SE529775C2 - Apparatus and method for detecting and locating laser beam sources - Google Patents

Abstract

The device has a detector (1) e.g. charge-coupled device camera, sensitive to a laser radiation, disposed in an image field of an imaging optical apparatus (2), and an electronic signal processing unit connected to the detector. A cross network (3) is disposed between a laser radiation source and the apparatus, such that the diffraction orders of the network are reproduced on the detector. A spectral filter (4) is placed upstream of the detector. An independent claim is also included for a method of detecting and localizing laser radiation source using a laser radiation source detection and localization device.

Description

If the frequency and low pulse power are used, the sensitivity of hitherto known laser warning sensors is generally not sufficient to detect this threat. In the aggravating direction, it is added that the weak laser radiation in the open air must be detected against the strong radiation background fi from daylight resp. a lighting from bright artificial light sources.

It is therefore an object of the present invention to provide a device as well as a method for detecting and locating laser beam sources which reliably detects not only the direct light from a pulsed laser or a constant beam laser incident but also indirectly, bent, reflected or scattered light laser's output aperture or from objects illuminated by the laser, makes a difference in relation to daylight or other radiation sources than laser light and possibly indicates the direction of the laser source with high accuracy. This task is solved by a device according to claim 1 resp. by a method according to claim 9. The invention is based on the use of a cross-grating, with which coherent and incoherent radiation is imaged in different ways on a radiation-loving detector. Through this measure, spectral broadband, ie. temporally incoherent point light sources, such as lamps or headlamps, no longer to be depicted as points in the detector's burner plane, e.g. in a CCD camera, but as bar images of its spectrum.

Lasers, as spectrally narrowband, ie. coherent sources, on the other hand, are imaged through the cross grid as a dot pattern and can thus be distinguished from the incoherent radiation sources. In order to determine the location of a laser-recognized light source, the location of the zero order of the diffraction image on the detector shall be determined. This can be done in different ways, depending on the type of cross grid used. In the simplest case, the point-shaped light med angle with the greatest intensity is the zero order of the bending image and thus identical to the position of the laser light source in the image field. With knowledge of the instantaneous direction of the optical axis of the optics and its focal length, the direction to the radiation source in the considered tube is also known. 10 15 20 25 30 529 775 3 A higher safety in position determination of a laser source is achieved by determining the symmetry center for resp. light fl corner pattern. This can in turn be done in a simple manner by the individual light fl corners being successively dimmed by means of an adjustable threshold value, e.g. by a pre-assembled gray wedge or by lowering the sensitivity of the detector. Since in a cross-grid the pixels of the same order also have the same intensity and these are arranged symmetrically around the zero order, the pixels of the same order disappear at increasing threshold value at the same time, so that from the respective vanished locations the symmetry center and thus the position for the zero order.

A further possibility for determining the position of the zero order consists in that the cross grid is rotated about the optical axis. In this case, all the pixels of higher order revolve around the pixel of the zero order, which stands still at its respective position and can thus be easily shielded.

Furthermore, the wavelength of the laser can be easily determined from the distance between the individual light beams in a symmetrical pattern, which enables a further characterization of the respective threat type.

The invention will in the following be described in more detail in connection with the exemplary embodiment shown in the diagrams. In this case: Fig. 1 shows the basic construction of a laser warning receiver with an fi-mounted cross-grid.

Fig. 2 shows the basic structure of a laser warning receiver with integrated residual light amplifier.

Fig. 3 shows the basic structure of a laser warning receiver with fi am-mounted deflection optics for circumferential detection, and Figs. 4a and 4b show the diffraction images generated with a card grating from a a) incoherent and b) coherent point-shaped radiation source. m 15 20 25 30 529 775 4 Det i fi g. 1 shows the exemplary embodiment of a laser warming receiver for the near and central infrared region has a camera with so-called "Focal Plane Array (FPA)", ie. a surface matrix detector 1, which is arranged in the focal plane of an imaging optics 2. Such a detector set typically consists of 256x256 individual detectors and is provided with an integrated readout electronics 5. The individual detectors in the FPA unit integrate the incident radiation under a fixed or variable integration time if e.g. 16 ms in parallel. As a result, these detectors differ from the individual detectors present in ordinary laser warning sensors, which, with a short time constant in the nanosecurity array, are adapted exclusively for the detection of pulsed radiation sources with the same pulse duration. Two different types of matrix detectors, which have different readout and transmission functions for the subsequent signal processing, are known, such as "Charge-Coupled-Devices (CCD" and "Complementary Metaloxide Semiconductor (ICMS)"), both of which can be used here. .

