FR2888333A1 - Laser radiation source detection and localization device for e.g. military field , has cross network disposed between laser radiation source and imaging optical apparatus, such that diffraction orders of network are reproduced on detector - 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

Detection and localization device

  The present invention relates to a device for detecting and locating laser radiation sources, comprising a radiation-sensitive detector disposed in the image field of a reproduction optic and an electronic signal processing means. linked to the detector. The invention also relates to a method for detecting and locating laser radiation sources making use of such a device.

  Since in the military field laser devices are used for a wide variety of purposes, detection means capable of discovering such laser sources are necessary for the protection against threats and for the triggering of counterfeits. measures against them. Devices of this type are known for example from DE 33 23 828 C2 or DE 35 25 518 C2. These devices are used for detecting and locating pulsed laser sources, as used, for example, in target illuminators or rangefinders. The laser radiation used in such cases is most often in the near infra-red domain and, therefore, is not perceptible by the human eye. The so-called known laser warning devices presuppose that the very short laser pulses arrive directly on the detector, so that they have a sufficient sensitivity for a scanning aperture of only a few millimeters.

  In another type of application, weapons such as shells and missiles are guided by a laser beam that initially is not directed directly at the target but at the shell or 2888333 2 the missile, inside which is placed a laser detector. Guiding the flight is performed by means of this detector along the ray axis, the shooter, by a suitable pointing radius, bringing the shell or missile to the target on a path chosen by him. This type of guidance is known as beam-rider guidance. The laser beam, in this type of application, being directed directly to the target only in the terminal phase, the laser radiation, seen from the target, during the approach phase of the projectile concerned, can not be detected only indirectly as scattered radiation or as reflected radiation. These indirect radiation fractions, however, are considerably lower than the direct radiation itself. Moreover, the lasers employed being most often low-power continuous lasers or pulse lasers with a high pulse frequency but with low peak pulse power, the sensitivity of the known laser warning detectors very often , is insufficient for a threat detection. This situation is further aggravated by the need to detect low power laser radiation in the open air, on the strong background of daylight radiation or among the bright light delivered by artificial light sources.

  From this point of view, the object of the present invention is to propose a device as well as a method for detecting and locating sources of laser radiation which surely detects not only the direct light of a pulsed laser or a laser beam. a continuous laser, but also indirect light, diffracted, reflected or scattered from the exit pupil of the laser or objects illuminated by the laser, isolates it, as laser light, from the light of day or from other sources of radiation and where appropriate indicates with great precision the direction of the laser source.

  This goal is achieved by means of a detection and localization device as described in the introduction, by arranging a cross-network between the laser radiation source and the optics, so that the diffracted orders of the network in crosses are reproduced on the detector. The cross network may be in the form of a transmission network or a reflection network, an amplitude network or a phase network.

  According to an advantageous characteristic of the invention, the cross network can rotate around the optical axis of the reproduction optics, the rotation being able to take place continuously with a predefinable rotational speed.

  According to another advantageous characteristic of the invention, a cross network is used which generates only the order 0 and the order one, for example a sine network.

  According to another advantageous characteristic of the invention, a residual light intensifier is arranged between the cross network and the reproduction optics. A spectral filter can also be placed in front of the cross network.

  According to another advantageous characteristic of the invention, a deflection optic, in particular a convex mirror, is arranged in front of the cross network and thus enables panoramic detection.

  In terms of the method, according to the invention, the image produced on the signal detector is analyzed for spot light spots, the position of which is recorded inside the image field and the location is determined. an image point corre- sponding to the zero order of the represented task model.

  According to an advantageous characteristic of the invention, by means of a variable threshold value, the images of the luminous spots which each have the lowest intensity are successively eliminated and, from the places of the light spots eliminated, determines the center of symmetry of the pattern of light spots.

  According to another advantageous characteristic of the invention, the position of the zero-order spot or the center of symmetry of the pattern of light spots with respect to the center of the image is determined and the angular position of the source is deduced therefrom. of laser radiation with respect to the optical axis of the reproduction optics.

