FR2706717A1 - High-resolution active laser imager system - Google Patents

Abstract

Imager comprising a continuous laser emitting an incident beam of electromagnetic radiation, an accousto-optical deflector for deflecting the said beam in a given direction, an oscillating mirror for deflecting the said beam in a direction perpendicular to the given direction, an optical system for focussing the beam, a heterodyne mixer of a fraction of the incident beam with the beam reflected by a target, a quadratic detector of the output beam from the said mixer and a device for displaying the image of the target. It is characterised in that the beam reflected by the target (27) passes through the acousto-optical deflector (7) and, at the exit from the said acousto-optical deflector, forms an angle with the incident beam (17) which depends on the range of the target, and in that the imager comprises duplexing means (6) separating the incoming incident beam in the acousto-optical deflector and the reflected beam exiting therefrom, and means (37) for orienting these duplexing means with respect to the reflected beam.

Description

- 1 -

  HIGH RESOLUTION ACTIVE LASER IMAGING SYSTEM

  The present invention relates to a target imager by beam

  layering of the latter by a laser beam and more particularly an imager of this type in which the line scanning is produced by an acousto-optical deflector (D.A.O.) and where the detection is a heterodyne detection. The object of the invention is to produce an imager of the type under consideration which has a high line scanning rate, a high resolution and a range of several kilometers. To fix the ideas, the imager of the invention has a line scanning rate of the order of 10 kHz, a resolution of 50 lprd and one hundred points per line and a

  range of around 5 kilometers.

  It is known (cf. Laser Focus, Acousto-Optic Scanning, December 1976, page 36) that if one applies to an acousto-optical device

  receiving an incoming optical beam of frequency fp a signal of ba

  layering with linear frequency variation, ft = fo + (t / T) Af0 (1)

  where fo is the minimum acoustic sweep frequency, ft this frequency

  quence at time t, Af0 the acoustic frequency excursion and T the scanning period, the beam (i) sees its frequency pass from fp to fp + ft fp + fo + (t / T) Af0 (2) and (ii) is deflected for the acousto-optic device by an angle at such that at = C + (t / T) As0 (3) - 2 -

  o t, as0 Aao correspond to ft 'f0 and AfO respectively.

  According to the invention, the return beam passes through the acousto-optical device. rl has on return the frequency fp + ft given by (2) and it crosses the acousto-optic device controlled by the frequency

  f (t + 2L / c) = fo + (t / T) Af0 + (2L / cT) Af0 -

  At the output of DA0., The return beam therefore has the frequency p ft + f (t + 2L / c) = fp + 2fo + (2t / T) AfO + (2L / cT) AfO (5) The change of heterodyne frequency between the frequency of the forward beam fp and the frequency of the return beam given by (5) gives the Doppler frequency 2f0 + (2t / T) Af0 + (2L / cT) Af0 (6) If the target has a radial speed v, the heterodyne frequency becomes 2f0 + (2t / T) Af0 + (2L / cT) Af0 + fv (7) o fv = (2r / X) 2v

  The tracking of the Dëppler frequency gives the radial speed of the -

  target. The deviation to the outward journey is given by expression (3); on return it is given by s (t + 2L / c) = 0 + (t / T) At0 + (2L / cT) aa0 (8) -3 - The return beam will therefore be offset from the incident beam by a (t + 2L /) - t = 'L = (2L / cT) Ac0 (9) If we assume that the beams are Gaussian beams, the response of the quadratic detector can be written 2 2 R (a) = exp. - (2a 2/2) (10) o = 2/2 X / rD

  and D is the diameter of the acousto-optical device.

  If we consider an increment of distance dL, there corresponds by equation (9) an increment of deviation 6a such that & a = (2dL / cT) Aa0 (11) The deviation FR of R corresponding to a variation 6O of a is 6R = 1exp. - 2 (6a / c) 2 2 (6a / o) 2 Precision on the measurement of the distance Assuming that we can distinguish two responses R (a) differing by 10%, we have 2 (da / c) 2 = 0.1 of o a = 0.22 a We get from equation (11) CL -cT X0.22 C 2Aua0 or

Y = N

  Cr N, number of points in a line, hence L = 0.11 (cT / N) Distance discrimination Assuming that two points belong to two different planes when R (a) in a plane is the hundredth of R (a) in the other, we have R (a) exp - 2 (ca / c) 2 = 0.01 s = 1.51 o FL = 0.75 (cT / N) Numerical application c = 3 x 108 m / s T = 1OO is N = 100 points Distance accuracy L = 33 m

  Distance discrimination L = 225 m.

