FR2514959A1 - HETERODYNE DETECTION TRANSMITTER-RECEIVER LASER DEVICE - Google Patents

Abstract

THIS DEVICE INCLUDES A LASER 1 EMITTING THROUGH A MIRROR 4 OF ITS CAVITY A FIRST BEAM 25 THROUGH A FREQUENCY OFFSET SYSTEM 7, 8, 9 TO ILLUMINATE A TARGET, THIS LASER EMITTING THROUGH THE OTHER MIRROR 3 OF ITS CAVITY A SECOND BEAM 26 CONCENTRATED ON A DETECTOR 20 WHICH ALSO RECEIVES AFTER CROSSING AN OPTICAL SYSTEM 17, 18 THE LASER RADIATION 30 RETURNED BY THE TARGET. APPLICATION TO THE MEASUREMENT OF THE SPEED OF A TARGET.

Description

1495 Heterodyne detection transceiver laser device

  The present invention relates to a transmitter-receiver laser device

heterodyne detector.

  It is known that it is possible to measure the distance of a target by sending to it a pulse delivered by a laser transmitter and by

  measuring the time interval between the departure of the impulse

  sion and the return of an echo of this pulse returned by the target.

  In heterodyne-detecting devices, the transmission frequency of the laser transmitter is slightly offset from that of a local oscillator, and the echo returned by the target is directed to a photoelectric receiver which also receives the delivered radiation. by the local oscillator The receiver then delivers modulated signals at a frequency equal to the difference between the frequency of the transmitter and that of the local oscillator. This results in a significant increase

  the signal-to-noise ratio at the reception.

  To avoid synchronization difficulties, we often use

  a single laser to perform the functions of the transmitter and oscillator-

local authority.

  For this, the energy of the beam delivered by the single laser is divided into two parts by means of an optical plate arranged at 45 degrees in the path of the beam. A first portion passes through an optical system which delivers a pulse of which the frequency is slightly offset from that of the laser, this pulse being directed towards the target The second part of the energy of the beam, delivered by the single laser is directed towards the receiver to be mixed with the echo

returned by the target.

  But the device described above has a drawback In

  effect the optical system causes parasitic reflections from the

  These reflections propagate in the opposite direction of the beam of emission towards the laser and are reflected by the optical plate on the receiver. This results in a high level of jamming which

  disrupts the measures significantly.

  The present invention aims to overcome this disadvantage.

  It relates to a laser transmitter-receiver device for detecting

heterodyne, having -2 -

  a continuous laser transmitter capable of delivering a radiation of frequency

  1, an optical system disposed at the output of the transmitter for receiving a first portion of the energy of the radiation of frequency f 1 and delivering in response a radiation pulse of offset frequency with respect to f 1, this pulse being transmitted in accordance with a beam oriented towards a target, the optical system comprising at least one dioptric surface

  able to send back to the transmitter reflections from the energy

  a photodetector disposed in the vicinity of the transmitter for receiving an echo, of frequency f 2, of the echo returned by the target, means for directing a second part of the energy towards the detector; frequency radiation f 1 in order to perform a heterodyne detection, the detector being able to deliver, in response to said frequency echo f 2 and to said second portion of the frequency radiation flp, an electrical signal modulated at the frequency f 1 -f 2 and a circuit for processing the modulated electrical signal delivered by the detector, characterized in that the laser transmitter comprises a resonant optical cavity comprising two opposite, partially transparent mirrors, the first part of the radiation of the emitter emerging from a first one of these mirrors, the second part of the radiation of the transmitter coming out of

  second mirror, the shifted frequency being equal to a predetermined value

  minus different from kc 2 LL being the distance between the first and second mirrors, c being the speed of light and k being a number that can take all the integer values, the amplitude of said parasitic reflections being decreased after crossing the cavity, the first mirror in the second mirror, A particular embodiment of the object of the present invention is described below, by way of example, with reference to the appended drawing in which the figure schematically represents a mode

  embodiment of the device according to the invention.

251 4959

  In this figure, a laser transmitter 1 of continuous type comprises a laser tube 2 whose end faces are respectively closed by two opposite mirrors 3 and 4 centered on an axis 6 and forming a resonant optical cavity According to a provision of the invention, the two mirrors 3 and 4 are partially transparent The tube 2 contains a laser active gas which can be excited by known means represented

schematically in 5.

  The input face of a cylindrical crystal 7 of germanium is dis-

  placed on the axis 6, so as to receive the laser radiation leaving the cavity by the mirror 4 The crystal 7 is provided with two transducers

  electro-acoustics 8 and 9 connected to an electrical power supply circuit.

  A diaphragm 11 is arranged so as to let through its opening 12 the radiation emerging from the other side of the crystal 7. This radiation then passes through an optical afocal emission system composed of a diverging lens 13 and a convergent lens 14

  centered on an axis 15 parallel to the axis 6.

  An afocal reception optical system is disposed along an axis 16 parallel to the axis 15 near the afocal optical system.

  The receiving system also has a different lens.

  Gente 17 is a convergent lens 18.

  An optical blade 19 is inclined at 450 on the axis 16 so as to

  send the reception radiation back to a photo detector

  20, through a concentration lens 21.

  A deflection mirror 22 is inclined at 450 on the axis 6 so as to

  return to the lens 21 and the detector 20, through the optical blade.

  than 19, the laser radiation coming out of the cavity through the mirror 3.

  The electrical output of the detector 20 is connected to a circuit of

  treatment 23 through a filter 24.

