achieving a distribution system which can distribute more than 250 HDTV (high definition television) signals or 1250 current-standard TV signals to about 8000 subscribers 10 km from the office, or a 40-Gb/s trunk-line system with a fiber span of more than 50 km     

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.     

Performance of linecoded optical heterodyne FSK systems withnonuniform laser FM response

A simple theoretical analysis for evaluating the performance of linecoded optical frequency shift keying (FSK) systems is presented. It accounts for the combined effects of laser phase noise, receiver noise, and nonuniform BM response of distributed feedback lasers. A close form expression for the random frequency noise due to the combined effect of laser nonuniform FM response and phase noise is developed. The analysis is carried out for three different linecoding schemes, i.e., alternate mark inversion, Miller code or delay modulation, and Manchester coding, to investigate the efficacy of the line coding schemes in counteracting the effect of nonflat FM response. Theoretical and simulation results show that the sensitivities of linecoded FSK systems are within 0.7 and 0.4 dB for single-branch and dual-branch detection, respectively, at a bit error probability of 10^-9 relative to the random non-return to zero FSK with flat FM response     

Over 1000 km FSK heterodyne transmission system experiment usingerbium-doped optical fiber amplifiers and conventional single-modefibers

The influence of spontaneous emission noise on coherent transmission systems using multistage erbium-doped optical fiber amplifiers is experimentally examined. A frequency-shift keying (FSK) heterodyne transmission experiment was successfully performed at 560 Mb/s through 1028 km of fiber using ten cascaded fiber amplifiers and conventional single-mode fibers with a zero dispersion wavelength of around 1.3 μm. In the experiment, no transmission penalty due to accumulated spontaneous emission noise or to fiber chromatic dispersion was observed     

System design and long-span transmission experiments on an optical FSK heterodyne single filter detection system

The influence of LD phase noise on a heterodyne noncoherent detection system was evaluated. Based on the evaluation, an optical FSK heterodyne single filter detection system with large frequency deviation and wide-band IF filter has been developed to allow use of stand-alone DFB LD's. In the system, a phase tunable DFB LD was used as an FSK transmitter light source to improve the FSK modulation characteristics. An IF filter with appropriate bandwidth evaded the influence of LD phase noise. With these configurations, long-span (243 km at 140 Mbit/s and 204 km at 280 Mhit/s) transmission experiments have been successfully carried out on this single filter detection system. To the contrary, influence of LD phase noise appeared in a limited IF bandwidth case, which agrees well with the theoretical evaluation.     

1.244 Gbit/s 32 channel 121 km transmission experiment using shelf-mounted continuous-phase FSK optical heterodyne system

A 1.244 Gbit/s 32 channel transmission experiment is demonstrated by using a shelf-mounted continuous-phase FSK optical heterodyne system. Stable 121 km fibre transmission is achieved with sensitivity ranging from -45.1 to -44.2 dBm at a bit error rate of 10/sup -9/. This is made possible by employing channel spacing stabilisation, optical tuning and polarisation diversity reception.<>     

140 Mbit/s optical FSK fibre heterodyne experiment at 1.54 ?m

An optical fibre heterodyne experiment using 2-FSK modulation at 140 Mbit/s has been operated in the 1.5 ?m loss window. A receiver sensitivity of ?55 dBm has been obtained; operation over a fibre link of 199.7 km showed no additional system degradation.     

Optical FSK signal detection in a heterodyne system using semiconductor lasers

A 10?8 bit error rate is achieved for 100 Mbit/s signals in an FSK heterodyne frequency-discrimination detection system, in which AlGaAs double-heterostructure lasers are used as both the FSK transmitter and the local oscillator. Receiving signal power level to achieve a low bit error rate is experimentally obtained, which confirms the principle of optical heterodyne detection. AM quantum noise in the laser local oscillator and FM quantum noise are important factors which determine the system performance.     

1 Gbit/s optical FSK heterodyne transmission experiment over 100 km of single-mode fibre

A 1 Gbit/s optical FSK modulation/heterodyne detection transmission experiment has been conducted at a wavelength of 1497 nm. The receiver sensitivity is ?37dBm and no degradation is observed after transmission through 100 km of single-mode fibre.     

4-ary FSK coherent optical communication system

Experimental verification of the sensitivity advantage of M-ary orthogonal signalling over binary signalling is reported. A 110 Mbits 4-ary FSK system using conventional Fabry-Perot lasers with a 7% IF linewidth to data-rate ratio has achieved a sensitivity of 37 detected photons per bit at a 10/sup -9/ bit error rate. This exceeds the performance possible using ideal binary orthogonal FSK with zero linewidth lasers.<>