GB2351813A

GB2351813A - Optical shield design

Info

Publication number: GB2351813A
Application number: GB8432070A
Authority: GB
Inventors: Paul Kevin Kimber; Simon Roger Hall; Peter Vincent Kolinsky; Graham Thomas Jenkin
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1983-12-19
Filing date: 1984-12-19
Publication date: 2001-01-10
Anticipated expiration: 2004-12-19
Also published as: GB8432070D0; GB2351813B; GB8333903D0

Abstract

Apparatus for protecting an optical sensor(s) against optical damage comprises a body (C) of optically anistropic material located in the optical of the sensor (preferably at a focal point (P)) and arranged to defocus an intense beam (R<SB>1</SB>, R<SB>2</SB>) from the sensor by an intensity-dependent refractive index charge. The apparatus may be incorporated in man-portable optical equipment for eye-protection against laser weapons.

Description

2351813 Optical Protection The present invention relates to the protection

of optical sensors against optical overload. The term optical is to be taken to include electromagnetic radiation of wavelength ranging from ultra-violet to infra-red and the

term optical sensor is to be construed accordingly. Thus I.R., visible and U.V. sensors are included within the field of the invention. Furthermore the retina of the human eye is to be considered as an optical sensor in the context of the present invention.

Highly sensitive optical sensors (particularly visible and I.R. detectors) have military applications and are vulnerable to destructive overload by high-power pulsed laser beams (from a laser range-finder for example), which can have peak powers of several kilowatts, i.e. many orders of magnitude higher than the power of typical scene-detected radiation. Further. military lasers of even higher power are being developed which can blind personnel and destroy optical systems. In such circumstances a typical optical sensor can be destroyed extremely rapidly, and an object of the present invention is to provide a fast-acting protective system which will give at least some measure of protection to an optical sensor without significantly impairing performance at normal light levels.

According to one aspect of the present invention an 25 optical protection arrangement comprises optically anisotropic -2liquid crystal material disposed in the optical path of an optical sensor such that said material changes its refractive index under the influence of an intense intercepted beam of electromagnetic radiation and thereby defocuses said beam from said sensor so as to limit the incident radiation intensity at said sensor. Preferably the liquid crystal material is located at a focal point of the arrangement.

By an optically anistropic material is meant a material whose molecules exhibit different polarisabilities about at least one pair of orthogonal axes. Preferably the arrangement includes apodising means located in said optical path so as to intercept electromagnetk radiation incident upon said optically anistropic liquid crystal material and generate a non-uniform optical intensity distribution in said material. Said apodising means may be arranged to cause the intensity of said incident light to increase or to decrease in the radial direction with respect to the beam axis. It is believed that in many cases self-focussing of an incident laser beam can be induced by this method,as is discussed below. Preferably said body of optically anistropic material is arranged to defocus an image of an external field of view from said sensor by transforming it into a virtual image.

Preferably the liquid crystal material is dissolved in a suitable solvent. Said solvent may itself be optically anistropic and may for example be carbon disulphide or nitrobenzene. Carbon disulphide is particularly suitable because it can readily dissolve commercially available liquid crystals to give concentrated, clear solutions.

Liquid crystals generally have large optical Kerr constants B 0 (defined by the well-known equation jOn = BXE, where An is the induced birefringence,,is 0 wavelength in-metres and E is the applied electric field strength of the optical radiation in volts per meter) and are therefore highly optically anistropic. Preferably the -3optical Kerr constant of the liquid crystal material is greater than 10- 13V-2 m. The required speed of response of the optical system to an incident laser pulse will depend on the pulse profile and the overload capability of the sensor, as well as the power of the laser beam. The speed of response will be determined by the relaxation time of the liquid crystal and in general the liquid crystal chosen will represent a compromise between high B 0 and acceptable relaxation time.

It is believed that optically anistropic molecules in a liquid (or a nematic or smectic liquid crystal) are electronically and molecularly oriented by the strong electric field associated with an intense light beam and thereby change the local refractive index experienced by that light beam. At low intensity levels the light has little effect on the orientations of the molecules. Therefore significant changes in the refractive index of optically anisotropic liquids and liquid cyrstals only occur at high light intensities.

