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.     

FSK heterodyne system experiments at 1.5 ?m using a DFB laser transmitter

140 and 70 Mbit/s FSK heterodyne system experiments are reported using single-filter detection with transmitter laser linewidths of 30 and 60 MHz. The system requirements that must be satisfied to avoid large performance penalties are considered.     

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.<>     

Coherent FSK transmitter using a negative feedback stabilised semiconductor laser

An electrical negative frequency feedback scheme is applied to a semiconductor laser to improve its direct frequency modulation performance in an FSK transmitter. An error signal caused by imperfect FM response is extracted through heterodyne discrimination detection with a stable master laser and fed back to the FM semiconductor laser.     

Optical heterodyne detection of directly frequency modulated semiconductor laser signals

Frequency modulated semiconductor laser signals are demodulated by optical heterodyne detection using an independently temperature stabilised semiconductor laser local oscillator and a square law detector followed by electrical frequency discrimination circuitry. Beat frequency stability of the free-running semiconductor laser transmitter and local oscillator and characteristics of direct frequency modulation and optical heterodyne detection are delineated.     

Optical DPSK heterodyne detection experiments using DBR laser diodes with external optical feedback

High receiver sensitivity, with 7 dB improvement over a direct detection system, has been achieved in a 400 Mbit/s optical DPSK heterodyne detection experiment using DBR laser diodes with external optical feedback. The influence of the laser output phase noise was evaluated experimentally in good agreement with theory.     

A 2-Gbit/s optical FSK heterodyne transmission experiment using a 1520-nm DFB laser transmitter

Optical FSK transmission at 2 Gbit/s using a directly modulated DFB laser at 1520-nm wavelength is reported. A receiver sensitivity ofbar{P} = -36.7dBm (etabar{P} = -39.2dBm) at 10^-9BER was achieved for transmission over 101 km of single-mode fiber with no additional penalty attributable to the fiber. The effect of the nonlinear phase of the transmitter FM response, and the system performance for discriminator demodulation, including the impact of laser phase noise, is analyzed and compared with experimental results.     

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.     

A stable submillimeter laser local oscillator for heterodyne radiometry and spectroscopy

A submillimeter laser with off-axis pump beam injection is described. This design concept achieves complete isolation of the pump laser with respect to the pump radiation reflected from the smm resonator. Active line independent stabilization of the pump laser is obtained by the use of an external tunable etalon as a frequency reference. The smm output power is constant to within 4% over periods of hours. Mean frequency drifts of less than 2 parts in 10⁸ per minute were measured by mixing with very high harmonics of an X-band synthesizer in a planar Schottky diode.     

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     

Optical FSK modulation using injection-locked laser diodes

A FSK (frequency shift keying) modulator based on the α-parameter induced frequency shift in injection-locked laser diodes is studied. The attainable frequency deviations can easily be set up to several gigahertz by acting on the injected power. A major limitation is found in the switching time which, in some anomalous cases, can become as large as 10 ns     

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     

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.     

Optical heterodyne detection

Optical heterodyne detection presents some difficult problems, but for certain applications it offers considerable advantage over the more conventional detection methods. Some of these advantages and problems, and how they are affected by the transmission medium involved, are discussed in this article. Experimental evidence has shown that although heterodyne reception is not desirable at the end of long atmospheric transmission paths, it is entirely feasible for long paths that are enclosed to isolate them from atmospheric disturbances.     

Frequency stabilisation of semiconductor laser using atomicabsorption line under direct FSK

The oscillation frequency of an AlGaAs laser diode is stabilised using the Rb-D2 absorption line (780 nm) under the direct frequency shift keying (FSK) condition. Only one side (mark or space) frequency signal is used to obtain the feedback signal for stabilisation in this system. The stability obtained is almost the same as that of the case without FSK     

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.     

Bit-error rate of PSK heterodyne optical communication system and its degradation due to spectral spread of transmitter and local oscillator

In a PSK heterodyne optical communication system, the FM noise of the transmitter and local oscillator degrades the bit-error rate (BER). A theory is presented giving the BER of such a system considering the FM-noise effect. To verify the theory, BER measurement is performed using a simulation model of the PSK heterodyne system.     

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     

Analogue baseband TV transmission experiments using semiconductor laser diodes

The transmission characteristics of analogue video signals are investigated by using semiconductor laser diodes. It is shown that the linearity degradation due to speckle noise depends on the optical-fibre types adopted. Based on this investigation, transmission over 32 km is carried out by using a 1·3 ?m laser diode and single-mode fibres, with 46 dB unweighted s.n.r.     

Optical time domain reflectometry using an M-ary FSK probe andcoherent detection

This paper proposes a new approach to enhancing the performance of optical time domain reflectometry (OTDR). This approach launches a probe signal modulated in the M-ary FSK format into a test fiber and detects the backscattering by coherent detection, which offers excellent frequency discrimination, followed by simple signal processing to recover the fiber impulse response. Fault location experiments verify the theoretical prediction of that the proposed approach reduces the measurement time to 1/M without sacrificing measurable dynamic range or spatial resolution. They also show that the approach is effective in reducing fluctuations in the OTDR trace. It is experimentally confirmed that OTDR with the proposed approach is feasible for long haul transmission systems