  According to another advantageous characteristic of the invention, the cross network being fixed, the image is examined on the detector in search of spot light spots arranged in a square, their distance is determined with respect to the closest neighboring spot and it is compared to a predefined range value.

  The invention is based on the use of a cross network, by means of which the coherent and incoherent radiations are reproduced in a different manner on a detector sensitive to radiation. By this arrangement, spectral wide point light sources, that is to say, incoherent light sources in time, such as lamps or projectors, are no longer reproduced in the form of points in the plane. focal point of the detector, for example a CCD camera, but in the form of image-strokes of their spectrum. On the other hand, lasers, as narrow-band sources, ie as coherent sources, are reproduced under the form of point models by the cross network and thus become identifiable among sources. incoherent radiation.

  To locate a light source identified as a laser, it is necessary to determine the location of the zero order of the image diffracted on the detector. This can be done in different ways, depending on the type of cross network used. In the simplest case, the point light spot with the highest intensity corresponds to the zero order of the diffracted image; it is therefore identical to the position of the laser light source in the image field. Knowing the instantaneous orientation of the optical axis and the focal length of the optics, we know the direction of the laser source in the space observed.

  Greater security in the determination of the position of a laser source can be obtained by determining the center of symmetry of the model of light spots concerned. This can be done in a simple manner by successively eliminating the different light spots by means of an adjustable threshold value, for example by means of a neutral corner placed upstream or by lowering the sensitivity of the detector. Since with a cross network, the image points of the same order also have the same intensity and that these are arranged symmetrically around the zero order, the image points of the same order disappear in same time when raising the threshold, which makes it possible to determine in a safe way the center of symmetry of the eliminated image points and consequently the position of the zero order.

  Another possibility for determining the position of the zero order is to rotate the grating around the optical axis. In this case, all the higher-order image points rotate around the zero-order image point which, in the image, remains quietly in its position and is easily identifiable.

  In addition, the laser wavelength can be easily determined from the distance of the different light spots of a symmetrical model, allowing further characterization of the threat type.

  The invention will be described in more detail in the following with the aid of exemplary embodiments shown schematically in the drawings. These show: FIG. 1, the basic arrangement of a laser warning receiver with an upstream cross filter, FIG. 2, the basic arrangement of an intensifier-type alert receiver 3, the basic arrangement of an upstream deflection optical laser warning receiver for a panoramic detection and FIGS. 4a and 4b, the diffracted images produced by a cross filter of a incoherent point light source (a)) and a coherent point light source (b)).

  The exemplary embodiment represented in FIG. 1 of a laser warning receiver for the near and mid-infrared range comprises a so-called focal plane array (FPA) camera, that is to say with a A 1-plane matrix detector which is disposed in the focal plane of a reproduction optic 2. A detector array of this type typically comprises 256 x 256 elementary detectors and includes an integrated scanning electronics. The elementary detectors of the focal target integrate the incident radiation with a fixed or variable integration time, for example 16 ms, in parallel. On this point, these detectors differ from the basic detectors of standard laser warning receivers which, with a short time constant, of the order of one nanosecond, are only suitable for the detection of pulsed sources having the same pulse duration. Two different types of matrix detectors are known which use different methods of analysis and transmission for signal processing, namely Charge-Coupled-Devices (CCD) and CMOS (Complementary Metal) -Oxid-Semiconductor), both of which can be used here.

  In the near-infrared range of 0.75 to 1.1 m, commercially available cameras equipped with silicon detectors, such as those used for shooting in the visible range (possibly with an intensifier device) can be used. residual light placed upstream). For the wavelength range between 1 and 5 m, infrared cameras equipped with platinum silicide (Pt: Si) or indium antimonide (In: Sb) detectors are available. in the wavelength range between 9 and 12 m, cameras equipped with mercury-cadmium telluride detectors. Some of these detectors require additional cooling.

  A cross network 3 and, if appropriate a spectral filter 4, is placed upstream of the camera, according to the invention. The spectral filter makes it possible to limit the optical bandwidth of the system to the range of the spectrum in which laser sources are supposed to exist. This in turn reduces the influence of the radiation of the background. Depending on the type of signal processing, the cross network 3 can be rotated around the optical axis of the camera by means of a driving means 6.

