FR2674342A1 - METHOD AND DEVICE FOR CONCENTRATING THE ENERGY OF A MONOCHROMATIC RADIATION BEAM. - Google Patents

Abstract

The method consists of capturing the radiation (16) emitted by the target, in response to its heating by the beam (15), in a frequency band excluding the frequency of the beam, and moving relative to each other the elements (4 and 5) of an optical system through which the beam passes, so as to increase the intensity of the radiation captured. Application to the destruction of military targets by laser.

Description

1 - Method and device for concentrating on a target the energy of a

  The present invention relates to a method and a device for concentrating on a target the energy of a monochromatic radiation beam such as a high energy laser beam. A device known to focus the energy of a laser beam on a target is described in the US publication entitled

  "Physics of quantum electronics" (C B HOGGE) vol 6 pages 69 to 75.

  This device called "starting wave" essentially comprises a laser generator whose beam is directed at the target after reflection on a deformable mirror and a receiving optical system oriented to capture the energy reflected by a bright spot of the target to concentrate the energy of the beam on the target, the mirror is deformed so as to increase the intensity

  light captured by the receiving optical system.

  This device has drawbacks. In particular, when the target is of large dimensions and thus has several bright spots, it is difficult to adjust the orientation of the receiving optical system on only one of the bright spots. Moreover, the reception of the radiation from of the target is disturbed by the atmospheric backscattering of the energy of the directed laser beam

towards the target.

  In the same American work cited, is described pages 67 to 69, another device for concentrating on a target the energy of a laser beam This device called "return wave" comprises a laser generator whose beam is directed to the target after passing through a dichroic optical separator and reflection on a deformable mirror The thermal radiation emitted by the target is reflected on the deformable mirror, following the same optical path in the opposite direction, then separated from the laser beam by reflection on the separator Ce radiation is finally received on a phase measuring system which delivers error signals to the elements of the deformable mirror so as to compensate for phase irregularities

of the return wave.

  The return wave device has the disadvantage of having an unstable operation, this instability being caused by 2 - the thermal defocusing phenomenon analyzed in the same work

pages 63 to 66.

  The present invention aims to overcome the disadvantages

  known devices mentioned above.

  The present invention relates to a method for concentrating on a target the energy of a monochromatic radiation beam, this beam coming out of an optical transmission system comprising at least two elements, of directing the beam towards the target, sensing the radiation from the target and moving the elements relative to each other so as to increase the intensity of the sensed radiation, characterized in that the radiation from the target is captured only in a predetermined band of frequencies transmitted by the target in response to receiving the beam, the frequency of the radiation

  monochromatic being located outside the predetermined band.

  The present invention also relates to a device for

  focus on a target the energy of a single beam of radiation

  chromatic, comprising a generator of a monochromatic wave, an optical transmission system receiving said monochromatic wave and delivering the beam, this optical system comprising at least two elements, means for orienting the optical transmission system to direct the beam to the target, a reception optical system disposed near the optical transmission system for sensing radiation from the target and means for displacing the elements relative to one another so as to increase intensity of the radiation picked up by the optical reception system, characterized in that the optical reception system comprises an optical filter which does not leave

  radiation from the target only in a predetermined band

  completed frequencies, the frequency of said monochromatic wave

  being outside the predetermined frequency band.

  Particular embodiments of the subject of the present invention are described below, by way of example with reference to the attached drawings, in which FIG. 1 schematically represents an embodiment of the device according to the invention. and FIG. 2 diagrammatically represents another embodiment of the device according to the invention. In FIG. 1, a laser generator 1 delivers a beam

  of monochromatic radiation 2 of high power, along an axis 3.

  The generator 1 may be of the carbon dioxide type emitting at the infrared wavelength of 10.6 microns. The beam 2 passes through a Cassegrain type optical emission system comprising two elements: a convex mirror 4 centered on the axis 3 to receive the beam 2 and a coaxial concave parabolic mirror 5 having an axial breakthrough 6 for passing the beam 2 The reflecting faces of the mirrors 4 and 5 are facing each other so that the beam 2 is reflected successively on the mirrors 4

and 5 to form a beam 15.

  The mirror 4 is integral with the plunger core 6 of an electro-

  magnet 7 via a mechanical connecting member shown

  schematically by a lever 8 The excitation winding of the electro-

  magnet 7 is connected to the output of a generator 9 of an electrical voltage

  The continuous input of the generator 9 is connected to the

  output of a control logic system 10.

