FR2523378A1 - TRANSMITTER-RECEIVER LASER DEVICE AND APPLICATION TO A REMOTE DETECTION SYSTEM - Google Patents

Abstract

LASER TRANSMITTER-RECEIVER DEVICE AND APPLICATION TO A REMOTE DETECTION SYSTEM. IT CONTAINS, SUCCESSIVELY ALIGNED ON A SAME AXIS, A LASER EMITTER 1, A SEPARATOR BLADE 6, A ROTARY SHUTTER 11 AND AN OPTICAL SYSTEM 10 IN ORDER TO FORM A BEAM OF PULSES 29 ILLUMINATE THE TARGET, AND ON THE REFERENCE AXIS 13 FROM THE SEPARATOR BLADE 6, A CRYSTAL 14 CAUSING A SLIP IN RADIATION FREQUENCY, THIS CRYSTAL BEING FOLLOWED BY A PHOTOELECTRIC DETECTOR 16 ALSO RECEIVING AN ECHO 30 OF THE LASER PULSES RETURNED BY THE TARGET. APPLICATION TO A REMOTE DETECTION SYSTEM.

Description

_ 1- Transceiver laser device and application to a system of

remote detection

  The present invention relates to a transmitter-receiver laser device

  and application of this device to a remote sensing system. A heterodyne detection transceiver laser device is known which can be used for example for the production of

  Remote sensing systems In this device, the frequency of

  laser transmitter is slightly offset from that of a local oscillator and the echo, returned by a target, is directed to a photoelectric detector which also receives the radiation delivered by the local oscillator The detector then delivers modulated signals at a frequency equal to the difference between the emission frequency and that of the local oscillator For this, the laser transmitter delivers a continuous beam which is divided into two parts by an optical blade inclined on the axis of emission A first part of the beam first passes through a frequency slip cell associated with a modulator, so as to obtain pulses of radiation whose frequency is

  slightly offset from that of the laser transmitter.

  then pass through an optical system capable of directing them to

  the target The second part of the beam constitutes the radiation of os-

  local oscillator, this radiation being directed towards the detector to be

  mixed with that of the echo returned by the target.

  The device described above has a drawback It is difficult to avoid, in fact, the formation of parasitic reflections of

  frequency shifted pulses on the diopters of the system.

  optical system and on the exit face of the

  frequency Some of these parasitic reflections pass

  the cell is poured without any frequency drift, and, after reflection on the mirrors of the laser cavity, are sent back by the optical plate towards the detector. This results in a high level of jamming which

  disrupts the operation of the device.

  The present invention aims to overcome this disadvantage.

  It relates to a laser transmitter-receiver device for detecting

  heterodyne, having 2 - a laser transmitter of a main continuous radiation, an optical separator arranged to take a part of the main radiation and transmit the other part, a low frequency modulator arranged to receive the part of the main radiation transmitted by the separator, this modulator being able to deliver, in response> laser pulses, an optical system disposed at the output of the modulator for directing the laser pulses towards the target, part of the energy of these pulses being back-diffused according to a return radiation of frequency f 1,

  a local oscillator capable of emitting laser radiation of

  The invention relates to an electro-optical receiver arranged to receive the frequency return radiation f and the frequency reference radiation f which is capable of delivering in response a modulated electrical signal at the intermediate frequency f 2 -f. 1 and an electrical signal processing circuit, characterized in that

  the local oscillator is constituted by a sliding member of frequency

  quence arranged between the separator and the receiver to receive the

  part of the main radiation taken by the separator, this radiation

  This element being of frequency f 1, this element being able to deliver in response the reference radiation of frequency f. In this device, the processing circuit may comprise, in

  series at the output of the receiver, successively an amplifier of the

  intermediate frequency signal, an envelope detector and an ampli-

  low frequency indicator connected to the modulator for detecting

  synchronous on the low frequency f 5.