  The importance of the cross network will be described later in detail with reference to FIGS. 4a and 4b.

  By placing two networks of identical, common features on one another, we obtain a two-dimensional cross network. If we project a point of light through a network of this type on a screen or in the focal plane of a camera, we obtain, in the case of a light at broadband, the diffracted image reproduced in FIG. 4a, in which a large number of colored diffracted spectra are grouped in a regular arrangement around a round spot, with their longitudinal axis directed towards the central spot, the fraction of the on-the-long spectrum. the weaker being located inside and the fraction with longer wavelength being located outside. In the case of monochromatic light, the image obtained is that shown in FIG. 4b, in which point light spots located at the inter-section points of a quasi-rectilinear square array are formed. The position of the image points and the intensity distribution of these, with a cross network, can be obtained by Fraunhofer network calculations. The points of intersection of two families of hyperbolas form the places of interference maxima for diffraction on a network of points. Let d be the lattice constant, oco the angle of incidence in the plane parallel to one of the gratings and R0 the angle of incidence in the plane parallel to the other lattice, we have the following relation for the angles a and (3 diffracted images in these two mutually perpendicular planes: sin a - sin ao = n * X / d (n = 0, + 1-1, + 1-2,) sin 13 sin 130 = m * d ( m = 0, +/- 1, +/- 2,).

  For the use according to the invention of a cross network in a laser warning detector, the following characteristics are important: É Monochromatic point sources are reproduced in the form of point networks with marked intensity maxima in the image plane of the camera, while the broadband punctual sources appear as elongated lines and are thus clearly differentiated.

  É extensive sources of broadband light produce a fuzzy mosaic over the entire image area; the radiation of the background is thus homogenized over the entire image area, which facilitates the identification of point images of laser sources.

  E the zero order of the diffracted model is located on the main ray, that is to say crosses the network without undergoing diffraction. This direction is also the axis of symmetry of higher orders. The direction of the radiation source can therefore be determined accurately from the diffracted model.

  The diffraction angle is shifted as a function of the wavelength AX according to the formula Aa = n / d * AX (same for the angle fi), that is to say that the wavelength The light source can be determined from the angular position of the diffraction maxima.

  When the cross is rotated, the pattern of light spots also rotates around the axis of symmetry. The direction of the light source with respect to the optical axis of the camera can then be determined accurately.

  We will take a ciphered example to describe the relationships when detecting two laser sources with different wavelengths: Let the wavelengths 2L1 = 1.064 m (for example Nd: YAG laser) and X2 = 0.904 m (by example GaAs laser diode), a lattice constant d = 10 m and angles of incidence a3 = (30 = 0, the diffraction angles are a = = 6.01, for long wavelengths, and 5.4, for shorter wavelengths, for the higher order, the diffraction angle is a multiple, and for weaker gratings the wavelength resolution increases, and with it the diffraction angle With about 600 lines and rows of a detector array and with a scan angle of the camera of 90, the angular resolution of one pixel is 0.15, for a lattice constant of 2 m The spectral resolution of a pixel in the first diffraction order is about 5 nm. Pectrale of a laser diode for a weapon guided by alignment on the spoke is about 3 nm.

  If the detector matrix now records several isolated light patterns, it is possible to know from the geometric distribution of these in the image, whether they are higher orders of a coherent laser source. This can be determined on the basis of the symmetry of the pattern of light spots and the identical brightness of all the light spots belonging to a given order. The problem can be electronically adjusted, for example by comparing in intensity the signals of each individual pixel in the focal plane to the signals of neighboring pixels. If it turns out that the intensity of a pixel is much greater than that of the neighboring pixels, the coordinates and the signal values thereof are recorded.

  The entire image can thus be reduced to a model of independent 988 pixels at relatively high intensity. Para-site signals can be eliminated at this stage by taking into account only the pixels that form concentric squares. When only a regular bridge model remains, it is very likely that it is a laser source.

