Optical: systems and elements – Holographic system or element – Using modulated or plural reference beams
Patent
1992-10-16
1994-11-08
Hsu, Alpus H.
Optical: systems and elements
Holographic system or element
Using modulated or plural reference beams
359115, 359124, 359285, 359298, 359305, 372 21, 372 33, 385 10, H04J 1400, G02F 111, H04B 1000
Patent
active
053632214
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to communications systems and in particular to such systems employing optical transmission links.
2. Discussion of the Prior Art
Modern computing and communications systems require ever higher data transmission rates. The speed of transmission is limited by the usable bandwidth of the transmission medium. In the case of transmission along coaxial cables, for example, data rates are limited to around a few hundred megahertz over large distances.
Optical transmission systems offer theoretical bandwidths of peta-hertz. If the transmission is in a dispersive medium, then this data rate is considerably reduced. In commercially available single mode optical fibres, for instance, the theoretical limit is greater than a few hundred gigahertz. This limitation arises from the maximum speeds at which the driving electronics can operate.
SUMMARY OF THE INVENTION
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: plurality of spatially separated output light beams, each beam having a different non-overlapping optical frequency bandwidth; and 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. is uniformly illuminated by the laser light. The drive signal to the A-O device is selected to be wide band to make use of the full range of channels of the spatial light modulating means. In a particular arrangement the A-O drive signal may be chosen to make use of only a selected number of the available channels.
Preferably the light modulation means is a 1-dimensional spatial light modulator (SLM) and the output light beams from the A-O cell are focussed by the lens onto the SLM. The SLM is located such that each element thereof defines an independent frequency channel. In one form the laser light may be incident on a plurality of Bragg cells aligned such that the output beams therefrom ape directed to respective lines of a 2-dimensional SLM. Alternatively the SLM could be an optically scattering or transmitting surface whose 2-dimensional structural details are to be scanned and transmitted.
In the preferred arrangement an unmodulated reference beam is also provided by the laser source, advantageously by means of a beamsplitter located between the source and the A-O cell. In an optical fibre communications system each signal beam in the communications channels may be heterodyned to the reference beam by coupling the signal beams into a first opt
REFERENCES:
patent: 3256443 (1966-06-01), Moore
patent: 4225938 (1980-09-01), Turpin
patent: 4328576 (1982-05-01), Oakley
patent: 4389093 (1983-06-01), Jackson
patent: 4468766 (1984-08-01), Spezio
patent: 4775972 (1988-10-01), Ih et al.
patent: 4809359 (1989-02-01), Dockery
patent: 4952010 (1990-08-01), Healey
patent: 4976520 (1990-12-01), Brandstetter et al.
patent: 5010346 (1991-04-01), Hamilton et al.
Shimazu et al., "Wavelength-Division-Multiplexing Optical Switch using Acoustooptic Deflector", pp. 1742-1747, Dec. 1987.
Harris et al., "Acoustic-Optic Photonic Switch", Nov. 1989, pp. 1177-1179.
Condon Brian
Dawber William
Hirst Peter F.
Shaw Andrew P.
Sutton Philip
Hsu Alpus H.
The Secretary of State for Defence in her Britannic Majesty's Go
LandOfFree
Optical multiplexer does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical multiplexer, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical multiplexer will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-1787043