WO2012038662A1

WO2012038662A1 - Telemetric measurement using a heterodyne-detection lidar device

Info

Publication number: WO2012038662A1
Application number: PCT/FR2011/052181
Authority: WO
Inventors: Matthieu Valla; François Lemaitre; Jean-Pierre Cariou; Béatrice Augere; Claudine Besson
Original assignee: ONERA (Office National d'Etudes et de Recherches Aérospatiales)
Priority date: 2010-09-22
Filing date: 2011-09-21
Publication date: 2012-03-29
Also published as: FR2965064B1; FR2965064A1

Abstract

The invention relates to a method for telemetric measurement using a heterodyne-detection LIDAR device, said method using a LIDAR transmission signal consisting of two optical wave components (SSE₁, SSE₂) that are produced simultaneously. Performing a subtraction between the beat frequencies which are produced by heterodyne detection and which correspond to the two components (SSE₁, SSE₂), respectively, makes it possible to obtain a measurement of the distance to a target at which the device is aimed. Such a measurement is exact, even when the target is in motion, and can withstand phase distortions that can negatively affect the two components of the LIDAR transmission signal.

Description

TELEMETRIC MEASUREMENT USING A LIDAR A DEVICE

HETERODYNE DETECTION

The present invention relates to a telemetric measurement which is carried out using a LIDAR device with heterodyne detection, also called coherent LIDAR. It relates to a method and a device for performing such a measurement.

LIDAR devices, for "Llght Detection And Ranging" in English, are widely used for many detection and measurement applications. These applications include measurement of airspeeds, the study of fluid flows, the study of vibration phenomena, the measurement of vehicle speeds, applications in the field of air transport, etc. Their operation consists in emitting a coherent optical wave towards a distant target to be studied, and in collecting a part of this wave which is backscattered by the target. In a coherent LIDAR device, the collected portion of the backscattered wave is subjected to heterodyne detection. The velocity measurement of the target is then deduced from a Doppler shift which is measured in a signal produced by the heterodyne detection.

It is also known to use these same devices to obtain a measurement of the distance away from the target. Such a measurement, called telemetry, is based on the detection of the propagation delay of the emitted and then backscattered wave, with respect to a frequency-modulated reference signal which is used for the heterodyne detection. In this case, the frequency ÎR _e f of the optical wave that is emitted towards the target is: f _Ref = f ₀ + Af _m od (t) (1 a) where fo is a basic optical frequency which is constant, Af _m0 d (t) represents the temporal modulation of the transmission frequency, and t denotes the time of emission. To obtain a measure of the distance D, Af _m0 d is a continuous function increasing or decreasing as a function of time t. The wave that is backscattered by the target then has the following frequency IR: f _R = f _R ef (t "2-D / C) + M Doppler = f 0 + Af _mod (t - 2 D / C) + Afpoppl 1, er (1 b) where Afooppie _r denotes the Doppler frequency shift that is produced by the displacement of the target, D is the distance between the optical transceiver head of the LIDAR device and the target, and C is the propagation velocity of the optical wave. The heterodyne detection produces a measure of the difference Δί between the frequencies ÎR _e f and f _R at the same time of reception, which is equal to:

Δί = f R - f Ref = Af _mod (t - 2 ^■ D / C) - Af _mod (t) + Δί Doppler (1 C)

This difference Δί corresponds to a signal beat frequency that is produced by the heterodyne detection. Now, the respective contributions of the distance and the speed of the target are mixed in this beat frequency Δ1

In order to separate these two contributions and to obtain measurements for both the distance of distance and the speed of movement of the target, it is known to modulate the frequency of the optical wave which is emitted in an opposite manner during windows of time. successive. Thus, during a first time window which is indicated by the index 1, the frequency R _e f of the optical wave that is emitted and the beat frequency of the heterodyne detection are respectively: f _Ref i = f ₀ + Af _mO d (ti) (2a)

Δίι = Af _mod (ti - 2 - D (ti) / C) - Af _mod (ti) + Af _Do ppier (ti) (2c) then for a second second window of time which is marked by the index 2 and during which the modulation of the transmitted optical wave is - Af _m od (t):

fRef 2 = fo - Af _mo d (t ₂ ) (2a ')

Af ₂ = - [Af _mod (t ₂ - 2 D (t ₂ ) / C) - Af _mod (t ₂ )] + Af _D0 ppier (t ₂ ) (2c ') In these relations (2a), (2c) and (2a '), (2c'), t ₁ and t ₂ denote two instants which respectively belong to the first and second time windows.

