Optical Switch

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 to pass through positions Pi or Pi+1 of the variable polarization rotating cell of the next macro-cell in the series, and (n-1) birefringent cells being orientated relative to light passing through so that light passing through a sub-cell (at position Pi) is switchable to pass through position Pi or Pi-1 of the variable polarization rotating cell of the next macro-cell in the series, the subcells being positioned to provide that light from any one of n inputs is switchable independently to any of n outputs, where n>1.

For convenience of manufacture the macro-cells may have an addressable sub-cell at each of the n positions even though redundant sub-cells may not be addressed. The redundant sub-cells can be readily determined and omitted before manufacture if desired. The macro-cells can be placed in any order. One convenient arrangement is to construct pairs of macro-cells of different birefringent orientation which can then be assembled with like pairs to form the optical switch. Another is to group like orientation macro-cells together.

As an alternative to liquid crystals the variable polarization rotating cell may be made of a material such as PLZT crystals.

A device in accordance with the present invention is now described in detail with reference to the accompanying drawings, in which:

FIG. 1 is a side elevation of a first embodiment of the present invention with light rays indicated diagrammatically;

FIG. 2 is a side elevation of an alternative embodiment of the present invention;

FIG. 3 is a side elevation of a further alternative embodiment of the present invention;

FIG. 4 is a ray diagram illustrating birefringence; and,

FIGS. 5A and 5B are graphs illustrating the birefringent properties of calcite and sodium nitrate respectively.

An optical space switch comprises two macro-cells 1, 2 arranged in series between inputs 3, 4 and outputs 5, 6. Each macro-cell 1, 2 comprises a substantially planar liquid crystal cell 7 and a calcite crystal 8 of rectangular cross-section positioned immediately adjacent the liquid crystal cell 7.

The liquid crystal cell 7 is formed from twisted-nematic liquid crystals. The area of each liquid crystal cell 7 is divided into sub-cells S, each sub-cell being aligned with an input 3, 4 of the switch. Electrodes (omitted for clarity) are used to apply fields to the sub-cells S in order to switch the sub-cells S from a state in which they rotate incident plane polarized light by 90.degree. to a state in which they transmit plane polarized light without rotation.

The optical axes of the macro-cells 1, 2 are arranged to lie in different directions. The direction of the optical axis of the first macro-cell 1 and the dimensions of the first macro-cell 1 are chosen so that plane polarized beams from either of the inputs 3, 4 incident upon the calcite crystal 8 via the liquid crystal 7 are ordinarily or extraordinarily refracted along a path such that they impinge upon either of the sub-cells S of the liquid crystal cell 7 of the second macro-cell 2. The orientation and dimensions of the second macro-cell 2 are in turn chosen so that beams from the sub-cells S of the liquid crystal cell 7 of the second macro-cell 2 are refracted along paths such that they impinge upon one or other of the outputs 5, 6 of the switch. When, as in FIG. 1, the macro-cells are of the same shape and dimensions, the angles a.sub.1, a.sub.2 between the optical axes of the macro-cells 1 and 2 respectively and the plane surfaces of the cells are such the a.sub.1 +a.sub.2 =180.degree..

The switch of FIG. 1 functions as a 2.times.2 cross point parallel-beam switch capable of independently switching light from any one of the inputs 3, 4 to any one of the outputs 5, 6. Consider a beam from the input 3, initially polarized in the plane of the paper. (For ease of reference in the following discussion the notation Snm is used where n is the macro-cell number, m the sub-cell number). When Sll is switched off, the plane of polarization of the beam is rotated by the sub-cell through 90.degree. to be normal to the plane of the paper. Light of this polarization state is extraordinarily refracted at an angle .theta. and propagates to impinge on sub-cell S22. When this sub-cell S22 is switched off the plane of polarization is again rotated through 90.degree.. The beam is ordinarily refracted at this cell S22 and, since its angle of incidence to the crystal of the second macro-cell 2 is zero, the beam continues to propagates parallel to the z-axis, finally leaving the switch through the output 6. Similarly it may be shown that with S12 switched on, light from input 4 is switched to output 5. This is the "crossed" switch condition. The "straight-through" condition is arranged by having all the sub-cells S11, S12, S21, and S22 switched on.

