GB2249822A - A method and apparatus for concentrating the energy of a monochromatic radiation beam onto a target - Google Patents

Abstract

The method consists in detecting the radiation (16) emitted by the target in response to heating thereof by the beam (15) passing through an optical system in a frequency band which excludes the frequency of the beam; and in moving the components (4 and 5) of an optical system in relation to each other to increase the intensity of the radiation picked up. Application to destruction of a military target by a laser. <IMAGE>

Description

"A METHOD AND APPARATUS FOR CONCENTRATING THE ENERGY OF A MONOCHROMATIC RADIATION BEAM ONTO A TARGET" The invention relates to a method and to apparatus for concentrating the energy of a monochromatic beam of radiation, such as a high-energy laser beam, onto a target.

A known apparatus for concentrating the energy of a laser beam onto a target is described in an article by C.B.Hogge on pages 69 to 75 of vol. 6 of the American work entitled "Physics of quantum electronics". Said apparatus, referred to as an "outgoing wave system", includes essentially a laser generator whose beam is aimed at the target after being reflected by a deformable mirror and an optical receiving system which is aimed to pick up the energy reflected by a bright point on the target. In order to concentrate the energy of the beam on the target, the mirror is deformed so as to increase the intensity of the light picked up by the optical receiving system.

Said apparatus has drawbacks. In particular, when the target is large and hence has several bright points, it is difficult to aim the optical receiving system at only one of the bright points. Further, the reception of the radiation returned by the target is disturbed by atmospheric retrodiffusion of the energy of the light beam aimed at the target.

Another apparatus for concentrating the energy of a laser beam onto a target is described on pages 67 to 69 of the same American work. This apparatus, referred to as a "return wave system" includes a laser generator whose beam is directed towards the target after passing through a dichroic optical separator and being reflected by a deformable mirror. The heat radiated by the target is reflected by the deformable mirror along the same optical path but in the opposite direction, and is separated from the laser beam by reflection at the separator. This radiated heat is finally received on a phase measuring system which delivers error signals to components of the deformable mirror so as to cause them to compensate phase irregularities in the return wave.

The return wave system has the disadvantage of being unstable in operation, this instability being caused by the thermal defocusing phenomenon analysed on pages 63 to 66 of the same work.

Preferred embodiments of the present invention mitigate the above-mentioned drawbacks of known apparatus.

The present invention provides a method of concentrating the energy of a monochromatic beam of radiation on a target, the beam emerging from an optical beam launching system which includes at least two components, said method consisting in: - aiming the beam at the target; - detecting the radiation returned by the target on being hit by the beam; and - moving the components relative to one another so as to increase the intensity of the detected radiation; wherein the radiation returned by the target is detected only in a predetermined band of frequencies, said band being chosen to exclude the frequency of the monochromatic beam of radiation aimed at the target.

The present invention also provides apparatus for concentrating the energy of a monochromatic radiation beam said apparatus including: - a monochromatic wave generator; - an optical beam launching system which receives said monochromatic wave and which delivers the beam, this optical system including at least two components; - means for rotating the optical beam launching system to aim the beam towards the target; - an optical receiver system disposed in the neighbourhood of the optical emission system to detect the radiation returned by the target;; and - means for making the components move in relation to one another so as to increase the intensity of the radiation detected by the optical receiving system, wherein the optical receiving system includes an optical filter which passes radiation returned by the target only in a predetermined frequency band, the frequency of said monochromatic wave being situated outside the predetermined frequency band.

Particular embodiments of the present invention are described hereinbelow by way of example with reference to the accompanying drawings in which: FIGURE 1 schematically illustrates one embodiment of apparatus in accordance with the invention; and FIGURE 2 schematically illustrates another embodiment of apparatus in accordance with the invention.

Figure 1 illustrates a laser generator 1 delivering a high-power monochromatic beam 2 of radiation along an axis 3. The generator may be of the carbon dioxide type which emits at an infrared wavelength of 10.6 microns. The beam 2 passes through an optical beam launching system of the cassegrain type which comprises two components : a convex mirror 4 centred on the axis 3 to receive the beam 2 and a coaxial concave parabolic mirror 5 with an axial opening to pass the beam 2. The reflecting surfaces of the mirrors 4 and 5 face 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 rigidly mounted on the plunger core 6 of an electro-magnet 7 via a mechanical connection member schematically illustrated as a lever 8. The excitation winding of the electromagnet 7 is connected to the output of a variable DC voltage generator 9. The input of the generator 9 is connected to the output of a logic control system 10.

An optical receiver system disposed in the neighbourhood of the optical beam launching system includes a detector 11 and an optical filter 12 disposed in front of a sensitive surface 13 of the detector 11.