  The device described above and illustrated by the functional figure

as follows.

  The continuous laser radiation frequency f 1 emitted by the emitter 1 comprises two beams 25 and 26 exiting respectively through the two partially transparent mirrors 4 and 3 of the resonant optical cavity.

  Circuit 10 sends a succession of successive electrical pulses

  4- sives at the input of the two transducers 8 and 9 so as to create an acoustic wave that propagates in the crystal 7 When the transducers 8

  and 9 are powered, the interaction of the acoustic wave and the

  penetrating beam through the input face of the crystal inclined on the axis 6 creates an offset of the beam frequency The transducers 8 and 9 are arranged so that the beam 27 leaving the crystal 7 is

  constantly in parallel with the beam 25 The beam 27 passes through the opening

12 of the diaphragm 1.

  When the transducers 8 and 9 are not powered, that is,

  in the time intervals separating the pulses delivered by the circuit 10, the beam 25 passes through the crystal 7 along a path illustrated in phantom in the figure, to be absorbed by the solid portion of the diaphragm 11. The beam 27 is therefore constituted by a series of successive pulses of laser radiation of frequency f D offset

  with respect to fi The beam 27 then passes through the afocal system 13-

  14 which delivers a beam 29 less divergent and larger section.

  The beam 29 is then oriented towards a target by known means

not shown.

  The target returns a portion of the energy of the laser pulses in the form of a beam 30 received on the lens 18 of the receiving afocal system. The beam 31, of frequency f 2 coming out of the lens 17, is reflected by the optical strip 19 and then is focused by the lens 21 on

  the sensitive surface of the detector 20.

  The beam 26 of frequency f 1 leaving the mirror 3 of the laser cavity is returned by the mirror 22 in a beam 32 which passes through the optical plate 19 to be also concentrated by the lens 21 on the sensitive surface of the photoelectric detector 20. which is of the quadratic type, delivers at its output an electrical signal modulated at the frequency f 2 -f 1 The filter 24 is of the bandpass type and allows

to extract the modulation.

  The device shown in the figure is intended to measure the

  radial velocity of a target In this case, the circuit 23 makes it possible to determine

  this speed as a function of the variations of the detected frequency f 2-f 1, these variations resulting directly from those of the

frequency f 2 by Doppler effect.

-5-

  It is difficult to avoid parasitic reflections

  either on the output face 33 of the crystal 7 or on a dioptric surface of the afocal emission system 13, 14. These reflections have the effect of returning a fraction of the energy of the offset frequency radiation, in the opposite direction to that of emission, that is to say towards

  the transmitter 1, on the exit face 34 of the mirror 4.

  To prevent these reflections from passing through the cavity 3-4 in order to mix with the beam 26 and to impress the sensitive surface of the detector 20, the frequency offset is determined so that the shifted frequency beam f D can be attenuated by the cavity laser. All that is required is that f D be different from k c

2 L

  where c is the speed of light, where L is the distance between the mirrors

  and k being a number that can take all the integer values.

  Preferably, the frequency offset is chosen such that f D is substantially equal to (2 k + 1) c 4 L

  Under these conditions, the parasitic impulses are strongly attenuated.

  clouds and their level after crossing the cavity is sufficiently weak

to not disturb the measurements.

  The present invention can be used in the realization of a speed measuring apparatus of a target; this apparatus may also comprise additional means known for measuring the distance of

target.

6 -

Claims (2)

  1 / Heterodyne transceiver laser device, comprising so   a continuous laser transmitter capable of delivering a radiation of frequency   quence f 1, an optical system disposed at the output of the transmitter for receiving a first portion of the energy of the radiation of frequency f 1 and in response delivering a radiation pulse of offset frequency with respect to f 1 being transmitted pulse in a beam directed towards a target, the optical system comprising at least one dioptric surface   able to send back to the transmitter reflections from the energy   a photodetector disposed in the vicinity of the transmitter for receiving an echo, of frequency f 2, of the echo returned by the target, means for directing a second part of the energy towards the detector; frequency radiation f 1 in order to carry out a heterodyne detection, the detector being able to deliver, in response to said frequency echo f 2 and to said second portion of the frequency radiation f 1, an electrical signal modulated at the frequency ff 2 and a circuit for processing the modulated electrical signal delivered by the detector, characterized in that the laser transmitter (1) comprises an optical cavity   resonant comprising two mirrors (3-4) opposite, partially transparent   the first part (25) of the radiation of the emitter emerging from a first (4) of these mirrors, the second part (26) of the radiation of the emitter emerging from the second mirror (3), the offset frequency being equal to a predetermined value different from kc 2 L   Where L is the distance between the first and second mirrors, where c is the speed of light and k is a number that can take all the integer values, the amplitude of said parasitic reflections being decreased after crossing.   of the cavity, from the first mirror (4) to the second mirror (3). 251 4959   2. The laser device as claimed in claim 1, characterized in that said predetermined value is substantially equal to (2 k + 1) c 4 L 3 / Laser device according to claim 1, characterized in that the optical system comprises a crystal (7) traversed by the radiation (25) of frequency f 1, this crystal being provided with at least one acousto-optical transducer (8) fed by an electric control circuit (10),   the radiation (27) leaving the crystal having said offset frequency.   4 / Apparatus according to claim 3, characterized in that said   optical system further comprises an afocal optical system (13-14)   poured by the radiation coming out of the crystal (7).   Device according to claim 1, characterized in that the circuit   processing circuit comprises a bandpass filter (24) centered on the frequency quence f 2-f 1-