FR2872293A1 - LASER MEASURING APPARATUS - Google Patents

Abstract

Laser measuring device (2) comprising a laser (4) for emitting pulses of light, a detection unit (42) with a number of detection elements (44), optical means (40) for directing pulses of light into a number of details of an object scene and for directing pulses of light reflected from the details onto the sensing elements (44), and having control means (48). It comprises at least one other laser (4) for the emission of light pulses, the control means (48) being designed for the control of the lasers (4) so that a second laser (4) does not emit a light pulse only when a first laser (4) has finished emitting a light pulse.

Description

The invention starts from a laser measuring apparatus comprising a laser for

  the emission of light pulses, a detection unit

  with a number of sensing elements, optical means for directing the light pulses in a number of details of a scene-object and for directing light pulses reflected by the details on the sensing elements, and comprising control means.

  A Laser Detection and Ranging (LADAR) laser is particularly suitable for obstacle signaling and high-resolution target recognition, particularly for aircraft and missiles. For medium and large measuring distances, the method of measuring pulse duration is accurate and particularly reliable. A laser pulse is emitted here in the direction of a selected detail of a surrounding scene-object, it is reflected or diffused on an object in detail and the time between transmission and return of the pulse is measured. light. From the measured time, the distance between the laser measuring device and the object is calculated.

  For an obstacle signaling or a precise target recognition, it is advantageous to implement a graphic method, the scene-object being divided into a matrix of details and the various details being analyzed either in parallel or sequentially at high speed with the method of measuring the pulse duration.

  The object of the present invention is to provide a laser measuring apparatus which allows measurements with a high resolution and a high repetition frequency.

  This object is achieved with a laser measuring apparatus of the aforementioned type which comprises, according to the invention, at least one other laser for the emission of light pulses, the control means being designed for the control of the lasers in a manner that a second laser emits a light pulse only when a first laser has completed the emission of a light pulse.

  The invention proceeds from the consideration that a high repetition rate can be achieved with parallel illumination of the scene-object each time by a laser pulse. Detection can then be performed in a matrix detector with an independent time measurement for each individual sensing element. Matrix detectors of this type with satisfactory resolution are very expensive to manufacture. In the case of an alternative sequential scanning of the scene-object, at least one laser pulse is required for each detail of the scene-object. To obtain a high repetition frequency, a high frequency of repetition of the pulses of the laser is needed here.

  The invention proceeds from this other consideration that a high distance resolution can be obtained with extremely short laser pulses of the order of a few nanoseconds. To obtain sufficient energy for reliable detection of the rediffused light pulse, the very short light pulse must be emitted with a very high power. Due to the thermal load of the laser, thus caused, the repetition frequency of the pulses and therefore the repetition rate of the images are limited. If several lasers are used for the emission of light pulses, the thermal load can be distributed among several lasers. By sequential control of the lasers, it is possible to obtain an unequivocal assignment of a re-scattered and detected light pulse to a transmission laser and thus a unambiguous measurement of the pulse duration. It is possible to guarantee a high distance resolution and a high image repetition rate.

  In addition to measuring the distance, it is also possible to measure other parameters by the laser measuring apparatus, for example the light frequency, the light polarization, the light phase, etc. The pulses of light can be emitted with any laser frequency possible, and preferably radiation is selected which is gentle on the eyes, in particular infrared radiation which protects the eyes. Lasers are advantageously designed as master oscillators. It is possible to obtain a particularly simple assignment of a received laser pulse, if at each moment at most one laser is active. The control unit is advantageously designed for an assignment of details to the detection elements. This can be achieved by a graphical laser measurement.

  The thermal load of the lasers can be kept low, in the case of high pulse power, if each laser is associated with a light amplifying element. For the laser meter as a whole a single light amplification element can be provided here which amplifies the light pulses of all the lasers. It is also possible to associate a light amplification element with one of several laser groups of the laser meter. In particular, the laser meter comprises a number of light amplifying elements, each light amplifying element being associated with a single laser and connected thereto optically. As a light amplifying element, an amplification fiber is particularly suitable because, due to its shape, it has a large surface and can therefore provide a large amount of energy.

  In one embodiment of the invention, the laser meter comprises a number of light amplifying elements and a plurality of pumping optical elements for exciting the light amplifying elements, each pumping element being optically connected to a plurality of light amplification elements for optical pumping. The pumping energy of the pumping elements can be used in this way by a plurality of light amplification elements. In addition, the energy of the pumping elements can be used particularly effectively in the case of sequential control of the lasers. The number of pumping elements can thus be kept reduced. The pumping elements may comprise a laser diode which is connected by an optical guide to the light amplification element t o.

