GB2278677A - Detection of laser radiation - Google Patents

Abstract

A laser radiation device includes two light guides 13, 14 of different optical activity, e.g. quartz rods of different lengths, through which the sampled radiation passes, and respective detector means 15/16, 15/17 to compare the polarisation characteristics of the output beams passed by the light guides. Laser radiation incident upon the device results in the output beams having different polarisation characteristics, but ambient radiation will result in beams having substantially the same polarisation characteristics. <IMAGE>

Description

DETECTION OF LASER RADIATION This invention relates to the detection of laser radiation and in particular, but not exclusively, to laser radiation detection apparatus for providing a warning signal when laser radiation is incident thereon. In military environments, such devices may be employed to alert personnel when laser radiation is directed towards them from, for example, a laser rangefinder or a laser target designator.

In known forms of laser detection device, radiation from the viewed scene is supplied to a spectral filter or coherence filter to separate out the radiation of interest before supplying it to a detector system. In each case, it will be appreciated that the system will be limited to one particular narrow range of wavelengths, and consequently if the device is to be able to detect more than one wavelength, separate filters and detectors will be necessary.

Furthermore, the signal to noise ratio of such detector systems is relatively high.

It is an aim of this invention to provide laser detection apparatus which overcomes at least some of the above disadvantages.

According to this invention, there is provided laser radiation detection apparatus including first and second spaced light guide means for receiving radiation from a viewed scene and each having a predetermined different degree of optical activity, and comparison means for determining the polarisation characteristics of the radiation passed by each of said light guide means.

Laser light is usually polarised and monochromatic.

Consequently, when such radiation is incident upon the apparatus, the differing degrees of optical activity of the first and second light guide means cause the beams leaving the light guides to have different polarisation vectors. When however ambient, randomly polarised light is incident upon the apparatus, although the light guides will twist the polarisation vectors of the respective beams, the extent of polarisation of the light emerging from each light guide will be substantially the same as that of the other beam.

Consequently, measurement of the polarisation characteristics will indicate whether laser radiation is incident on the apparatus.

Preferably said comparison means incl udes first and second detector means for receiving radiation passed by said first and second light guide means, and polarisation means interposed between said detector means and said light guide means.

Preferably said comparison means further includes a comparator to which are supplied the outputs of each of the first and second detector means.

Preferably each of said first and second light guide means comprises a rod element having a different degree of optical activity. This may be achieved having elements formed of the same material but being of different lengths. A particularly suitable material has been found to be quartz.

In order to give the radiation passing into each of the light guide means a fixed orientation spatially with respect to the apparatus, further polarising means may be provided, through which radiation from the viewed scene passes, prior to entering said light guides. In this case, in order that laser radiation having a polarisation vector disposed at 90D to the polarising plane of the further polarising means is still detected, it is preferred for two similar laser radiation detecting apparatus to be provided, each apparatus including a further polarising means located in front of the respective light guide means, the polarisation axis of each further polarising means being disposed at an angle to that of the other.

Further aspects will become apparent from the following description which is by way of example only, in which reference will be made to the accompanying drawing which is a schematic representation of an embodiment of laser detecting device.

By way of introduction, it should be noted that the angle through which the plane of polarisation of a beam of monochromatic light is twisted by passage through an optically active material is dependent on the wavelength of the light and the length of the light path through the material. By way of example, a 1 cm. quartz rod will rotate the plane of polarisation of light of wavelength 600 n.m.

through 217 .

Referring now to the drawing, the device 10 comprises an inlet 11 for radiation, a first polarising sheet 12, two quartz rods 13, 14 arranged side-by-side and each of different length and a second polarising sheet 15. A first radiation detector 16 receives radiation passing out of first rod 13 through polarising sheet 15 and a second radiation detector 17 receives radiation passing out of second rod 14 through the same polarising sheet 15. The radiation detectors comprise photodiodes and their output signals are supplied to a differential amplifier 18 which supplies an output signal representative of the difference, if any, between the intensities sensed by the detectors. The detectors 16 and 17 are responsive to radiation in the band from visible light to near I.R. radiation.

Laser light impinging on the device 10 passes via inlet 11 through first polarising sheet 12 and into the body of the device.

This gives the radiation a fixed orientation spatially with respect to the device. The radiation then passes into each rod 13, 14 where, due to the different lengths of the rods - and thus their different optical activities - the polarisation vector is twisted by a different amount. Thus on leaving the rods the polarisation vectors will have different orientations from one another and this will result in a difference in intensity after passing through the second polarising sheet or analyser 15, and the difference in intensity will be indicated at the output of the differential amplifier 18.

When ambient radiation impinges on the device, since it will be randomly polarised and will also have a much broader spectrum, light passing through the first polarising sheet will be given a fixed orientation, but when the polarised broad spectrum beam passes through each of the rods, the polarisation planes of each of the frequencies of the spectrum will be twisted by different amounts and the beams will leave the rods substantially randomly polarised. Thus the intensities sensed by the detectors 16 and 17 after the light has passed through the second polarising sheet 15 will be similar, and little or no signal will be output by differential amplifier 18. The device may include an electronic thresholding circuit to negate any slight intensity differences caused by the background.With an appreciable amount of laser radiation incident on the device, the intensities sensed by the detectors 16, 17 will not be equal and the differential amplifier output will rise above threshold.

In order to prevent the above described laser detecting device from giving a false null reading should the incident laser radiation be polarised in a plane at 900 to the polarisation plane of the polarising means 12, it is preferred for a laser detecting system to include two such devices as described above, each disposed with their rod elements disposed parallel and side by side, but with their respective first polarising sheets 12 oriented with their polarising planes at 90" to one another.

Light from a surveyed scene may be caused to enter the inlet by a wide angle lens, or a telescope arrangement, together with a scanning system, may be employed, and it is believed that the design of such a system is within the competence of one skilled in the art.

Claims (8)

1. Laser radiation detection apparatus including first and second spaced light guide means for receiving radiation from a viewed scene and each having a predetermined different degree of optical activity, and comparison means for determining the polarisation characteristics of the radiation passed by each of said light guide means.

2. Laser radiation detecting apparatus according to Claim 1, wherein said comparison means includes first and second detector means for receiving radiation passed by said first and second light guide means, and polarisation means interposed between said detector means and said light guide means.

3. Laser radiation detecting apparatus according to Claim 2, wherein said comparison means further includes a comparator to which are supplied the outputs of each of the first and second detector means.

4. Laser radiation detecting apparatus according to any of the preceding Claims, wherein each of said first and second light guide means comprises a rod element having a different degree of optical activity.

5. Laser raiation detecting apparatus according to Claim 4, wherein each of said rod elements is formed of the same material but is of different length.

6. Laser radiation detecting apparatus according to any of the preceding Claims and including further polarising means through which radiation from the viewed scene passes prior to passing through said first and second light guide means.

7. A laser radiation detecting system, including two laser radiation detecting apparatus according to Claim 6, wherein each of the further polarising means is oriented with its polarising axis disposed at an angle to that of the other polarising axis.

8. Laser radiation apparatus substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.