The filter 12 is a passband filter; it passes radiation whose wavelength lies between 3 and 5 microns and stops radiation outside said infrared band. The electric output of the detector 11 is connected to the input of the control system 10.

The assembly constituted by parts 1 to 8 is installed on a support, not illustrated, which can be rotated horizontally and vertically to aim the axis 3 at a target (which is not illustrated in Figure 1). The apparatus also includes orientation means for pointing receiving axis 16 of the detector 11 towards the target.

When illuminated by laser beam 15, the target is heated and thus radiates an incoherent infrared wave, part of which is detected by the detector 11 in the 3 to 5 micron infrared band as passed by the filter 12.

The emission wavelength of the beam 15 must be situated outside the receiving band. The emission wavelength of the laser 1 and the receiving band are preferably both chosen to lie in the infrared range as in the example described. In a variant, the laser generator 1 may be a hydrofluoric acid chemical laser which emits at a wavelength of 3.8 microns, in which case the filter 12 then defines an infrared receiving band in the 8 to 10 micron range.

The apparatus 14 measures the intensity of the infrared radiation detected by the detector 11, and the data from this measurement is transmitted to the input of the control system 10.

The system 10 regulates the voltage of the generator 9 so as to cause the mirror 4 to move along the axis 3 in translation by moving the core 6, the movement being in a direction such that it varies the focal length- of the optical beam launching system. The direction of this is chosen to increase the intensity of the radiation picked up by the detector 11.

Preferably, the system 10 is of a type which is capable of causing successively constant amplitude small movements of the mirror 4, in one direction or the other along the axis 3, depending on which direction increases the intensity of the radiation picked up by the detector 11.

Figure 2 illustrates a laser generator 17 which delivers a monochromatic beam 18. A cassegrain type afocal optical system 19 analogous to that in Figure 1 is constituted by a convex mirror 20 and by a concave mirror 21. Said optical system receives the light beam 18 and delivers a light beam 22 of increased cross-section, the beam 22 being reflected on a deformable mirror 24 which sends a light beam 25 towards a target which is not illustrated in the figure.

The deformable mirror 24 is shown fixed on three piezoelectric components 26, 27 and 28 each equipped with two electrodes and rigidly mounted on a rigid plate 29. Of course, in practice there are many more than just three piezoelectric components.

The apparatus further includes an electric signal generator 30 with three outputs each connected to a pair of electrodes on a respective one of the piezoelectric components 26, 27 and 28. The input of the generator 30 is connected to the output of a logic control system 31 which is itself connected to a memory 32.

In the neighbourhood of the deformable mirror 24, a receiving optical system has a detector 33 with an optical filter 34 disposed in front of its sensitive surface. The filter 34 is a passband filter and is analogous to filter 12 (Figure 1). It passes radiation only in a frequency band which excludes the emission frequency of the laser 17. The electric output of the detector 33 is connected to the input of a measuring unit 35 whose output is connected to the input of the system 31.

The apparatus illustrated in Figure 2 operates as follows.

The preferably infrared beam 25 heats the target which in return radiates an incoherent wave. The detector 33 detects part 36 of this wave via the filter 34 which only passes radiation in a predetermined frequency band, said band excluding the emission frequency of the laser 17. Data concerning the intensity of said radiation which intensity is measured by the measuring unit 35 is transmitted to the input of the system 31.

The system 31 controls the generator of electric control signals by the generator 30.

In a first scan, the generator 30 applies an electric voltage El to each of its outputs in succession. Said voltage drives the corresponding piezoelectric components to cause a small movement of amplitude D in an arbitrary direction at a point on the surface of the deformable mirror 24. When the movement D increases the intensity of the radiation detected by the detector 33, the value +D is stored in a memory 32 and the voltage E is maintained at the corresponding output e.g. by capacitance. In contrast, when the initial movement D reduces the intensity of the detected radiation, the generator 30 emits an extra signal - El which causes the piezoelectric component to return with a movement of amplitude D in the opposite direction to its initial movement and the value -D is stored in the memory.

In a second scan, the generator 30 again delivers signals of amplitude E2, the signs of these signals being identical to those of the values stored in the memory during the first step. These signals cause movements of amplitude D whose magnitudes and signs are added to those of the first scan. When a piezoelectric component receives a signal which increases the intensity of the radiation picked up by the detector 33, the piezoelectric component is left in its new position and the value in the memory is not modified. If, in contrast, a piezoelectric component receives a signal which reduces the intensity of the detected radiation the generator 30 emits an extra signal of opposite sign and the value previously stored in the memory is then replaced by that which corresponds to the last movement and so on.