According to another aspect of the invention in a method of protecting the human eye from damage by a pulsed laser beam a body of optically anisotropic material is interposed in the field of view to intercept said beam and arranged to limit the observed optical intensity by an intensity-dependent change in refractive index. Besides liquid crystals, suitable optically anisotropic materials include 4 oly (toluene sulphonate diacetylene) and 'Me for example. 0 92 In a particularly preferred form of the invention the body of optically anisotropic material is arranged to undergo self- focussing. This phenomenon is described by H.T.T. Loy and Y.R. Shen, I.E. E.E. Journal of Quantum Electronics QE9 p.409(1973) and can usually be induced by -4ensuring that the incident light intensity decreases sharply away from the centre of the beam. However a few materials exhibit a decrease in refractive index with increasing light intensity and in such cases self- focussing is facilitated by ensuring that the light intensity increases away from the centre of the beam. The distance (z(f)) from the input plane (boundary of the non-linear medium) to the self-focussing point is given by the approximate relationship (for a Gaussian beam, in the case that the molecular reorientation time is very much shorter than the pulse length of the light beam) ZM k.d 2 1 where d is the beam radius at the input plane, P is the focussing parameter C>O for a converging beam), k = 2'Vt1>,with),,the wavelength. Due to the d 2 term a lens will be necessary to first reduce the beam radius at input plane to a suitable value. P C is a constant. The complementary behaviour, namely self -defocussing is also known and results in divergence of the light from the centre of the beam. Arrangements In accordance with the invention may utilise either self- focussing or self-defocussing behaviour, which should be distinguished from the defocussing of the image of the field of view from the sensor which necessarily occurs in use of the invention.

Suitable optically anistropic liquid crystal materials include the liquid crystals E18, K15 and C15 obtainable from B.D.H. p.l.c.

It is envisaged that either the pure materials or solutions of the materials may be used.

One embodiment of the invention will now be described with reference to the accompanying drawing, which is a schematic representation of an optical system designed to protect an optical sensor from a high power pulsed laser beam, the rise time of the pulses being of the order of a microsecond.

The drawing shows an objective lens 1-1 which focuses light fil, R2 from a distant field of view onto a point P in a cell C containing a 95% (by volume) solution of K15 liquid crystals (obtained from B.D.B. p.l.c.) in carbon disulphide. An apodising filter F1 ensures that any uniform beam (e.g. from a distant laser) is nonuniform when it enters the cell and a complementary apodising lens F2 restores the light distribution to what it would have been if F1 were absent. Filter F1 ensures that the light intensity at the centre of the beam is greater than at its edges. At normal light levels (such as would be obtained from scenery in daylight) the light is focussed into a sensor S by a lens 4_ 2.

At high, potentially destructive light levels such as would be produced by a pulsed laser beam shone into objective lens Ll, self-focussing occurs in the liquid in C causing the focal point P to move. Consequently, L2 no longer forms a real image on sensor 5, but forms a virtual image P"', as indicated by dashed ray lines 5, 6. Besides the reduction in light intensity at the sensor caused by movement of the focal point P, the transmission of visible and near I.R. light through the cell is also drastically reduced by a quite different mechanism. namely plasmainduced absorption. The self-focussing effect generates extremely high temperatures in the path of the laser beam through the liquid crystal solution. As a result, small quantities of the solution in this path are instantaneously converted into plasma, which strongly absorbs visible and near I.R. light. This effect may be enhanced by providing a metal sheet (not shown) at focal po int P S pierced at its intersection with the optical axis by a pin-hole. The sharp metal edges of the pin- hole concentrate the electric field component of the self-focussed laser pule and thereby facilitate plasma generation. Also the sheet contributes metal ions to the plasma. In some cases a metal grid may -6be used rather than a metal sheet with a single pin-hole.

In an alternative embodiment, the apodising filter F1 way be chosen to cause the intensity of the light beam to increase towards its edges, so that a self -defocussing effect is obtained. Most of the high intensity light will then scatter from the sides of cell C and will not even reach the sensor S. Cell C may be only a few tens of millimetres long if a material with a suitably high optical Kerr constant is chosen, so that the entire apparatus may be adapted to be worn by, or carried by a person as eye protection.

Claims

1. An optical protection arrangement comprising optically anisotropic liquid crystal material disposed in the optical path of an optical sensor such that said material changes its refractive index under the influence of an intense intercepted beam of electromagnetic radiation and thereby defocuses said beam from said sensor so as to limit the incident radiation intensity at said sensor.

2. An arrangement as claimed in Claim 1 wherein said material is located at a focal point of the arrangement.

3. An arrangement as claimed in Claim 2 wherein said material is arranged to defocus an image of an external field of view from said sensor by transforming said image into a virtual image.

