Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1995-11-15
1999-05-04
Pascal, Leslie
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359117, 359140, 359165, H04B 1000
Patent
active
059009560
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND TO THE INVENTION
The present invention relates to a method and apparatus for processing optically encoded signals, and in particular to the recognition of an optically encoded binary word.
The need to compare a digitally encoded word with a target word arises in a variety of different contexts within the overall field of optical communication and signal processing systems. One important application for such techniques is in the recognition of addresses in the header of a frame carried on an optical communications network. To enable use of such a network at very high bit is rates, e.g. 100 Gbit/s, it is important that the recognition can be carried out at ultrafast speed, e.g. 400 ps for a 40-bit word. To this end, a number of different approaches have previously been proposed. These include ultrashort pulse spectral phase encoding, optical correlation using spread spectrum and time division encoding techniques, and all-optical logic techniques using, for example, a soliton-dragging gate or a non-linear optical loop mirror. In the case of techniques of this latter sort, using non-linear fibre devices, it has been possible to process ultrafast optical signals. However, such devices suffer a significant disadvantage, in that they have long latency times, that is a long delay between input and output signals.
One approach to codeword recognition is disclosed in PCT/CH91/00122. Whilst recognising the need to provide speed and sensitivity, this prior system relies upon a hybrid electrical/optical system in which the word to be recognized W is applied in the optical domain to a gate switched by an electrical control sequence corresponding to the target word T. The output from the gate is summed incoherently in a photodetector. In practice, the speed of recognition achievable with such a system is severely limited.
SUMMARY OF THE INVENTION
According to the present invention, in a first aspect there is provided a method of recognising an optically encoded binary word W characterised in that the word is recognised by carrying out in the optical domain a bit-wise Boolean AND operation on the word W to be recognised, or a word derived therefrom and the target word T or a word derived therefrom.
Preferably the word encoded is chosen from the subset of binary words for which
In the notation adopted in the present specification, WT is the operation ##EQU1## where w.sub.i and t.sub.i are the ith bits of the respective binary words and, as is customary in Boolean algebra, sums and products represent the OR and AND operations, respectively, and t.sub.i denotes the logical complement of t.sub.i.
The present inventor has realised that with an appropriate choice of codewords it is possible to carry out recognition entirely in the optical domain, using an optical AND operation.
The preferred embodiment restricts the encoding words to a sub-set of all the possible n-bit binary words and then carries out word recognition using a method based entirely on AND operations at the bit level. As discussed in further detail below, the AND operation, as represented by the simple scalar product of two optical fields, is a fundamental capability of many non-linear optical devices. Accordingly, it can be implemented far more straightforwardly than other logical operations, such as the exclusive-OR operation previously used for optical code word recognition. This approach will therefore allow the processing of optical code words at ultrafast speeds far beyond the capability of electronic logic circuitry, but with the possibility of modest energy requirements, high stability and low latency in the signal processing devices, together with small physical volume and potential for integration. Bit rates in excess of 10 Gbit/s and potentially as high as 100 Gbit/s and beyond can be achieved.
The use of ultrafast non-linear optical AND gates has previously been proposed as a means of demultiplexing very high speed optical data. In such a scheme, if A is a very high speed data stream and B is a lower frequency optical clock signal, the
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British Telecommunications public limited company
Pascal Leslie
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