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

Satellite-based communication systems of today are increasingly capacity limited. Based on radio frequency or microwave (generically RF) technologies, current state-of-the-art satellite communications (satcom) are often constrained by hardware and spectrum allocation limitations. Consequently, mobile payload sensors and instruments (on satellite or aircraft) are often implemented with restricted capacity to better match that of the host platform. Such limitations are expected to worsen as future interplanetary, deep-space, and manned missions use more sophisticated data-intensive sensors and as the demand for information—and a bigger return on the space-exploration investment—continues to increase.

Optical Satellite Communications

An optical heterodyne subcarrier-multiplexed (SCM) quadrature-phase-shift keying (QPSK) system is described that employs differential encoding to transmit multiple video channels. With this differential encoding scheme, only a simple delay-and-multiply demodulator is necessary, considerably reducing the signal processing complexity. The demodulators reproduce the data in a parallel fashion so no baseband demultiplexing or four-level decision circuit is required. This approach also avoids the complications of implementing a carrier recovery circuit (CRC) and a phase-locked loop associated with coherent microwave demodulation schemes. In addition to bandwidth efficiency and simplicity, differential QPSK also provides very good receiver sensitivity. With heterodyne detection, a receiver sensitivity of -41 dBm is achieved. >

Optical DQPSK video system with heterodyne detection

This paper focuses on some heretofore overlooked advantages of coherent detection (CD) over direct detection (DD) in optical fiber communication systems. The basis for these sensitivity improvements is the fact that the local laser in coherent fiber optic systems adds a large number of photons to the net received optical signal. Under these conditions, the Bohr correspondence principle enables us to treat the photodiode output signal classically. Specifically, the usual CD environment yields a photodiode output that can be regarded as a classical signal disturbed by Gaussian noise (central limit theorem). This detection process has been studied extensively, and there exist many powerful signal processing techniques that may be used to enhance system sensitivity. In particular, we utilize a sum of digitally modulated subcarriers to frequency modulate an optical carrier along with the aid of coherent detection to achieve 11.1 photons/b sensitivity as determined by computer simulation. >

Sensitivity enhancement using coherent heterodyne detection

Heterodyne transmission experiments of multiple video channels using differentially encoded quadrature phase shift keying (QPSK) are described. Combining the QPSK microwave modulation format with the subcarrier multiplexing technique results in a total of 20 video channels at 50 Mb/s being transmitted on 10 microwave subcarriers. Using a differential encoding approach, this system provides good receiver sensitivity and bandwidth efficiency, while maintaining a simple receiver design. No carrier recovery circuit or phase-locked loop is required for demodulation, which greatly reduces the receiver complexity. It is demonstrated that only a standard delay and multiply demodulator are necessary. With optical heterodyne detection, a receiver sensitivity of -37.5 dBm is achieved for the 20 channel system. The extension of this technique to intensity modulated direct detection subcarrier systems along with migration scenarios for digital video and broadband services is discussed. >

Heterodyne video transmission with differentially encoded quadrature phase shift keying

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

Frequency shift keying (FSK) has proved one of the most popular modulations for use in coherent optical communication systems. FSK systems typically use a binary signaling format where one of two possible tones is transmitted corresponding to data bits one and zero. Demodulation can be accomplished with a frequency discriminator, a two-filter demodulator, or if a 3-dB loss in theoretical performance is acceptable, a single filter receiver.

Demonstration of a 4-ary FSK coherent optical communication system

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4-ary FSK coherent optical communication system

Demonstration of a 4-ary FSK coherent optical communication system