  The present invention also relates to a remote detection system, this system comprising a transceiver laser device defined above, characterized in that it further comprises, an additional laser transmitter capable of delivering a continuous main beam of frequency f 3 offset from fly

  optical pickup means for directing part of the radiation

  a frequency modulator at the input of the frequency slip member, an additional modulator at a low frequency different from the frequency of receiving the other portion of the main frequency radiation f 3,

  an additional optical system to direct the beam to the target

  from the additional modulator, a filter circuit whose input is connected to the output of the envelope detector, this circuit having two connected outputs

  respectively at the input of the low-frequency amplifier and at the

  an additional low frequency amplifier connected to the modula-

additional driver,

  a divider circuit whose two inputs are respectively connected

  the outputs of the low-frequency amplifier and ampli-

  additional low frequency

  and a recording system connected to the output of the dividing circuit

  sor. Particular forms of the subject of the present invention are described below, by way of example, with reference to the accompanying drawings in which

  FIG. 1 diagrammatically represents an embodiment of the arrangement

  According to the invention, FIGS. 2A, 2B, 2C, 2D, and 2E are amplitude diagrams.

  impulse as a function of time, illustrating the functioning of the

  FIG. 3 diagrammatically represents another embodiment of the device according to the invention, and FIG. 4 schematically represents a remote detection system comprising a laser transmitter-receiver device of the following type.

illustrated in Figure 1.

  FIG. 1 shows a continuous laser transmitter 1 which can

  for example of the waveguide type, carbon dioxide This laser comprises

  along an axis 2, two mirrors 3 and 4 forming a resonant optical cavity The mirror 3 is partially transparent and the mirror 4 is

  constituted by a network whose inclination on the axis 2 is adjustable.

  Between the mirrors 3 and 4 is disposed a laser tube 5 provided with transparent windows inclined at the Brewster incidence At the output of the transmitter 1, is disposed a separating optical blade 6 partially reflecting and inclined at 450 on the axis 2 Beyond the blade 6 are arranged two convergent lenses 9 and 10 centered on the axis 2 and forming an afocal optical system A low frequency modulator such

  that a shutter is placed at the common focus of the two lenses.

  The rotor comprises a disc 11 provided with openings, driven in rotation about its axis by a motor 12. On the return axis 13 of the blade 6 is disposed a germanium crystal 14 provided with an acousto-optical transducer in contact with the

  crystal and powered by an electric generator 15 A photo detector

  toelectric 16 receives, along an axis 17, the radiation coming out of

  crystal 14 An afocal optical system formed of a convergent lens

  18 and a diverging lens 19 is centered on an axis 20 intersecting the axis 17 between the crystal 14 and the detector 16 at the intersection point

  axes 17 and 20, is disposed a partial optical return blade 21-

  The reflective and inclined reflection of the radiation passing through the afocal optical system 18-19 is transmitted to the detector 16. At the output of the detector 16, there is arranged a processing circuit which successively comprises an amplifier 22, an envelope detector 23

  and an amplifier 24 connected in series.

  A light generator 25 and a photoelectric receiver 26

  mounted on a support 27 and aligned along an axis parallel to the axis 2 and passing through the openings of the disk 11 form a low frequency modulation sensor The electrical output of the receiver 26 is

connected to the amplifier 24.

  The device shown in FIG. 1 operates in the manner

next.

  With the motor 12 switched on, the continuous radiation 28 of frequency f 1 emitted by the transmitter 1 is interrupted by the running of the openings of the disc 11 at a low frequency of a few kilohertz, for example. at the exit of the lens 10, the formation of a pulse beam 29 directed towards a not shown target. A portion 30 of the energy of these pulses is returned by the target and, after passing through the optical system 18-19, is reflected by the blade 21

  towards the sensitive surface of the detector 16.

  Under the action of the acousto-optical transducer associated with

  14, the portion 31 of the radiation 28, reflected by the blade 6, is deflected by the crystal 14 and undergoes a frequency shift f 2 -f 1 which can be of the order of 100 M Hz The beam 32, at the frequency f 2, which exits the crystal 14 acts as a reference beam emitted by a local oscillator and comes to illuminate the sensitive surface of the detector 16 which also receives the return radiation 30 at the frequency fi. quadratic type affected a heterodyne detection and

  delivers at its output an electrical signal modulated at the intermediate frequency

diary f 2-f 1.