  From the size of the squares, one can calculate the wavelength of the laser source and compare it for example to va-them stored in a library of threats, in order to obtain an additional confirmation of the threat. By comparing the point models thus obtained from a series of images of a camera, it is possible to calculate and track the movements of the laser source relative to the target. According to the same principle, it is possible to rapidly isolate several laser sources, classify them and examine them separately. For a specialist trained in electronic signal processing, this problem can be solved with a simple microprocessor without any specific image processing in a computer.

  Cross networks can be configured as transmission networks or reflection networks. These can be in the form of amplitude networks as well as phase networks. The superiority of the phase gratings lies in their much greater transmission, the radiation being lost on the screen network portion in the amplitude gratings.

  A particular case of networks is the so-called sine network with a locally cos2-like evolution of the amplitude transmission with an amplitude network or the refractive index with a phase grating. With this type of network only the zero order and the +/- 1 order are produced in the diffracted spectrum. Moreover, these networks, because of their high efficiency in the transmission of light in the order one, are particularly well suited for the detection of weak laser sources.

  With regard to the realization of networks, holography manufacturing is becoming more and more widespread. Two waves with a small deviation of direction, produced by division of a laser beam, strike a layer of photosensitive lacquer and produce on it a pattern of interference bands which can be transformed into a network structure. Cross networks can thus be produced by a double illumination of models of interference bands of this type arranged at right angles. This technique makes it possible to manufacture both phase gratings and amplitude gratings for the transmitted light mode. Phase gratings are obtained by the known method of discoloration of the amplitude structure. The difference of illumination of the layer in the light bands and the dark bands can also be converted a variation of thickness (profile with furrows) and used as phase network in transmission. The application of an aluminum layer in the gas phase makes it possible to obtain a reflection network.

  The sinewave phase gratings are particularly suitable for application in a laser warning device in the visible range and the near infrared range between 0.35 and 2.5 m. These can be applied for example on silica glass and used, for example as a transmission lens placed in front of a camera. In the infrared range beyond 2 m one can use either amplitude transmission holograms for the transmission mode, or reflection gratings for the reflection mode, which are placed in front of a camera. In the infrared range around 10 m (CO2 laser), reflection networks are mainly used. Networks called ladders with a sawtooth groove profile are particularly well suited for a dis-positive laser warning. The inclination of the grooves is chosen so that for a wavelength corresponding to a maximum intensity, the reflection direction and the diffraction direction coincide. The corresponding order n is also preferred.

  Another possibility of cross-network optimization lies in the sizing of the modulation depth and the network constants or even the network location frequency. Low-frequency location networks have, as we know, a large number of diffracted orders whose intensities behave like the Bessel function squares. With strong modulation, the intensity increases in higher orders. If we reduce the modulation, the intensity in the higher orders decreases in favor of the weaker orders.

  The optimization work for laser warning devices then consists in obtaining a maximum intensity in the first orders. This maximum is theoretically 33% for a linear network. For a cross network it is deduced that each time 10% remains for the four interesting orders. The remaining 60% of incident light is spread over the other orders.

  The diffraction efficiency can be considerably improved in cross networks by increasing the frequency of location. For location frequencies of about 400 to 500 line / mm pairs in holographic phase gratings, the intermediate region is between thin holograms and thick holograms. Higher orders are already far fewer. By using even higher place frequencies, for example 700 pairs of lines / mm, it is possible to eliminate the higher orders almost completely. The zero order light fraction can be kept below 20% so that each of the four first order diffracted images receives about 20% of the light.

  Lasers used for military and security applications are limited to a few relatively narrow wavelength ranges between 800 and 850 nm, 1050 and 1070 nm, 1450 and 1650 nm and 9.5 and 11.5 m. To limit the annoying radiation of the background it is advantageous to place a spectral filter in front of the lens of the camera, in addition to the cross network used. With a filter width of, for example, 10-20 nm in the near-infrared range, it is possible to decrease the background by a factor of 10 to 20.

