GB2223600A

GB2223600A - An optical switch

Info

Publication number: GB2223600A
Application number: GB8818978A
Authority: GB
Inventors: Peter Healey
Original assignee: British Telecommunications PLC
Current assignee: British Telecommunications PLC
Priority date: 1988-08-10
Filing date: 1988-08-10
Publication date: 1990-04-11
Also published as: GB8818978D0

Abstract

An optical space switch provides a single stage device formed by a three-state polarisation rotator (1) and a polarisation sensitive deflector (2), such as a birefringent crystal. The polarisation rotator (1) and the deflector (2) can be arranged so that there is in addition to the two conventional states, i.e. the straight through and deflected states, a third state in which input light is divided into two components one of which is deflected and the other transmitted straight through. This new, distributed state considerably enhances the possible range of applications of the switch. A larger optical switch may be formed comprising a plurality of macro-cells. Each macro-cell includes a polarisation rotator (1) and a polarisation sensitive defector (2). The polarisation rotator (1) of at least one of the macro-cells is a three state polarisation rotator. <IMAGE>

Description

AN OPTICAL SWITCH The present invention relates to optical space switches. It is known to construct such switches from a combination of variable polarisation cells and polarisation sensitive deflectors which cause incident rays to follow different paths according to their polarisation states. The paper by W. Kulcke et al at pages 64 to 67 of the IBM Journal January 1964 discloses one example of such a switch. In this switch the variable polariser is a KDP Pockels cell and a birefringent calcite crystal functions as a polarisation sensitive deflector. As with other known switches the variable polarisation cell in this case is used simply as a two-state ON-OFF device. When the cell is OFF it leaves the polarisation state of the linearly polarised incident ray unchanged. When the cell is ON it rotates the plane of polarisation of the incident ray by 900.

Rays therefore leave the variable polarisation cell in one of two linear polarisation states such that they either pass straight through the calcite crystal or are deflected according to their polarisation state. A single stage comprising one KDP cell and one calcite crystal can therefore switch an input between one of two outputs. A number n of similar stages arranged in series n can switch one input to any one cf 2n outputs. Thus to achieve a desired number of outputs the number of stages n is increased to an appropriate value, up to practical limits set by such factors as insertion losses.

According to the present invention, an optical switch comprises a three-state polarisation rotator and a polarisation sensitive deflector adjacent the polarisation rotator, the polarisation sensitie deflector being arranged to direct light fre the polarisation rotator along one or other o two paths, or to distribute light between the two paths in accordance with the polarisation state of the light.

Preferably in a first state the three-state polarisation rotator transmits light with its plane of polarisation unchanged, in a second state the three-state polarisation rotator rotates the plane of polarisation of incident light through a first fixed angle and in a third state the three-state polarisation rotator rotates the plane of polarisation of incident light through a second fixed angle.

Preferably the three-state polarisation rotator is oriented with respect to the optical axis of the polarisation sensitive deflector and the plane of polarisation of light at the input of the switch so that when the three-state polarisation rotator is in the first state substantially all of the light is transmitted through one of the two paths when the three-state polarisation rotator is in the second state substantially all the light is transmitted through the other of the two paths, and when the three-state polarisation rctator is in the third state the light is distributed substantially equally between the two paths.

The present invention provides an optical switch with an increased number of states. A single stage device formed by one three-state polarisation rotator and a birefrinaent crystal can be arranged so that there is in addition to the two conventional states, i.e. the straight-throuah and deflected states, a third state in which input light is divided into two components one of which is elected and the other transmitted straight-through. This new, distributed state considerably enhances the possible range of applications of a switch in accordance with the present invention.For example, an optical distributor including such a switch can be dynamically reconfigured to ensure maximum power efficiency, the switch bering used selectively te enable different branches of the distributor according to the number of inputs and outputs which have to be connected at any one time. When an n-stage switch is used the total number of states scales with 3n.

Preferably the three-state polarisation rotator is a smectic C phase ferro-electric liquid crystal cell.

