1988-09-23
1990-05-08
Sikes, William L.
350334, 350347V, 350 366, G02B 626
Patent
active
049232690
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a bidirectional optical space switch which is capable of use in a centralised switching system for an optical network. A centralised switching system is the simplest active network which is compatible with optical and electronic multiplexing, offsets the maximum network size, range, and flexibility and is also compatible with the existing wire-networks.
SUMMARY OF THE INVENTION
A paper to entitled "4.times.4 optical--Gate matrix switch" by A. Himeno and M. Kobayashi (Journal of lightwave Technology Vol LT-3 No. 2 April '85) discloses an optical gate matrix switch in which optical signals entering each input port I.sub.i of four input ports are distributed by an optical splitter S.sub.i to each of gate elements G.sub.i1 to G.sub.i4. When a desired single gate G.sub.ij is opened a light signal which can pass through the gate to an output port O.sub.j via a combiner C.sub.j, which combines the output of all gates G.sub.1j to G.sub.4j, to an optical receiver. This arrangement permits multiconnections between any input port I.sub.j and any plural output ports O.sub.j to be obtained. Each optical gate G.sub.ij is formed by a polarisation rotator placed between its own polariser and crossed analyser to form an on-off optical gate controllable by a driving voltage applied to the polarisation rotator. A disadvantage of this arrangement is that it is uni-directional.
According to the present invention a bi-directional optical space switch comprises:
a first and a second array of inputs for emitting optical signals to be switched;
a first and a second array of outputs for receiving switched optical signals from the first and second array of inputs, respectively;
an active array of cells having first and second sides each cell being selectively capable of varying the polarisation state of light passing through from one side of the cell to the other in response to an applied control signal;
a first optical coupling means for optically coupling the first sides of each of a plurality of groups of cells with a corresponding inputs of the first array of inputs and with a corresponding output of the second array of outputs;
a second optical coupling means for optically coupling the second sides of each of a plurality of groups of cells witha corresponding input of the second array of inputs and with a corresponding output of the second array of outputs;
the first and second optical coupling means including a respective first and second polarising beam splitter each arranged both to polarise light travelling from each input to the array of cells and to direct appropriately polarised light, only, passing through a cell away from the inputs and to the outputs.
The polarising beam splitters may, for example, comprise two right angle prisms having their hypotenuse faces cemented together and sandwiching a dielectric multilayer so that light polarised in the hypotenuse plane is reflected by the film and turned through 90.degree./o whereas light polarised transverse to the hypotenuse plane passes straight through the splitter. The use of a polarising beam splitter in the bidirectional switch according to the present invention ensures that there is substantially no loss of light save for that lost on discrimination by its polarisation state since the polarising beam splitters provide both the polarising means and the beam splitters. If a non-polarisation state selective beam splitter were used in series with a separate polariser there would be typically a 50.degree./o loss, at both beam splitters leading to a minimum of a 75.degree./o power loss through the switch. The present invention therefore provides a particularly effective bi-directional optical switch.
Preferably the active array of cells has the form of a matrix array with the optical coupling means being arranged to connect each input to its respective column, or row, of the matrix array of cells and to couple each row, or column, to its respective output. Preferably the input and output arrays are formed by linear arrays arranged perpe
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British Telecommunications public limited company
Sikes William L.
Ullah Akm E.
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