The displacement amplitudes El, E2, ...EN of the successive scans preferably decrease from one scan to the next to speed up convergence of the deformable mirror on an optimum deformation, however such reduction is not essential and the amplitude may be the same at each scan. In either case the displacement amplitude of the last scan and the number N of scans are chosen as a function of the smallest detectable change in intensity of the radiation returned by the target in such a manner that there is substantially no detectable change between the intensity detected at the end of the (N-l)th scan and the intensity detected at the end of the Nth scan.

In the two examples described hereinabove of the apparatus in accordance with the invention, it is not necessary to point the receiving optical system at a bright point of the target since the radiation emitted by the target heated by the energy of the laser beam has a very wide emission lobe of the Lambert type. Further, reception is not hindered by atmospheric retrodiffusion of the laser beam since the wavelength of the radiation picked up is different from that of the laser beam.

Lastly, it is observed that the operation of the apparatus in accordance with the invention is not affected by the instability which occurs in the known return-wave apparatus due to thermal defocusing.

The apparatus in accordance with the invention can be applied to destroying military targets with lasers.

Of course, the invention is in no way limited to the embodiments described and illustrated which are given only by way of example. In particular, without going beyond the scope of the invention, some technical means can be replaced by equivalent means.

Thus, in the apparatus illustrated in Figure 1, the electro-magnet 7 can be replaced by an electric motor which, via a reduction gear system, can modify the angle of the surface of the mirror 4 relative to the axis 3 so as to cause the beam 15 to perform a scanning action.

Claims (9)

Claims

1. A method of concentrating the energy of a monochromatic beam of radiation on a target, the beam emerging from an optical beam launching system which includes at least two components, said method consisting in: - aiming the beam at the target; - detecting the radiation returned by the target on being hit by the beam; and - moving the components relative to one another so as to increase the intensity of the detected radiation; wherein the radiation returned by the target is detected only in a predetermined band of frequencies, said band being chosen to exclude the frequency of the monochromatic beam of radiation aimed at the target.

2. A method according to claim 1, wherein the monochromatic radiation is infrared radiation.

3. A method according to claim 1, wherein the predetermined band lies in the infrared region of the spectrum.

4. A method according to claim 1, wherein the optical beam launching system includes a plurality of movable components each capable of moving in a first direction or in a second direction opposite to the first, and further including a memory with a storage location corresponding to each of said movable components with each storage location storing a value indicative of movement in one of said first and second directions; and wherein said components are scanned in N successive scans in such a manner that each of said components is caused to move in N successive steps of predetermined amplitudes to increase said detected intensity, with each step of each component being performed according to the following cycle of operations: firstly the component is caused to move by the predetermined amplitude for the step in a direction determined by the value stored in the memory which corresponds to the component; secondly the effect of such movement on said detected intensity is observed; and thirdly, if said effect is an increase in intensity, the movable component is left in its new position and the value stored in the corresponding memory location is unchanged, while if said effect is a decrease in intensity, the movable component is returned to the position it occupied at the beginning of the cycle and the value stored in the corresponding memory location is changed to a value indicative of movement in the opposite direction to that stored at the beginning of the cycle; the number N of scans and the predetermined amplitude for at least the Nth step of each movable component being chosen as a function of the minimum detectable change in intensity such that there is substantially no detectable change between the intensity detected at the end of the (N-l)th scan and the intensity detected at the end of the Nth scan.

5. Apparatus for concentrating the energy of a monochromatic radiation beam onto a target, said apparatus including: - a monochromatic wave generator; - an optical beam launching system which receives said monochromatic wave and which delivers the beam, this optical system including at least two components; - means for rotating the optical beam launching system to aim the beam towards the target; - an optical receiver system disposed in the neighbourhood of the optical emission system to detect the radiation returned by the target; and - means for making the components move in relation to one another so as to increase the intensity of the radiation detected by the optical receiving system, wherein the optical receiving system includes an optical filter which passes radiation returned by the target only in a predetermined frequency band, the frequency of said monochromatic wave being situated outside the predetermined frequency band.

6. Apparatus according to claim 5, wherein the optical beam launching system includes a deformable mirror fixed on a plurality of piezoelectric elements each equipped with two electrodes and wherein the means for moving the elements are electrode polarization means.

7. An apparatus according to claim 5, wherein the two elements are constituted respectively by a concave mirror which includes an axial opening to pass the monochromatic wave emitted by the generator and by a convex mirror whose reflecting surface faces that of the concave mirror, the convex mirror being disposed to receive the monochromatic wave which passes through the opening and to reflect this wave on the reflecting surface of the concave mirror.

8. A method of concentrating the energy of a monochromatic beam of radiation substantially as hereinbefore described.

9. Apparatus for concentrating the energy of a monochromatic radiation beam substantially as hereinbefore described with reference to Figure 1 or 2 of the accompanying drawings.