The preferred embodiment of the present invention uses a ferro-electric liquid crystal cell arranged to function as a 3-state polariser in place of the simple ON-OFF polarisers known in the prior art. Such a cell may be arranged so that in response to a bipolar bias voltage applied to electrodes on the surface of the cell its optical axis switches between directions with an angular separation of 45". If the cell is oriented so that one of the voltage selected optical axes is parallel to the plane of polarisation of incident light then the light will be rotated by 0 or 900 according to the polarity of the applied bias. A suitably oriented polarisation sensitive deflector following the cell will then direct the light via one or other of a pair of paths corresponding to the first and second states of the switch.If zero bias is applied to the cell then the plane of polarisation of the light is rotated by just 450 so that it impinges on the polarisation sensitive deflector at an angle such that it is divided into orthogonal polarisation components which are split between the twc paths. This provides the third, distributed, state of the switch.

Preferably the optical switch comprises n macro-cells where n is an integer greater than 1, each macro-cell including a polarisation rotator and a polarisation sensitive deflector, the polarisation rotator of at least one of the macro-cells being a three-state polarisation rotator.

A switch in accordance with the present invention is now described in detail with reference to the accompanying drawings in which: Figure 1 is a side elevation of a first example; Figure 2 is a diagram showing polarisation states at successive interfaces of the first example; Figure 3 is a side elevation of an alternative example; Figure 4 is a diagram showing polarisation states at successive interfaces of the alternative example; and, Figure 5 is a perspective view of a connection network incorporating the switch of Figure 1.

An optical switch comprises a number of macro-cells MC arranged in series between an input side and an output side. Each macro-cell MC includes a polarisation rotator 1 and a polarisation sensitive deflector 2.

The polarisation rctators are three-state devices formed from smectic C phase ferro-electric liquid crystal material. A control voltage is applied to each cell via surface electrodes (omitted for clarity). A suitable liquid crystal cell is described in the paper by K. M.

Johnson et al at pages 385 to 391 of Optical Engineering, May 1987, Volume 26 No. 5. When a bipolar bias voltage is applied to such a cell it switches between two optic axis orientations separated by an angle of 2p, where p is the tilt angle. Typically p is equal to 22F degrees. In the preferred embodiment the liquid crystal cell is oriented so that one of the voltage-selected optical axes is parallel to the plane of polarisation of the incident light. In this condition, light passes through the liquid crystal cell with its plane of polarisation unaltered. When a bias voltage of opposite polarity is applied the optic axis of the cell then switches so that it is at an angle of 45C from the plane of polarisation of the incident light.As a result linearly polarised light incident on the cell has its plane of polarisation rotated by 900, i.e. 2 x 45".

The polarisation sensitive deflector following the liquid crystal cell is typically formed from calcite.

The optic axis of the deflector is oriented so that light polarised at 0 passes straight through the deflector to a first output and light polarised at 900 is deflected to leave the cell at a second spatially separate output.

In addition to the two switched states described above there is a third distributed state in which no bias voltage is applied to the liquid crystal cell. The optic axis of the cell then has a tilt angle of zero. Since the cell as a whole is oriented with one of the voltage selected axes parallel to the plane of polarisation of the incident light there is an angle of 22F between the plane of polarisation of the light and the optic axis of the cell in its unbiased state. The light is therefore rotated through a total angle of 2 x 22, i.e. 450. It is therefore divided by the polarisation sensitive deflector into two substantially equal orthogonal polarisation components which are distributed between the two outputs.

Figure 2 shows the polarisation states of light at successive interfaces a b of the switch. Light enters each macro cell MC plane polarise along the x or y axis.

Although for clarity only one input beam, polarised along the x-axis, has been shown, in practice the number of beams at each successive stage may double and each beam may be polarised along the x-axis or the y-axis. The thickness of the calcite crystal cell of successive macro cells double to give a corresponding doubling of the distance through which the outputs of the cell are displaced. It is acted upon by the liquid crystal cell 1 in the manner described above so that at the interface b with the polarisation sensitive deflector 2 the plane of polarisation lies at 00, 450 or 900 to the x-axis.

Although in this example the liquid crystal cell rotates the plane of polarisation in the clockwise sense it is also possible to configure the switch so that the plane of polarisation is rotated anti-clockwise.

