GB2317459A

GB2317459A - Protecting optical systems

Info

Publication number: GB2317459A
Application number: GB8332135A
Authority: GB
Inventors: Peter Reginald Hall
Original assignee: British Aerospace PLC; Matra Bae Dynamics UK Ltd
Current assignee: BAE Systems PLC; Matra Bae Dynamics UK Ltd
Priority date: 1982-12-01
Filing date: 1983-12-01
Publication date: 1998-03-25
Anticipated expiration: 2003-12-01
Also published as: GB2317459B; GB8234013D0; GB8332135D0

Abstract

An optical or quasi-optical system includes within its optical path a component selected for rapid failure, and reflection and/or refraction means which concentrate a low proportion of the radiation in the optical path at said component whereby in normal usage a high proportion of radiation in the optical path is transmitted and the performance of the system is not excessively degraded, but in abnormal usage when a high energy beam irradiates the system, sufficient energy is concentrated upon the selected component to effect failure.

Description

PROTECTING OPTICAL SYSTEMS The present invention relates to the protection of optical and qauasi optical systems, for example surveillance systems, from serious damage resulting from an attack by a high energy optical or quasi optical beam of an aggressor.

To effect rapid failure of a component of the type described it is desirable to direct a high power density to that component, that is to say to effect a concentration of energy, so that for example, differential absorption of that energy by the component takes place with an attendant enhancement of thermal stress. This can lead to the well-known avalanch effect of rapid and catastrophic disruption of the component.

Accordingly, an objective of the present invention is to provide arrangements in which when irradiation by a high energy beam takes place a suitably high power density can be at least locally effected but which in normal use does not excessively degrade the optical system.

According to one aspect of the invention an optical system includes within its optical path a component selected for rapid failure, and reflection and/or refraction means which concentrate a low proportion of the radiation in the optical path at said component.

The reflection and/or refraction means may or may not be included in said selected component.

By this arrangement, in normal usage a high proportion of radiation in the optical path is transmitted and the performance of the system is not excessively degraded, but when a high energy beam irradiates the system, sufficient energy is concentrated upon the selected component to effect failure.

The concentration can be in the form of a focus or caustic surface, for example.

The selected component can have energy absorption aiding arrangements to further enhance differential stress and facilitate failure.

Furthermore, if it is desired to re-direct the incident beam rather than merely diffuse it, the selected component may include two portions which together do not materially degrade, or even enhance, the system, but if one is caused to fail, for example by concentration of a low proportion of the radiation in the optical path thereon, then the remaining portion effects such redirection.

Some examples of optical systems are now described with reference to the accompanying drawings, in which: Figure 1A is a known optical system, Figure 1B is a further known optical system, Figure 2A is part of an optical system according to one aspect of the invention, Figure 2B is an arrangement having a similar effect to that of Figure 2, Figure 3 is an optical component, Figure 4 is an optical component, Figure 5A is a further optical component, and Figure 5B is an enlarged view of Figure 5A.

Referring to Figure 1, a schematic optical system is shown in which light rays 1 are shown passing through a telescope having various optical components 2, 3, 4 and 5 for focussing on a detector 6.

The component 3 is selected to be destroyed when irradiated by a high energy beam for an aggressor. When the component 3 fails and disappears, the light rays follow the path denoted by broken lines 7 so that the ability of the system to focus a high energy beam onto the detector 6 is removed.

In Figure 1B, a further schematic optical system is shown in which light rays 1 are shown passing through a similar telescope to that of Figure 1A, but including inclined Mangin mirrors 8, 9 instead of component 3.

In this case, the coating of the mirrors 8, 9 is selected to be destroyed when irradiated by a high energy beam. When this happens, the light rays 1 follow the path denoted by broken lines 10 since the coating, when removed, allows the mirror to become light transmitting.

Although the use of Mangin mirrors in converging light beams is not a sound design feature, this arrangement is useful because the radiation passes twice through the component thereby doubling the energy absorbed. Moreover, special mirror coatings could be used which are selectively absorptive.

Referring now to Figure 2A, an optical system includes an optical component 11, having two devices lla and llb.

Light rays 1 pass through that referenced lla to that referenced llb and from thence to a detector, not shown.

The device lla is selected for rapid failure when the optical system is irradiated by the high energy beam of an aggressor. Such failure is effected by arranging that the device llb and from thence to a detector, not shown.

The device lla is selected for rapid failure when the optical system is irradiated by the high energy beam of an aggressor. Such failure is effected by arranging that the device llb reflects back a small proportion of the incident light, say 3%, onto a region 12 of the device lla. The reflected rays are shown in broken outline 13 as focussing on a rear face of the device 11.

The reflection may be provided by a slightly reflecting surface and the focussing arranged, if necessary, by small changes to the contour of that surface.

Alternatively, a wholly-reflective region 14 of annular form could be accommodated around the periphery of the device llb by increasing the aperture of the system beyond that necessary for the function which the system is to perform.

This wholly-reflective region would be contoured to provide focus in the same region 12 of the device gila.

An alternative to Figure 2A, shown in Figure 2B, is suitable when the component selected to be rapidly failed is of a thickness capable of accommodating both the part-reflective surface and that on which the reflected rays are focussed.

Thus, an optical system includes an optical component 15 having a face 16 and a face 17. Light rays 1 pass through faces 16 and 17 and from thence to a detector, not shown.