The difference is then made and then the sum of the two beat frequencies Δίι and Δί ₂ which are respectively measured for the two successive time windows. If the distance to the target and its speed are the same during both windows of time, these differences and sum are reduced to:

Δίι - Δί ₂ = 2 χ [Af _mod (t - 2 - D / C) - Af _mod (t)] (3a)

Δίι + Δί ₂ = 2 x Afooppier (3b)

Knowing the function Af _m0 d (t), the relation (3a) makes it possible to obtain a measurement of the distance D away from the result of the subtraction Δίι - Δί ₂ between the two beat frequencies. For example, when the modulation function Af _m0 d (t) is a linear ramp, the difference Δίι - Δί ₂ is directly proportional to the distance of distance D. More generally, a measurement of the distance D is obtained in performing a matched filtering of the signal that is produced by the heterodyne detection. Simultaneously, the sum Δίι + Δί ₂ of the two beat frequencies according to the equation (3b) is equal to 4-V / A, where V is the displacement velocity of the target and λ is the optical wavelength used: λ = C / fo.

But the relations (3a) and (3b) above are inaccurate when the target moves so that the distance of distance D and the velocity V vary between the two successive time windows: D (ti) ≠ D (t ₂ ) and Af _Do ppier (ti) ≠ Δίοορρΐβ _Γ (t ₂ ). In particular, the operation of subtraction between the two beat frequencies Δίι and Δί ₂ of the relation (3a) no longer makes it possible to suppress the contribution of the speed of displacement to obtain a measured quantity which depends only on the distance d However, by applying the relations (3a) and (3b) to such a moving target, the values obtained for the distance D and the velocity V are only approximate or even inaccurate.

Other phenomena can also modify the results of such measurements with respect to the real values of the distance of distance D and the speed V of the target. In particular, a phase noise that alters the optical wave emitted and then backscattered in a manner that is different during the two successive time windows is particularly harmful. However, such a phase noise can be produced during the amplification of the optical wave which is intended to be emitted, during its emission itself, and / or by turbulence of the propagation medium of the optical wave between the LIDAR device and the target. In particular, atmospheric turbulence disrupts such measures telemetry and velocimetry which are performed for air traffic or long-range control applications.

Under these conditions, an object of the invention is to provide telemetry measurements whose accuracy is improved, especially for a moving target.

Another object of the invention is to provide such telemetry measurements, which still use a LIDAR device with heterodyne detection, but which are robust with respect to parasitic phenomena which can alter the optical wave which is implemented.

To achieve these and other objects, the invention provides a range measurement method that uses a heterodyne-type LIDAR device, in which an optical wave emission signal is produced towards a target from an optical head of the device, and a backscattered signal which originates from the target is collected by this optical head and then detected by heterodyne detection so as to produce a heterodyne detection signal, the method comprising the following steps:

IM producing a frequency modulation of the optical wave in the transmit signal, said modulation comprising continuous growth segments and continuous decay segments of the frequency; Subtracting from each other two beat frequencies of the heterodyne detection signal which are obtained respectively for the growth segment and the decay segment; and

13 / deduce a value of a distance away from the target, from a result of the subtraction of beat frequencies.

The method of the invention is characterized in that step IM is executed so that the transmission signal simultaneously has two optical wave components, with a first of these optical wave components being frequency modulated according to the continuous growth segments for the frequency of this first component, at the same time as a second of these optical wave components is frequency modulated according to the segments of continuous decay for the frequency of this second component.

Step 121 is then performed by subtracting a first beat frequency that is obtained for the first optical wave component within the heterodyne detection signal, at a second beat frequency that is obtained for the second component of optical wave inside the same heterodyne detection signal, these first and second beat frequencies being relative to the same time of detection of the backscattered signal. Thus, the invention consists in transmitting a LIDAR optical wave which is composite in the direction of the target, this emission wave simultaneously comprising the two optical wave components at certain times at least of the operation of the device. The measurement of the distance away from the target is then deduced from a difference between the beat frequencies of the heterodyne detection which are obtained respectively for the two components of the emission wave, and for the same instant of program.