In the near infrared range of 0.75 - 1.1 μm, commercially available cameras with silicon detectors can be used, such as those used for capturing images within the visible range (possibly with a pre-connected residual light amplifier).

For the wavelength range between 1 and 5 μm, there are infrared cameras with platinum silicide (Pt: Si) or indium antimony (ln: Sb) detectors, and in the wavelength range between 9 and 12 μm mercury-cadmium-telluride detectors . Some of these detectors require additional cooling.

In front of such a camera, a cross grating 3 and possibly a spectral alter 4 is now mounted in accordance with the invention. The latter makes the bandwidth of the system narrower, covering the spectral range in which the laser source is assumed to lie. This also reduces the surface radiation input. Depending on the type of signal analysis, the cross grating 3 can be rotated about the optical axis of the camera by means of a drive unit 6.

The significance of the cross letter should be described in more detail below using fi g. 4 a and b: 10 15 20 25 529 775 5 If you lay two identical, ordinary bar grids on top of each other, you get a two-dimensional cross grid. If you project a point of light through such a grid on a screen resp. on the focal plane of a camera, in broadband light, a bending image, reproduced in fi g, occurs. 4a, where a large number of colored bending spectra are grouped around a round spot in regular order, so that their longitudinal direction points towards the central spot, the short-wavelength part of the spectrum lying inwards and the longer-waving part outwards. In monochromatic light, the phenomenon in the image changes according to fi g. 4b, where point-shaped light fl angles occur, which lie in the intersection points of an almost rectilinear square grid. The position of the pixels and their intensity distribution at a cross lattice can be determined using lattice calculations according to Fraunhofer. The intersection points of two hyperbola masses form the locations of interference maxima when bending in a flat point grid. If the lattice constant is denoted b and the angle of incidence in the plane parallel to one lattice is denoted by concern and the angle of incidence to the plane parallel to the other lattice is denoted by ßo, one obtains for the angles ot resp. ß in the inflectional forms in these two, perpendicular to each other planes the following expression: sinct-sinot0 = n'X / d (n = 0, il, i2, .... ..) sin ß -sin [30 = m '? t / d (m = 0, il, i2, .....) For use in accordance with the invention of a cross-grating in a laser warning sensor, the following properties are now important: the maxi in the camera's image plane, broadband point sources are depicted as solid lines, and can thus be separated from each other. o Superficial, broadband light sources produce a smeared mosaic over the entire image surface; the background radiation is thus homogenized over the entire image surface, which facilitates the detection of point-by-point images of laser sources. o The zero order of the bending pattern lies on the main beam and thus goes without bending through the grid. This direction is also the symmetry rim of the higher order bending pattern. The direction to the radiation source can thus be unambiguously determined from the bending pattern. ø The bending angle shifts with the wavelength A7 »according to the formula Aot = n / d 'Alt (corresponding to the angle ß), ie. the wavelength of the light source can be determined from the angular positions of the bending maxima. 0 When rotating the card grid, the light fl corner pattern also rotates around the axis of symmetry. The direction of the light source in relation to the camera's optical axis can thus be unambiguously determined.

To clarify the conditions when detecting two laser sources with different wavelengths, a numerical example shall be given: For assumed wavelengths of M = 1,064 μm (eg 'Nd: YAG laser) and X2 = 0.904 μm (eg a GaAs laser diode), a grating constant d = 10 μm and angles of incidence ao = ßo = 0, the bending angle ot = ß = 6.1 ° for the longer wavelength and 5.4 ° for the shorter wavelength. At higher orders, the bending angle multiplies. At shorter grid distances, the wavelength resolution becomes larger and at the same time the bending angle.

With about 600 lines and rows in a detector set and a camera shooting angle of 90 °, the angular resolution of a pixel is 0.1 5 °. At a lattice constant of 2 μm, the spectral resolution of a pixel in the first bending order amounts to about 5 mn. By comparison, the spectral bandwidth of a laser diode in a radiant weapon is about 3 nm.

