GB2229288A - Optical device - Google Patents

Abstract

An optical device includes an optically active medium such as a cholesteric liquid crystal 10 which undergoes a reversal of the sense of optical rotation at a given temperature. The temperature of the medium is maintained adjacent said given temperature so that a variation in the local temperature causes a change in the amount and/or sense of the optical rotation. Such variation may be brought about by an incident beam of radiation. <IMAGE>

Description

"Optical Device" This invention relates to optical devices and in particular, but not exclusively, to optical bistable switch devices which use a cholesteric liquid crystal.

A typical existing optical bistable switch employs the Fabry-Perot interferometer or etalon architecture in which a cavity bounded by two semitransparent substrates is filled with a non-linear material. The non-linearity can be thermal, excitonic, magnetic, electric etc.

Broadly stated, according to one aspect of this invention, there is provided an optical device comprising an optically active medium which undergoes a reversal of the sense of optical rotation at a given temperature in use, whereby a variation in the local temperature of the medium causes a change in the extent and/or sense of the optical rotation.

A preferred form of optically active medium includes a cholesteric liquid crystal but the invention extends to devices which includè other optically active materials.

The invention will now be described by way of non-limiting example, reference being made to the accompanying drawings, in which: Figure 1 is à schematic illustration of a first example of optical device addressed by plane polarised light; Figure 2 is a schematic illustration of a second example of optical device addressed by righthand and left-hand circularly polarised inputs; Figure 3 is a schematic illustration of a third example of optical device addressed by a single circularly polarised input; Figures 4a and 4b illustrate possible constructions for a Fabry-Perot etalon and a simple transparence respectively, and Figure 5 is a schematic graph illustrating the variation of rotatory power with temperature for a typical cholesteric liquid crystal for use in the examples illustrated in Figures 1 to 4.

A cholesteric liquid crystal (CLC) is one whose molecules arrange themselves into helical chains. An interaction between the dLc and light incident parallel to the helical axis takes place when the following equation is satisfied: \= nP where is the wavelength of the incident light, n is the average refractive index of the CLC and P is the pitch of the helix. The type of interaction of the light will depend on its state of polarisation and there is no noticeable effect with unpolarised light.

If plane polarised, the light is resolved by the helical structure into righthand and lefthand circular components H1 and H2 as shown in Figure 1. Selective, diffuse reflection will take place upon the polarisation state (H2) of the same handedness as the helix of the CLC, but the opposite polarisation state (H1) is transmitted.

In each of the examples illustrated in the Figures, the cavity between the walls of the substrate is filled with a CLC mixture of a special form in which the optical rotary power changes with temperature and, at a given "helix reversal temperature" THI there is an inversion of the optical rotatory power, i.e. the pitch of the helix switches handedness. The variation of rotatory power with temperature relative to the helix reversal temperature TH is illustrated in Figure 5.

Referring to Figure 1, an optical device is made up by aligning the CLC medium 10, a quarter wave retardation plate 12, and an analyser 14.

The arrangement illustrated in Figure 1 functions in the following manner. The temperature of the CLC medium 10 fs maintained at a level close to but just below the helix reversal temperature of the CLC when plane polarised light of a relatively low intensity I1 is at normal distance to it. As mentioned above, the beam will be resolved into circularly polarised components H1 and H2. In this example, H1 is transmitted and H2 is reflected. The transmitted beam is passed through the quarter wave retardation plate 12 and emerges as linearly polarised light at a polarisation angle 61 to the vertical.When the intensity of the incident beam is increased by a predetermined amount to I2, the temperature of the CLC medium 10 is increased taking it beyond the helix reversal temperature TH SO reversing the handedness of the pitch of the CLC medium and inverting its optical power. The reflected beam is now Hn nd H2 is transmitted to be passed through the quarter wave retardation plate so that it becomes plane polarised at angle e2 to the vertical. The angle between 61 and a2 is 900, so that, by suitable adjustment, the analyser 14 will block one emergent beam and transmit the other.

Reducing the intensity of the input beam back to I1 will have the reverse effect, so that H1 is again transmitted and H2 reflected. Thus, changing the input intensity between the two values I1 and I2 causes the plane of polarisation of the plane polarised light emerging from the quarter wave retardation plate 12 to be rotated through 900. The output from the analyser 14 may be detected by a detector (not shown) incorporating suitable thresholding, if necessary.

Figures 2 and 3 show examples of optical device comprising a cell of CLC medium 10 in which the contrast between switching states is improved by using either or both left-handed (LH) or right-handed (RH) circularly polarised light as the inputs. If the handedness of the input matches that of the helix of the CLC medium 10, it will be reflected. If the intensity of the input beam is increased sufficiently to raise the temperature of the CLC medium above the helix reversal temperature THI the input previously reflected by the CtC medium will now be transmitted. As in the example of Figure 1, detection would be by means of a quarter wave retardation plate and analyser or other suitable detection apparatus.