In an alternative embodiment the liquid crystal cell 1 is arranged to rotate the plane of polarisation of the linearly polarised incident light by i450 in response to the bipolar bias voltage and to pass the light with its plane of polarisation unaltered when the cell is in its unbiased state. In this embodiment light enters the ferro-electric liquid crystal at a given stage at 450 to the polarisation axis of the polarisation sensitive deflector. Then if no bias voltage is applied to the ferro-electric liquid crystal the light passes with its plane of polarisation unaltered at 450 through to the calcite crystal where it is split into its two orthogonal polarisation components at 0 and 900 polarisation. One of the two components passes through the crystal undeviated while the other is deflected.The light input to the stage is therefore distributed between the two outputs.

If a control voltage of + V is applied to the liquid crystal cell, a typical value for V being in the range 16 to 30 volts, the light incident on the cell has its plane of polarisation shifted by + 450. The light therefore impinges on the calcite crystal at 00 or 900 to the polarisation axis of the crystal and so either all the light passes straight through the crystal or all the light is deflected. In this manner all the light entering the stage is directed to one or other of the outputs.

This example requires a half-wave retarder, which may be a standard quartz or mica half-wave plate, to rotate the plane of polarisatlon of the light so that it enters the ferro-electric liquid crystal at an appropriate angle. A half wave retarder rotates linearly polarised light through twice the angle at which the light meets its axis.In the present embodiment each half wave retarder is set at 220 to the 0 axes. 0 polarised light impinging on the macro-cell MC from a preceding cell or from the input of the switch is rotated to 450, that is through twice the angle of incidence and similarly light which is initially polarised at 900 is rotated through 1350, that is 2x 67h . The light at 450 and 1350 can then be arranged to leave the same output of the macro-cell by applying control voltages of opposite polarities to the variable polarisation cell as light with the respective polarisation states passes through it.

If the switch is to be used in an optical system where its input is already plane polarised at 450 then the half wave retarder is omitted from the first input-side macro-cell.

Figure 4 shows the state of polarisation of an input signal at successive interfaces (a,b,c,) of a macro cell MC of the alternative embodiment. The input beam is polarised along the x or y axis and the plane of polarisation subsequently rotated by the liquid crystal cell 1 as shown. Although for clarity only two input beams have been shown, in practice the number o beams at each successive stage may double. The thickness of the calcite crystal cell of successive macro cells double to give a corresponding doubling of the distance through which the cuputs cf the cell are displaced.

Although in the embodiments discussed the calcite cells are arranged in order of increasing thickness from the input side to the output side of the switch other arrangements may equally be used. Equally it is possible to use alternative forms of polarisation sensitive deflectors, such as suitably aligned beamsplitter cubes and right angle prism combinations.

A switch in accordance with the present invention may be used in a variety of applications. Where appropriate, macro-cells incorporating conventional two-state variable polarisers may be used in conjunction with three-state macro cells as described as above to give a desired switching/distribution characteristic.

Such a switch may be used as a beam splitter/distributor for distributing a single input between a number of outputs in a 1 to N configuration, in a N to 1 selector arranged to select a single output from N inputs such as that described in our co-pending U.K application number 8804202 or in an N to N switch such as that disclosed in our pending U.K. application number 8713043. The incorporation in such distributors and selectors of a switch or switches in accordance with the present invention enables the configuration of the distributor or selector to be varied as necessary to provide optimum optical power efficiency. For example, if each stage of a 1 to N distributor is formed from the three-state switches of the present invention then an optical sianal m can be shared dynamically amongst 2m locations, where m is an integer in the range 0 to n. Thus if at a given instant the signal needs to be distributed between only a few outputs then a suitably low value for m may be selected so that the power at each output is maximised.

Similarly when used in an N to 1 selector the switch m of the present invention enables up to 2 signals to be selected at a given output port at one instant. This facility to control the configuration of the switch is valuable in such applications as the dynamic multiplexing of telecommunications traffic to transmit several inputs along a single common path.