  - 5 - In accordance with the invention, the imager comprises a continuous laser emitting an incident beam of electromagnetic radiation, an acousto-optical deflector for deflecting said beam in a given direction, an oscillating mirror for deflecting said beam in a direction perpendicular to the given direction, a heterodyne mixer of a fraction of the incident beam and of the beam reflected by a target, a quadratic detector of the output beam of said

  mixer and a target image display device, character-

  terized in that the beam reflected by the target crosses the deflec-

  acousto-optic sensor and made at the outlet of said acousto-deflector

  optics an angle with the incident beam which depends on the distance from the target; from which it follows that the imager of the invention makes it possible to

  determine the direction, distance and speed of the target.

  The invention will now be described in detail in connection with the accompanying drawings in which: - FIG. 1 is a block diagram of the imager of the invention; - Fig. 2 is a block diagram of the electronic circuits of the imager; and

  - Fig. 3 is a representation of the optical part of the imager.

  Referring to FIG. 1, a continuous wave CO 2 laser 1 has been shown which emits an incident beam 2 at 10.6 μm. This beam is separated by a separator 3 and is applied on the one hand to an afocal system 4 and on the other hand to a heterodyne mixer 5. The beam leaving the afocal system 4 is applied to a separator or duplexer 6 and from there. to an acousto-optical deflector 7 controlled by a scanning control device 8 which generates a linear variation signal which it applies to DA0. 7. The beam leaving the D.A.0. 7 falls on a galvanometric deflector 9 also controlled by the device

  scan command 8 then passes through an afocal system 10.

  The beam reflected or scattered by the target returns in the direction of return in the afocal system 10, the two image and line deflectors 9 and 7 then is returned to the heterodyne mixer 5 by the

duplexer 6.

  The two outward and return beams mixed in the heterodyne mixer 5 are applied to a quadratic detector 11. The output - 6 of this quadratic detector is connected to a circuit 12 comprising a

  second mixer and a filter, itself connected to an ampli

  tude 13 followed by a filter 13 'and an image processing circuit 14

  controlling a display device 15.

  The sweep control device 8 comprises a ramp generator 80 (FIG. 2) which is connected to a voltage-controlled oscillator (VCO), 84, and to a power amplifier 85. This amplifier is

  connected to the acousto-optic deflector 7 by connection 81.

  The sweep control device 8 is also connected to a frequency divider 86 and to a device for controlling the galvanometric deflector 87, itself connected to the galvanometric deflector 9 by connection 82. The sweep control device 8 transmits to the circuit processing 14 the line scan signal through the connection

  83 and the image scanning signal via connection 88.

  The output signal from the quadratic detector 11 which is the signal to

  intermediate frequency given by expression (7) has a frequency va-

  reliable because of the term (2t / T) AfO. To make the fixed frequency fixed

  intermediate, a second frequency change is made by means of a local wave of variable frequency whose frequency is that given by (7) increased by a fixed intermediate wire frequency The frequency of the local wave is therefore: flocal = 2fo + (2t / T) Af0 + (2L / cT) Afo + fv + fi (11) The local wave which we will see how it is produced is applied

  to the intermediate frequency mixer 121 and filtered in the filter 122.

  This filter is centered on a filter with a width equal to twice the frequency

video footage.

  The output of the filter 122 is connected to the detector 13 followed by the low frequency filter 13 '. The output of the low frequency filter 13 ′ is applied on the one hand to the image processing circuit 14 and on the other hand to the input of an operational amplifier 123 in order to ensure DÈppler tracking. This operational amplifier also receives the line scanning ramp. The output of the operational amplifier 123 is connected to the input of a voltage controlled oscillator (VCO), 124. The output of the VCO, 124, is connected to the mixer 121 through a voltage controlled attenuator, 125. The attenuator 125 is controlled by the

  filter output signal 13 ', which forms a control circuit

  gain tomatic. We can see that the output of the attenuator 125 provides

- 7-

  denies the local heterodyne signal of the second frequency change.

  The output signal of the low frequency filter 13 'is also applied to a strip of photodetectors coupled to devices

  coupled charges which together form an image integrator 16.