The face 17 is arranged to internally reflect and focus a small proportion of the incident light, say 3%, onto the face 16 at 18, the reflected rays being shown at 19. As with the embodiment of Figure 2A, the partial reflection and focus effect of face 17 could be replaced in an over-apertured embodiment by a wholly-reflective region similar to that referenced 14.

Figure 3 illustrates a similar component to that of Figure 2B. In this embodiment, however, instead of the face 17 both partially reflecting and focussing the rays, it is arranged to effect a caustic surface or region, shown generally at 20. The caustic surface 20 constitutes a region of anomalously high power density.

The arrangements exemplified by Figures 2A, 2B and 3 merely ensure that by damage and consequent effective removal of the component, any incident high energy beam is prevented from reaching other components and the detector in the optical path of any system. If it is desired to positively direct such a beam away from the optical path, then the embodiment of Figure 4 can be used. In this arrangement, in general, the component selected for failure includes two portions which do not materially degrade, or even enhance, the system but if one is caused to fail, for example by focussing as at points 12 and 18 in Figures 2A and 2B, respectively, or by providing a caustic surface 20 as in Figure 3, then the beam is redirected.

Specifically the component selected for failure is a parallel-surfaced block 21 comprising two matching prisms 22, 23 closely positioned together so that a plane of separation 24 is formed. In the Figure, incident light rays 1 pass through the block 21 as shown in hard outline, but if one or other of the prisms is removed, e.g. that referenced 23, total internal reflection, as shown by broken outline 25 occurs in that remaining referenced 22.

The embodiments described with respect to Figures 2A, 2B and 3, illustrate how a region of high power density can be placed upon an optical component or part thereof, thereby causing differential heating and sufficient thermal stress to cause rapid failure.

Naturally, to effect such failure, massive structural disruption must take place and it is considered desirable to engineer this by inducing stresses other than by relying upon absorption by merely a coating on the component.

To provide a wideband slightly absorbing coating on a germanium component, a single layer of carbon can be used, but it is thought that this coating may be merely blasted off the component.

Therefore, Figures 5A and 5B illustrate by way of example certain stress enhancing means.

In Figure 5A, a component is formed with conducting tracks 26, the higher energy absorption of such tracks compared with that of the material of the body of the component resulting in uneven temperature distributions and hence high thermal stresses.

Additionally, as illustrated in Figure 5B, or alternatively, crack initiating discontinuities may be formed in the body of the component. These will, in general, have stress concentrating regions formed by sharp edges or points. In Figure 5B, a conical hole 27 is one example, whilst a tapered V-shaped notch 28 is another example more suitable for an edge region.

As before intimated, the conducting tracks 26 may be combined with such holes 27 and notches 28.

Claims

CLAIMS:

1. An optical or quasi optical system includes within its optical path a component selected for rapid failure, and reflection and/or refraction means which concentrate a low proportion of the radiation in the optical path at said component whereby in normal usage a high proportion of radiation in the optical path is transmitted and the performance of the system is not excessively degraded, but in abnormal usage when a high energy beam irradiates the system, sufficient energy is concentrated upon the selected component to effect failure.

2. An optical or quasi optical system according to Claim 1, wherein said reflection and/or refraction means effects an energy concentration in the form of a caustic surface.

3. An optical or quasi optical system according to Claim 1, wherein said reflection and/or refraction means effects an energy concentration in the form of a focus.

4. An optical or quasi optical system according to any one of the previous Claims, wherein said selected component includes at least one energy absorption aiding arrangement to enhance differential stress and facilitate failure.

5. An optical or quasi optical system according to any one of the previous Claims, wherein said selected component is optically associated with a further component, the two being such that together they do not materially degrade the system, but on failure of the selected component, the remaining component redirects any beam from the optical path such that no further damage is caused.

6. An optical or quasi optical system according to any one of the previous Claims, wherein said selected component is provided with thermal stress enhancing means.

7. An optical or quasi optical system substantially as described with reference to Figures 2A, 2B, 3 or 4.

8. An optical or quasi-optical system substantially as described with reference to Figures 5A and 5B.

8. An optical or quasi optical system substantially as described with reference to Figures 5A and 5B.

Amendments to the claims have been filed as follows 1. An optical or quasi-optical system having a single optical path including within its optical path a component selected for rapid failure, and reflection and/or refraction means which concentrates a low proportion of the radiation in said single optical path at said component whereby in normal usage a high proportion of radiation in the optical path is transmitted and the performance of the system is not excessively degraded, but in abnormal usage when a high energy beam irradiates the system, sufficient energy is concentrated upon the selected component to effect failure.

2. An optical or quasi-opticai system according to Claim 1, wherein said reflection and/or refraction means effects an energy concentration in the form of a caustic surface.

3. An optical or quasi-optical system according to Claim 1, wherein said reflection and/or refraction means effects an energy concentration in the form of a focus.

4. An optical or quasi-optical system according to any one of the previous Claims, wherein said selected component includes at least one energy absorption aiding arrangement to enhance differential stress and facilitate failure.

5. An optical or quasi-optical system according to any one of the previous Claims, wherein said selected component is optically associated with a further component, the two being such that together they do not materially degrade the system, but on failure of the selected component, the remaining component redirects any beam from the optical path such that no further damage is caused.

6. An optical or quasi-optical system according to any one of the previous Claims, wherein said selected component is provided with thermal stress enhancing means.

7. An optical or quasi-optical system substantially as described with reference to Figures 2A, 2B, 3 or 4.