  Near the optical transmission system is disposed a reception optical system comprising a detector 11 and an optical filter 12 disposed in front of the sensitive surface 13 of the detector 11. The filter 12 is of the bandpass type: it passes the radiation whose wavelength is between 3 and 5 microns and stops the radiation outside this infrared band The electrical output of the detector 11 is connected to a measuring apparatus 14 whose output is connected

at the entrance of the system 10.

  The assembly constituted by the elements 1 to 8 is mounted on a support not shown movable in site and in deposit, by means of which

  the transmission axis 3 is oriented towards a non-visible target in FIG. 1.

  The device also comprises orientation means for directing the reception axis 16 of the detector 11 towards the target illuminated by the laser beam 15, the target heats up and emits back incoherent infrared radiation 4 Part of this radiation is captured by the detector 11 in the infrared band of 3 to 5 microns that leaves

pass the filter 12.

  It is necessary that the emission wavelength of the beam 15 is located outside the reception band. The emission wavelength of the laser 1 and the reception band are preferably chosen in the infrared range, as in the example described In an alternative embodiment, the laser generator 1 may be a hydrofluoric acid chemical laser emitting at the wavelength of 3.8 microns, the filter 12 then defining an infrared reception band

  in the range of 8 to 10 microns.

  The apparatus 14 measures the intensity of the infrared radiation

  detected by the detector 11, the information of this measurement being transmitted

  put at the input of the control system 10.

  The system 10 regulates the voltage of the generator 9 so as to cause, by displacement of the core 6, a translation of the mirror 4 along the axis 3, in a direction such that it causes a variation in the focal length of the optical system. emission, this variation being apt to increase

  the intensity of the radiation captured by the detector 11.

  Preferably, the system 10 and of a type capable of controlling small successive displacements of the mirror 4, these displacements being of constant amplitude and occurring in one direction or the other of the axis 3 as they cause an increase or

  decrease in the intensity of the radiation captured by the detector 11.

  FIG. 2 shows a laser generator 17 delivering a monochromatic beam 18. An afocal optical system 19 of the Cassegrain type and similar to that of FIG. 1 consists of a convex mirror 20 and a concave mirror 21. optical receives the beam 18 and delivers a beam 22 of increased section, the beam 22 being reflected on a deformable mirror 24 which returns

  a beam 25 to a target not visible in the figure.

  The deformable mirror 24 is fixed on three piezoelectric elements

  electric 26, 27 and 28 each provided with two electrodes and solidarity

  of a rigid plate 29 Of course the number of piezoelectric elements

  In practice, triques is much more than three.

  The device further comprises an electronic signal generator

  30 having three outputs respectively connected to the two electrodes of the piezoelectric elements 26, 27 and 28.

  of the generator 30 is connected to the output of a logic control system 31 connected itself to a memory 32.

  In the vicinity of the deformable mirror 24 is disposed a reception optical system comprising a detector 33 and an optical filter 34 disposed in front of the sensitive surface of the detector 33. The filter 34 is a band-pass filter quite similar to the filter 12 (FIG. and allows the radiation to pass only in a frequency band excluding the emission frequency of the laser 17 The electrical output

  of the detector 33 is connected to the input of a measuring apparatus whose output is connected to the input of the system 31.

  The device shown in Figure 2 operates in the following manner.

  The beam 25, preferably infrared, heats the target which emits back incoherent radiation The detector 33 captures a portion 36 of this radiation through the filter 34 which leaves

  In this case, the radiation is transmitted to the input of the system 31. The irradiation intensity of the radiation measured by the apparatus 3520 is transmitted to the input of the system 31.

  The system 31 controls the transmission of electrical signals by the generator 30.

  In a first step, the generator 30 delivers at each of its outputs a voltage E that causes, by action

  on a piezoelectric element, a small displacement of amplitude D at a point on the surface of the deformable mirror 24, in an arbitrary direction

  When the displacement D causes an increase in the intensity of the radiation picked up by the detector 33, the value + D is stored in the memory 32. On the other hand, when the initial displacement D30 causes a decrease in the intensity of the radiation captured, the generator 30 emits an additional signal -E which causes an additional displacement of the piezoelectric element of amplitude D, but of direction opposite to the initial direction, the value -D then being stored. In the second step the generator 30 delivers signals of amplitude E, the signs of these signals being identical to that of the values stored in the first step These signals 6 - cause displacements of amplitude D, these displacements adding in magnitude and in sign to those of the first step When a piezoelectric element receives a signal which causes an increase of the intensity of the captured radiation, the displacement is maintained, and the value in memory is not modified If by against an element

  piezoelectric receives a signal which causes a decrease in the inten-

  the detected radiation, the generator 30 emits an additional signal

  opposite value and the value previously stored in memory is then replaced by that corresponding to the last displacement, and

and so on.