4. An arrangement as claimed in any preceding Claim wherein said material is dissolved in a liquid.

5. An arrangement as claimed in Claim 4 wherein said liquid is itself optically anisotropic

6. An arrangement as claimed in Claim 5 wherein said liquid is carbon disulphide.

7. An arrangement as claimed in Claim 5 wherein said liquid is notrobenzene.

8. An arrangement as claimed in Claim 5 or Claim 6 wherein said material is K15 liquid crystal as identified hereinabove.

9. An arrangement as claimed in any of Claims 1 to 5 wherein apodising means are located in said optical path so as to intercept electromagnetic radiation incident upon said material and generate a non-uniform optical intensity distribution in said material.

10, An arrangement as claimed in any preceding Claim wherein the optical Kerr constant of said liquid crystal material is greater than 10-13V-2 M.

-B- An arrangement as claimed in any of Claims 2, 3 and 9 wherein in use, self-focussing of said beam is induced in said material.

12. An arrangement as claimed in any of Claims 2, 3 and 9 wherein In use, self-defocussing of said beam Is induced in said material.

13. An arrangement as claimed in any preceding Claim wherein a metal body is located within said liquid crystal material with one or more edges thereof closely adjacent to said optical path.

14. An arrangement as claimed in any preceding Claim adapted for use as eye protection against a pulsed laser beam, the rotational relaxation time of said optically anisotropic liquid crystal material being shorter than the rise time of the pulses of said laser beam.

15- An optical protection arrangement substantially as described hereinabove with reference to the accompanying drawing.

16. A method of protecting the human eye against a pulsed laser beam wherein a body of optically anisotropic material is interposed in the field of view to intercept said beam and arranged to limit the observed optical intensity by an intensity-dependent change in refractive index.

17. A method as claimed in Claim 16 wherein said body of optically anisotropic material comprises a liquid crystal phase.

C Amendments to the claims have been filed as follows A optical protection arrangement comprising optically anisotropic liquid crystal material disposed in the optical path of an optical sensor such that said material changes its refractive index under the influence of an intense intercepted beam of electromagnetic radiation and thereby defocusses said beam from said sensor so as to limit the incident radiation intensity at said sensor.

^]aim 1 wherein said material 2. An arrangement as claimed in C is located at a focal point of the arrangement. 3. An arrangement as claimed in Claim 2 wherein said material is arranged to defocus an image of an external field of view from said sensor by transforming said image into a virutal image. 4. An arrangement as claimed in any preceding Claim wherein said material is dissolved in a liquid. 5. An arrangement as claimed in Claim 4 wherein said liquid is itself optically anisotropic. 6. An arrangement as claimed in Claim 5 wherein said liquid is carbon disulphide. 7. An arrangement as claimed in Claim 5 wherein said liquid is nitrobpnzene. 8. An arrangement as claimed in Claim 5 or Claim 6 wherein said material is K15 liquid crystal as identified hereinabove. 9. An arrangement as claimed in any of Claims 1 to 5 wherein apodising means are located in said optical path so as to intercept electromagnetic radiation Incident upon said material and generate a non-uniform optical intensity distribution in said material. 10. An arrangement as claimed in any preceding Claim wherein the optical Kerr constant of said liquid crystal material is greater than 10-13y-2m. 11. An arrangement as claimed in any of Claims 2, 3 and 9 wherein in use, self-focussing of said beam is induced in some material.

4 1 ri 12. An arrangement as claimed in any of Claims 2, 3 and 9 wherein in use, self defocussing of said beam is induced in said material. 13. An arrangement as claimed in any preceding Claim wherein a metal body is located within said liquid crystal material with one or more edges thereof closely adjacent to said optical path. 14. An arrangement as claimed in any preceding Claim adapted to be held in front of the eyes to provide protection against a pulsed laser beam, having a known minimum pulse rise time, the rotational relaxation time of said optically anisotropic liquid crystal material being shorter than said minimum rise time of the pulses of said laser beam. 15. An optical protection arrangement substantially as described hereinabove with reference to the accompanying drawing. 16. A method of protecting the human eye against a pulsed laser beam wherein a body of optically anisotropic material is interposed in the field of view to intercept said beam and arranged to limit the observed optical intensity by an intensity-dependent change in refractive index. 17. A method as claimed in Claim 16 wherein said body of optically anisotropic material comprises a liquid crystal phase.