  This signal is amplified by the intermediate frequency amplifier 22

  diary, then passes successively through the envelope detector 23 and

  the low frequency amplifier 24 so as to undergo a synchronous detection

  chrone on the low frequency f 5 modulation created by the shutter

  For this purpose, the radiation emitted by the generator 25 is

  interrupted by the scrolling of the openings of the disk 11 and

  the amplifier 24 receives the electrical signals emitted by the detection

  26, these signals being representative of the low frequency of

  This type of processing circuit makes it possible to significantly reduce the reception bandwidth and therefore to increase the ratio

signal on noise at the reception.

  A parasitic retroreflection of the laser pulses on one of the dioptric faces of the lens 10 crosses in the opposite direction the lens 9 and the blade 6 and then, after reflection on the output mirror 3 of the laser 1 by

  example, is returned along the axis 13 by the blade 6 This retrore-

  parasite bending passes through the crystal 14 o it undergoes a slip of

frequency f 2-f 1.

  Thus, at the exit of the crystal 14, along the axis 17, the beam

  32 at the frequency f 2, which is superimposed on the retrore-

  bending constituted by a radiation of the same frequency f 2 but modulated at

  the low frequency f 5 of the rotary shutter.

  Figure 2 A shows the variations, as a function of time t, of

  the amplitude A of the pulses resulting from this superposition, observed

  at the exit of the crystal 14, in the case of a retroreflection

site.

  FIG. 2B shows the shape of the return pulses 30,

  quence fi, returned by the target These pulses are also modu-

  At the same low frequency f, with a delay corresponding to the optical path back and forth to the target, this delay being in practice negligible. FIG. 2C shows the shape of the electrical signal delivered by the receiver 16, this signal being modulated at the intermediate frequency f 2 -f 1. The amplifier 22 amplifies the intermediate frequency signal and, in practice, blocks the low frequency La Figure 2 D shows the shape

  signals obtained at the output of the amplifier 22.

  After passing through the detector 23, only the envelope of these signals is retained, as shown in FIG. 2E. The amplifier 24 makes it possible to perform synchronous detection on the low frequency signals.

from the detector 26.

  It thus appears that a parasitic retroreflection on the optical transmission system does not interfere with the reception signals. This result essentially stems from the fact that such parasitic reflection occurs.

  at the frequency f 2 on the receiver 16, after passing through the crystal 14.

  In addition, the reference beam 32 leaving the crystal is continuous and can not give rise to a parasitic retroreflection on the

exit face of the crystal 14.

  The device represented in FIG. 3 is preferably used when the amplitude of the low frequency modulation introduced by the rotary shutter is high: in this case, it is very difficult to produce an amplifier 22 that performs the signal amplification without saturation. intermediate frequency The device illustrated by the

  FIG. 3 has the same elements as that of FIG.

  The elements are designated by the same references. It furthermore comprises a polarizer 7 disposed on the axis 2 between the plate 6 and the lens 9, and a quarter-wave plate 8 also disposed on the axis 2 between the polarizer 7.

and the lens 9.

  The polarizer 7 creates a rectilinear polarization, in a first plane, of the laser radiation transmitted by the plate 6 This rectilinear polarization is converted into a circular polarization by the quarter-wave plate 8 If there then occurs a parasitic retroreflection of the laser pulses on one of the dioptric faces of the lens 10, this retroreflection traverses in the opposite direction the lens 9 and the quarter-wave plate 8, which then transforms the circular polarization of the radiation into a rectilinear polarization in a second plane perpendicular to the first plane. parasitic reflection on the faces of the lens 10

  is blocked by the polarizer 7.

  Moreover, as in the previous case, the beam of

  laser returned by the blade 6 and through the crystal 14 to constitute

  kill the reference beam is continuous and retroreflection of this radiation on the faces of the crystal is not likely to cause a signal

parasite at the reception.

  FIG. 4 shows a system making it possible to detect at

  distance the presence of a gas such as ozone in the atmosphere.