  In a particularly preferred embodiment of the invention, the pumping elements comprise conductive pumping optical fibers and the light amplifying elements and the pumping optical fibers are combined into a bundle of fibers, the amplification elements of light in the fiber bundle being optically connected, along their longitudinal fiber faces, to pumping optical fibers. The light from the pump optical fibers can be transmitted uniformly to the light amplifying elements, the pump light being available to a plurality of light amplifying elements by each pumping element. The optical fibers for pumping may be ordinary fibers of optical guides, such as, for example, glass fibers. In order to obtain as uniform a passage as possible from the pumping light from the pumping optical fibers to the light amplifying elements, the light amplifying elements and the pumping optical fibers advantageously extend in parallel and thus are united by as tightly as possible in the fiber bundle. The fiber bundle advantageously comprises a sheath of a material having a low refractive index, which surrounds the pump optical fibers and the light amplification elements.

  In another embodiment of the invention, each light amplifying element is directly surrounded in the fiber bundle by a plurality of pumping optical fibers and is optically connected thereto in the fiber bundle. The light of multiple pumping optical fibers can be transmitted directly from the pumping optical fibers to the amplifying elements, thereby keeping the loss of pumping light low.

  A particularly intensive passage of the pumping light from the pumping optical fibers to the light amplifying elements, and thus a short fiber bundle, can be achieved if at least one pumping optical fiber and one pumping element are present. Each of the light amplifiers has a longitudinal face of flattened fiber and the pumping light fiber and the light amplifying element are joined to each other along the longitudinal face of flattened fiber. A large passage surface may be used for passage of the pump light from the pump optical fiber to the light amplification element. Advantageously, all the pump optical fibers and the light amplification elements are equipped with a longitudinal face of flattened fiber and are assembled to each other by this flattened face. The passage of light can be further improved if the pump optical fibers and the light amplification elements have a hexagonal or square cross-section and are thus clamped as close together as possible.

  Advantageously, two light amplifying elements are separated from each other in the space in the fiber bundle by at least one optical pumping fiber. The direct passage of the pumping light from one light amplifying element to another can be effectively countered, and thus a sample can be taken from the pumping light in an amplification element. from light. With this same advantage, the light amplification elements and the optical pumping fibers are advantageously arranged in the fiber bundle so as to obtain an optical connection between two light amplification elements, only indirectly by an optical fiber for pumping. .

  In another embodiment of the invention, it is proposed that all the optical pump fibers are optically connected to each other in the fiber bundle. All light amplification elements can access the total pump light.

  It is furthermore proposed that the light amplifying elements are connected to optical guides which are guided in parallel in a plane, at least in part, on the side of the light amplification elements turned away from the lasers. . The optical guides are arranged in this part, with their cross-section in a line, so that the guiding of the light pulses from the optical guide in details, arranged in line, of the scene-object, is particularly simple.

  With an additional mobile optical unit, which is provided for guiding light pulses from an optical guide in columnar details, the light pulses can be directed from the light amplifying elements into a matrix of details of the light. the scene-object. With the light pulses reflected from this matrix, it is possible to produce an image of the scene-object with, for example, information relating to the distance.

  In another embodiment of the invention, the control means for controlling the lasers are designed so that a third laser emits a light pulse together with the first laser, and a fourth laser emits a light pulse together with the second laser. It is possible to obtain a particularly high frequency of repetition of the images. The separation of the light pulses for the purpose of the unambiguous recognition of the signals can be effected for example by coding or optical separation on different details or detection elements of the detector. The details of the scene-object are thus illuminated both sequentially and in parallel.

  Advantageously, at least two pumping optical fibers are arranged between a light amplification element associated with the third laser and a light amplification element associated with the first laser. Optical contact between the light amplifying elements in the fiber bundle is only possible by at least two pumping optical fibers. This can result in a new rapid excitation of the light amplification elements simultaneously de-energized by the light pulses, without the need for a pumping optical fiber to serve the two light amplification elements or even for large amounts of light. pumping light from one light amplifying element undesirably pass into the other light amplifying element.

  An emission of light pulses from the lasers into selected details of the scene-object can be obtained if the lasers are each optically connected to an optical guide, if the optical guides each comprise an optical guide end and if the control unit is designed for an assignment of optical guide ends to details. The optical guide ends can be directed in detail by an optical unit of the optical means, which is able, for example, by a tilting, to direct light pulses from one optical guide end, sequentially, to several selected details of the scene-object.

  Other advantages are apparent from the following description of the drawings. The drawings show exemplary embodiments of the invention.