If fl your isolated light spots are detected by the detector set, their location distribution in the image can be deduced in the case of the higher orders to a coherent laser source. This can be determined from the symmetry of the light ck corner pattern and the identical brightness of all light ck corners belonging to a specific order. The task can e.g. is solved electronically in such a way that the signals for each individual pixel in the focal plane are compared in their intensity with the signals for the respective adjacent pixels. If it appears that the intensity of a pixel is clearly higher than that of the neighbor, its coordinates and signal values are noted. The overall image can thus be reduced to a dot pattern of individual pixels with higher intensity. Now signal interference can be eliminated in such a way that only those pixels which form concentric squares are considered. If a regular dot pattern then remains, the presence of a laser source is very likely.

From the dimension of the squares, the wavelength of the laser source can now also be calculated and e.g. compared with values in a threat library to obtain further confirmation of the threat.

If the dot pattern thus obtained is compared with a series of images from a carnera. the movements of the laser source in relation to the target can be calculated and followed. According to this simple regulation, several laser sources can also be quickly kept apart, classified and viewed separately. For a person skilled in the art, familiar with electronic signal processing, this task can be solved with a simple microprocessor without the need for any special image processing in a computer.

Cross gratings can be made either as transmission gratings or reaction gratings.

These can be designed both as amplitude and phase gratings. The advantage of phase gratings is their significantly higher transmission, as with amplitude gratings the radiation at the respective shading part of the grating is lost.

A special case of lattice is the so-called the sine grating with local cosz orbit for the amplitude transmission when using an amplitude grating or for the refractive index with a phase grating. With this lattice type, only zero and the order il occur in the bending spectrum. In addition, due to the high efficiency of first-order light transmission, these gratings are particularly suitable for the detection of weak laser sources. i When it comes to the manufacture of lattices, the holographic slår will make the manufacture of lattices more and more effective. In this case, two waves arising from laser beam division with a small difference in direction are dropped on a photoresist layer and an interference strip pattern is obtained, which can be converted into lattice beam structures. Thus, cross gratings can be manufactured by lighting twice with such perpendicular interference patterns. With this technology, both amplitude and phase gratings can be manufactured for transmission operation; the latter arise from the known bleaching of the amplitude structure. The different illumination of the layer in the light and dark strips can also be converted into a layer thickness change (groove profile) and used as a phase grating in transmission. By attaching with aluminum, a reaction grating is obtained analogously.

For use in a laser detector for visible and near infrared range of 0.35 to 2.5 μm, the transmission sine phase gratings are particularly leather-lined. These can e.g. placed on quartz glass and used as a transmission attachment device. In the infrared ring above 2 μm, either amplitude transmission holography can be used for transmission operation or reaction grating for reaction operation; remove a camera. In the infrared range at 10 μm (CO 2 laser) predominantly reaction grating was used. Particularly suitable for a laser warmer are so-called Echelett grids, which have a serrated drawing profile.

The drawing angle is selected so that for a desired “Blaze” wavelength (Blaze = maximum intensity) the direction of reflection and bending coincide. Then the corresponding order n will also be fi enforced.

An additional possibility for optimizing cross-lattice consists in the arrangement of the modulation depth and the lattice constants resp. the local sequence of the gratings. Grating with a low local frequency has, as is well known, many bending schemes, the intensities of which relate to the squares of the Hessel functions. Through strong modulation, the intensity of the higher orders thus increases. If the modulation is reduced, the intensity of the higher orders decreases in favor of the lower orders. The optimization task for laser detectors now consists of maximizing the intensity in the first orders. This maximum is theoretically 33% for a linear grid. For a cross-grid, follow from this that for the interesting four schemes, 10% each remains. The remaining 60% of the incident light is distributed among the other schemes. 15 210 25 529 775 9 The bending efficiency can be greatly increased with cross gratings by increasing the local sequence. For local frequencies of approx. 400-500 line pairs / mm at holographic transmission phase gratings, the transition orbit between thin and thick holograms must be consulted.

Significantly fewer higher orders already appear here. If even higher local frequencies were used, e.g. 700 lines epar / mm, the higher orders can be almost completely suppressed. The zero order light share can be kept below 20% so that each of the four first order bending images receives about 20% of the light.

The lasers used for military and security purposes are limited to a few, relatively narrow wavelength zone ranges between 800-850 nm, 1050-1070 nm, 1450-1650 nm and 9.5-11.5 μm. In order to dampen the disturbing background radiation accordingly, it is advantageous to, in addition to the cross grating used, also mount a spectral filter in front of the camera's imaging lens. With an alter width of e.g. 10-20 nm in the near-red area, the background can be lowered by a factor of 10-20.