  Referring now to FIG. 3, we see a continuous wave C02 laser of 3 watts of power, for example the 941 S laser from Spectra Physics. The beam 2 emitted by the laser falls on the separator 3 in ZnSe which separates it into an incident beam 17 and a local beam 18. The incident beam 17 passes through a lens 19 of germanium which, with the lens 20 of the same material, form the elements of the afocal input system 4 of FIG. 1. This afocal system has a magnification of 11. Between the two lenses 19 and 20, the incident beam 17 crosses the duplexer 6, the structure of which will be explained later. The germanium lens 22 and the mirror 29 form the afocal output system 10 of FIG. 1. This afocal system has an enlargement of 2. Between the lenses 20 and 22 is arranged the acousto-optical deflector 7. As this deflector acts only in one plane, cylindrical lenses 24 and 25 are associated with the lenses 21 and 22 They ensure sufficient angular resolution of the DA0., And form a system

  with variable astygmatism. The D.A.0. 7 has an opening of 45 mm by 3mm.

  The lens 23 and the lens 28 respectively provide the conju-

  pupil gation between D.A.0. and the mirror g and between the mirror 9

and the mirror 29.

  On leaving the lens 23, the beam falls on the galvanometric deflector 9 controlled by the drive device 87. The beam then falls on a deflection plane mirror 26, passes through a lens 28 and falls on the spherical mirror 29 which is the projector

directed towards the target.

  The return beam 27 superimposed on the incident beam 17 follows the same path as the latter to the duplexer 6. It is returned by the

  duplexer 6 on the lens 30 which focuses it on the quadrant detector

  dratic 11 through the mixing blade 31 in ZnSe.

  The local beam 18 is similarly focused on the quadrant detector.

  dratic 11 by the lens 32 through the mixing blade 31.

  -8 - The duplexer 6.is a polarization duplexer. It is a network letting a polarized wave cross it perpendicular to the lines of the network and reflecting a polarized wave parallel to the lines of the network. The lines of the network formed on the duplexer 6 are perpendicular to the plane of FIG. 3. The polarization of the beam

  incident 17 is parallel to the plane of FIG. 3 and the beam 17 tra-

  therefore pours the polarizing duplexer 6. On the beam path 17, between the lens 28 and the spherical mirror 29 is arranged a quarter wave plate 33 in CdS. This plate converts the incident beam with rectilinear polarization into a beam with circular polarization. At

  back, the beam 27 which has the reverse circular polarization retre-

  pours the quarter wave plate 33 and the direction of polarization rec-

  beam 27 is thus at right angles to the polar

  sation of the beam 17. The duplexer 6 then behaves like a mirror

  and returns the return beam to the mixing blade.

  A half-wave plate 34 is disposed on the path of the local heterodyne beam 18 in order to rotate the polarization of this beam

  of 90 and make it parallel to that of the return beam 27. A pola-

  -risateur 35 is arranged in front of the quadratic detector 11.

  We have seen (formula (9)) that the return beam 27 formed with the

  incident beam 17 an angle aL depending on the distance from the target.

  Duplexer 6 can rotate around an axis to adjust the imager

  at a given distance. Targets located in a plane other than the plane -

  on which the imager is adjusted are eliminated by the diaphragm 36. We

  shown in FIG. 3 a button 37 which turns the duplexer 6.

-9-

Claims (4)

R e v e n d i c a t i o n s   1 - Imager comprising a continuous laser emitting an incident beam of electromagnetic radiation, an acousto-optical deflector for deflecting said beam in a given direction, an oscillating mirror for deflecting said beam in a direction perpendicular to the given direction, an optical focusing system of the beam, a heterodyne mixer of a fraction of the incident beam with the beam reflected by a target, a quadratic detector of the output beam of said mixer and a device for displaying the image of the target, characterized in that the beam reflected by the target (27) passes through the acousto-optic deflector (7) and makes at the exit of said acousto-optic deflector an angle with the i-ncident beam (17) which depends on the distance from the target and that the imager comprises duplexing means (6) separating the incident beam entering the acousto-optical deflector and the reflected beam leaving it and means ( 37) to orient these duplexing means with respect to the reflected beam. 2 - Imager according to claim 1, characterized in that the duplexer (6) is a polarization duplexer passing a beam of direction of given polarization and reflecting a beam of direction of polarization perpendicular to said given direction and that the imager includes means (33, 34) for assigning to the beam   incident and the reflected beam of perpendicular polarizations.   3 - Imager according to claim 1, characterized in that the acousto-optical deflector (7) has the shape of a flat lamp and that the incident beam (17) is made flat in its passage through the deflector   acousto-optics by two cylindrical lenses (24, 25).   4 - Imager according to claim 1, characterized in that the optical beam focusing system is constituted by a first afocal (19, 20) of adaptation of the beam to D.A.0. and a second afocal (22, 29) which ensures the pupil conjugation and fixes the resolution of the imager.