  It is shown: Fig. 1 a laser measuring apparatus intended to be arranged in an aircraft, FIG. 2 a cross-sectional view of a bundle of fibers with pumping fibers and light amplifying elements, FIG. 3 a detail of a cross-section of a fiber bundle variant, FIG. 4 a diagram of the variation over time of a light pulse emission, and FIG. Another diagram of a variation variant of a light pulse emission FIG. 1 shows a laser measuring apparatus 2 with a number, for example seven lasers 4, only three of which are schematically represented in FIG. form of boxes, for clarity. The lasers 4 are master oscillators for the emission of excitation light pulses which are directed, by optical guides 6 and a connecting element 8, into light amplification elements Al-A7 (FIG. 2). In the connecting element 8, the optical guides 6 are connected to seven light amplification elements Al-A7.

  Each of the seven lasers 4 is connected in this way to only one of the seven light amplifying elements Ai-A7.

  The light amplification elements A1-A7 are shown in FIG. 2 in cross section. They comprise an erbium-doped active laser core which is surrounded by an optically conductive pumping sheath 12. Each of the light amplification elements A1-A7 is in turn surrounded by six pumping optical fibers 14 which are each optically connected to laser diodes 18 by an optical guide 16. The laser diodes 18 form an integral part, together with the guides. 16 optical and pumping optical fibers 14, pumping elements 20 for optical pumping light amplification elements A1-A7.

  For optical pumping of the light amplifying elements A1-A7, the laser diodes 18 emit pumping light in a pumping frequency, which radiates in the pumping optical fibers 14. Inside the pumping optical fibers 14, the pumping light is reflected. In order to prevent pumping optical fibers 14 from escaping from the pumping light 14, the pump optical fibers 14 and the light amplifying elements A1-A7 are surrounded by a fiber sheath 22 which is made of a material having an optical refractive index which is less than the refractive index of the material of the pump optical fibers 14 or the pump casings 12. By an optical contact of the pump optical fibers 14 with each other and with the amplification elements After a certain distance through a pumping optical fiber 14, the pumping light is located in one of the pumping ducts 12 of the light amplifying elements A1-A7 and reflected back to the light source A1-A7. inside the pump sheath 12 until it enters the core 10 and acts on the laser activity material of the core 10.

  The light amplifying elements A1-A7, the pump optical fibers 14, the fiber sheath 22 and a fiber jacket 24 protecting against external influences form an integral part of a bundle of fibers 26. The length of the bundle The fiber is selected so as to obtain as complete a passage as possible of the pumping light from the pump optical fibers 14 in the light amplification elements A1-A7. The bundle of fibers 26 is wound for the most part, over several meters in length, in a receptacle 28.

  On the opposite side of the lasers 4 there is provided a connecting element 30 on the fiber bundle 26, in which the light amplifying elements A1-A7 are each connected to another optical guide 32. pumping optical fibers terminate near the connecting element 30.

  The seven optical guides 32 each include an optical guide end 34 which is disposed in a radiation unit 36. The seven optical guide ends 34 are disposed in a plane and optionally coated to reduce radiation losses. In FIG. 1, above the radiation unit 36, optical means 40 are shown which are indicated only schematically. Optical means 40 serves to direct light pulses through the optical guide ends 34, each in a detail of a scene-object surrounding the laser meter 2, and to guide pulses of reflected light. on detection elements 44 of a detection unit 42. The radiation unit 36, together with optical means 40 and the detection unit 42, is part of an optical device 38 represented as a whole schematically only . The optical device 38 can be pivoted by cardan suspension 46 in two sphere coordinates, respectively 180, so that the radiation unit 36 and the detection unit 42 can be tilted for example with respect to the container. 28 or to control means 48. For displacement of the optical device 38 there is provided a displacement means 50, for example in the form of electric motors which are controlled by the control means 48 which comprise one or more control units .

  The optical guides 32 are guided in parallel at least in the region of the radiation unit 36, on the side of the light amplifying elements Al-A7 turned away from the lasers 4, so that in combination with the optical means 40, they can illuminate a line of details of the scene-object. By a simple tilting of the optical device 38, transversely to the line, a second line of details of the scene-object, etc., can be illuminated, until a desired matrix of details of the scene-object is illuminated by pulses of light.