  The number N of the steps is chosen to be large enough so that the intensities picked up at the end of the rank N and N-1 stages have substantially the same value. Moreover, the value E of the electrical signal is chosen sufficiently small to obtain a precise convergence on the maximum radiation intensity But it is obvious

  that plus E is small, plus N must be large.

  In the two examples described above of the device according to the invention, it is not necessary to adjust the orientation of the optical reception system to a bright point of the target, since the radiation emitted by the target heated by the The energy of the laser beam has a very broad emission lobe, of the Lambert type. Moreover, the reception is not hindered by the atmospheric backscattering of the laser beam since the radiation captured has a wavelength different from that of the laser beam. finally that the operation of the device according to the invention is not affected by the instabilities that occur in known devices

  said to return wave due to thermal defocus.

  The device according to the invention can be applied to the destruction

  military targets by laser.

  Of course, the invention is not limited to the embodiments described or shown which have been given by way of example. In particular, it is possible, without departing from the scope of the invention,

  replace certain technical means by equivalent means.

  Thus, in the device shown in FIG. 1, the electromagnet 7 can be replaced by an electric motor which, through a reduction system, can modify the angle of the surface of the mirror 4 with respect to the magnet. axis 3, so as to control

a scan of the beam 15.

-7-

Claims (5)

  1 / A method for concentrating on a target the energy of a monochromatic radiation beam, said beam emerging from an optical transmission system comprising at least two elements, consisting of directing the beam towards the target, to capture the radiation coming from of the target and moving the elements relative to each other so as to increase the intensity of the radiation captured, characterized in that the radiation from the target is captured only in a predetermined band of frequencies transmitted by the target in response to receiving the beam, the frequency of the radiation   monochromatic being located outside the predetermined band. 2 / A method according to claim 1, characterized in that the monochromatic radiation is infrared radiation.   3 / A method according to claim 1, characterized in that the predetermined band is an infrared band.   4 / A method according to claim 1, characterized in that the transmission optical system comprising a plurality of elements, the displacement of the elements is carried out in N steps and that during a step of any rank lower than N , the elements successively undergo a displacement of predetermined amplitude D in an arbitrary direction, the displacement of an element being maintained when it causes an increase in the intensity of the captured radiation, the value + D then being stored, the displacement of an element being followed by an additional displacement of amplitude D in the direction opposite to the arbitrary direction if the initial displacement of this element causes a decrease in the intensity of the radiation picked up, the value D being then stored in memory, and that during the next step the elements successively undergo identical displacements in amplitude and in direction to the values stored in the step preceding, an element undergoing an additional displacement of the same amplitude but of opposite direction when the initial displacement causes a decrease in the intensity of the radiation captured, the value previously stored is then replaced by that corresponding to the last displacement, N being an integer chosen large enough that the intensities picked up at the end of the rank N 8 -   and N-1 have substantially the same value.   A device for concentrating on a target the energy of a monochromatic radiation beam, comprising a generator of a monochromatic wave, an optical transmission system receiving said monochromatic wave and delivering the beam, said optical system comprising at least two elements. , means for orienting the optical transmission system to direct the beam towards the target, a reception optical system disposed near the optical transmission system for capturing radiation from the target, and means for displacing the to the elements with respect to one another so as to increase the intensity of the radiation picked up by the reception optical system, characterized in that the optical reception system comprises an optical filter which does not leave   radiation from the target only in a predetermined band   completed frequencies, the frequency of said monochromatic wave   being outside the predetermined frequency band.   6 / Apparatus according to claim 5, characterized in that the optical transmission system comprises a deformable mirror attached to a plurality of piezoelectric elements each provided with two electrodes and the means for displacing the elements   are polarization means of the electrodes.   7 / Apparatus according to claim 5, characterized in that the two elements are respectively constituted by a concave mirror   having an axial opening to allow the monochromatic wave to pass   generated by the generator, and by a convex mirror whose reflective surface is opposite that of the concave mirror, this convex mirror being arranged to receive the monochromatic wave passing through the opening and to reflect this wave on the   reflective side of the concave mirror.