GB2162653A

GB2162653A - Optical switching arrangement

Info

Publication number: GB2162653A
Application number: GB08519269A
Authority: GB
Inventors: Roger Michael Wood; Shiv Kumar Sharma
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1984-08-01
Filing date: 1985-07-31
Publication date: 1986-02-05
Also published as: GB8519269D0

Abstract

An optical switching arrangement for controlling the transmission of an incident light beam in dependence on its intensity consists of a slab (1) of a photo-emissive material positioned in the light path of the beam such that an increase in the intensity of the beam above a predetermined threshold will cause a photo-electron to be emitted by the slab (1). A plasma in the neighbourhood of the slab (1) is thereby initiated, the plasma being effective to restrict further transmission of the beam through the slab (1). The photo-emissive material may be a solid e.g. germanium or sodium chloride, a coating on a solid, a fluid, gas or colloidal liquid, or a sharp edge of a material having a low work function. The plasma may be produced from ions of the photo-emissive material or from ions of a fluid surrounding the photo-emissive material. <IMAGE>

Description

SPECIFICATION Optical switching arrangement This invention relates to optical switching arrangements. More particularly the invention relates to optical switching arrangements for controlling the transmission of an incident light beam in dependence on its intensity.

Such an arrangement finds application for example in the protection of a detector arranged to receive light from a laser from sudden increases in the output of the laser.

Whilst some such arrangements already exist, these are often incapable of responding quickly enough to protect such a detector from a very sudden pulse of high intensity light.

It is an object of the present invention to provide an optical switching arrangement for controlling the transmission of an incident light beam in dependence on its intensity which is capable of fast response times.

According to the present invention an optical switching arrangement for controlling the transmission of an incident light beam in dependence on its intensity comprises a quantity of a photoemissive material positioned in the light path of the beam such that an increase in the intensity of the beam above a predetermined threshold will cause a photo-electron to be emitted by the quantity thereby to initiate a plasma in the neighbourhood of said quantity effective to restrict further transmission of the beam through the quantity Preferably the arrangement includes a focussing lens positioned in the light path of the beam so as to adjust the area of the beam incident on the quantity.

One optical switching arrangement in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawing which is a schematic diagram of the arrangement in operation.

Referring to the drawing, the arrangement includes a slab of germanium 1 positioned in the light beam path between a pulsed carbon dioxide laser (not shown) and a detector 3, a convex lens 5 being interposed between the laser and the slab 1.

The lens 5, slab 1 and detector 3 are positioned along the light path such that in normal operation of the laser, the pulsed beam of light from the laser is focussed by the lens 5 onto the detector after passing through the siab 1. The lens 5 and slab 1 are mutually positioned along the light path such that the size of the focal spot of the laser light on the surface 7 of the slab 1 nearer the lens 5 is such that if the intensity of the laser light increases beyond that expected during normal operation of the laser the light intensity threshold for the formation of photo-electrons will be reached.

If the intensity of the laser light does increase such that a photo-electron is emitted by the surface 7, the photo-electron will initiate an avalanche breakdown of the surface 7. This in turn will ionise the neighbouring atmosphere such that an atmospheric plasma is formed adjacent the surface 7.

Any further incident light will then be absorbed or reflected by the plasma such that up to about 99% of the incident laser light will be prevented from reaching the detector 3 until the intensity of the incident laser light falls beneath a level at which the plasma becomes extinguished, this level being typically of the order of one per cent of the threshold intensity.

The optimum size of the focal spot of the laser light beam on the surface 7 of the slab 1 is a function of the threshold intensity of incident laser light from which it is required to protect the detector 3 and will generally be in the order of millimetres. If the slab 1 is moved to a position nearer the lens 5 such that the spot size increases beyond this optimum size the power energy density of the laser light on the surface will decrease. Thus a higher intensity of incident laser light will be needed to initiate the avalanche break down of the surface. If however the slab 1 is moved to a position well away from the lens 5 such that the spot size is smaller than the optimum size the probability of the photo-emission of an electron from the surface and the subsequent avalanche breakdown will be increased.The laser light is however, likely to cause radiation damage of the slab 1 for small spot sizes.