Figure 5 shows an 8 x 8 connection network comprising an array D of eight 1 x 8 distributors on the input side cascaded with an array S of eight 8 x 1 selectors on the output side. In this example each of the selectors and distributors is formed entirely from three-state switching stages.

Table 1 shows one setting of the connection network.

The lefthand side of the table shows the mapping of inputs to outputs resulting from this setting. On the right hand side in each row the states for switches a, b and c corresponding to a given input and switches d, e, f corresponding to a given output are shown. In this table "-" indicates the state in which the switch transmits light with its plane of polarisation unchanged, "+" indicates the state in which the switch rotates the plane of polarisation through 900 and "0" indicates the state in which the plane of polarisation of the light is rotated through 450, that is the distributed state.

In Figure 5 ray paths for input ports 2 and 8 are shown. These paths pass through stages all of which occupy the conventional "+" or "-" states. The light in this case is therefore transmitted from the selected input to the selected output without being split or combined with other paths. At the same time however other inputs are switched using the distributed "0" state. Consider, for example, input port 3. As shown on the left hand side of the table the input at port 3 is simultaneously accessed by output ports 4 and 5. This is achieved by splitting the input light at the corresponding switch a which is in the distributed state and then using conventional "+" and "-" states in the succeeding switches to direct the two resulting rays to the desired outputs.This type of distribution function is useful for one to many type connections such as occur in television distribution network.

Similarly it is possible to use distributed states in the selectors on the output side of the connection network. Output port 6, for example, is connected to both input port 4 and input port 5. Switch d corresponding to output port 6 is in the distributed "0" state and so combines light from two output paths from the input side distributors. This multiple selection distribution function is particularly useful in signal multiplexing operations such as combining suitably modulated and encoded signals onto a common transmission highway.

Many other settings of the switch are possible and although different settings have differing efficiencies these will all in general be superior to the efficiency achieved with an equivalent simple n-way distributor such as that described in our co-pending U.K. application 8804202.

Claims

1. An optical switch comprising a three-state polarisation rotator and a polarisation sensitive deflector adjacent the polarisation rotator, the polarisation sensitive deflector being arranged to direct light from the polarisation rotator along one or other of two paths, or to distribute light between the two paths, in accordance with the polarisation state of the light.

2. A switch according to claim 1, in which the three-state pclarisation rotator is arranged so that in a first state the three-state polarisation rotator transmits light with its plane of polarisation unchanged, in a second state the three-state polarisation rotator rotates the plane of polarisation of incident light through a first fixed angle and in a third state the three-state polarisation rotator rotates the plane of polarisation of incident light through a second fixed angle.

3. A switch according to claim 2, in which the three-state polarisation rotator is oriented with respect to the optical axis of the polarisation sensitive deflector and the plane of polarisation of light at the input of the switch so that when the three-state polarisation rotator is in the first state substantially all of the light is transmitted through one of the two paths when the three-state polarisation rotator is in the second state substantially all the light is transmitted through the other of the two paths, and when the three-state polarisation rotator is in the third state the light is distributed substantially equally between the two paths.

4. A switch according to any one of the preceding claims, in which the three-state polarisation rotator is a smectic C phase ferro-electric liquid crystal cell.

5. A switch according to any one of the preceding claims, comprising n macro-cells where n is an integer greater than 1, each macro-cell including a polarisation rotator and a polarisation sensitive deflector, the polarisation rotator of at least one of the macro-cells being a three-state polarisation rotator.

6. A switch substantially as described with reference to Figures 1 to 4 of the accompanying drawings.

7. A 1 x N distributor, where N = 2k, k an integer greater than 1, including a switch according to any one of the preceding claims.

8. An N x 1 selector, where N = =, k an integer greater than 1, including a switch according to any one of claims 1 to 7.

9. An N x N connection network comprising a distributor array and a selector array arranged in series between an input side and an output side, the distributor array comprising N distributors according to claim 7 arranged in parallel in a first direction, the selector array comprising N selectors according to claim 8 arranged in parallel in a second direction perpendicular to the first direction.

10. A connection network substantially as described with reference to Figure 5 of the accompanying drawings.