GB2256764A

GB2256764A - Laser system with heterodyne detection

Info

Publication number: GB2256764A
Application number: GB8708269A
Authority: GB
Inventors: Jean Louis Meyzonnette; Remy Bertrand
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1986-04-08
Filing date: 1987-04-07
Publication date: 1992-12-16
Anticipated expiration: 2007-04-07
Also published as: DE3711634A1; FR2677456A1; GB2256764B; GB8708269D0; FR2677456B1

Description

- f - Laser system_ with heterodyne detection 22 _) 67 54

Background of the invention

Field of the invention

The present invention relates to improvements to systems using one (or more) laser associated with a receiver with heterodyne detection. Such systems can be used in particular for laser ranging or active imaging in the military field.

Description of the prior art

In such laser sys tems for which the present invention is 1 intended, one wants to get some information on an object, a target or a landscape element, by illuminating the scene ny means of a transmitt,er including a laser, and by detecting the flux backscatikered by the object, the target or the landscape element, by means oil a receiver using the heLerodyne d e ' e c t i o n In this type of reception, the fltjx,factised or not, bac'<scattered by the target is caused to interfere, by means of a mixing device, on the photosensitive surface of an optical detector, with the aid of a local oscillator, with a reference optical signal. The latter can be the beam a second laser cons tituting the local oscillator laser, or alternatively a frac tion of the beam from the transmitting laser. The optical de tector of the receiver produces an electrical signal with a beat frequency, and the analysis of this signal allows to de- 1.

Lermine certain characteristics related to the scene illuminated by the transmitter. These characteristics can'be, for example, the distance of a point, the albedo of a surface or the frequency shift related to the motion velocity of the target (shift due to the Doppler effect).

Such a system is described in the French patent application NII '22 462 717 that relates more specifically to electromagnetic detection with pulse compression and is intended to construct a laser range-finder. This type of system makes it possible to measure both the distance and the motion velocity of the target.

This system uses a laser transmitter set that sends periodic pulses towards the target, the duration of each pulse being of about 20 microseconds. An optical system of modu- lation inclUded in the transmitter allows to modulate the optical beam, for exemple in frequency. The laser system also uses a receiver set receiving the backscattered wave that it causes to interfere with a local wave so as to obtain a beat phenomenon to render visible the characteLcistic component of tne target.

The result of this eat is analyzed ny an optical detector that furnishes a useful electrical signal.

A disadvantage appears in this type ol system using a Laser transmitter delivering pulses of relatively long duration.

This disadvantage lies in the fact that a fraction of the laser flux scattered)y the elements of the transmission system, for exemple the optical modulator or optical elements such as lenses or mirrors, falls on the optical detector of the reception system. T Lt then appears a stray additional signal 4 signal analogous to that which would come from a target at a quasi-zero distance and with a zero relative velocity. This signal may be very disturbing for it is superimposed on the useful signal and often is much more powerful that the latter that comes from a more distant target.

This phenomenon is particularly disturbing in the case where an optical suL-Jassemi-tly common to the transmission channel and the reception channel is used, which promotes stray reflections.

This phenomenon will be better understood from the detai- led description given in the following pages.

An object of the present invention is to remedy this disadvantage. It is proposed to add to this kind of system means permitting to eliminate the stray signal by superimposing, at the sensitive surface of the Optical detector, an optical signal coherent with the stray signal. An adjustment oF the amplitude and the phase oF this additional signal allows, through interference, to highly reduce and even to suppress the detrimental effect of the Stray signal.

Summary of the invention

An object of the invention is a laser system with heterodyne detection including, firstly, transmission means for sen- ding towards a target a modulated transmission wave and including a laser transmitter and a modulator, and, secondly, reception means to receive the wave reflected by the target and including a first mixer for mixing the wave coming from a local oscillator and the reflected wave to produce a -beat wave, and beat detector furnishing an electrical useful signal that is characteristic of the beat wave and the stray waves due to the elements making up the transmission system.

The laser-system according to the present invention is characterized in that it comprises means to tap a fraction of the modulated wave coming from the transmission system, means for modulating in phase and in amplitude said modulated wave to produce a correction wave, a second mixer for mixing the beat wave with the correction wave, and a control circuit for controlling the phase and amplitude mcdulation means re- ceiving 'the useful signal, SO 2S to control the phase and amplitude modulation circuit to eliminate the Stray component superimposed to the beat wave.