It will be appreciated that where a slab of photoemissive material is used the choice of the material from which the slab is made will depend on the wavelength of the laser light, it being desirable for the slab to be transparent to the incident laser light during normal operation of the laser. Many other materials other than germanium are however possible, for example sodium chloride. In some circumstances however it may be desirable to use a slab of material which is transparent to the laser light but not photo-emissive in itself, the slab carrying a thin coating of a photo-emissive material which is not necessarily transparent to the laser light, but is so thin, generally in the order of a fraction of the wavelength of the laser light so as not to cause significant attenuation of the light.This then widens the choice of photo-emissive material to include, for example, oxide materials such as MgO on a glass substrate, or metal layers such as silver or aluminium on a variety of transparent substrates.

It will also be appreciated that where the incident light beam width is of a suitable size, the lens may be omitted from the arrangement.

It will also be appreciated that whilst the optical switching arrangement described herebefore relates to the switching of a pulsed laser beam, the arrangement may be used with any intense light beam. In particular, in principle the arrangement may be used with a continuous light beam, although it may be more difficult to initiate the emission of a photo-electron as this generally requires a higher incident light threshold to initiate the process, and furthermore it may be difficult to extinguish a plasma once initiated.

Although the switching arrangement described above by way of example requires the initiation of an atmospheric plasma in order to operate, it will be appreciated that in some arrangements in accordance with the invention, the quantity of photoemissive material may be situated in a vacuum chamber, the ions required for the production of the plasma being obtained from the quantity itself.

Alternatively, the quantity of photo-emissive material may be situated in an environment other than an atmospheric environment which environment provides the ions for the plasma.

It will also be appreciated that the quantity of photo-emissive material may itself be a fluid such as a gas or a colloidal liquid. Such an arrangement has the advantage that the dynamic range of the incident light which may be switched is extended as there is less likelihood of permanent damage to the quantity of photo-emissive material than for a solid material.

It will also be appreciated that in some arrangements it may be advantageous to use a material having a low work function and having the form of a sharp edge, such as an actual edge, a point or a mesh in order to produce a photoelectron at relatively low incident light intensities in order to initiate the plasma, as there will be a high electromagnetic field concentration on such an edge.

It will also be appreciated that whilst the optical switching arrangement described herebefore is designed for the protection of a detector against increases in incident light intensity, an arrangement in accordance with the invention also finds application in other situations such as optical computing, and controls for illumination.

Claims

1. An optical switching arrangement for controlling the transmission of an incident light beam in dependence on its intensity comprising: a quantity of a photo- emissive material positioned in the light path of the beam such that an increase in the intensity of the beam above a predetermined threshold will cause a photo- electron to be emitted by the quantity thereby to initiate a plasma in the neighbourhood of said quantity effective to restrict further transmission of the beam through the quantity.

2. An arrangement according to Claim 1 in which the quantity is a surface layer of a solid.

3. An arrangement according to Claim 2 in which the quantity is a slab of germanium.

4. An arrangement according to Claim 2 in which the quantity is a quantity of sodium cloride.

5. An arrangement according to Claim 2 in which the quantity is a coating of a photo-emissive material sufficiently thin so as not to cause signifi cant attenuation of the beam on an optically trans parent substrate.

6. An arrangement according to Claim 5 in which the coating is of an oxide material.

7. An arrangement according to Claim 5 in which the coating is of a metallic material.

8. An arrangement according to Claim 2 in which the quantity is a sharp edge of a material having a low work function.

9. An arrangement according to Claim 1 in which the quantity is a fluid.

10. An arrangement according to Claim 9 in which the fluid is a gas.

11. An arrangement according to Claim 9 in which the fluid is a colloidai liquid.

12. An arrangement according to any one of Claims 1 to 11 in which the plasma is produced from ions of the quantity.

13. An arrangement according to any one of Claims 1 to 11 in which the plasma is produced from ions of a fluid surrounding the quantity.

14. An arrangement according to Claim 13 in which the fluid is a gas.

15. An arrangement according to Claim 13 in which the fluid is a colloidal liquid.

16. An arrangement according to any one of the preceding Claims including a focussing lens positioned in the light path of the beam so as to adjust the area of the beam incident on the quantity.

17. An optical switching arrangement for controlling the transmission of an incident light beam in dependence on its intensity substantially as herein-before described with reference to the accompanying drawing.