The respective contributions of the distance of distance and the velocity of the target at the beat frequencies are identical for the two components of the optical wave which are implemented, since these beat frequencies are produced for the same instant of time. heterodyne detection. Thus, the values of the distance of distance and the speed of the target are unambiguous for each measurement. The subtraction operation of step 121, between the two beat frequencies that are produced by the heterodyne detection, therefore makes it possible to accurately suppress the contribution of the target displacement speed. In other words, the relation (3a) which is given above remains exact even when the target is mobile, and the value which is thus obtained for the distance from the target, from the result of this subtraction, corresponds exactly at the actual distance at the moment of measurement. In addition, disturbances that may affect the LIDAR wave that is emitted, including its phase, similarly alter the two optical wave components. This is particularly the case for a phase noise that would be produced during the amplification or emission of the LIDAR wave, or by atmospheric turbulence that would occur between the LIDAR device and the target. The contributions of such disturbances are then mutually compensated between the two optical wave components during the subtraction operation of step 121. In this way, the result of the telemetric measurement which is obtained at the end of the step 131 is robust with respect to such disturbances.

In implementations of the invention which are preferred, in particular for their simplicity, the following improvements may be used, each in isolation or by a combination of several of them:

the frequency variations in the continuous growth segments and in the continuous decay segments, respectively for the first and the second optical wave component of the emission signal, may be opposite; the continuous growth segments of the frequency and the continuous decay segments of the frequency, respectively for the first and the second optical wave component of the emission signal, can be linear ramps of constant slopes;

the continuous growth segments of the frequency and the continuous decay segments of the frequency, respectively for the first and the second optical wave component of the transmission signal, can be synchronous at each instant of the operation of the device; and

the modulated frequency of the first optical wave component can be obtained at step IM by applying the continuous growth segments from a first initial optical frequency, and the modulated frequency of the second optical wave component can be obtained by applying the continuous decay segments from a second initial optical frequency, with the first initial optical frequency being greater than the second initial optical frequency by a fixed deviation.

The invention also proposes a detection type LIDAR device heterodyne adapted for performing telemetry measurements, and comprising:

a laser oscillator, which is adapted to produce an optical wave;

an optical splitter, which is arranged to divide the optical wave into an emission source signal and a reference signal;

an optical amplifier, which is adapted to produce a transmission signal from the transmission source signal;

an optical head, which is adapted to transmit the transmission signal to a target and to receive a backscattered signal from the target;

a mixing-detection unit, which is adapted to transmit the transmission signal to the optical head, and to produce a heterodyne detection signal from the backscattered signal received by the optical head and the reference signal;

means for analyzing the heterodyne detection signal, which are adapted to perform a spectral processing of the heterodyne detection signal;

a modulator, which is arranged to modulate in frequency at least the transmission source signal; and

a control unit which is connected to a control input of the modulator.

The device of the invention is characterized in that the control unit is adapted to control an operation of the modulator to implement a method as described above. For this, the emission source signal is modulated to generate the two optical wave components in the transmission signal, with frequencies of these components which respectively vary according to segments of continuous growth or continuous decay. In addition, the analysis means are adapted to provide a result of the subtraction of the first beat frequency obtained for the first optical wave component within the heterodyne detection signal, at the second beat frequency obtained for the second optical wave component within the same detection signal heterodyne, these first and second beat frequencies being relative to the same time of detection of the backscattered signal. They are further adapted to derive the value of the distance away from the target from the result of the subtraction of the two beat frequencies.

According to one possibility, the analysis means can be adapted to combine a first part of the heterodyne detection signal which has the first beat frequency and which corresponds to the first optical wave component of the emission signal, with a second part the same heterodyne detection signal which has the second beat frequency and which corresponds to the second optical wave component of the transmission signal. The same analysis means can then measure a difference between the first and second beat frequencies as the frequency of a signal resulting from the combination.

Alternatively, the first and second beat frequencies can be determined separately, and the subtraction operation between them can be performed numerically.

In an improvement of a device according to the invention, the analysis means may also be adapted to add the first beat frequency which is obtained for the first optical wave component inside the heterodyne detection signal. at the second beat frequency which is obtained for the second optical wave component within the same heterodyne detection signal, these first and second beat frequencies being relative to the same detection time of the backscattered signal, and to perform a Doppler effect analysis for deriving a value for the target moving speed from the result of adding the first and second beat frequencies. The device can then simultaneously provide measurements of the distance of distance and the speed of the target, which are relative to the same moment during a displacement of the target.

Other features and advantages of the present invention will emerge in the following description of nonlimiting exemplary embodiments, with reference to the appended drawings, in which: FIGS. 1a and 1b show two architectures of LIDAR devices with heterodyne detection, with which the invention can be implemented;

FIGS. 2a and 2b illustrate two possible compositions for a suitable LIDAR transmission signal according to the invention; and

FIGS. 3a and 3b illustrate two embodiments of a frequency modulator that can be used alternately in each architecture of FIGS. 1a and 1b.

