Abstract: An optical source for a communications system includes an optical signal source whose output is modulated to contain sidebands. The sidebands interact to produce beat frequencies and modifying apparatus, such as a length of dispersive optical fibre, is used to modify characteristics of the sidebands so that the beat frequency can be detected at a receiver such as a photodetector. In this way, high frequency signals can be generated and photodetected, the output of the receiver being suitable to drive a microwave antenna, thus providing an optically fed microwave communications link. Using a video signal generator a video signal can be superimposed on the received beat frequency and therefore carried by the microwave link.

Abstract: A method and apparatus for controlling the polarization of an optical signal has many applications, including in coherent systems, e.g. heterodyne detectors. Four birefringent elements (19, 21, 23, 25) in series are arranged to rotate the state of polarization (SOP) alternately about orthogonal axes on a Poincare sphere. A controller enables time varying initial and final polarization states (S, S') to be tracked. It also ensures that the birefringence limits of the elements are never reached by carrying out an adjustment procedure. A reduction by a full revolution (2.pi.) for one element (e.g. 19) can be achieved by varying the transformations of the other elements by not more than .pi..

Abstract: Apparatus for processing one or more optical signals to produce a desired polarization transformation on a Poincare sphere has particular application in a receiver for a coherent communication system. One or more birefringent elements (11) receive a polarized optical signal from a local oscillator (12), and vary the state of polarization of the local optical signal in order to match it to a received optical signal from a remote source on a line (13). The transformed local signal is fed on a line (14) to a directional coupler (15) which compares the local and remote optical signals. A receiver (16) extracts data by interference between the two signals in the directional coupler (15), and feeds data out at a line (17). A level detector (18) detects the level of the output signal (17) and supplies a feed-back signal to a polarization controller (19), which controls the birefringent element or elements (11).