Method and arrangement for determining the direction to a laser beam source

Abstract: Method and arrangement for determining the direction to a laser beam source. Amplitude peaks in the light occurring when a pulsed laser beam generated by the laser beam source passes through an air space are detect­ ed. The detection is carried out with the aid of at least one rotating receiver lobe which images the scattered light on an associated fixed detector. The detected amplitude peaks are compared in order to determine the greatest amplitude peak. The angle of rotation value of the receiver lobe, when the greatest amplitude peak is detected, is set as belonging to a limited angle range within which the direction to the laser beam source is assumed to lie. The arrangement comprises a rotatable optics system, a fixed detector system which detects the magnitude of the light transmitted through the optics system to the fixed detector system, and an evaluation part 9 which compares detected amplitude peaks in order to establish the direction to the laser beam source. The optics system comprises at least one light input with, counting from each light input, a first cylindrical lens, prism, reflector, filter, second cylindrical lens, diaphragm and glass block. By using a rotating receiver lobe instead of conventional laser-warning systems with a number of fixed receiver lobes, a low-component and thus uncomplicated direction-determining arrangement is obtained.

Description: The present invention relates to a method for determining the direction to a laser beam source emitting a pulsed laser beam, by detecting the light occurring within the scattering area of the laser beam when the pulsed laser beam generated by the laser beam source passes through an air space, which light has a pulsed character, the light being detected by one or more fixed detectors with the aid of at least one rotating receiver lobe with a given limited lobe breadth in the plane of rotation of the lobe for each detector. The invention also relates to an arrangement for carrying out the method. A method determining the position of a laser beam is previously known from EP - 0 269 142. According to a described embodiment the time distance between the front and rear flank of the scattered radiation reaching the detector device, i. e. the pulse width or pulse length of the scattered radiation is determined at two different angular positions of a rotatable sensitivity or receiver lobe. The method involves rather complex mathematical calculations requiring use of approximative methods, for example the Newton-Raphsons iteration method. The position of the laser beam is expressed as an angle between a reference line and the optical axis of the laser beam and as the shortest distance between the measuring point and the optical axis. There is, however, no determining of the direction to the laser beam source. Information about the direction to a laser beam source is a very valuable information when defeating an enemy and which is provided in accordence with the method applied for a patent. In a conventional method for determining the direction to a laser beam source, use is made of a great number of fixed receiver lobes facing in different directions. Since each receiver lobe is allocated a specific direction, an evaluation circuit can, on the basis of the light signal picked up essentially at the same time by the respective receiver lobes, establish the direction to the laser beam source. Such a method is known from for example US-A-4 687 923. The conventional method known from said US patent is inter alia used in so-called laser-warning systems. Such laser-warning systems are mounted on tanks for example, with the purpose of warning the crew that they may be exposed to a threat. The threat can consist of a missile which is directed towards its target by a laser pointer. In order to be able to accurately determine the direction to the laser beam source, a large number of fixed receiver lobes with associated optics and detectors are required. Known laser-warning systems constructed in accordance with the conventional method with fixed receiver lobes are therefore both expensive and complicated. The light from a laser beam source can be divided into three types of radiation, namely scattered, direct and reflected radiation. It is the scattered radiation which is of greatest interest for the invention. The scattered radiation from a laser beam source with a narrow beam has a weakly decreasing amplitude with increasing distance from the laser within the area in question (for example 0.2 km to a distance which is less than the meterological visibility prevailing at the time), when the measurement of the amplitude is carried out along a line which is parallel to the midline of the laser beam and when the angle of the receiver lobe to the laser lobe is constant. Also as regards this radiation, when the said angle is constant, the power amplitude is halved when the perpendicular distance from the midline of the laser lobe to the receiver is doubled. Finally, the power amplitude is halved when the said angle is doubled within the range from about 0° to over 60° . The aim of the present invention is to provide a method for determining the direction to a laser beam source, which method affords high reliability of directional determination and is easy to implement at low cost in the form of a reliable, low- component and therefore uncomplicated direction-determining arrangement. The aim of the invention is achieved by means of a method which is characterized in that the amplitude peaks of the pulsed character of the light are detected during the rotation of the rotating receiver lobe, the amplitude peaks detected being related to the angle of rotation, which is the direction of the receiver lobe relative to the direction of the receiver to the laser beam source and separated in time, in that the detected and time separated amplitude peaks are compared in order to determine the greatest amplitude peak, and in that the angle of rotation value of the associated receiver lobe, when the greatest amplitude peak is detected, is used to determine, relative to the said angle of rotation value and corresponding to the said greatest amplitude