In Figures 1a and 1b, elements that are designated by identical references are themselves identical or have functions that are identical. In addition, the following references denote the components which are indicated below, and which are used in the same way as in a heterodyne detection LIDAR device, of the monostatic type, as known before the present invention: 100: the LIDAR device with heterodyne detection as a whole;

1: a laser oscillator, which is adapted to produce an optical wave OL;

2: an optical splitter, which is arranged to divide the optical wave OL into a transmission source signal SSE and a reference signal SR _e f; 3: an optical amplifier, which is adapted to produce an emission signal SE from the source signal SSE;

1 1: a mixing-detection unit ("mixing and detection unit");

12: an optical head, which is adapted to transmit the transmission signal SE towards a target and to receive a backscattered signal R; and

20: analysis means.

In addition, T designates the target whose distance of distance D with respect to the optical head 12 must be measured, as well as its radial velocity V, possibly. The direction which connects the optical head 12 to the target T is the pointing direction of the device 100. By radial velocity V the target T the component of the speed of this target which is parallel to the pointing direction.

The laser oscillator 1, the optical separator 2 and the amplifier 3 may belong to a transmission laser unit 10, which produces the transmission signal SE. In a known and particularly advantageous manner, each of these components can be made from at least one optical fiber which conducts the optical wave OL or the signals SSE, SR _e f or SE. Currently, the optical wave OL and the signals SSE, SR _e f and SE belong to the infrared electromagnetic radiation band, for which the wavelength is between 1.535 μιτι and 1.565 μιτι (micrometer). .

The mixing-detecting unit 11 is adapted to transmit the transmission signal SE to the optical head 12, and to produce a heterodyne detection signal SRF from the backscattered signal R which is received by the optical head 12 and the reference signal SRef.

The analysis means 20 are adapted to analyze the heterodyne detection signal SRF. In a manner that is commonly used, these analysis means 20 perform a combination of filtering operations, signal composition, spectral analysis, as well as a Doppler effect analysis, from the detection signal. heterodyne SRF.

The device 100 may also include other components, in a manner that is customary for heterodyne detection LIDAR devices. Such components are not included in the present description, insofar as they have no direct connection with the object of the invention.

In addition to the foregoing components, a device 100 according to the invention comprises:

a frequency modulator 4 for modulating at least the SSE emission source signal; and

a control unit 40 for controlling an operation of the frequency modulator 4.

In the device 100 of FIG. 1a, the modulator 4 is arranged to receive as input the SSE emission source signal which is produced by the optical separator 2, and for outputting the modulated emission source signal to the optical amplifier 3. In other words, the modulator 4 is located between the separator 2 and the amplifier 3. In this first case, only the signal SSE emission source comprises the frequency modulation that is produced by the modulator 4.

In the alternative device 100 of FIG. 1b, the modulator 4 is arranged to receive as input the optical wave OL which is produced by the laser oscillator 1, and to output this modulated optical wave to the optical separator 2. Otherwise said, the modulator 4 is located between the laser oscillator 1 and the separator 2. In this second case, the emission source signal SSE and the reference signal SR _ef are modulated in an identical manner.

The two devices 100 of FIGS. 1a and 1b are equivalent to the invention, so that all the implementations of the invention which are described below can use either one or the other of these devices.

The modulator 4 produces the temporal modulation of the SSE emission source signal which is illustrated in FIG. 2a. This modulation generates two components inside the SSE signal, which are called optical wave components and noted respectively SSE1 and SSE ₂ . Each of these components is itself modulated in frequency within successive time periods. In the diagram of Figure 2a, the abscissa axis identifies the time t, and the ordinate axis locates the instantaneous frequency f of each component. The frequency of the first SSEI component increases continuously within each time period Wi, and the frequency of the second SSE component ₂ decreases continuously within each time period W ₂ . In general, for the invention, it is not necessary for the periods Wi and W _{2 to} have durations that are identical or synchronous. Similarly, changes in the frequency of each SSEI or SSE component ₂ may be any in each period, provided that the frequency is increased for the first SSEI and decreasing component to the second component ₂ SSE. Variations of the frequency of the SSEI component within each Wi period are then called continuous growth segments and denoted Af _m0C M, and those of the SSE component ₂ within each period W ₂ continuous decay segments and denoted Af _m0C Due to these different directions of variation, the variations in the frequency of the SSEI component are always different from those of the frequency of the SSE component ₂ , between two instants which belong to both the same period Wi and the same period W ₂ .