There may still be some residual transmission in the lower intensity state of the above device because the optical path length of the system is only finite, but detector levels could be thresholded to account for this.

The efficiency of the device in Figures 2 and 3 could be introduced further by introducing an absorbing dye into the CLC medium in the cell. Antireflection coatings may be provided on the outer and inner surfaces of the substrates to cut down multiple reflections and subsequent scatter. The backplate of the substrate may be replaced by the quarter wave retardation plate.

Referring now to Figures 4a and 4b, there are illustrated two possible detailed designs of optical device. In Figure 4a, the CLC medium is confined between two alignment layers 16,17 which cause homogeneous alignment of the liquid crystal molecules at each surface. A pair of semi-transparent surfaces 18,19 are provided to either side of the alignment areas and the whole cell is enclosed by front and rear transparent substrates 20,21. The rear transparent substrates 21 may be replaced by a quarter wave retardation plate or its equivalent.

Referring now to Figure 4b, there is shown a simple transparent cell similar to the Fabry-Perot etalon except that the semi-transparent mirror surfaces 18 and 19 are removed. Again, the rear substrate 21 may be replaced by a quarter wave retardation plate.

As mentioned above, Figure 5 illustrates the dramatic reversal of the rotary power of the CLC material around the helix reversal temperature. In the examples given above, the devices have operated on the basis of the rotatory power of the CLC flipping its rotary sense to change the state of the optical device.

The examples given may operate as simple on/off switches or, by rotating the analyser by 900, as an inverter. By having two inputs incident on the devices, they may be operated in "transphasor" mode; i.e. by modulating one input beam by changing the temperature and hence the optical rotary power of the CtC medium with a second.

In such an arrangement, the second, gate, beam need not be polarised. In this way, the device is capable of performing any of the gate operations (AND,OR,XAND,NOR, etc) by using two plane polarised or circularly polarised inputs.

As will be seen from Figure 5, by operating the devices either just above or just below the helix reversal temperature TH they may exhibit grey levels.

Also, by having two circularly polarised beams of different wavelengths incident on the device, transmission of one or the other, or both could be arranged by increasing the intensity of either beam to change the pitch of the helix of the CLC medium. Thus one or other of the beams would be reflected in accordance with the equation = nP given above.

The devices described above are highly parallel and thus have great advantages over existing electronic switches. The local thermal heating of the CLC medium can be confined to a small spot due to the low thermal conductivity of the CLC medium so that a single device would have the capacity to process many independent inputs over its working area. It should be noted however that, where large, array-type devices are constructed, care should be taken to ensure that the substrates have sufficient flatness over the area of the array. The device possesses high cascadability and, because the "HIGH" output state of the device is almost completely transparent, it will be possible to operate a series of cells with only a low insertion loss. In a typical cell, the helix reversal temperature is about 42do, and the thickness of the cell is 10 am or greater.

The invention also extends to any novel combination of features substantially as disclosed herein.

Claims (10)

1. An optical device comprising an optically active medium which undergoes a reversal of the sense of optical rotation at a given temperature, said medium being at or near said given temperature in use, whereby a variation in the local temperature of the medium causes a change in the extent and/or sense of the optical rotation.

2. An optical device according to Claim 1, wherein the temperature of said medium in use is adjusted by means of an incident radiation beam.

3. An optical device according to Claim 2., wherein the temperature of said medium is caused to rise above said given tempesature when the intensity of said incident radiation beam exceeds a given level, whereby said optical device operates as a bistable switch.

4. An optical device according to any preceding claim, wherein said optically active material includes a cholesteric material.

5 An optical device according to any preceding claim, wherein said optically active medium is a cholesteric liquid crystal material contained between two substantially transparent substrate means each including an alignment layer for providing homogeneous planar adjustment of the liquid crystal material.

6. An optical device according to any of Claims 1 to 4, wherein said optically active material comprises a free-standing film or slab.

7. An optical device according to any of Claims 1 to 4, wherein said optically active material is contained between two partially transparent substrate means together defining a Fabry-Perot cavity.

8. An optical device according to any of the preceding claims, which includes a circularly or elliptically polarised input beam of wavelength selected with respect to the pitch of the optically active medium such that Bragg reflection may occur when the optical rotation applied by said medium is in the appropriate sense.

9. An optical device according to any preceding claims, wherein the thickness of the medium is at least 10

10. An optical device substantially as hereinbefore described with reference to,and as illustrated in, any of the accompanying drawings.