peak, a limited, non-symmetrical angle range within which the direction of the receiver to the laser beam source is assumed to be included, and an arrangement for carrying out the method according to claim 1 characterized in that the rotatable optics system for the generation of said at least one receiver lobe comprises, counting from each light input, a first cylindrical lens, prism, reflector, filter, second cylindrical lens, diaphragm and glass block, that the fixed detectors are adapted to detect amplitude peaks occurring within the receiver lobes, and that the evaluation unit, on the basis of the detected amplitude peaks together with associated angle of rotation values, which respective value indicates the direction of the receiver lobe relative to the direction of the receiver to the laser beam source, is adapted to determine relative to the angle of rotation value corresponding to the greatest amplitude peak a limited, non-symmetrical angle range within which the direction to the laser beam source is assumed to be included. It is thus possible, with the aid of a single receiver lobe which is rotated, to determine the direction to the laser beam source by studying the amplitude peaks which occur in the detector during rotation of the receiver lobe. A limited angular range in association with the direction of the greatest amplitude peak is assumed to include the actual direction to the laser beam source. In accordance with an advantageous embodiment of the method according to the invention, the angle of rotation to the laser beam source is set as the angle of rotation corresponding to the greatest amplitude peak corrected by an angle of rotation corresponding to half the distance between two adjoining amplitude peaks. By providing the optics system with two light inputs and thus two receiver lobes, an arrangement can be obtained which permits faster determination of the direction to the laser beam source. The invention will be described in greater detail below by means of examples and with reference to the attached drawings, in which Figure 1 illustrates a method for determining the direction to a laser beam source, Figure 2 shows an example of amplitude peaks which can occur during scanning of one turn, Figure 3 shows a side view of a first exemplary embodiment of an arrangement according to the invention for determining the direction to a laser beam source, Figure 4 shows a side view of a second exemplary embodiment of an arrangement according to the invention for determining the direction to a laser beam source, and Figure 5 shows a side view of a third exemplary embodiment of an arrangement according to the invention for determining the direction to a laser beam source. In Figure 1 a laser beam source has been designated by 1. The laser beam source 1 can consist, for example, of a Nd YAG-laser which emits pulsed laser radiation with a pulse length of the order of magnitude of 15ns and a pulse spacing of the order of magnitude of 0.1s. The transmitter lobe of the laser beam source is designated by 2. A receiver lobe 3 rotating preferably in the horizontal plane scans the air space. The receiver lobe 3 can advantageously have a horizontal lobe breadth of the order of magnitude of 10° and a vertical lobe breadth of between 60° and 80°. The field strength of the transmitter lobe is made up of scattered light and direct light. The detected scattered light varies with the direction of rotation of the sensitivity lobe and shows an insignificant quantity of speckles, in contrast to the direct beam which shows a number of speckles upon thermal atmospheric turbulence. In the invention it is the scattered light which is primarily of interest for the directional determination. When the receiver lobe is rotated for scanning the air space and the receiver lobe scans part of the transmitter lobe 2, the receiver lobe can detect any amplitude peaks occurring in the transmitter lobe as scattered light. Figure 2 shows examples of amplitude peaks A which can be obtained during rotation of the receiver lobe about one turn in the horizontal plane as a function of the angle of rotation  of the receiver lobe. The direction to the laser beam source is established on the basis of the greatest amplitude peak. Since the laser beam source does not transmit continuously, the direction corresponding to the greatest amplitude peak is not necessarily the exact direction to the laser beam source. It is possible that a greater amplitude peak could have been detected if the time relations between the laser pulses of the laser beam source and the rotation of the receiver lobe had been different. On average, the greatest amplitude peak is situated half way between two neighbouring amplitude peaks from the correct direction to the laser beam source. The angle of rotation corresponding to the greatest amplitude peak is therefore corrected by an angle value corresponding to half the distance between two amplitude peaks in order to obtain a probably more correct determination of direction to the laser beam source. The detected amplitude peaks are amplified in an amplifier 4 and are advantageously converted to a digital signal in an analog-to-digital convertor 5. The digital signal is evaluated in an evaluation unit 6 which determines the greatest amplitude peak during a rotational turn of the receiver lobe and corrects the angle of rotation corresponding to the greatest amplitude peak by an angle value corresponding to half the distance between two amplitude peaks. An arrangement for determining the direction to the laser beam source is shown in Figure 3 in a first exemplary embodiment. The arrangement includes a rotatable optics system 7, a fixed detector 8 and an evaluation part 9. The optics system is made up of a cylindrical lens 10 which receives incident light, which can originate from a laser beam source, and whose cylinder axis lies essentially in the plane of the drawing. The main purpose of the cylindrical lens 10 is to form the receiver lobe of the rotating optics system, with a lobe breadth in the horizontal plane of for example 10°. The cylindrical lens 10 is arranged on the input surface of a prism 11. A reflector 12 is arranged on a second surface of the prism 11, and on a third surface, the