For a reason which will be explained below, the SBS _ï frequency component variations can advantageously start from a first frequency faith at the beginning of each period Wi, and those of the SBS component ₂ from a second frequency fo ₂ at the beginning of each period W ₂ , f ₀ i being greater than fo ₂ by a fixed distance Δί ₀ : faith - ½ = Δί ₀ .

Figure 2b corresponds to Figure 2a for a preferred embodiment of the invention. In this embodiment, the two components SSE1 and SSE ₂ have respective frequency variations which are opposite, synchronous and linear within common time periods W. In other words, the frequency of the SSE1 component varies according to rectilinear, or linear, increasing ramps with a positive slope which is denoted by K _m0C i, and the frequency of the SSE component ₂ simultaneously varies according to linear decreasing ramps of slope -K _m0C i. By way of example, the period W may be of the order of a few milliseconds when the modulator 4 is of the electro-optical type. The optical frequency difference Af ₀ may be of the order of 1 GHz (gigahertz).

To realize such modulations of the emission source signal

SSE, which are in accordance with Figure 2a or 2b, the frequency modulator 4 can be achieved using an intensity modulator or a phase modulator.

FIG. 3a illustrates an embodiment of the frequency modulator 4 from an electro-optical intensity modulator. Such intensity modulator may comprise two Pockels cells 41 and 42 which are connected optically in parallel, so as to receive each approximately half of the intensity of the SSE emission source signal in the case of a device 100 which is in accordance with FIG. 1a, or about half of the intensity of the optical wave OL in the case of a device 100 which is in accordance with Figure 1b. In a known manner, a Pockels cell produces a variable electric field inside an active material which is adapted to modify the phase of an optical wave as a function of the electric field. The Pockels cells 41 and 42 may be based on lithium niobate and are preferably identical. The modulator 4 further comprises an electrical modulation signal generator 43, which is connected to respective control inputs of the two Pockels cells 41 and 42. In addition, the two cells 41 and 42 are oriented so that the same signal modulation produced by the generator 43 generates optical phase shifts which are opposed between the two cells, at each instant of an operation of the device 100. Furthermore, an additional DC voltage generator 44 can be inserted between the output of the generator 43 and one of the two Pockels cells, for example the cell 42. The generator 43 is controlled by the unit 40 to produce a sinusoidal modulation voltage V _m0C i whose phase varies quadratically with time according to the relation:

V _mod = β ^■ Vi _rad "COS (TT K t ² ) (4) starting from 0 at the beginning of the period W. β is the ratio between the amplitude of the modulation voltage and the voltage Vi _ra d which is necessary for a Pockels cell to change the phase of the optical wave portion passing through it by 1 radian In a manner superimposed on the variable optical wave phase shift that is produced by the generator 43, the generator 44 produces a constant additional phase shift a, between the waves which respectively pass through the two cells 41 and 42. In known manner, the Bessel series development of the total field of the optical wave at the output S of the modulator 4, after the superposition of the parts of the wave which have passed through the cells 41 and 42 respectively, shows that the resulting optical wave has the two-component form of FIG. 2b, for the pair of values a = 0 and β = 2.4048 with the slope K _m0 d = 2 x K, or for the pair of values α = π and β = 1, 8 412 with the slope K _m0 d = K. In practice, the second of these pairs of values for a and β is preferred because the modulation of the wave The optical gain obtained is then less sensitive to modulator parameter fluctuations.

Those skilled in the art also know how to modify the relation (4) of the modulation voltage to introduce the difference Δί ₀ between the starting values of the respective frequency modulation ramps of the two components SSE1 and SSE ₂ .

The inventors report, however, that it is preferable that the difference between the respective frequencies of the two components SSE ₁ and SSE ₂ be greater than the inverse of a reaction time of the amplifier 3. In this way, the amplification of the transmission source signal does not generate distortion of the transmission signal SE.

FIG. 3b illustrates an embodiment of the frequency modulator 4 from an electro-optical phase modulator. Such a phase modulator is adapted to produce a frequency modulation of an optical wave according to a control signal which is periodic two-state. It may comprise a Pockels cell 45 which is connected to receive as input the SSE emission source signal (FIG. 1a) or the OL optical waveform (FIG. 1b). The modulator 4 then comprises a modulation signal electrical generator 46 which is connected to a control input of the cell 45. When the generator 46 is controlled by the unit 40 to produce the two-state periodic control signal corresponding respectively to at the voltage values V _{n 2} and V- _{TT / 2} , and with a switching frequency that itself varies linearly with time, the modulated optical wave still has the two-component form of Figure 2b. In this control signal, the values V _{n 2} and V _{-TT 2} are selected so that the Pockels cell 45 produces variations equal to ττ / 2 and -π / 2, respectively, of the phase of the optical wave which the crossing.

