A LASER WARNING DEVICE

Abstract: The present invention relates to a laser warning device. In a military connection, such devices have the task to register laser radiation from e.g. laser rangefinders or laser designators and by a warning signal make clear to the illuminated person that laser illumination has occurred or continues. In addition to indicating this it is also desirable that the laser warning device is able to establish the direction of incidence. The present invention solves the raised problem by comprising a position sensitive detector or a position sensitive detector arrangement having a detector surface (1), that is protected from radiation that falls directly onto it, and a reflector (2). The reflector is designed to reflect towards said detector surface, while focusing (7) towards the line of intersection between the detector surface and a plane that is perpendicular to said surface and in which the direction of incidence for the incoming radiation lies, the radiation that falls on the laser warning device and is on the hole parallel to the plane of the detector surface. The latter is defined as diverging not more than +/- 30 DEG from said plane. Further, the radiation may come from an arbitrary angle within the detector plane, up to 360 DEG . This leads to said arbitrary angle being determined by the position for the concentrated radiation energy.

Description: A laser warning device The present invention relates to a laser warning device. In a militar connection, such devices have the task to register laser radiation fr e.g. laser rangefinders or laser designators and by a warning signal clear to the illuminated person that laser illumination has occurred continues. The laser warning device can be located on or in close vici of military objects that are especially exposed to systems using a las for target acquisition and the like, for instance tanks, ships and pe nent installations, such as airfields etc. Laser beams for the mention reasons have very little divergence. Typically the cross-section of th main beam is 1 after 1 km. It could therefore be necessary to place several laser warning devices in different places around an object in to get a reasonable security of detection, if the object is large. Due the atmospheric conditions there is also a weeker more or less scatter radiation of considerably greater cross-section. The laser warning device shall indicate that laser illumination has oc red and it is also desirable that it is able to establish the directio (bearing and elevation) and the type of laser (laser wavelength, pulse length and pulse repetition frequency). There are several previous types of laser warning devices. Most of the require a great number of light collection elements or detectors in or to determine the direction to the light source with high resolution. T what is shown in the German Offenlegungsschrift 3300849, the America Patent 4,868,378 and the Swedish Patent 459529 can be mentioned as examples. They all have different drawbacks. The present invention, however, solves the raised problem to determine the direction to a las radiation source, without having the drawbacks of the previous laser warning devices, by being designed in the way that is evident from the following independent claims. In the following the invention will be described in more detail in rel to the accompanying drawings where, fig 1 shows a principal sketch of a laser warning device according to the invention, fig 2 shows a cross-section of said laser warning device whe the laser radiation source is located above the plane the detector surface and fig 3 shows the same cross-section when the laser radiation source is located below said plane. The laser warning device comprises a position sensitive photodetector o position sensitive photodetector arrangement. Position sensitive detecto are previously known and in the Swedish publ shed patent application 368741 such a detector is described and direction is also given about basic knowledge about them. Instead of one such detector, a detector arrangement, a detector array or the like, which gives a signal that depends on the position of the illuminated spot, can be used. Therefore, when in the following a position sensitive detector is mentioned, both a single one and a position sensitive detector arrangement is referred to. Basis for the invention is that the laser warning device is provided wit reflector, which focus the incoming laser radiation towards the photodet tor, and that the detector surface must be protected against directly in coming radiation. The focusing results in a larger signal from the detec than would otherwise have been the case. In fig 1 it is shown how the pr tection against directly incoming radiation can take the form of a roof above the laser warning device. In the figure it is further shown how it possible to implement the invention by locating the detector 1, in this case an annular position sensitive photodetector, in the focus of a sur¬ rounding reflector 2, in this case located on the inner surface of a spherical zone. The radiation that falls on the detector 2 and that is mo or less parallel to the plane of the detector surface is thereby focused towards the detector as seen both in a plane parallel to the detector surface and in planes perpendicular thereto. Limits for the deviation of the radiation from a plane that is parallel to the detector surface shoul in practical applications not be greater than - 30°. Within said zone on the other hand, the laser warning device can be sensitive to incoming radiation within a great angle, which is an advantage with the invention. Normally the laser warning device is designed to be sensitive within 90° and up to 360°. If it is only the bearing that is of interest and not the elevation of incoming radiation, it is possible to restrict oneself to focusing in o direction and use a reflector in the form of the inner side of a trunca cone instead of the previously mentioned spherical zone. The radiation then focused towards a line on the detector surface, namely the line of intersection between the detector surface and a plane that is perpendic to said surface and in which the direction of incidence of the incoming radiation lies. If the reflector is made on the inner side of a closed dome above the detector surface, the detector gets on the same time, in a simple way, necessary protection against the environmental influence of the surroun world, that is rain etc. The dome can be provided with a coating that partially reflects the wave length of the radiation in question, so that one part 5 of the radiation that falls on the laser warning device, passes through the dome 2, while another part 6 is reflected away. The part 5 that passes the dome, then passes across the warning device and hits the inner side of the opposite side of the dome 2. Then once again one part 7 of the beam is reflected a focused way onto the detector 1, while another part 8 passes through t dome. From the figure it is evident that the laser radiation is focused toward an area on the detector that is diametrically opposite the one from whic it comes, which consequently can be used to determine the bearing to the laser source. In fig 2 and 3 the beam path for the laser radiation from two different directions of elevation has been drawn in principle. In figure 2 the las radiation source is located above the plane of the detector surface, whi result in the radiation being focused further away from the centre than the case, when the laser radiation source is located below the plane of detector surface, which is shown in figure 3. Thus it is possible to determine the bearing and the elevation of the la source by using a photosensitive detector, which is able to register the position of the radiation that is focused in two dimensions. It is also possible to restrict the function only to the registration of for exa the bearing.. The focusing of the laser radiation towards the detector will not be o timal within the entire sensitivity range of the laser warning device, to the simple design of the collection optics. This is to a large exte compensated for by the detection being designed in such a way that the position of the central point of the collected radiation is determined It is also evident from the figures that parts of the laser radiation reflected or transmitted away in the different boundary surfaces in th example shown, which reduces the sensitivity of the laser warning devi This is, however, compensated for by the device being active within a angular range with a large light collecting area. As an alternative to the example shown, it is possible to use a l ser warning device that has a completely reflecting coating on the inner si of one or more sectors of the dome, that together take up up to half o circumference, and opposite this sector or these sectors the dome is completely transparent. Hereby the light yield will, of course, increas considerably, as in principle all radiation in certain angles will reac the detector 1. Naturally, the drawback is that the laser warning devic only sensitive to radiation in said angles. When there is a need for coverage of a larger angle, up to the full circle, it is then possible use two or more warning devices located close to each other, for instan stacked on each other. In order to determine the approximate wavelength of the radiation that falls on the laser warning device, the dome can be made as a filter tha blocks radiation outside a certain wavelength interval. If, then, sever such laser warning devices, which are sensitive to different wavelength intervals, are located in the vicinity of each other, for instance stac on each other, it is possible to cover a large wavelength interval at t same time as one finds out, to which wavelength interval the radiation belongs. In order to make sure that the detector 1 is not reached by directly inc ing radiation, it might be suitable to provide the dome 2 with an annula screen 9 of adapted height along its base. Further, concerning detecto that have a central hole, it is possible to place a cylindrical tap of adapted height near the hole as an additional screen.

