1989-01-23
1990-11-13
Miller, Stanley D.
350334, 350347R, 350338, 350378, 350400, G02F 113, G02B 530
Patent
active
049697170
DESCRIPTION:
BRIEF SUMMARY
Optical signal processing and transmission techniques, such as those used in telephony, give rise to the need for devices capable of switching light between points separated in space. Although space switching can be achieved using waveguides such as fiber optic cables in combination with suitable couplers there are many situations in which it would be advantageous to use free space switching of unguided beams for example in image switching. However, the known free space optical switches involve the use of complex systems of precisely aligned optical components such as lenses and are not suitable for industrial application.
It is well known that certain classes of material, such as calcite, are birefringent. When an object is viewed in unpolarized light through a cell formed of such a material two spatially separated images are observed separated by an amount determined by the thickness and orientation of the cell. The two images observed are polarized in orthogonal planes and the observed separation of the images results from the property of birefringent materials that light is refracted through different angles, that is it is either ordinarily or extraordinarily refracted, according to its polarization state.
A cell of birefringent material is used in conjunction with a variable polarization rotating cell such as a liquid crystal cell of the type used, for example, in liquid crystal display devices. The liquid crystal cell may be of the twisted nematic crystal type. When a nematic liquid crystal is aligned homogeneously in a thin cell with 90.degree. between the direction of orientation of crystals in the upper and lower surfaces it exhibits a twisted planar structure which has the property that the plane of polarization of plane polarized light incident upon the cell is rotated through 90.degree.. When an electric field is applied across the cell the molecules of the nematic liquid crystal align themselves along the field lines thus destroying the twisted structure and removing the polarization rotation property.
Alternatively a liquid crystal cell with crystals uniformly aligned in a single direction may be used. If the liquid crystal molecules of such a cell are aligned at an angle T to the plane of polarization of incident light then the cell rotates the plane of polarization of the light through a total angle of 2T. Orienting the crystals at 45.degree. to the plane of polarization therefore results in rotation of incident plane polarized light through 90.degree.. If the thickness and birefringence of the cell are chosen appropriately the cell functions as a .lambda./2 phase plate. The required birefringence can be achieved by applying a bias voltage across the cell to tilt the molecules. The cell is switched "OFF", i.e. into the state in which the plane of polarization of incident light is not rotated, by applying a large voltage across the cell to align the crystals in such a direction that the birefringence of the cell is effectively removed.
Such a cell can switch light between two orthogonal polarization states in response to a field applied to the liquid crystal cell. By arranging a birefringent cell adjacent the liquid crystal cell with its optical axis oriented so that ordinarily and extraordinarily refracted rays are directed to two different positions it is possible to switch the light between these two positions in response to the switching between orthogonal polarization states effected by the liquid crystal cell. Such an arrangement is disclosed in a short communication in the IBM Journal of Jan. 1964 entitled "A Fast, Digital-Indexed Light Deflector" by Kulke et al. (pages 64 to 670).
According to the present invention an optical switch comprising:
a series of 2(n-1) macro-cells, each having a variable polarization rotating cell with up to n individually addressable sub-cells each located at one of n positions Pl to Pn and a birefringent cell;
(n-1) birefringent cells being orientated relative to light passing through so that light passing through a subcell (at position Pi) is switchable
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Soref & McMahon; "Calcite 2.times.2 Optical Bypass Switch Controlled by Liquid Crystal Cells", Optics Letters, vol. 7, No. 4, 1982.
Schmidt; "A High Speed Digital Light Beam Deflector", Physics Letters, vol. 12, No. 3, 1964.
Kulcke et al.; "A Fast Digital Indexed Light Deflector", IBM Journal, vol. 6, No. 10, Mar. 1964.
Schmidt; "Electro-Optic Deflection of a Laser Beam", Philips Technical Review, vol. 36, No. 5, 1976.
Nelson; "Digital Light Deflection", The Bell System Technical Journal, vol. XLIII, No. 3, May 1964.
British Telecommunications public limited company
Mai Huy K.
Miller Stanley D.
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