When the emission source signal SSE consists of the two components SSE ₁ and SSE ₂ , the heterodyne detection signal SRF, which is transmitted by the detection unit 11 to the analysis means 20, comprises superimposition terms of each component backscattered with the reference signal SR _ef used. These terms have a beat frequency that is equal to the difference between the frequencies of the optical waves that are superimposed. Thus, for a device 100 which is in accordance with FIG. 1a, that is to say when the reference wave SR _e f is not itself modulated, the frequency thereof is f ₀ , and the frequencies of the two optical wave components SSE1 and SSE ₂ retrodiff used by the target T are: f 1 = fo + Af _m od 1 (t - 2 ^■ D / C) + Afooppier (5a) f 2 = fo + Δί mod 2 (t - 2 ^■ D / C) + Af Doppler (5b) where Afmod 1 and Af _m0 d 2 are the two continuous modulation functions, increasing and decreasing in segments respectively for the optical wave components SSE1 and SSE ₂ . The first beat frequency measured for the SSEI component is then:

Afi = f 1 - fo = faith - fo + Af _m od i (t - 2 D / C) + Af _Do ppier (6a) and the second beat frequency, which is measured for SSE component ₂ , is: Af ₂ = f ₂ - f ₀ = ½ "fo + Afmod ₂ (t - 2" D / C) + Af Doppler (6b)

Thanks to the method of the invention, the values of the distance of distance D of the target T and the Doppler shift Af _Do ppier are strictly the same in the two relations (6a) and (6b), since these values relate to detection signals that are simultaneous.

During a combination by the analysis means 20 of the two heterodyne detection beats which have the frequencies given by the relations (6a) and (6b), the following frequencies appear by subtraction and by addition, assuming that fo is equal to the average faith and f ₀₂ :

Afi - Af ₂ = Af ₀ + [Af _mod 1 - Af _{mod 2} ] (t - 2 - D / C) (7a) Afi + Af ₂ = [Afmod 1 + Afmod ₂ ] (t - 2 "D / C) + 2 "Af Doppler (7b)

The signal components which respectively have the frequency Afi-Af ₂ and the frequency Afi + Af ₂ can easily be separated by frequency filtering, thanks to the presence of the term Af ₀ in the relation (7a).

In this case of Figure 1a, with the composite function [Afmod 1 - Afmod 2] which is not necessarily linear but which is nevertheless Known, the skilled person can deduce from the relation (7a) a measure of the distance of distance D. Indeed, it is possible in particular to compress temporally the component of the combination signal which corresponds to the subtraction of beat frequencies Δίι and Δί ₂ , using the well-known technique of pulse compression. Such pulse compression is usually carried out using a matched filter, that is to say by filtering the component of the signal corresponding to the frequency Δίι - Δί ₂ with a replica reversed temporally of this same component. The temporally reversed replica has an instantaneous frequency which is equal to [Af _m0 d 1 - Af _m0 d 2] (- t). This type of filtering reshapes the spectral components of the signal at the input of the filter, so that the temporal response of the signal at the filter output is compressed until reaching the Fourier limit. Thus, the target T appears in the filtered signal as if the LIDAR transmission signal had been produced by a pulse-operated laser whose pulse duration is equal to the inverse of the frequency shift of the function. composite [Af _m0 di - Af _m0 d 2] - The temporal response thus compressed makes it possible, by measuring the propagation delay, to obtain a value for the distance D away from the target T.

For a device 100 which corresponds to FIG. 1 b, the reference wave SR _e f itself comprises the two components SSE ₁ and SSE ₂ . In this case, the first beat frequency results from the superposition of the SSE1 component as emitted with the same component SSE1 backscattered by the target T and then received by the optical head 12. This first beat frequency is then: Afi = Af _m od i (t - 2 - D / C) - Af _m od i (t) + Afooppier (6a ')

Similarly, the second beat frequency results from the superposition of the SSE component ₂ as emitted with the same SSE component ₂ backscattered by the target T then received by the optical head 12:

Af ₂ = Af _m od ₂ (t-2-D / C) - Af _m od ₂ (t) + Afooppler (6b ') As previously, the values of the distance of distance D and the Doppler shift Af _Do ppier which intervene in the relation (6a ') are strictly equal to those which intervene in the relation (6b'), since these values relate to detection times which are identical for the two optical wave components SSE1 and SSE ₂ , thanks to the invention.

