GB2301883A

GB2301883A - Laser scatter detector

Info

Publication number: GB2301883A
Application number: GB8613297A
Authority: GB
Inventors: Paul William Walland
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1986-06-02
Filing date: 1986-06-02
Publication date: 1996-12-18
Anticipated expiration: 2006-06-02
Also published as: GB8613297D0; GB2301883B

Abstract

The scatter detector comprises a sensor 1 which is arranged to respond to reflected or scattered laser light from an incident beam 4 striking a surface 3 of a potential target, and in which sensor 1 is operatively associated with a signal processor capable of rejecting any received pulses the durations of which exceed a predetermined time interval, thus eliminating sun-glint, explosion flashes or flares.

Description

LASER SCATTER DETECTOR This invention relates to a laser scatter detector.

It relates particularly to a device which permits laser radiation falling on a protected vehicle or installation to be detected and differentiated fram other possible sources of illumination. The detector can thus give a warning of the presence of a hostile laser beam by detecting the incident radiation.

The laser warning receivers which are available at present comprise detectors which are mounted on a protected vehicle in such a way that, in use, they will have a direct line of sight to the illuminating laser source. This technique has the disadvantage that, for a nearby laser source having only a small divergence in the output beam, the incident laser power may fail to fall on the detector and thus the required warning response will not be triggered.

The present invention was devised to avoid this disadvantage by separating the detector from the body of the protected vehicle and then monitoring radiation scattered from the vehicle skin. Any radiation falling on the skin will thus produce scattering so that a line of sight from the detector to the laser source is no longer necessary.

According to the invention, there is provided a laser scatter detector arranged to detect the presence of a laser beam, the scatter detector comprising a sensor which is arranged to respond to reflected or scattered laser light from a suitable surface, and which is operatively associated with a signal processor capable of rejecting any received pulses the durations of which exceed a predetermined time interval. The sensor may be supported on a mounting which maintains it at a fixed distance from the said surface.

Preferably, the signal processor includes a timer capable of producing a timing pulse of a predetermined length. The processor may further include means capable of comparing the timing pulse length with the length of a received signal pulse so that a pulse will be rejected if the duration of the said pulse exceeds that of the timing pulse.

In a different embodiment, the signal processor may operate at the optical frequency and this may include the timing operation which is done optically which will enable noise and bandwidth limitations which are inherent in electronics systems to be avoided.

By way of example, a particular embodiment of the invention will now be described with reference to the accompanying drawing, in which: Figure 1 is a schematic diagram showing the scatter detector in operation, and, Figure 2 is a block diagram showing the elements of the pulse detector.

As depicted in Figure 1, a laser scatter detector 1 is mounted a short distance away fram a protected surface 2 and the detector is able to view a protected area 3 of the surface 2. When a threat laser beam 4 is directed onto the protected area 3, the beam is scattered in all directions and a small proportion of the scattered beam forms a detection sample 6 which enters the scatter detector 1.

Since the detector 1 is positioned to observe a comparatively large area of the protected surface fram a close distance, a laser beam striking anywhere within this large area will trigger the required response. This is clearly more effective than attempting to sense a point source with a direct line of sight observation.

For a good scattering surface (that is, one whose scattering profile approximates to Lambertian scatter) the power incident upon the detector is independent of the illuminating beam diameter, so long as all of the beam falls within the protected area. The solid angle enclosed by the extremes of the field of view of the detector can be large (of the order of 4 steradians) for a highly scattering surface, but this could be reduced and more detectors utilized if the surface has poor scattering properties with a strong specular component. If necessary, the surface can be finished with a paint coating which is formulated to have good scattering properties.

The laser scatter detector 1 is connected to form an input to a pulse detector as shown in Figure 2. The pulse detector comprises an optical filter 7, a sensor 8 which is a photodetector and which is AC-coupled to an amplifier 9. The amplifier 9 also has an input to which a reference voltage can be applied on the line 11 in order to set a required output voltage threshold value.

The circuit also includes a timer 12 which is driven by a timing generator 13 and which produces pulses of a predetermined duration.

In operation, when a laser pulse 14 which has a duration of T (pulse) seconds strikes the sensor 8, this produces a corresponding electrical pulse which passes through to the timer 12 to appear on an output lead 16.

The timer 12 produces its own pulse having a duration of T (max) seconds when triggered by the appearance of the laser pulse 14. If T (max) is greater than T (pulse) then the timer will recognise that the laser pulse indicates a possible threat and an output signal will appear on the output lead 16. On the other hand, if T (max) is less than T (pulse) indicating that there was no sharp drop in pulse intensity within the predetermined timed period, then no signal will appear on the output lead 16.

The circuit thus enables possible false alarms due to sun glints, explosions, flares, etc. to be discriminated against in favour of extremely short pulses which are very likely to be due to a laser source. In practice, a suitable period for the timing pulse has been found to be about one microsecond.

The foregoing description of an embodiment of the invention has been given by way of example only and a number of modifications may be made without departing from the scope of the invention as defined in the appended claims. For instance, in some applications, the use of an optical filter through which the incoming pulse passes before it enters the sensor 8 may not be necessary and this component could be omitted.

The laser scatter detector of the invention has been found to be particularly suitable for use in the protection of helicopters, but it could equally well be used to protect other possible targets such as ships, radar installations etc. A pod-mounted version could provide protection for a fixed wing aircraft.

The present embodiment performs temporal signal processing in the electronic domain. An alternative embodiment might incorporate an optical fibre or fibres to direct illumination to a remote processor which might analyse the signal in the optical frequency and phase domains to provide improved discrimination against non-laser pulses. Additionally, temporal signal processing may be performed optically to avoid noise and bandwidth limitations inherent in electronics systems.

Claims

CLAIMS:

1. A laser scatter detector arranged to detect the presence of a laser beam, the scatter detector comprising a sensor which is arranged to respond to reflected or scattered laser light from a suitable surface, and which is operatively associated with a signal processor capable of rejecting any received pulses the durations of which exceed a predetermined time interval.

2. A scatter detector as claimed in Claim 1, in which the sensor is supported on a mounting which maintains it at a fixed distance fran the said surface.

3. A scatter detector as claimed in Claim 1 or 2, in which the said signal processor includes a timer capable of producing a timing pulse of a predetermined length.

4. A scatter detector as claimed in Claim 3, in which the said signal processor includes comparator means effective to reject any received signal pulses the durations of which exceed that of the timing pulse.

5. A scatter detector as claimed in any one of Claims 1 to 4, in which the said sensor includes an optical filter located in front of a photodetector.

6. A laser scatter detector substantially as hereinbefore described with reference to the accompanying drawing.

Amendments to the claims have been filed as follows 1. A laser scatter detector arranged to give a warning of the presence of a hostile laser beam if such a beam should fall onto a vehicle or installation having a surface which will scatter the incident radiation, the scatter detector comprising a sensor which is arranged to respond to scattered laser light from the said surface, and which is operatively associated with a signal processor capable of rejecting any received pulses the durations of which exceed a predetermined time interval, such that laser pulses are able to be distinguished from non-laser sources of radiation, 2. A scatter detector as claimed in Claim 1, in which the sensor is supported on a mounting which maintains it at a fixed distance from the said surface.