Optical multiplexer

DETAILED DESCRIPTION The object of the present invention is to provide a communications system capable of operation at high data rates by exploitation of the available bandwidth in optical transmission links.
The invention provides an optical multiplexer for a communications system comprising: a substantially monochromatic light source, preferably a laser; moving grating means to disperse light from the source to provide a plurality of spatially separated output light beams, each beam having a different non-overlapping optical frequency bandwidth; and a spatial light modulation means having a plurality of electrical signal input connections corresponding to spatially separated modulation channels, each channel having incident thereon a respective one of the output light beams to form thereby a plurality of parallel signal beams providing orthogonal communications channels.
The principle of operation of the invention relies on the Doppler shift in frequency of optical radiation diffracted by an acousto-optic Bragg cell or other moving grating.
The optical multiplexer thus uses frequency division multiplexing, providing parallel optical channels which can be transmitted through free space or combined in a single optical fibre.
Preferably the light dispersing means comprises an acousto-optic (A-O) device having an input transducer and a source of polychromatic radio frequency signals (RF) connected to the input transducer and a Fourier Transforming (FT) lens providing an FT of the A-O output in a focal plane in which the light modulation means is located.
Advantageously the RF source generates a chirp frequency function.
Light transmitted through the A-O device is diffracted such that the angles of diffraction of the light are proportional to the frequency components of the RF signals.
This produces a spanial distribution in the focal plane of the lens where the transverse displacement from the optic axis is proportional to the frequency of the RF signal.
In the preferred arrangement a beam expander is provided such that the A-O device, preferably a Bragg cell