Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Patent
1995-11-13
1998-02-17
Sikes, William L.
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
349172, 349198, 359260, G02F 11333, G02F 113, G02F 103, G02F 107
Patent
active
057196540
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to optical devices and particularly to a liquid crystal shutter for operation in the infrared or other long light wavelength region.
Liquid crystal (LC) shutters operating in the visible region of the light spectrum are well known, and there are several alternative types, which have different characteristics. The simplest is the common twisted nematic shutter which has a switching time of about 30 msec. Fast response using a nematic liquid crystal material can be obtained from the II-cell (British Patent Application No. 2184860A) which achieves a switching time of about 1 m sec. However, the fastest commonly available liquid crystal shutter relies on ferroelectric liquid crystal technology and switches in less than 100 .mu.sec. In all cases, a critical parameter of the design of the device is the product of birefringence (.DELTA.n).times.thickness (d) and in particular its ratio with the wavelength of operation. All of the above devices ideally require and to equal .sup..lambda. /2 where .lambda. is the light wavelength of interest, normally taken as 550 nm for operation in the visible light spectrum.
It will therefore be apparent that there is considerable difficulty in implementing a fast shutter to operate at long wavelengths, such as 10.6 .mu.m, the most common wavelength of the CO.sub.2 laser. This is because the product .DELTA.nd is now required to be about 20 times larger than is required for visible light operation. Since the birefringence An varies only slightly, and if anything it decreases at longer wavelengths, the thickness of the liquid crystal layer must be increased. However, in nematic devices, the response time varies as the square of the LC layer thickness, whereas ferroelectric LC devices (FLCDs) cease to function correctly if the layer becomes too thick. Hence, no fast-acting shutter can be implemented using conventional LC shutter technology which can be used at long wavelengths.
Shutters can be classified into two types, namely those which operate over a broad range of wavelengths, and those which are effective over only a restricted range of wavelengths. All of the devices described above are broadband operating; although they are most efficient at the design wavelength, .lambda..sub.0, they have useful performance for a range, say, .lambda..sub.0 .+-..lambda..sub.0 /4. However, in some cases such a broadband capability is not needed, for example when modulating a laser line. Then the shutter needs to be effective only at a single wavelength, or over a very narrow band of wavelengths. Such a device would, for example, be of great value in controlling the output of the CO.sub.2 laser mentioned above, which is used for a variety of marking, cutting and welding operations in fields ranging from surgery to ceramic circuit board manufacture.
It is an object of the present invention to provide a fast-acting LC shutter suitable for use with lasers or other narrow-band light sources.
According to the invention there is provided an electrically-controllable liquid crystal filter device comprising two partially-reflecting substrate structures mounted substantially parallel to each other with a space therebetween to form a Fabry-Perot filter; a layer of ferroelectric liquid crystal material disposed in said space, said liquid crystal material being switchable, by application of an electric field thereto, between different refractive index values whereby the device is switchable between transmissive and reflective states for light of a particular wavelength, characterised in that, the spacing of the substrate structures is such that the wavelength of light to be transmitted is 10.6 .mu.m, and in that the Fabry-Perot filter operates in a low order state (as hereinbefore defined).
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which
FIG. 1 is a schematic cross-sectional view of a liquid crystal shutter device in accordance with the invent
REFERENCES:
patent: 4926177 (1990-05-01), Sakata
patent: 5150236 (1992-09-01), Patel
patent: 5381253 (1995-01-01), Sharp et al.
patent: 5510914 (1996-04-01), Liu et al.
Masterson et al., "Ferroelectric Liquid-Crystal Tunable Filter", Optics Letters, vol. 14, No. 22, Nov. 15 1989, pp. 1249-1251.
OPTICS LETTERS, vol. 17, No. 6, Mar. 1992, New York, US, pp. 456-458, XPOOO258278, J. S. Patel, Electrically tunable ferroelectric liquid-crystal Fabry-Perot filter.
Taylor Gregory Stuart
Wiltshire Michael Charles Keogh
Malinowski Walter
Sikes William L.
Videojet Systems International Inc.
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