output surface of the prism, there is a filter 13. A laser beam incident on the input surface of the prism passes through the prism 11 and is reflected on the reflector 12 in order to then leave the prism 11 through the output surface. The filter 13 limits the wavelength range to be considered and preferably has a transmission breadth of < 50 nm for a wavelength range of incident laser light of the order of magnitude of 1µm. When the laser light has passed the filter 13, the light goes through a second cylindrical lens 14, a diaphragm 15 and a glass block 16 so as to finally strike the fixed detector 8. The cylindrical lenses 10 and 14, which both have their cylinder axes essentially in the plane of the drawing, have light-collecting tasks. The diaphragm 15, together with the first cylindrical lens 10, essentially defines the form of the sensitivity lobe. The prism and glass block contribute to a favourable field of view in the vertical direction. The optics system described affords an even distribution of the received energy across the whole surface of the detector 8. The output signal from the detector 8 is coupled to an evaluation part 9 in which there is an amplifier 4, an analog-to-digital convertor 5 and an evaluation unit 6. The evaluation unit 6 determines the greatest amplitude peak for each rotational turn of the rotatable optics system and performs a correction of the angle of rotation value corresponding to the greatest amplitude peak by an angle value corresponding to half the distance between two adjoining amplitude peaks. The light which is detected by the detector 8 can have a strength which varies very considerably. In order to be able to deal with such variations in strength, that is to say a large dynamics range, parts of the detector can be provided with damping arrangements so that the damped part of the detector contributes to the output signal of the detector when the incident radiation exceeds the saturation value for the non-damped detector part. In this context reference is made in particular to US 3,193,687 and US 3,962,577. A second exemplary embodiment of an arrangement for determining the direction to the laser beam source is shown in Figure 4. The arrangement includes a rotatable optics system 7, fixed detectors 8a and 8b, and an evaluation part 9. The rotatable optics system 7 is divided up into two separate parts a, b which are each made up of a cylindrical lens 10a/10b which receives incident light, which can originate from a laser beam source. The main task of the cylindrical lens 10a/10b is to form the receiver lobe of the rotatable optics system, with a lobe breadth in the horizontal plane of for example 10°. The cylindrical lens is arranged on the input surface of a prism 11a/11b with the cylinder axis essentially in the plane of the drawing. A reflector 12a/12b is arranged on a second surface of the prism 11a/11b, and on a third surface, the output surface of the prism, there is a filter 13a/13b. The two separate parts a, b are separated by a block 17. A light beam incident on the input surface of the prism passes through the prism 11a/11b and is reflected on the reflector 12a/12b in order to then leave the prism 11a/11b through the output surface. The filter 13a/13b limits the wavelength range to be considered and preferably has a transmission breadth of < 50 nm for a wavelength range of incident laser light of the order of magnitude of 1µm. When the laser light has passed the filter 13a/13b, the light passes through a second cylindrical lens 14a/14b, a diaphragm 15a/15b and a glass block 16a/16b so as to finally strike the fixed detector 8a/8b. The cylindrical lenses 10a/10b and 14a/14b have light-collecting tasks, and their cylinder axes are essentially in the plane of the drawing. The diaphragm 15a/15b, together with the first cylindrical lens 10a/10b, essentially defines the form of the sensitivity lobe. The prism and glass block contribute to a favourable field of view in the vertical direction. The optics system described affords an even distribution of the received energy across the whole surfaces of the detectors 8a, 8b. The output signal from the detectors 8a, 8b is coupled to an evaluation part 9 in which there is an amplifier 4, an analog-to-digital convertor 5 and an evaluation unit 6. The evaluation unit 6 determines the greatest amplitude peak for each turn scanned by the rotatable optics system and performs a correction of the angle of rotation value corresponding to the greatest amplitude peak by an angle value corresponding to half the distance between two adjoining amplitude peaks. Figure 5 shows a third exemplary embodiment of an arrangement for determining the direction to the laser beam source. The arrangement includes a rotatable optics system 7, a fixed detector 8 and an evaluation part 9. The optics system has two light inputs. From the light inputs the light is conveyed towards the fixed detector 8 via a separate part for each light input consisting of a first cylindrical lens 10a/10b, a prism 11a/11b and a reflector 12a/12b, and a common part consisting of a filter 13, a second cylindrical lens 14, a diaphragm 15 and glass block 16. In contrast to the exemplary embodiment according to Figure 4, the incoming light is detected here in a single detector. This means that the construction requires fewer components. In Figure 5 the input surfaces have been shown turned 180° relative to each other in the plane of rotation, primarily for purposes of clarification. In the case where a common detector is used and a rotatable optics system with partly common components, a 180° relative turning is normally not suitable, since it is not possible to determine on which light input the incoming light is incident. By changing the relative turning to a value different from 180°, for example 135°, this problem can be avoided. In order to prevent reflection-susceptible objects situated in the vicinity of the direction-determining arrangement from interfering with the arrangement, it is possible to arrange around the direction-determining arrangement a casing (not shown) with maskings which prevents reflections from the reflection-susceptible objects. An example of a reflection-susceptible object is the turret of a submarine.

