JPH02308130A - Optical apparatus - Google Patents

Abstract

PURPOSE: To make it possible to bring about a change of the degree and direction of polarized light by using an optically active medium to be subjected to the inversion of the direction of the polarized light by a given temp. CONSTITUTION: The optically active medium to be subjected to the inversion of the direction of the polarized light by the given temp. is used. The polarized light is separated to circularly polarized components H1 and H2 when the medium is maintained at the level immediate below the temp. near the spiral inversion temp. of CLC(cholesteric liquid crystals). H1 is transmitted and H2 is reflected. The transmitted beam passes a quarter-wave plate 12 and is emitted as the light linearly polarized at a polarization angle θ1 with the perpendicular. When the intensity of the incident beam increases, the temp. of the CLC medium 10 increases until the temp. exceeds the spiral inversion temp. TH. The beam reflected at this time is H1 and H2 passes the quarter-wave plate and is plane polarized at an angle θ2 with the perpendicular. As a result, an analyzer 14 is capable of blocking the one emerging beam and allowing the transmission of the other if a suitable adjustment is executed.

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to, but is not limited to, an optical device;
This invention relates to optical bistable switches using cholesteric liquid crystals.

A typical existing optical bistable switch is a Fabry
A Perot interferometer or etalon structure is employed in which a cavity bounded by two translucent substrates is filled with a nonlinear material. Nonlinearities can be thermal, excitonic, magnetic, electrical, etc.

Broadly speaking, one aspect of the invention includes an optically active medium that undergoes a reversal of direction of optical rotation at a given temperature;
An optical device is provided in which, in use, a change in the local temperature of a medium causes a change in the degree and/or orientation of optical rotation.

Although the preferred form of optically active medium includes cholesteric liquid crystals, the invention extends to devices incorporating other optically active materials.

The invention will now be described by way of non-limiting example with reference to the drawings.

Cholesteric liquid crystals (CLCs) are those whose molecules are arranged in helical chains. λ is the wavelength of the incident light, h
When P is the average refractive index of the CLC and P is the pitch of the helix, an interaction occurs between the incident light and the incident light. The form of light interaction depends on its state of polarization; with unpolarized light there is no significant effect. When plane polarized, the light has a clockwise component H1, as shown in FIG.
It is decomposed into the same component H2 with a counterclockwise rotation due to the helical structure. Selective diffuse reflection occurs with a polarization state (H2) of the same rotation (clockwise or counterclockwise) as the CLC helix, but the opposite polarization state (Hl) is transmitted.

In each of the illustrated examples, the cavities between the walls of the substrate are formed such that the optical power varies with temperature, and at a given "helix reversal temperature J TH , the optical power, i.e., the reversal of the pitch around the helical switch. A special form of C such that
Fill with LC mixture.

The change in optical power with temperature with respect to the helical inversion temperature TH is illustrated in FIG.

Referring to FIG. 1, the optical apparatus includes a CLC medium 10.1.
It is created by a straight array of /4 wave plate 12 and analyzer 14.

The arrangement illustrated in FIG. 1 operates in the following manner. The temperature of the CLC medium 10 is such that when plane-polarized light of relatively low intensity 11 is at a normal distance to it,
It is maintained at a level close to, but just below, the helical inversion temperature of C. As mentioned above, the beam is split into circularly polarized components H and H3. In this example, H+ is transmitted and H! is reflected. The transmitted beam is l
The light passes through the /4 wavelength plate 12 and exits as light that is linearly polarized at a polarization angle θ1 with respect to the vertical. When the intensity of the incident beam increases by a predetermined amount to I2, the CLC medium's degree increases until it exceeds the helical reversal temperature TH, so that the rotation of the pitch of the CLC medium is reversed. , its optical power is reversed. Now the reflected beam is H+
, H3 is transmitted through the 1/4 wave plate so that it becomes plane polarized at an angle θ'2 with respect to the vertical. The angle between θ1 and θ2 is 90', so that by appropriate adjustment the analyzer 14 blocks one emerging beam and transmits the other.

(2) Reducing the intensity of the input beam back to , has the opposite effect, so that again H+ is transmitted and H2 is reflected.

Thus, the two values 11 and 1. The change in the input intensity during this period causes the plane of polarization of the plane-polarized light exiting the quarter-wave plate 12 to rotate by 90°. The output from the analyzer 14 is passed to a detector (if necessary) incorporating suitable thresholding means.
(not shown).

2 and 3 show an example of an optical device with a cell of CLC medium 10, in which the contrast between the switching states is circularly counterclockwise (LH) or clockwise (RH) as an input. Improvements can be made by using either or both polarized light. If the rotation of the input matches that of the helix of the CLC medium 10, it will be reflected. The intensity of the input beam has increased sufficiently so that
When the temperature of the CLC medium increases above the helical reversal temperature TH, the input that was previously reflected by the CLC medium is transmitted. As in the example of FIG.
This may be done by a wave plate and analyzer or other suitable detection device. Since the optical path length of the system is only finite, there will still be some residual transmission in the low-intensity state of the device described above.
Detector levels could be thresholded to take this into account.