  The light amplification elements A1-A7 and the pump optical fibers 14 are respectively parallel and very tight in the fiber bundle 26. The light amplifying elements A1-A7 are optically connected here, along their longitudinal faces of fibers 52, to the optical fibers for pumping 14. All the optical pump fibers 14 are connected to each other optically in the fiber bundle 26. From the pumping light, guided in a pumping optical fiber 14, can reach in this way all the light amplification elements A1-A7. To achieve a particularly intensive optical connection, the cross sections of the pump optical fibers 14 and the light amplifying elements Al-A7 may have a geometry that differs from the circular cross-section shown in FIG. 2, for example a hexagonal section. As shown in FIG. 3, the light amplifying elements A 'and the pumping optical fibers 14' are flattened against each other along their longitudinal fiber faces 52 ', which makes that the pumping light from the pump optical fibers 14 'penetrates very rapidly from the pump optical fibers 14' into pump ducts 12 'of the light amplifying elements A'. In place of the hexagonal geometry shown in FIG. 3, a square geometry of the cross-section of the pump optical fibers and the light amplification elements can also be imagined.

  For the emission of a laser pulse by the laser measuring apparatus 2, the light amplifying elements Ai-A7 are pumped as described above. At a first instant ti, fixed by the control means 48, one of the lasers 4 emits an excitation light pulse (see FIG. 4). This laser 4 is optically connected for example to the light amplification element A1-A2. The excitation light pulse is introduced into the core 10 of the light amplifying element A1 and at this point causes a massive deexcitation of the core material. A strong laser pulse with a laser frequency different from the pumping frequency is produced by the light amplifying element A1 and guided by one of the optical guides 32 to the optical guide end 34 of the optical guide 32, and there is emitted in a detail of the scene-object. The control means 48 are designed for this purpose, for an assignment of the optical guide ends 34 and the detection elements 44 to details of the scene-object. A reflected part of the emitted light pulse is recorded at a slightly later time by one or more detection elements 44 which emit a signal corresponding to the control means 48. The duration which elapses between the first instant tl and the subsequent moment, a distance from an object, reflecting the impulse of light, is determined in the detail of the scene-object.

  Then, at a second time t2 the control means 48 control another laser 4 which now in turn emits a pulse of excitation light. This laser 4 is connected to the light amplification element A2. This light amplifying element A2 amplifies the laser excitation light pulse as described above, so that a generated light pulse is emitted by the radiation unit 36 in combination with the means optics 40 in a second detail of the scene-object, and is reflected therein. The reflected light pulse is recorded at a later time by another detection element 44 of the detection unit 42, and a signal is transmitted by this detection element 44 to the control means 48. From the duration which flows between the third instant t3 and the subsequent instant, a distance is determined with respect to an object in the second detail of the scene-object. Then, at a third instant t3, a signal is again emitted by the control means 48 to a third laser 4 which excites the light amplification element A3. An object in a third detail of the scene-object is illuminated and its distance is calculated. In this way, all the lasers 4 are activated successively by the control means 48 and light pulses are emitted by the seven light amplifying elements A 1 -A 7 one after the other, in seven different details of the scene. object.

  FIG. 4 shows in a schematic diagram the progress of this sequential emission of light pulses in details of the scene-object. At the first instant t1, the first laser 4, made as master oscillator Mi, is controlled by the control means 48. During a first duration Ait, this master oscillator Mi emits a pulse of excitation light. Before at the second instant t2 one of the lasers 4, designed as the second master oscillator M2, is controlled by the control means 48, a second duration Ait has elapsed within which the first pulse of light emitted has been reflected and detected by the detection unit 42. It is also possible to choose the second duration Ait smaller, the second time t2 or the third time t3 is however chosen in principle so that a light pulse the next excitation pulse is emitted before the previous excitation light pulse is completed.

  In this way, all the Mi-M7 master oscillators are sequentially controlled and the A1A7 light amplifying elements are de-energized sequentially. The succession of the de-excitation of the light amplifying elements A1-Ai is chosen here so that between a first light amplifying element A1 and a light amplification element A2 subsequently de-energized, there is also a distance as large as possible and as many as possible of pumping optical fibers 14 be disposed. In this way, the de-excitation of the light amplifying element A1 does not influence or insensibly only the continuing excitation of the next light amplifying element A2. The master oscillators M1-M7 are associated with the light amplification elements A1-A7 so that between light amplifying elements A1, A2 or A3, A4 respectively A5, A6, de-energized one after the other, at least two pumping optical fibers 14 are arranged. An optical connection between de-energized light amplifiers is then produced in the fiber bundle 6 only by at least two pumping optical fibers 14. Between the sixth and the seventh this is not the case exceptionally, and in the case of a fiber bundle having a larger number of light amplifying elements, a spacing of at least two Optical fiber pumping is usually possible.

