Dual channel optical homodyne receiver

#This invention relates to wideband communication receivers and more specifically to homodyne receivers operable at wavelengths in the "optical" region.

#In the past, many communications systems have been suggested which use the coherent output of a laser oscillator as the information carrier. The vast spectrum made available by laser communications is, of course, well known. At the optical wavelengths of lasers, which include not only the visible region but the infrared region as well, the generation of extremely narrow beams is highly conservative of the transmitted energy and at the same time provides a great deal of spatial security against undesired reception.

#While the potential of wideband laser communications has been long recognized, the availability of components necessary to realize such systems has been limited. In recent years, however, a great number of improvements have made such systems practical. These improvements have included the development of highly reliable CO.sub.2 lasers, the development of wideband optical modulators, the development of efficient wideband detectors, and the development of suitable optical components and subsystems.

#In order that the invention may be clearly understood and readily carried into effect, it will now be described with reference by way of example to the accompanying drawings, wherein like reference numerals correspond to like structural elements and, in which:

#FIG. 1 is a block diagram of a simplified dual channel synchronous receiver useful in the understanding of the principles of the present invention;

#FIG. 2 is a block diagram of a preferred embodiment of the present invention;

#In FIG. 1 there is shown a simplified block diagram of a dual-channel synchronous receiver useful in understanding of the principles of the present invention. Such receivers have been used for many years in the radio frequency region for the synchronous reception of suppressed-carried amplitude modulated signals. This receiver, generally referred to as a Costas receiver or Costas demodulator, is described in an article entitled "Synchronous Communications", by John P. Costas, Proceedings of the IRE, December 1956, on pages 1713-1718.

#In the circuit of FIG. 1, an input signal, typically derived from a receiving antenna, is split into two substantially equal-amplitude components and coupled to first and second synchronous detectors 10 and 11 respectively. Although not shown in the above-cited article the usual Costas receiver employs at least one stage of radio frequency amplification in the signal path between the antenna and the synchronous detectors. In any event, the output of a controllable local oscillator 12 is coupled to a second input of synchronous detector 11. The output of local oscillator 12 is also coupled through a 90.degree. phase shift network 13 to a second input of synchronous detector 10. The outputs of synchronous detectors 10 and 11 are coupled through respective amplifiers 14 and 15 to separate inputs of a video correlator 16.

#The output of video correlator 16 is, in turn, coupled to a frequency control circuit 17 which controls the phase and frequency of local oscillator 12. The output of the synchronous receiver is obtained from the output of video amplifier 15 and is coupled to the appropriate utilization circuitry, not shown. It is understood that other circuit elements would be employed in a practical realization of such a receiver, but for the sake of clarity have been omitted in the circuit of FIG. 1.