The efficiency of the apparatus in Figures 2 and 3 is increased by introducing an absorbing dye into the CLC medium in the cell.
More could be introduced. To reduce multiple reflections and therefore scattering, antireflection coatings can be provided on the interior and exterior surfaces of the substrate. The back plate of the substrate can be replaced with a quarter wave plate.

Referring to Figures 4a and 4b, two possible detailed designs of the optical device are illustrated. In FIG. 4a, the CLC medium is confined between two straight-aligned layers 16, 17, straight-aligned in a helical directional member on each side. A pair of translucent surfaces 18,19 can be replaced with 174 wave plates or the like on either side of the straight placement area.

Referring to FIG. 4b, there is illustrated a simple transparent cell similar to the Fabry-Perot etalon, except that the translucent mirror surfaces 18 and 19 have been removed.

As mentioned above, FIG. 5 illustrates the dramatic reversal of the optical power of a CLC material around the helical reversal temperature.

In the example given above, the device operates on the optical rotation power of the CLC, which moves the direction of optical rotation to change the state of the optical device. The example given operates as a simple on-off switch or as an inverter by rotating the analyzer by 906. By having two inputs incident on the device, this can be done in "Transphasor" mode, i.e. conditioning one input beam by changing the temperature and then optically rotating the CLC medium by the second input beam. In such a deployment, the second gate beam need not be polarized.

According to this method, the device can perform any gating (AND, OR, XAND, NOR, etc.) by using two plane-polarized or circularly polarized inputs.

As can be seen in FIG. 5, by operating the device just above or just below the helical reversal temperature TH, this has a gray level. Also, by having two circularly polarized beams of different wavelengths incident on the device,
Transmission of one or the other or both can be arranged to change the pitch of the helix of the CLC medium by changing the intensity of either beam. Thus, one or the other of the beams will be reflected according to the equation λ=F+P given above.

The device described above is highly parallel and thus has significant advantages over existing electronic switches. Localized heating of the CLC medium is difficult due to the low thermal conductivity of the CLC medium.
Because it can be confined to a small location, a single device has the ability to process many independent inputs across its work area.

It should be noted, however, that when large array-type devices are constructed, care should be taken to ensure that the substrate has sufficient flatness across the area of the array. Since the device has a high degree of cascading capability and the "high" power state of the device is almost completely transparent, it is possible to operate cells in series with only low insertion losses. In a typical cell, the helical inversion temperature is about 42° and the cell thickness is 10 μm or more.

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

[Brief explanation of drawings]

FIG. 1 is a schematic illustration of a first example of an optical device for receiving plane-polarized light. FIG. 2 is a schematic illustration of a second example of an optical device receiving clockwise and counterclockwise circularly polarized inputs. FIG. 3 is a schematic illustration of a third example of an optical device receiving a single circularly polarized input. Figures 4a and 4b illustrate possible configurations for the Fabry-Bello etalon and simple transparency, respectively. FIG. 5 is a diagrammatic graph illustrating the variation of optical power with rotation temperature for a typical cholesteric liquid crystal for use in the examples illustrated in FIGS. 1-4. In the drawing, lO is the CLC medium, 12 is the quarter-wave plate, 14 is the analyzer, 16.17 is the straight layer, 18.19
indicates a translucent surface and 20.21 indicates a transparent substrate. Increasing intensity of LH or RH
Degree (T H H Increase in soil input strength 2) τH H

Claims (1)

Claims: 1. An optically active medium that undergoes a reversal of the direction of optical rotation at a given temperature, said medium being at or near said given temperature in use. , whereby a change in the local temperature of the medium causes a change in the degree and/or orientation of optical rotation. 2. Optical device according to claim 1, wherein the temperature of the medium in use is adjusted by an incident radiation beam. 3. When the intensity of said incident radiation beam exceeds a given level, the temperature of said medium is raised above said given temperature, thereby causing said optical device to
3. Optical device according to claim 2, which operates as a bistable switch. 4. Optical device according to any one of claims 1 to 3, wherein the optically active medium comprises a cholesteric material. 5. said optically active medium is a cholesteric liquid crystal material sandwiched between two substantially transparent substrate means, each of said substrate means providing homogeneous planar control of the liquid crystal material; Claim 1 comprising a straight alignment layer for
5. The optical device according to any one of 4 to 4. 6. Optical device according to any one of claims 1 to 4, wherein said optically active medium consists of a free-standing film or slab. 7. An optical system according to any one of claims 1 to 4, wherein said optically active medium is sandwiched between two partially transparent substrate means which together form a Fabry-Perot cavity. Device. 8. A circularly or elliptically polarized input beam of a wavelength selected with respect to the pitch of the optically active medium such that when the optical rotation exerted by said medium is in the appropriate orientation, Black reflection can occur. The optical device according to any one of claims 1 to 7, comprising: 9. The optical device according to any one of claims 1 to 8, wherein the thickness of the medium is at least 10 μm.