Claims: Method for determining the direction to a laser beam source (1) emitting a pulsed laser beam, by detecting the light occurring within the scattering area of the laser beam when the pulsed laser beam generated by the laser beam source passes through an air space, which light has a pulsed character, the light being detected by one or more fixed detectors (8; 8a, 8b) with the aid of at least one rotating receiver lobe (3) with a given limited lobe breadth in the plane of rotation of the lobe for each detector, characterized in that the amplitude the pulsed character of the light are detected during amplitude peaks of the pulsed character of the light are detected during the rotation of the at least one rotating receiver lobe (3), the amplitude peaks detected being related to the angle of rotation, which is the direction of the receiver lobe (3) relative to the direction of the receiver to the laser beam source (1) and separated in time, in that the detected and time separated amplitude peaks are compared in order to determine the greatest amplitude peak, and in that the angle of rotation value of the associated receiver lobe, when the greatest amplitude peak is detected, is used to determine, relative to the said angle of rotation value and corresponding to the said greatest amplitude peak, a limited, non-symmetrical angle range within which the direction of the receiver to the laser beam source is assumed to be included. Method according to claim 1, characterized in that the angle of rotation to the laser beam source (1) is set as the said angle of rotation value corresponding to the said greatest amplitude peak corrected by an angle of rotation corresponding to half the distance between two detected adjoining amplitude peaks. Arrangement for carrying out the method according to claim 1 for determining the direction to a laser beam source (1) emitting a pulsed laser beam, the arrangement being adapted to detect the light occurring within the scattering area of the laser beam when the pulsed laser beam generated by the laser beam source passes through an air space, which light has a pulsed character, the light being detected by one or more fixed detectors (8; 8a, 8b) with the aid of at least one rotating receiver lobe (3) with a given limited lobe breadth in the plane of rotation of the lobe for each detector (8; 8a, 8b), the rotating receiver lobes (3) being generated by a rotatable optics system (7) with one light input for each receiver lobe and including lenses and at least one reflector, the detected light being evaluated in an evaluation unit (6), characterized in that the rotatable optics system (7) for the generation of said at least one receiver lobe comprises, counting from each light input, a first cylindrical lens(10; 10a, 10b), prism (11; 11a, 11b), reflector (12; 12a, 12b), filter (13), second cylindrical lens (14), diaphragm (15) and glass block (16), that the fixed detectors (8; 8a, 8b) are adapted to detect amplitude peaks occurring within the receiver lobes (3), and that the evaluation unit (6), on the basis of the detected amplitude peaks together with associated angle of rotation values, which respective value indicates the direction of the receiver lobe (3) relative to the direction of the receiver to the laser beam source (1), is adapted to determine relative to the angle of rotation value corresponding to the greatest amplitude peak a limited, non-symmetrical angle range within which the direction to the laser beam source is assumed to be included. Arrangement according to claim 3, characterized in that the optics system is provided with two light inputs. Arrangement according to claim 4, characterized in that the said filter (13), second cylindrical lens (14), diaphragm (15) and glass block (16) which are incorporated in the rotatable optics system (7) are common to the two light inputs, and in that the detector system consists of a single fixed detector (8). Arrangement according to claim 4, characterized in that the light inputs in the plane of rotation of the rotatable optics system (7) are turned 135° relative to each other. Arrangement according to claim 4, characterized in that the optics system (7) is made up of two completely separated parts which are designed to cooperate each with a light input, and in that the detector system consists of a fixed detector (8a resp.8b) for each one of the two separate parts. Arrangement according to claim 7, characterized in that the light inputs in the plane of rotation of the rotatable optics system (7) are turned 180° relative to each other.

Inventors: BJOERKMAN BENGT, BJOERKMAN, BENGT, Björkman, Bengt
Applicants: CELSIUSTECH ELECTRONICS AB
Application year: 1990
Main classification: G01S3
IPC codes: G01S3, G01S7, G01C15