  This system comprises a laser transmitter-receiver device identical

  1 in this device, the laser emitter 1 emits radiation whose frequency corresponds exactly to a line of absorption of the gas to be detected, this frequency being

  adjusted by rotation of the network 4 The beam 29 coming out of the optical system

  that 10 undergoes partial absorption as it passes through a layer of

desired gas.

  This system further comprises another laser transmitter 33 similar to

  the laser transmitter 1 The radiation 34 emitted by the transmitter 33 has a frequency of

  close to the frequency f 1 of the radiation emitted by the laser 1, but this frequency f 3 does not correspond to an absorption line of the gas to be detected. The radiation 34 propagates along an axis 35 parallel to

  the axis 2 and successively passes through a separating optical blade 36 par-

  partially reflective and an afocal optical system comprising two convergent lenses 37 and 38 At the common focus of the lenses 37 and 38

  a rotary shutter formed of a disc 39 similar to the disc

  than 11, this disc being rotated about its axis by a

  The disc 39 is provided with openings arranged so as to pro-

  when the motor 40 rotates, a periodic shutter of the

  Thus, at the exit of the lens 38, a series of laser pulses 41 directed in the same direction as the laser pulses 29 is obtained. The beam 41 also encounters the gas to be detected but does not undergo no absorption on contact with this gas Part of the energy of the beam 41 is partially returned 8

  by the target along a path coinciding with that of beam 30.

  This return energy is received on the receiver 16 after crossing the

  optical system 18-19 and return by the blade 21.

  The separating blade 36 reflects at right angles part of the

  energy of the beam 34 in a beam 42 which is superimposed on the beam 31 after return by a mirror 43 and reflection on a partially reflective optical plate 44, disposed between the blade 6 and the crystal 14 The portion of the energy of the beam 34 returned by the blade 6 thus passes through the crystal 14 and undergoes a frequency shift f 4-f 3 equal to

fi.

  The receiver 16 therefore receives, on the one hand, two reference radiations.

  continuity of crystal 14, frequencies f 2 and f 4, and

  secondly, two radiations formed by pulses of frequency feedback.

  quences f 1 and f 3 returned by the blade 21.

  The receiver 16 thus delivers at its output two types of electrical signals: pulses of radiation of frequency f 2 -f, modulated at the low frequency f 5 of the rotary shutter 11-12 and pulses of a radiation of frequency f 4-f 3, modulated at the

  low frequency f 6 of the rotary shutter 39-40.

  It should be noted that the difference f 3-f 1 is sufficiently

  large (in practice of the order of 1015 Hertz) for electrical signals to be

  frequency f 2-f 3 or f 4-f 1 can not be issued by the

receiver 16.

  The electrical pulses of frequencies f 2 -f 1 and f 4 -f 3 delivered by the receiver 16 first pass through the amplifier 22 which blocks the low modulation frequencies, then the envelope detector 23, to enter an organ filter 45 which separates and delivers on an output 46 the frequency signals f 5, and on an output 47 the signals

of frequency f 6.

  The output 46 of the filtering member 45 is connected to the input of the amplifier 24 which performs a synchronous detection on the signals

  low frequency f 5 from the detector 26.

  The output 47 is connected to the input of another amplifier 48 which

  performs a synchronous detection on the low frequency f 6 of the shutter

  For this purpose, this shutter comprises a sensor of

  frequency, similar to that of the shutter 11-12, comprising a general

  light source 49 delivering a beam to a detector 50, this beam

  being periodically interrupted by the scrolling of the openings of the dis-

  The electrical output of the detector 50 is connected to

the amplifier 48.