  An alternative time sequence for the emission of light pulses is shown diagrammatically in FIG. 5. The first and second master oscillators M1, M2 are here controlled simultaneously by the control means 48 for the transmission of light. an excitation light pulse. The light amplification elements A1, A2 respectively connected to the master oscillators MI, M2 are opposite each other inside the fiber bundle 26, so that the excitations of the light amplification elements A1, A2 interfering with each other only to a negligible extent. Then, the third and fourth master oscillators M3, M4 and with them the third light amplifying element As and the light amplifying element A4 facing it, are excited. In the arrangement shown in FIG. 2, there remains finally the seventh light amplifying element A7 with the seventh master oscillator M7 which alone is controlled, and it is possible to imagine fiber bundle geometries 26 which allow a control of principle by pair or group of master oscillators or light amplification elements.

  Due to the distant arrangement of simultaneously de-energized Al-A6 light amplifying elements and the fact of at least two pumping optical fibers 14 disposed therebetween, a strong absorption of the light amplifying elements Ai, A3, A5, occurring after de-excitation, does not interfere or to a negligible extent only the simultaneous absorption of the light amplifying elements A2, A4, A6.

  To facilitate the assignment of reflected light pulses received by the detection unit 42, two light pulses transmitted simultaneously may be designed as pulse trains which are modulated differently in frequency or amplitude. It is also possible to emit pulses of light produced simultaneously in details, as far as possible from each other, from the scene-object, so that the light pulses reflected by the optical means 40 reach the elements. detection 44 very far from each other, and can thus be distinguished from each other. This greatly increases the repetition rate of the images.

Claims (12)

  A laser measuring apparatus (2) comprising a laser (4) for emitting light pulses, a detection unit (42) with a number of detection elements (44), optical means ( 40) for directing the light pulses in a number of details of a scene-object and for directing light pulses reflected by the details on the detection elements (44), and having control means (48), characterized by at least one other laser (4) for emitting light pulses, the control means (48) being adapted for controlling the lasers (4) so that a second laser (4) is emits a light pulse only when a first laser (4) has completed the emission of a light pulse.   Laser measuring apparatus (2) according to claim 1, characterized in that each laser (4) is associated with a light amplification element (A1-A7), in particular an amplification fiber.   Laser measuring apparatus (2) according to claim 1 or 2, characterized by a number of light amplifying elements (A1-A7) and a plurality of pumping optical elements (20) for exciting the elements. light amplification unit (A1-A7), each pumping element (20) being optically connected to a plurality of light amplification elements (A1-A7) for optical pumping.   Laser measuring apparatus (2) according to claim 3, characterized in that the pumping elements (20) comprise conductive pumping optical fibers (14) and the light amplifying elements (A1A7) and the fibers pumping optical elements (14) are combined into a bundle of fibers (26), the light amplifying elements (A1-A7) in the bundle of fibers (26) being optically connected along their longitudinal fiber faces ( 52) to pumping optical fibers (14).   Laser measuring apparatus (2) according to claim 4, characterized in that in the fiber bundle (26) each light amplifying element (A1-A7) is directly surrounded by a plurality of optical pump fibers (14). ) and is optically connected thereto in the fiber bundle (26).   6. Laser measuring apparatus (2) according to claim 4 or 5, characterized in that at least one pumping optical fiber (14 ') and one light amplifying element (A') each have a face longitudinally of flattened fiber (52 ') and the pumping optical fibers (14') and the light amplifying element (A ') are joined to each other along the longitudinal fiber face (52') ) flattened.   Laser measuring apparatus (2) according to one of claims 4 to 6, characterized in that two light amplifying elements (A1-A7) are separated from each other in the space in the fiber bundle by at least one pumping optical fiber (14).   8. Laser measuring apparatus (2) according to one of claims 4 to 7, characterized in that all the optical pump fibers (14) are optically connected to one another in the fiber bundle (26).   Laser measuring apparatus (2) according to one of claims 4 to 8, characterized in that the light amplifying elements (A1-A7) are connected to optical guides (32) which are guided in parallel in a plane, at least in part, on the side of the light amplification elements (A1-A7) turned away from the lasers (4).   Laser measuring apparatus (2) according to one of the preceding claims, characterized in that the control means (48) are designed for controlling the lasers (4) so that a third laser (4) emits a light pulse together with the first laser (4), and a fourth laser (4) emits a light pulse together with the second laser (4).   Laser measuring apparatus (2) according to claims 4 and 10, characterized in that at least two pumping optical fibers (14) are arranged between a light amplification element (A1-A7) associated with the third laser (4), and a light amplifying element (A1-A7) associated with the first laser (4).   Laser measuring apparatus (2) according to one of the preceding claims, characterized in that the lasers (4) are each optically connected to an optical guide (32), the optical guides (32) each having one end of optical guide (34) and the control means (48) are adapted to affect the optical guide ends (34) to details.