Claims: Claims: 1. A laser warning device, which gives the direction of incidence tow the warning device for certain laser radiation or its perpendicular p tion on a defined plane through the warning device, characterized in t it comprises a position sensitive detector or a position sensitive det arrangement having a detector surface (1) that is protected from radia that falls directly onto it and that lies in said defined plane, and a flector (2) that is designed to reflect towards said detector surface, while focusing (7) towards the line of intersection between the detect surface and a plane that is perpendicular to said surface and in which direction of incidence for the incoming radiation lies, the radiation falls on the laser warning device and is on the hole parallel to the p of the detector surface, defined as falling within - 30° of said plane comes from an arbitrary angle within said plane, up to 360°, so that t position for the concentrated radiation energy determines said arbitra angle. 2) Laser warning device according to claim 1, characterized in that th reflector (2) is designed to focus (7) the radiation towards a point in detector surface (1), so that the position for the concentrated radiati energy also determines the angle of incidence for the radiation towards plane of the detector surface. 3) Laser warning device according to claim 1 or 2, characterized in tha the reflector (2) is made on the inner surface of a rotationally symmet cal dome that surrounds the detector surface (1) and provides it with protection from the surrounding world. 4) Laser warning device according to claim 3, characterized in that the rotationally symmetrical dome is a spherical zone. 5) Laser warning device according to claim 3 or 4, characterized in tha sector of the dome is completely transparent to the radiation in questi and another sector, opposite the first, is completely reflective to sai radiation. 6) Laser warning device according to claim 3 or 4, characterized in tha the dome is coated with a coating, that is partially reflective to the radiation in question, so that one part (5) of the radiation (4) that f on the laser warning device passes from outside in through the dome and after having passed across the warning device, is reflected by the inne side of the dome towards the detector surface. 7) Laser warning device according to claims 3-6, characterized in that dome is proviced with an impervious screen (9) in the form of a ring at base, near the plane of the detector surface. 8) Laser warning device according to any of the claims 3-6, characteriz in that the detector surface (1) has a central hole, near which a cylin cal tap of adapted height is located as a screen. 9) Laser warning device according to any of the above-mentioned claims, characterized in that it comprises a filter that blocks the radiation o side a certain wavelength interval, and that it is intended to be mount together with, for instance stacked on, other similar laser warning devi that are, in the same way, sensitive to other wavelength intervals.

Inventors: BOLANDER GOERAN, NISSBORG KURT, BOLANDER, GOERAN, NISSBORG, KURT
Applicants: FOERSVARETS FORSKNINGSANSTALT
Application year: 1992
Main classification: G01S3
IPC codes: G01S3