Additional terms appear further in the heterodyne detection signal SRF for this second case according to Figure 1b, which correspond to the superposition of the SSE1 component as emitted by the optical head 12 with the SSE component ₂ as backscattered then received, and the superposition of the SSE component ₂ as emitted by the optical head 12 with the component SSE1 as backscattered and then received. But these terms have a beat frequency that is close to Δί ₀ , and are eliminated by frequency filtering.

Relations (7a) and (7b) then become in the case of Figure 1b:

Δίι - Δί ₂ = [Af _mod 1 - Af _{mod 2} ] (t - 2 - D / C) - [Af _mod 1 - Af _{mod 2} ] (t) (7a ')

Δίι + Δί ₂ = [Af _mod 1 + Af _{mod 2} ] (t-2-D / C) - [Af _mod 1 + Af _{mod 2} ] (t) + 2-Af _Do pp (7b ')

When the two modulation functions Af _mod i and Af _{mod 2} are linear, the result of the operation of subtraction of the beat frequencies according to the relation (7a ') is a linear function of the distance of distance D. It thus provides directly a measure of the distance D.

In general, since the modulation functions Af _mod 1 and Af _{mod 2} are respectively increasing and decreasing, their difference in the relations (7a) and (7a ') is never constantly zero.

According to an improvement of the invention, the result of the operation of adding the beat frequencies according to the relation (7b) or (7b ') provides a measurement of the frequency shift by Doppler effect Af _Do ppier- For example, when the two modulation functions Af _mod i and Af _{mod 2} are opposite, the result of the operation of adding the beat frequencies according to the relation (7b) or (7b ') is directly equal to twice the frequency shift by Doppler effect Af _do ppier- an analysis of this shift, performed one of the ways known to the skilled person then provides a measure of the removal speed V of the target T.

It is understood that the invention may be reproduced by modifying aspects of implementation in relation to the detailed description given below. above, while retaining some of the benefits that have been mentioned. In particular, the optical wave modulator of the electro-optical type can be replaced by an acousto-optical type modulator. In known manner, such an acousto-optic modulator may consist of a refractive index grating whose characteristics are varied using a piezoelectric element control.

Claims

1. Telemetric measurement method using a device of the type

Heterodyne detection LIDAR (100), wherein an optical wave emission signal (SE) is produced towards a target (T) from an optical head (12) of said device, and a backscattered signal (R) from the target is collected by said optical head and then detected by heterodyne detection to produce a heterodyne detection signal (SRF),

said method comprising the following steps:

IM producing a frequency modulation of the optical wave in the transmit signal (SE), said modulation comprising continuous growth segments and continuous decay segments of the frequency;

Subtracting from each other two beat frequencies of the heterodyne detection signal (SRF) respectively obtained for said growth segment and said decay segment; and

13 / deriving a value of a distance (D) away from the target, from a result of the subtraction of the beat frequencies, the method being characterized in that the step IM is executed so that the emission signal (SE) simultaneously has two optical wave components, a first of said optical wave components (SSE1) being frequency-modulated according to the continuous growth segments together with a second of said wave components optical (SSE ₂ ) is frequency modulated according to the continuous decay segments, and step 121 is performed by subtracting a first beat frequency obtained for the first optical wave component (SSE1) within the detection signal heterodyne (SRF), at a second beat frequency obtained for the second optical wave component (SSE ₂ ) within said heterodyne detection signal (SRF), said first and second frequencies of e beat being relative to the same time of detection of the backscattered signal (R).

The method according to claim 1, wherein the frequency variations in the continuous growth segments for the first optical wave component (SSE1) of the transmission signal (SE) are opposite to the frequency variations in the decay segments. continuous for the second optical wave component (SSE ₂ ) of said transmission signal.

3. The method of claim 1 or 2, wherein the continuous growth segments of the frequency and the continuous decay segments of the frequency, respectively for the first (SSE1) and second (SSE ₂ ) optical wave component of the emission signal (SE), are linear ramps of constant slopes.

4. Method according to any one of claims 1 to 3, wherein the continuous growth segments of the frequency and the continuous decay segments of the frequency, respectively for the first (SSEI) and the second (SSE ₂ ) component d optical wave of the transmission signal (SE), are synchronous at each moment of operation of the device.