  The outputs of amplifiers 24 and 48 are respectively connected

  at the two inputs 51 and 52 of a dividing circuit 53 The amplitude of the signals arriving at the input 51 is smaller than that of the signals

  arriving at input 52, since beam 29 has been absorbed by

  The divider circuit 53 thus delivers to the input of a recorder 54 a signal representative of the product of the concentration of the gas by the length of the layer traversed by the beam. -

Claims (8)

  1 / Heterodyne transceiver laser device, comprising   both a laser transmitter of a main continuous radiation, an optical separator arranged to take a part of the main radiation and transmit the other part, a low frequency modulator arranged to receive the part of the main radiation transmitted by the separator, this modulator being able to deliver, in response, laser pulses, an optical system disposed at the output of the modulator for directing the laser pulses towards the target, part of the energy of these pulses being back-scattered according to a return radiation frequency fi,   a local oscillator capable of emitting laser radiation of   frequency, of a frequency different from f 1, an electro-optical receiver arranged to receive the return radiation of frequency f and the reference radiation of frequency f 2 'and capable of delivering in response an electrical signal modulated at the intermediate frequency f 2-f 1 and an electrical signal processing circuit, characterized in that the local oscillator is constituted by a frequency slip member (14) arranged between the separator (6) and the receiver (16) to receive the part (31) of the main radiation (28) taken by the separator (6), this radiation (28) being of frequency f 1, this   organ (14) being able to deliver in response the reference radiation. this (32) of frequency f 2.   2 / Apparatus according to claim 1, characterized in that the circuit   processing device comprises, in series at the output of the receiver (16), succes-   an amplifier (22) of the intermediate frequency signal, an envelope detector (23) and a low frequency amplifier (24) connected to the modulator (11-12) for synchronous detection on the low frequency.   3 / Apparatus according to claim 2, characterized in that the amplifica-   the intermediate frequency signal (22) is able to block the low frequency.   4 / Apparatus according to one of claims 1 and 2, characterized in that that it contains 11 -   a polarizer (7) disposed between the separator (6) and the modulator   (11-12), so as to create a rectilinear polarization of the portion of the main radiation transmitted by the separator and a quarter wave plate (8) disposed between the polarizer (7) and the modulator (11-12) for transform the linear polarization into a cicular polarization.   Device according to claim 1, characterized in that the frequency sliding member (14) comprises a crossed crystal.   part (31) of the main radiation (28) taken by the separator   tor (6) and an acousto-optic transducer in contact with the crystal.   6 / Apparatus according to claim 2, characterized in that the modula-   tor has a rotating shutter disposed at the common focus of a system   afocal constituted by said optical system (10) and an optical element   convergent (9) disposed between the shutter (11-12) and the separator   (6) 1 7 / Device according to claim 6, characterized in that the connection between the low frequency amplifier (24) and the modulator (11-12)   includes an opto-electronic sensor (25-26) associated with the rotational shutter   to output electrical signals at the low modulation frequency, the output of this sensor being connected to the amplifier low frequency (24).   8 / Apparatus according to claim 1, characterized in that it comprises   an optical plate (21) disposed in the path of the reference radiation.   (32) between the frequency slip member (14) and the receiver   tor (16), said blade (21) being oriented to return to the receiver   (16) the return radiation (30) and being able to transmit the reference radiation (32).   9 / Apparatus according to claim 1, characterized in that the transmitter   laser (1) -is a carbon dioxide laser of the waveguide type.   10 / Remote sensing system comprising a laser device emits   receiver according to claim 2, characterized in that it further comprises   an additional laser transmitter (33) capable of delivering radiation   continuous main beam (34) of frequency f 3 offset from fly optical pickup means (36) for directing a portion (42) of the main radiation (34) of frequency f 3 to the input of the 12 - frequency slip (14),   an additional modulator (39-40) at a low frequency f 6 different   rent of f 5, arranged to receive the other part of the main radiation   pal (34) frequency f 3, an additional optical system (38) for directing the radiation emerging from the additional modulator (39-40) towards the target, a filter circuit (45) whose input is connected to the output of the envelope detector (23), this circuit having two outputs (46-47)   connected respectively to the input of the low frequency amplifier   (24) and at the input of an additional low-frequency amplifier.   wall (48) connected to the additional modulator (39-40),   a divider circuit (53) whose two inputs (51, 52) are connected   respectively to the outputs of the low frequency amplifier (24) and the additional low frequency amplifier (48) and a recording system (54) connected to the output of the circuit divider (53).