Brief description of the drawings

The present invention and its various Features will be better understood From the fo-1lowing description given with reference to the accompanying drawings, in which: Figures 1, 2 and 3 illustrate signal waveForms allowing to --3etter underSLand the disadvantages of the prior art devices; - Figure 4 is a diagram of the system according to the present invention; - Fiaure 5 is a schematic diagram of an enDodiment of the device according to the present invention.

Preferred embodiments of the invention The signal shown in figure 1 illustrates the wave OE transmitted!)y a prior art device. This wave consists of successive th a duration pulses wit &I.

In Figure 2, the reflected wave OR of the transmitted wave OE is illustrated. This reflected wave is made up of pulses of smaller intensities than those of the transmitted Wave but of same duration AI. Moreover, these pulses are shifted by an interval AT wiEh respect to the transmitted wave, 'this in- terval representing the time necessary for the transmitted wave to reach the target and come back to the device.

Assuming that the durationLl of the pulses transmitted by the laser tranSMitteC is 20 microsecondes, the velocity of light being 3x10 a meter/second and the target being loca- ted at 3 km, the reflected Wave will come back after 2 time of 20 microsecondes. Hence we infer for this example that, for any target located at less than 3 km from the device, the receiver will receive the beginning of the back pulse when the transmitter is sending the end of the same pulse. This example is the case illustrated in figures 1 and 2.

In Figure 3, there is illustrated in heavy line the electrical signal delivered by the optical detector al the receiver, related to the stray wave and corresponding to a pulse of the transmitted W2Ve. There is represented in fine 1 tne electrical signal delivered by the optical detector and related to the pulses of the reflected wave. In view oil the fact that this reflected wave has an intensity markedly lower, in practice 1000 times lower, then that of the transmitted wave, it is easier to understand, in this particular case, the necessity to eliminate the stray waves dUe to the transmission system. AS 2 Matter Of fact, this stray wave Can be construed as the reflected wave of a target whose velocity and distance are zero. 30 With reference to Figure 4, the system according to the present invention uses some elements common to the prior art system. The transmission wave is produced by a laser generator 11 I and modulated by a modulator 2 according to a mod0lation signal SM. The modulated wave OM is applied to a transmitter 11 that de'Livers the wave OE transmitted in the direction of a potential target. These three elements make up the transmission system of the laser system.

The reception system is made up in the receiver 12 that receives the reflected wave be able to use the information constituted cillator 3. The I transmitted of these two waves represents only the gain in a known optical detector 9 ful signal is of ranging and active imaging.

As explained above, the stray wave constitutes the main it component of the reflected wave, the pulses of the transm Lted wave OE being in Most cases no longer distinguishable.

According to the present invention, to the beat wave OB, before detection in the detector 9, tnere is superimposed a correction optical wave OC coherent with the beat'. -wave 06 so as to produce 2 second beat. This correction Wave is the modu12tion wave OM modulated in phase and in amplitude so as to reduce to minimum this beat phenomenon, allowing thus the de30 tector to receive only a negligible or even zero fraction of irst place of a OP. in order to by the pulses of the transmitted wave and, consequently, by the reflected wave, the received wave is directed by the receiver to a first mixer 4 'that also receives a 10C21 wave OL generated by a local os- ocal wave OL is of the same nature as the wave OE before it is modulated. The interference in a first mixer 4 produces a beat that pulses carried by the reflected wave. Amanner, this beat wave 06 is applied to an that delivers a useful signal SU. This;jsethe signal that is exploited for tne purposes the stray waves.

One therefore uses to this end means 6 For 'Lapping a fraction oF the modulated wave so as to have an accurat replica of it. These tapping means 6 are located at the output of the modulator 2 before the transmission by the transmitter 11.

There are then used in series a phase modulator 7 and an amplitude modulator 8. The order of these modulators is practically not important.

The phase modulator can be built according to conventional optics by using either the electro-optical effect of a mirror mounted on a shim, or the piezoelectric effect. The amplitude modulator can be built as an optical modulator using either the electro-optical effect, associated or not with polarizers, or the guided-wave optics. The wave tnus modulated constitutes the correction wave OC that is then mixed in an optical manner in a second mixer 5 with the beat wave OB containing the stray wave. The output wave from the second mixer is applied to the detector 9 nal SU.

According trolling the means of phase modulation 7 and Of aMplitide modulatinn 8 receives 'the useful signal. This control circuit generates, as a lunction of the useful signal, control VOltages VP and VA applied respectively to the phase modulator anti the amplitude modulator. This control circuit includes a memo- ry in which is stored a program of variation of these two control V01Lages VP and VA so as to modulate the correction wave OC in order for the mixing of the latter with the beat wave 06 to oppose the stray component.