The cut-off sensitivity of commercially available CCD cameras for a constant needle laser source at an irritation time of 20 ms to detect through the lattice is at about 6 pW. The signals due to scattered radiation for a jet rider at a distance of 10-1 km are comparatively in the range 1 pW-1 nW, ie. within the detection range of a laser variator according to the invention. A further increase in sensitivity is possible for CCD cameras by extending the integration time, cooling the detector and by placing an electron multiplier stage (eg lvlicro-Channel-Plate with 10,000 times gain) in front of the detector. The latter possibility is shown in the exemplary embodiment according to Fig. 2. in front of a first lens 22.1, in the focal plane of which lies a detector set 21 connected to a microprocessor 25, a light screen 26, an electron multiplier 27, a photocathode 28, a further lens 22.2, a cross grating 23 and a spectral 24.lter 24. Hereby the elements 26, 27 and 28 form a so-called residual light amplifier, the image of which is then imaged on the detector set 21. The required angular recording range for a laser vane may be different depending on the application. In many applications, a normal lens with a field angle of 40 ° -55 ° is sufficient. For an all-around recording, e.g. for a helicopter, four such laser detectors are normally installed in different places in the outer plate, each laser detector covering an angle of 90 °. i Another option for all-around laser threat detection is shown in fi g. 3. Here frarnfór is a laser vanier according to fi g. 1 or 2 with a detector set 31, an imaging optics 32 and a cross grating 33 arranged a convex mirror 34, which senses the light from a horizontally directed plane I at a horizontal angle of view of 90 ° (eg 60 ° above the plane I and 30 ° below plane I) and images the area thus sensed in an annular surface II on the detector 31. Each direction from the area thus sensed then corresponds to a pixel on the annular surface H.

The method described here for detection of laser sources can be realized resp. combined with the most diverse imaging equipment. In particular, instead of detector sets, specially designed single detectors with an amphora-arranged optical sensing device (scarmer) can also be used.

Claims (1)

Device for detecting and locating laser beam sources with a radiation-sensitive detector arranged in the image field of an imaging optics and a signal analyzer connected to the detector, a cross grating (3) being arranged between the laser and the optics. 2) in such a way that the bending orders of the cross grating (3) are imaged on the detector (1), characterized in that the cross grating (3) is rotatable about the optical axis of the imaging optics. . Device according to Claim 1, characterized in that the cross grating (3) is designed as a transmission or reaction grating. . Device according to Claim 1 or 2, characterized in that the cross grating (3) is designed as an amplitude or phase grating. Device according to Claim 1, 2 or 3, characterized in that the rotation of the cross grid (3) takes place continuously at a predeterminable speed. . Device according to one of Claims 1 to 4, characterized in that a cross grid is used which produces only the 0th and first order, e.g. a sine lattice. . Device according to one of Claims 1 to 5, characterized in that a residual light amplifier is arranged between the cross grating (23) and the imaging optics (22.1). . Device according to one of Claims 1 to 6, characterized in that a spectral filter (4, 24) is arranged in front of the cross grid (3, 23). . Device according to one of Claims 1 to 7, characterized in that a deflection optics (34), in particular a convex mirror, is arranged in front of the cross-grid (33). 10 15 20 9. 10. ll. Method for detecting and locating laser beam sources with a device according to claim 1, characterized in that the image generated on the detector is scanned by the signal analyzer for point light ljus, whose position within the field is recorded, and that the location of a pixel is detected , which represents the 0th order of the imaged light spot pattern. Method according to Claim 9, characterized in that, by means of a variable threshold value, the images of the light som angles having the respective lowest intensity are successively dimmed, whereby the symmetry center of the light ck pattern is obtained from the locations of the dimmed light fl angles. Method for detecting and locating laser beam sources by means of a device according to claim 1, characterized in that the image of the signal analyzer generated on the detector is scanned for at least one point-shaped light fl angle, which determines the 0th order of an imaged light fl angle pattern, the position of which within the image field. registered. Method according to one of Claims 9 to 11, characterized in that the position of the 0 ord order of light fl or the fl angle of symmetry of the light fl pattern is determined in relation to the center of the image and from this the angular position of the laser beam source in relation to the optical axis of the imaged optics is obtained. Method according to one of Claims 9 to 12, characterized in that in the case of dormant cross-grids, the image on the detector is scanned for square-arranged point-shaped light spots, the mutual distances of which to the nearest neighbors are determined and compared with a predetermined interval value.