The principle upon which the 2.times.2 switch is constructed may be extended to linear arays of n inputs and outputs. Non-blocking (i.e. independent) switching is then achievable through the use of 2(n-1) macro-cells. Thus, for example, an 8.times.8 switch as shown in FIG. 2 comprises fourteen macro-cells arranged in series, the first seven macro-cells having optical axes in one direction and the other seven macro-cells having optical axes in a second direction. Alternatively a switch for directing light from one of the eight inputs back to one of the other inputs may be constructed using 7 macro-cells with axes in one direction and a mirror surface 9 adjacent the seventh macro-cell, as shown in FIG. 3. The same calcite blocks present different orientations to light passing in the different directions thus light passing from the inputs and back again will pass through fourteen macro-cells, seven of each orientation.

The choice of an appropriate orientation and dimensions for the macro-cells of a switch in accordance with this invention requires knowledge of the angular offset r between the extraordinary and ordinary rays characteristic of a particular orientation of the birefringent material. Applying the method of secondary wavelets due to Huygens to a ray normally incident upon a crystal plane as shown in FIG. 4 it may be shown that the angular offset r is related to the angle .psi. characteristic of the orientation of the crystal by the equation ##EQU1## where v.perp. is the velocity of light perpendicular to the optical axis and v.parallel. is the velocity of light parallel to the optical axis. The ratio v.parallel./v.perp. is constant characteristic of a particular birefringent material and so, provided this characteristic constant is known, r may be calculated for any given value of .psi.. FIGS. 5a and 5b show r as a function of .psi. for calcite and sodium nitrate respectively.

Once the angular offset for the birefringent material of the switch is known it is possible to calculate the appropriate dimensions for the macro-cells. A typical value for the 1/e width of a beam being switched is 500 microns, this width being achievable through the use of rod lenses. To allow independent switching of the beams it is necessary that after passing through a macro-cell a displaced beam should be shifted by at least one beam diameter laterally. Referring to FIG. 5 the maximum angular offset available in calcite is found to be 6.2.degree. which occurs when the optical axis is at an angle of 48.degree. to the surface. In sodium nitrate, which may be used as an alternative to calcite, the maximum angular offset R is increased to 10.degree.. Using calcite, which has the advantage that large defect-free crystals are easier to manufacture, and allowing 800 microns beam offset at the exit, the necessary thickness of the calcite crystal for each macro-cell is 800/tan(6.2.degree.), i.e. 7.4 mm. Twisted nematic liquid crystal cells can be fabricated with a thickness of approximately 2 mm giving a total thickness for the macro-cell of approximately 10 mm. Thus the total thickness t for an 8.times.8 switch is given by ##EQU2##

It is possible to calculate from the properties of the materials used the performance of an 8.times.8 switch in terms of insertion loss, cross talk and switch speed.

Assuming that anti-reflection coatings or index-matching materials between each dielectric interface and high impedence ITO (Indium Tin Oxide) coatings in the liquid crystal cell are used, the insertion loss should be reducible to 0.2 dB per cell. The propagation loss within the calcite crystal is typically less than 0.1 dB per centimetre. Thus the total insertion loss for an 8.times.8 switch using calcite crystals as the birefringent material is typically 4 dB (neglecting losses on coupling to external components). This figure may be further improved by the use of stressed polymer sheets in place of the birefringent crystals of calcite, such sheets being capable of offering both high transparency and high birefringence.

The cross-talk within the switch will largely be determined by the efficiency of the twisted nematic liquid crystal cells. Gooch and Parry have shown in a paper published in Elect Lett 10,1, page 2, 1974 that the extinction ratio between parallel polarizers is found to be better than 1% and at periodic values of wavelength the extinction is found to be indistinguishable from zero. The distribution of cross-talk in the switch will be strongly affected by the particular switching state of the matrix of elements making up the switch. Total cross-talk is typically less than 30 dB.

The dominant factor in limiting the speed of the switch is the nematic switch-off time, which is determined by bulk fluid flow considerations within the liquid crystal. Using thin twisted nematic liquid crystal cells with commercially available nematic mixtures a switch-off period of 30 ms is obtainable. A further improvement of a few tens of per cent may be attained by doping the nematic with a cholesteric. Further improvements in the performance of standard liquid crystal cells may be obtained by suitable surface treatments and the use of appropriate nematic mixtures. However to gain a significant increase in speed with a switch-off period of 30 .mu.s or less it is desirable to use smectic C ("ferro-electric") liquid crystals.

The optical switch according to the present invention can switch optical images and thus has application to the switching of spatially multiplexed optical buses for example.