Optical: systems and elements – Optical modulator – Light wave directional modulation
Patent
1990-12-27
1993-05-18
Lerner, Martin
Optical: systems and elements
Optical modulator
Light wave directional modulation
359303, 365 63, 365 64, 395 24, G11C 506, G02F 131
Patent
active
052125874
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to binary tree switching networks of particular but not exclusive application to optical space switches suitable, for example, for use in optical telephony.
Stage binary tree switching networks are employed in many types of telecommunications switching systems to select 1 from N=2.sup.n inputs, or to connect an input to one of N=2.sup.n outputs. For example, they find application in the well known splitter/combiner networks, beam steering networks employing digital light deflection, and time division switching/multiplexing networks.
It is known to construct a binary tree switching network in the form of an optical space switch from a number of stages, each stage comprising a variable polarisation cell (rotator) and a polarisation sensitive deflector. The stages are arranged such that when appropriate control signals are applied to the variable polarisation cells they change the polarisation state of light passing through the cells so that light is routed from a selected one of a number of spatially separate inputs to the output or from an input to one of a number of spatially separate outputs. The applicant's earlier application number GB8804202 discloses one example of such an optical switch.
In such switches the variable polarisation cells are typically formed from twisted nematic liquid crystal material. Such a cell rotates the plane of polarisation of incident light by 90.degree. when switched OFF but when switched ON by a control signal allows light to pass through with its plane of polarisation unaltered. In practice the behaviour of such cells departs from the ideal. When the cell is switched ON or OFF a certain fraction of the incident light is left with its plane of polarisation rotated and unrotated respectively. At each switching stage therefore whilst most of the light is deflected to the correct port a certain fraction passes through to the wrong port. If subsequent stages route this portion of light to the output port it then appears as crosstalk in the output signal.
Each stage of a binary tree switching network of 2.sup.n inputs or outputs can be considered to be a multi-pole crosspoint array followed by an interconnection pattern. If at each crosspoint a small fraction (c<<1) of any signal on the open contact is coupled onto the closed contact path, where it is assumed that c is the same for all crosspoint devices and that it is independent of switch state, 1-st order crosstalk arises at or from unselected output or input ports that are routed directly (or via closed contacts) to the unselected contacts of the crosspoints used in the selected circuit.
According to the present invention a binary tree switching network having a plurality of first ports and a second port includes two or more switching stages which are configured to switch the first ports to the second port by means of a control code set having a minimum Hamming distance greater than 1.
The applicant's invention may serve as a selector, in which a plurality of inputs are each switched to a selected output. Equally, due to its symmetry the present invention may serve as a connector in which a single input is connected to a plurality of outputs. For the sake of clarity, the invention will be hereinafter described in terms of a selector, although it is to be understood that it could also be used as a connector.
The applicant's invention is based on the realisation that all unselected input ports whose control code-words differ from the selected circuit control code-word in only one bit position will generate 1-st order crosstalk on the selected switching input. Since each connection must use n crosspoints, it follows that there exists n potential sources of 1-st order crosstalk for every connection, regardless of switch setting and that, generalising, m-th order crosstalk will originate from all unselected input ports that are m open crosspoints away from the selected path, that is, whose control code-words differ from the selected circuit code-word in m bit positions. A switching network accor
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British Telecommunications PLC
Lerner Martin
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