With recent successes of laboratory, inatmosphere, and space demonstrations of free-space optical communications, there is no doubt that the technology is ready for operational deployment. While these successes have shown that there are no laws of physics against such systems, their estimated system costs are still much too high for serious considerations. Two types of development can reduce the cost dramatically. The first is via the improvement of physical-link communication efficiency by an order of magnitude using photon-counting receivers for vacuum channels, system complexity, weight, and power for space systems can be greatly reduced. The second is through the use of coherent systems in links where clear-air turbulence impairs communication efficiency, and in multiple access applications where coherent processing can reduce the level of interference, significant reduction in system costs can be realized

Free-Space Optical Communications

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 bandwidth B is optimized. The value of B opt and the corresponding optimum loop performance are evaluated in this paper. Further, the maximum permissible laser linewidth \delta\nu is evaluated and found to be 5.9 \times 10^{-6} times R b , where R b (bit/s) is the system bit rate. This number corresponds to BER = 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 only \delta\nu = 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.

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

Given the RMS transmitter pointing error and the desired probability of bit error (PBE), it can be shown that an optimal transmitter antenna gain exists which minimizes the required transmitter power. Given the RMS local oscillator tracking error, an optimum receiver antenna gain can be found which optimizes the receiver performance. The impact of pointing and tracking errors on the design of direct-detection pulse-position modulation (PPM) and heterodyne noncoherent frequency-shift keying (NCFSK) systems is then analyzed in terms of constraints on the antenna size and the power penalty incurred. It is shown that in the limit of large spatial tracking errors, the advantage in receiver sensitivity for the heterodyne system is quickly offset by the smaller antenna gain and the higher power penalty due to tracking errors. In contrast, for systems with small spatial tracking errors, the heterodyne system is superior because of the higher receiver sensitivity. >

Impact of random pointing and tracking errors on the design of coherent and incoherent optical intersatellite communication links

Optical homodyne receivers based on decision-driven phase-locked loops are investigated. The performance of these receivers is affected by two phase noises due to the laser transmitter and laser local oscillator, and by two shot noises due to the two detectors employed in the receiver. The impact of these noises is minimized if the loop bandwidth B is chosen optimally. The value of B opt and the corresponding optimum loop performance are evaluated in this paper. It is shown that second-order phase-locked loops require at least 0.8 pW of signal power per every kilohertz of laser linewidth (this number refers to the system with the detector responsivity 1 A/W, dumping factor 0.7, and rms phase error 10°). This signal power is used for phase locking, and is, therefore, lost from the data receiver. Further, the maximum permissible laser linewidth \Delta\nu is evaluated and for second order loops with the dumping factor 0.7 found to be 3.1 \times 10^{-4} \cdot R_{b} , where R b (bit/s) is the system bit rate. For R_{b} = 100 Mbit/s, this leads to \Delta\nu = 31 kHz. For comparison, heterodyne receivers with noncoherent postdetection processing only require \Delta\nu = 0.72-9 MHz for R_{b} = 100 Mbit/s. Thus, the homodyne systems impose much more stringent requirements on the laser linewidth than the heterodyne systems. However, homodyne systems have several important advantages over heterodyne systems, and the progress of laser technology may make homodyning increasingly attractive. Even today, homodyne reception is feasible with experimental external cavity lasers, which have been demonstrated to have \Delta\nu as low as 10 kHz.

Decision-driven phase-locked loop for optical homodyne receivers: Performance analysis and laser linewidth requirements

An optical PSK heterodyne communications receiver is investigated. The receiver is based on the decision-directed phase-locked loop. The performance of the phase-locked loop subsystem is analyzed taking into account both shot noise and laser phase noise. It is shown that for reliable phase locking (rms phase error less than 10°), heterodyne second-order loops require at least 6771 electrons/s per volt every hertz of the laser linewidth. This number corresponds to the limit when the loop dumping factor η is infinitely large; if \eta = 0.7 , then the loop needs 10 157 electrons/(s . Hz). If the detector has a unity quantum efficiency and \lambda = 1.5 \mu m, the above quoted numberers give 0.9 pW/ kHz for \eta \rightarrow \infty and 1.35 pW/kHz for \nu = 0.7 . The loop bandwidth required is also evaluated and found to be 155 \Delta\nu , where \Delta\nu is the laser linewidth. Finally, the linewidth permitted for PSK heterodyne recievers is evaluated and found to be 2.26 \cdot 10^{-3} R_{b} where R b is the system bit rate. For R_{b}=100 Mbit/s, this leads to \Delta\nu kHz. Such and better linewidths have been demonstrated with laboratory external cavity lasers. For comparison, ASK and FSK heterodyne receivers are much more tolerant to phase noise,-they can tolerate \Delta\nu up to 0.09 R b . At the same time, homodyne receivers impose much more stringent requirements on the laser linewidth ( \Delta\nu ).

Performance analysis and laser linewidth requirements for optical PSK heterodyne communications systems

This paper describes the design and performance of an M ary 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 of M -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.

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

Heterodyne lasercom systems using GaAs lasers for ISL applications

Heterodyne LASERCOM systems using GaAs lasers for ISL applications

M-ary FSK performance for coherent optical communication systems using semiconductor lasers

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

Heterodyne lasercom systems using GaAs lasers for ISL applications

Heterodyne LASERCOM systems using GaAs lasers for ISL applications

M-ary FSK performance for coherent optical communication systems using semiconductor lasers