The response time of this feedback control loop must be compatible with the characteristic evolution times of the op- that delivers, as in the prior art, the useful sig- to the invention, a c ntrol circ-uit 10 for con- - 8 tical stray wave at the sensitive surface of the optical d- -L Lector 9. This evolution is related to the variatiorfs of the optical path followed by the stray wave, that are due to the internal displacements of the various elements of the system.

These dispLacements, of the mechanical type, can be relatively slow, which allows to follow, by means of the feedback control loop, the evolution in phase and in amplitude of the stray wave at the sensitive surface of the detector 9.

An embodiment of this system is shown in Figure 5. With relerence to this figure, there is the laser transmitter 1 followed by 'the modulator 2, that can be an acousto-optical modulator producing the modulated wave ON. As in Figure 4, there are the phase modulator 7 followed by the amplitude modulator 8, the detector 9 and the control circuit 10 that will be described in detail in the following paragraphs.

The transmitter and the receiver are implemented in the lorm of two common elements 13 and 16. The first one is an optical device that performs bcth Lransmission and reception, and includes 'Ln particular a common scanning device. A separa- tion device 14 flor separating the transmission wave OE From the reflected Wave OR directs tne latter towards the mixer 24 that can be implemented in the form of a semitransparent plate. This semitransparent plate receives an the other side the local wave OL from the local oscillator that, here, is constituted by a fraction of the wave transmitted by the laser transmitter 1. To this end, another semi'Lransparent plate 23 is used that taps a fraction of tlie laser t)eam. The means for 'Lapping a fraction of the modulated wave and the means for mixing the correction wave OC with the be2t wave OB constituted by the semitransparent plate 24 can also be constituted by other semitransparent plates, respectively 26 and 15. The useful signal SU from the detector 9 is applied to 'the control circuit 10 that is described, as well as its operation, in the following paragraphs.

The useful signal is fi of processing cir-CUits 25.

pression, spectral analysis and possibly amplification sta ges. At the output of these processing circuits, the useful signal is then suitable for controlling the phase and ampli tude modulators. To this end, it is applied to a processor 2 :_7 that controls, according to a procedure described later, two ramp generators 28 and 29 that generate each a Voltage varying in a predetermined manner. The VOltages from these ramp generators constitute, after amplification in the ampli fiers 31 and 32, respectively, the control voltages VP and VA for controlling the phGSe and amplitude modulators.

The procedure for controlling the modulation of the correction wave OC is as follows.

For the phase modulation, one wants to have phase Opposition oetween the correction wave OC and the beat wave OB. To this end, the first step consists in varying, by means of the ramp generator 28, the phase in a detemined manner. The second step consists in determining the minimum value of the useful signal after processing, as a function of the variaticns of the phase control value VP. The phase control value VP 0 corresponding to the minimum useful signal is then stored and applied to the phase modulator 7 during the amplitude modulation.

rst processed in an arrangement This processing can 6e pulse com- This amplitude modulation begins with a first step consisting in varying, by means of tne ramp generator 29,-the value of the amplitude control voltage VA. The second step consists in determining the minimum amplitude of the useful signal af- ter processing and noting 'the corresponding value VA 0 of the amplitude control voltage.

The cycle is repeated in this Manner by performing again a phase modulation by varying the phase control voltage VP, the amplitude control voltage VA being set at the previous value VA 0. The cycle is repeated indefinitely, the value of the useful signal being stabilized after 2 few cycles. In each cycle, the particular values of the control voltages VP 0 and VA may vary slightly as the stray waves evolve with time.

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Claims

1. A laser system with heterodyne detection, including first 1 y: transmission means to send towards a target a modulated transmission wave, comprising a laser transmitter and a modu- lator; and secondly:

- reception means for receiving the wave reflected by said target, comprising a first mixer for mixing the local wave generated by a local oscillator with said reflected wave for producing a beat' wave, a detector of said beat wave fur- nishing an electrical useful signal, characteristic of said beat wave and the stray waves due to the optical elements of the system, tion of said modulated wave coming - means For tapping a Fract from the transmission system; - means for modulating in phase and in amplitude said modulated wave to produce a correction wave; - a second mixer for mixing said beat wave With said optical correction wave modulated in phase and in amplitude; - a circuit for controlling said means of modulation in phase and in amplitude, receiving said useful signal, so as to control said means of modulation in phase and in amplitude to eliminate the stray component superimposed to said beat wave.