The method according to any one of claims 1 to 4, wherein the modulated frequency of the first optical wave component (SSE1) is obtained at step IM by applying the continuous growth segments from a first initial optical frequency (faith), and the modulated frequency of the second optical wave component (SSE ₂ ) is obtained by applying the continuous decay segments from a second initial optical frequency (fo ₂ ), said first optical frequency initial value being greater than said second initial optical frequency with a fixed difference (Δί ₀ ).

6. LIDAR device with heterodyne detection (100) adapted to perform telemetric measurements, and comprising:

a laser oscillator (1) adapted to produce an optical wave (OL);

an optical splitter (2) arranged to divide the optical wave into a transmission source signal (SSE) and a reference signal (SR _e f);

an optical amplifier (3) adapted to produce a transmission signal (SE) from the transmission source signal; an optical head (12) adapted to transmit the transmission signal towards a target (T) and to receive a backscattered signal (R) from the target;

a mixing-detection unit (1 1), adapted to transmit the transmission signal to the optical head, and to produce a heterodyne detection signal (SRF) from the backscattered signal received by said optical head and the signal of reference; and

means for analyzing (20) the heterodyne detection signal, adapted to perform a spectral processing of said heterodyne detection signal,

the device being characterized in that it further comprises:

- a modulator (4) arranged to frequency modulate at least the transmission source signal (SSE); and

a control unit (40), connected to a control input of the modulator, and adapted to control an operation of said modulator to implement a method according to any one of the preceding claims; and

the analysis means (20) being adapted to provide a result of subtracting the first beat frequency obtained for the first optical wave component (SSE1) within the heterodyne detection signal (SRF), at the second beat frequency obtained for the second optical wave component (SSE ₂ ) within said heterodyne detection signal (SRF), said first and second beat frequencies being relative to the same detection time of the backscattered signal (R ), and to deduce the value of the distance (D) away from the target (T) from the result of the subtraction of the two beat frequencies.

7. Device according to claim 6, wherein the analysis means

(20) are adapted to combine a first portion of the heterodyne detection signal (SRF) having the first beat frequency and corresponding to the first optical wave component (SSE1) of the transmit signal (SE), with a second portion said heterodyne detection signal (SRF) having the second beat frequency and corresponding to the second optical wave component (SSE ₂ ) of said transmission signal (SE), and for measuring a difference between the first and second beat frequencies as a frequency of a signal resulting from the combination.

8. Device according to claim 6 or 7, wherein the modulator

(4) is an electro-optical intensity modulator.

The device of claim 8, wherein the electro-optical intensity modulator comprises two Pockels cells (41, 42) optically connected in parallel, and an electrical modulating signal generator (43) connected to control inputs. respective of the two Pockels cells, and adapted to produce a sinusoidal modulation voltage (V _m0d ) whose phase varies quadratically with time, said two Pockels cells being oriented so that the same modulation signal generates optical phase shifts which are opposed between the two cells, at each moment of operation of the device (100).

The apparatus of claim 6 or 7, wherein the modulator (4) is an electro-optic phase modulator adapted to produce frequency modulation of an optical wave in accordance with a two-state periodic control signal.

1 1. An apparatus according to claim 10, wherein the electro-optical phase modulator comprises a Pockels cell (45) and an electrical modulation signal generator (46), said modulation signal electrical generator being connected to a control input of the Pockels cell and adapted to produce the two-state periodic control signal with a switching frequency which itself varies linearly with time.

12. Device according to any one of claims 6 to 11, wherein the modulator (4) is arranged to receive as input the transmission source signal (SSE) produced by the optical separator (2), and to transmit outputting said modulated emission source signal to the optical amplifier (3).

13. Device according to any one of claims 6 to 1 1, wherein the modulator (4) is arranged to receive the input optical wave (OL) produced by the laser oscillator (1), and to output said optical wave modulated to the optical separator (2), so that the transmission source signal (SSE) and the reference signal (SR _ef ) are modulated in an identical manner.

Apparatus according to any one of claims 6 to 13, wherein the analyzing means (20) is further adapted to add the first beat frequency obtained for the first optical wave component (SSE1) to the inside the heterodyne detection signal (SRF), at the second beat frequency obtained for the second optical wave component (SSE ₂ ) within said heterodyne detection signal (SRF), said first and second beat frequencies being relating to the same detection time of the backscattered signal (R), and to perform a Doppler effect analysis so as to deduce a value of a displacement speed (V) from the target (T), from a result of the addition of the first and second beat frequencies.