  The sensitivity threshold of a commercial CCD camera for detecting a continuous laser source with an integration time of 20 ms across the cross network is about 6 pW. Signals resulting from the scattered radiation of a machine guided by alignment at a distance of 10-1 km are in the range of 1 pW to 1 nW, ie in the detection range of a device laser warning device according to the invention. A further increase in the sensitivity of CCD cameras is possible by increasing the integration time, by cooling the detector or by placing an electron multiplier stage (eg microchannel chip with amplification of 2888333 12 10000 times) in front of the network. detector. This latter possibility is represented in the embodiment of FIG. 2. In this case, a luminescent screen 26, an electron multiplier 27, a photocathode 28, an additional lens 22.2, a cross network 23 and a spectral filter 24 are arranged upstream of a first lens 22.1 in the focal plane of which is disposed a detector array 21 connected to a microprocessor 25. The elements 26, 27, 28 constitute what is called a residual light intensifier whose the image is then reproduced on the detector array 21.

  The angular sweep range of a necessary laser warning device will be different depending on the application. For many applications, normal lenses with an angle of view of 40 - 55 will suffice. For a panoramic detection, for example for a helicopter, 4 laser warning devices of this type are generally installed at different points of the cabin, each device covering an angle of 90.

  Another possibility of panoramic detection of laser threats is shown in FIG. 3. In this, a convex mirror 34 is placed in front of a laser warning device according to FIG. 1 or 2 which comprises a detector array 31, an optical system. of reproduction 32 and a cross network 33; the convex mirror receives light from a horizontal plane I at a horizontal image angle of 90 (for example 60 above plane I and 30 below plane I) and reproduces the area covered inside an annular surface II on the detector 31.

  Each direction of the covered area corresponds to an image point on the annular surface II.

  The laser source detection method described herein can be implemented and combined with very different cameras. In particular, instead of detector arrays, individual detectors specifically designed with an upstream optical scanning device (scanner) can be used.

Claims (2)

13 Claims   1. A device for detecting and locating laser radiation sources comprising a radiation-sensitive detector disposed in the image field of a reproduction optics and an electronic signal processing means connected to the detector, characterized in that a cross network (3) is arranged between the laser radiation source and the optics (2), so that the diffracted orders of the cross network (3) are reproduced on the detector (1).   2. Device according to claim 1, characterized in that the cross network (3) is shaped transmission network or reflection network.   3. Device according to claim 1 or 2, characterized in that the cross network (3) is in amplitude network or in phase network.   4. Device according to any one of claims 1 to 3, characterized in that the cross network (3) can rotate around the optical axis of the reproduction optics.   5. Device according to claim 4, characterized in that the rotation of the cross network (3) takes place continuously with a predefinable rotational speed.   6. Device according to any one of claims 1 to 5, characterized in that a cross network is used which generates only the order 0 and the order one, for example a sine network.   7. Device according to any one of claims 1 to 6, characterized in that a residual light intensifier is disposed between the cross network (23) and the reproduction optics (22.1).   8. Device according to any one of claims 1 to 7, characterized in that a spectral filter (4, 24) is placed in front of the cross network (3, 23).   9. Device according to any one of claims 1 to 8, characterized in that a deflection optic (34), in particular a convex mirror, is disposed in front of the cross network (33).   A method for detecting and locating laser radiation sources using a device as claimed in claim 1, characterized in that by means of the processing means, the analysis is performed. image produced on the signal detector for spot light spots, the position of which is recorded within the image field and the location of an image point corresponding to the image zero order of the stain pattern shown.   11. The method of claim 10, characterized in that using a variable threshold value is successively eliminated the images of the light spots which each have the lowest intensity and the fact that at From the places of the light spots removed, the center of symmetry of the light spot pattern is determined.   12. A method for detecting and locating laser radiation sources using a device according to claim 4, characterized in that by means of the signal processing means, the image produced on the detector for searching for at least one point light spot determining the zero order of a reproduced pattern of light spots, and recording its position within the image field.   13. Method according to any one of claims 10 to 12, characterized in that the position of the zero-order spot or the center of symmetry of the light spot pattern relative to the center of the image is determined. and the angular position of the laser radiation source with respect to the optical axis of the reproduction optics is deduced therefrom.   14. Method according to any one of claims 10 to 13, characterized in that the cross network is fixed, the image is examined on the detector for spot spots-the squares arranged in a square, which are determined distance from the nearest neighbor spot and compare it to a predefined range value.