2. A laser system according to claim 1, wherein said tapping transparent plate.

means are constituted by a semiL

3. A laser system according to claim 1, wherein said second mixer, for mixing said beat wave with said correction wave, is constituted by a semitransparent plate.

4. A laser system according to claim 1, wherein said first mixer, for mixing said local wave with said reflected viave is a semitransparent plate.

5. A laser system according to claim 1, wherein said local oscillator is constituted by a semitransparent plate located at the output of said laser transmitter.

-em according to clain 1, wherein said means 6. A laser syst of modulation in phase and in amplitude are made up of a phase modulator controlled by a phase control voltage furnished by said control circuit, and an amplitude modulator controlled by an amplitude control voltage furnished by said control circuit, these modulators being arranged in series.

7. A laser system according to claim 6, wherein said control circuit comprises two ramp generators to Furnish respectively phase and amplitude control V01tages, and a processor that C011Lrols alternately said two ramp generators so that, when one of the t,,.4o control valt:e is fixed, the other varies in a determined manner depending on the characteristics of the ramp generator that controls it, then to 'set t control value to a constant value for which ful signal is minimum, and to vary the control value that is remained Fixed, depending on 'the characteristics ul' the ramp generator that controls it, then to set this varying second control value to a determined control value for which said useful signal is minimum, and to go again and again through this control cycle of said ramp generators during the opera- tion of the system.

S. sor ter passing through processing circuits.

his varying fir said received use A laser system according to claim 7, wherein said proces- receives said useful signal coming from said detector af- 9. A laser system according to claim 1, wherein the transmission and the recepLiOn take place by means of an optical devi- 13) common to transmission and reception, a scanning device and a separation system (14) the transmission wave and the reception wave, receiving said modulated wave (OM) coming from said tapping means (6, 26) and directing said reflected wave (OR) towards said first mixer (4, 24). 10. A laser system substantially as described hereinbefore with reference to the accompanying drawings and as illustrated in Figure 4 or in Figure 5 of those drawings.

and comprising For separating - i; Aniendments to the c have been fled as fog 1. A laser system with heterodyne detection, including firstly:

- transmission means to send towards a target a modulated transmission wave, comprising a laser transmitter and a modulator; and secondly: reception means for receiving the wave reflected by said target, comprising optical elements in common with the transmission means, and a first mixer for mixing said reflected wave with a local wave of the same nature as the transmission provided by the laser transmitter, for producing a beat wave, a detector of said beat wave furnishing an electrical useful signal, character- istic of said beat wave and of stray waves due to unwanted feedback by optical elements of the system; -ion of said modulated wave coming - means For tapping a fract fron, the modulator; - means for modulating in phase and in amplitude said fraction of said modulated wave to produce a correction wave; t - a second mixer for mixing said beat wave WiLI1 said optical correction wave modulated in phase and in amplitude; - a circuit for controlling said means of modulation in phase and in amplitude, receiving said useful signal, so as to con- trol said means of modulation in phase and in amplitude to minimise the amplitude of said useful signal.

2. A laser system according to claim 1, wherein said tapping means are constituted by a semitransparent plate.

-Is 4. A laser system according to claim 1, wherein said first mixer, for mixing said local wave with said reflected wave, is a semitransparent plate.

6. A laser system according to clain 1, wherein said means of modulation in phase and in amplitude are made up of a phase modulator controlled by a phase control voltage furnished by said control circuit, and an amplitude modulator controlled by an amplitude control voltage furnished by said control circuit, these modulators being arranged in series.

7. A laser system according to claim 6, wherein said control circuit comprises two ramp generators to furnish respectivel-y phase and amplitude control voltages, and a processor to control said two ramp generators for alternately varying the phase control voltage, while maintaining constant the ampli tude control voltage, and varying the amplitude control voltage while maintaining constant the phase control voltage.

8. A laser system according to claim 7, wherein said processor receives said useful signal coming from said detector after passing through processing circuits.

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9. A laser system according to claim 1, wherein the transmission and the reception take place by means of an optical device (13) common to transmission and reception, and comprising a scanning device and a separation system (14) For separating the transmission wave and the reception wave, receiving said modulated wave (OM) coming from said tapping means (6, 26)and directing said reflected wave (OR) towards said first mixer (4, 24).

10. A laser system substantially as described hereinbefore with reference to the accompanying drawings and as illustrated in Figure 4 or in Figure 5 of those drawings.

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