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

We describe some of the results of the Advanced Research Projects Agency (ARPA) sponsored Consortium on Wideband All-Optical Networks in developing architectures, technology components, and applications for the realization of scaleable, wideband, and transparent optical wavelength-division multiplexing (WDM) networks. Our architecture addresses all-optical transport over the wide, metropolitan, and local areas. It utilizes wavelength partitioning, routing, and active multiwavelength cross-connect switches to achieve a network that is scaleable in the number of users, data rates, and geographic span. The network supports two services which can be point-to-multipoint or multipoint-to-multipoint simplex or duplex connections. The A service is a transparent physically circuit-switched service and the B-service is a scheduled time-slotted circuit which is transparent within its time slots. We have developed a 20-channel local and metropolitan area WDM testbed deployed in the Boston area, now undergoing characterization and experimental applications.

A wideband all-optical WDM network

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

The chief objective of this paper is to develop a fundamental understanding of the effects of laser phase noise on the performance of coherent lightwave communication systems. A comprehensive treatment applicable to a wide variety of coherent receiver designs under a broad range of conditions is provided. Our models and analytical tools are developed in sufficient detail to encompass a broad range of applications. Formulas are derived for the bit error rate in homodyne and heterodyne Phase Shift Keying (PSK), Differential Phase Shift Keying (DPSK), Frequency Shift Keying (FSK) and on-off keying. Estimates are provided of the penalties accrued due to phase noise. Based on detailed mathematical analysis and estimates, we made several findings. Near quantum-limited receiver sensitivity can be achieved with PSK using homodyne detection only at signaling rates 3000 times greater than the laser linewidth. A receiver sensitivity 3 to 6 decibels poorer than the quantum limit can be achieved with heterodyne rather than homodyne detection. DPSK, for example, can operate at rates only 300 times greater than the laser linewidth. At lower rates, FSK is an attractive candidate. It can be designed to be extremely tolerant of phase noise by using wide frequency deviations.

Coherent lightwave communications

The major impairments in lightwave systems and the signal-processing techniques that can be used to reduce these impairments are discussed. Guidelines for the use of these techniques are presented. These guidelines are used to determine the techniques with the most potential for providing improved performance in long-haul systems, local-area networks, and local loop systems. Although many of the techniques have been previously considered for these applications, few have been implemented in commercial products. Some of the most promising techniques include dispersion-compensating fibers, polarization control, nonlinear cancellation (NLC), and coding for reducing the effect of distortion in long-haul single-frequency-laser direct-detection systems. >

Reducing the effects of transmission impairments in digital fiber optic systems

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 10−9 bit error rate. This exceeds the performance possible using ideal binary orthogonal FSK with zero linewidth lasers.

4-ary FSK coherent optical communication system

Coherent optical communication systems that employ semiconductor lasers have exhibited degraded performance caused by laser phase noise. A rate 1/2, constraint length of seven, convolutional code with hard decision, Viterbi decoding has been applied to an experimental 220 Mbit/s optical FSK channel. Code testing has demonstrated the ability to improve system efficiency and significantly reduce the effect of laser linewidth on bit error rate performance, verifying prior theoretical work.

Error correcting coding of a 220 Mbit/s coherent optical communication channel

Threshold decoding of a rate 4/5 convolutional code is shown to improve the performance of a 1 Gbit/s optically preamplified on-off keyed (OOK) system by 3.7 dB to achieve a receiver sensitivity of 37 photon/bit (−56.2 dBm). The encoder/decoder is implemented as a single monolithic integrated circuit.

Performance of a 1 Gbit/s optically preamplified communication system with error correcting coding

Applications such as high resolution image transmission and the aggregation of multiple bit streams have increased the interest in very high speed (Gbit/s) data communications for space applications. The ability to perform error correction on a data link can yield significant improvements in channel efficiency and can mitigate the effects of various bit error rate (BER) impairments afflicting many communications systems. A codec integrated circuit (IC) has been constructed which is capable of supporting up to 2 Gbit/s of data throughput. The device has been demonstrated in two optical communications systems, a 1 Gbit/s binary optically preamplified OOK system at 1.5 Â´m and an 800 Mbit/s FSK MOPA system at 0.98 Â´m. At a BER of 10-9, this coding provided a 3.7 dB sensitivity improvement for the OOK system, and a 6.2 dB improvement with the FSK system. The measured sensitivity of the coded OOK system was 37 photons/bit, which is better than could be realized with an ideal uncoded system. The viability of applying error correcting coding to higher data rate systems is discussed and a method of utilizing these ICs to provide four Gbit/s operation is shown.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

High-data-rate error correcting coding

Performance estimates for a coherent optical communication system

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