Multiplex communications – Wide area network – Packet switching
Patent
1988-02-29
1990-09-25
Crosland, Donnie L.
Multiplex communications
Wide area network
Packet switching
455619, H04J 1400
Patent
active
049598260
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to optical transmission systems and especially to multiple-access systems.
2. Description of Related Art
Transmitting a large number of channels of information over a given link requires the signals to be separable at the receiving end. This may be achieved by dividing the available band into disjoint slots for each user or by permitting each user to occupy the entire band and rendering the various transmitted signals orthogonal in some way so that the signals will not interfere with one another.
This latter spread spectrum technique has received attention particularly with radio frequency transmission. Proposals for utilising spread spectrum techniques in optical systems have been made, involving the generation of pseudo-random electrical signals which are modulated with information to be transmitted and then applied to an optical source such as a laser. The different channels are then code division multiplexed. Unfortunately both the generation of pseudo-random signals and code multiplexing are complex techniques.
If the coherence time of an optical signal is relatively short then the channels can be multiplexed by imposing a time delay greater than the coherence time. A scheme has been proposed for optical transmission systems in which the signal from a short coherence interval source is split into several components. Each component is then divided in two and one part modulated and one part delayed. These parts are recombined with the other components which have been split and provided with different modulations and delays, and transmitted. At the receiving end the combined signal is re-divided, each division split into two and one part delayed, and the parts then recombined so that equally delayed parts of the modulated and unmodulated signal are combined and detected. The disadvantage of this system is that there are high losses at the detection stage; half of the modulated signal and half of the unmodulated signal are lost. The associated direct detection techniques are also high in losses.
SUMMARY OF THE INVENTION
The present invention is directed towards providing a more efficient optical spread spectrum system.
According to a first aspect of the invention there is provided a method of transmitting information using a broad spectrum optical carrier signal, comprising dividing the carrier signal over at least two paths, modulating the carrier signal in at least one of the paths in accordance with information and recombining the divided signals with delay multiplexing, and receiving the information by combining a portion of the recombined signal in heterodyne detection with a local oscillator signal having a frequency shifted with respect to the carrier, and in which the local oscillator signal is delayed prior to combination by a delay corresponding to a delay of the delay multiplexing.
The local oscillator signal must have the same spread spectrum characteristics as the optical carrier. The local oscillator signal may be generated remotely at a receiver or distribution point, or may consist of a signal divided from the carrier and transmitted along a separate path or multiplexed onto the same path as the recombined modulated paths. Preferably the local oscillator signal is divided from the carrier signal and is subject to a frequency shift. The frequency shift is conveniently imposed prior to transmission in order to keep most of the active components at the transmitter. Preferably the local oscillator is polarisation multiplexed for transmission.
The paths preferably comprise optical waveguides, such as, for example, optical fibres.
In a preferred embodiment of the invention a return message is transmitted by mixing the divided and recombined signals and the local oscillator signal to provide a return message carrier signal and the return message carrier signal is divided over two paths one of which is modulated in accordance with a return message and then recombined with the other return message path with delay multi
REFERENCES:
patent: 4335463 (1982-06-01), Foucard
patent: 4390247 (1983-06-01), Freye
patent: 4644267 (1987-02-01), Tsui
patent: 4666295 (1987-05-01), Duvall
patent: 4742576 (1988-05-01), McMahon
patent: 4748686 (1988-05-01), Glomb
Journal of Lightwave Technology, vol. LT-3, No. 5, Oct. 1985 (New York, U.S.), L. L. Brooks et al; "Coherence Multiplexing of Fiberoptic Interferometric Sensors", pp. 1062-1071, see p. 1062.
Beek L. Van
British Telecommunications public limited company
Crosland Donnie L.
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