enhanced processing coherent frequency modulation lidar

Abstract

The invention relates to a coherent lidar system (10) comprising: a laser source (SL) configured to generate laser radiation (L) with a laser optical frequency fopt-l varying linearly over a plurality of N successive frequency ranges indexed i, a first optical device (DO1) configured to spatially separate the laser radiation (L), a detection device (Det), a second optical device (DO2) configured to simultaneously provide said pixel with a recombined beam (Lrecomb), a frequency shift device (FSD) disposed in the path of the reference beam and configured to shift the laser optical frequency by a shift frequency (fd0) within the interval [fRmax, fRmin], a processing unit (UT), the coherent lidar imaging system being further configured to determine distance information from a signal detected (Spix) by said pixel (P). Figure 9

Description

Title of the invention: Frequency-modulated coherent lidar with improved processing FIELD OF THE INVENTION

[0001] The present invention relates to the field of active imaging and in particular 3-dimensional scene imaging (distance and possibly radial velocity of objects). More particularly, the invention relates to frequency-modulated coherent lidars for determining distance information, and where appropriate the velocity of points in a scene to be observed. STATE OF THE ART

[0002] Measuring a distance involves using a light source to illuminate the scene and a detector (more generally an image sensor) having the capacity to codify the distance (and / or speed) values ​​in order to obtain three-dimensional information of the entire observed scene. More specifically, at any point (x,y) of the scene, a depth value (z) and possibly a speed value (vz) are assigned to it, so as to obtain a depth map z = f(x,y). and possibly a radial speed map vz = g(x, y).

[0003] Several active imaging techniques exist in the literature. We can cite the techniques of structured light, direct and indirect time of flight. We are more interested here in techniques using a coherent light source (Laser) and deflecting a part of the beam (local oscillator beam) in order to use it as an amplifier of the signal backscattered by the scene once it is illuminated (point by point or globally, see below) by the rest of the undeflected beam.

[0004] The principle of a coherent lidar is well known in the state of the art and illustrated [Fig.l]. A coherent lidar 1 comprises a coherent source, typically a laser Las, which emits a coherent light wave modulated in frequency L (IR, visible or near UV domain) and a detection device DetO comprising at least one pixel P comprising a photo-detector component PD. A transmission device illuminates a volume of space, and a reception device collects a fraction of the light wave backscattered by a scene or a target to be observed. At reception, a mixture is carried out between the backscattered light wave and a part of the emitted wave which has not passed through the scene, called the "local oscillator". The interference of these two waves is detected by the photo-detector, and the electrical signal at the output of the detector has an oscillating term proportional to the product of the local oscillator power by the power of the backscattered signal from the scene.This signal is digitized and distance and / or speed information is extracted. The offset of .

[0005]

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[0016]

[0017] backscattered Fonde Doppler frequency is a function of the radial velocity of the point having backscattered the wave: The lidar 1 thus comprises a first optical device DO1 configured to spatially separate the laser radiation L into a reference beam Lref (local oscillator) and an object beam Lo directed towards a scene to be observed Sc. The lidar 1 also comprises a second optical device DO2 configured to simultaneously provide the pixel P with a recombined beam Lrecomb corresponding to a superposition of the reference beam Lref and a beam reflected by the scene Lo,r when it is illuminated by the object beam. The coherent lidar imaging system is further configured to determine distance information from a signal detected Spix by the pixel P. In [Fig.l], only one pixel is shown but the DetO detector can in practice have a large number of pixels, arranged in a line or in a matrix. Frequency modulated coherent lidars, commonly referred to as FMCW (Frequency Modulated Continuous Wave) lidars, are known for determining a depth map. [Fig. 2] schematically illustrates the implementation of the FMCW lidar 1 known from the state of the art. The laser source LAS is configured to emit laser radiation L with an optical frequency fopt_LO varying linearly over at least one frequency range of width B for a duration T, as illustrated [Fig.2]. B is commonly referred to as a "chirp". The backscattered wave, which has traveled twice the distance z from the lidar 2 to the scene, has a frequency fopt_sc. The two interfering paths (Lref of frequency fopt LO and Lo,r of frequency fopt sc) on the photodetector produce beats whose frequency is proportional to the delay At between the two paths, therefore to the distance z. These beats are found in the detected electrical signal iPD. More precisely, for a linear ramp, the frequency of the oscillations is: / «=><» with B the optical frequency excursion or “chirp” during the duration T of the ramp, c the speed of light. The distance z can be deduced from the number k (k ~ Tf ) of periods measured during the duration T (so-called counting method): 2B The range resolution of the lidar is & ~ . (Ibis) It is also possible to measure f R by spectral analysis by Fourier transform of the beat signal (so-called FFT method). To make a complete image of the scene, a first known type of FMCW lidar sequentially illuminates the scene using a scanning device and the recombined beam is detected by a single photodetector. In practice, it is difficult to access distance image acquisition at video rate (typically 50Hz) for high resolution images (e.g. VGA or XGA) because the time available for distance measurement at each point is very short.

[0018] Thus, a second type of known FMCW lidar, as described in document WO2021144357 and illustrated [Fig.3], allows parallel acquisition of information from the points of the scene by a large number of pixels. The single photodetector is replaced by a matrix imager 41 where each pixel Pij contains a photoreceptor PDij which detects the information corresponding to the image of a point of the scene, the image being produced by suitable optics. The beam Lo heading towards the scene illuminates the entire targeted surface in one go without any scanning. The local oscillator beam Lref is also adapted to illuminate the entire surface of the imager. Since the distance is proportional to the beat frequency of the heterodyne signal, it is therefore necessary to measure this frequency on each pixel if we want to obtain the distance information of the entire scene simultaneously.Typically, the measurement can be carried out simultaneously for all pixels (“global shutter” mode) or line by line (“rolling shutter” mode).

[0019] The lidar 3 of the aforementioned document comprises the laser source LAS modulated temporally by the periodic ramp of excursion B and duration T mentioned above, and the coherence length of the laser radiation is at least twice the maximum predetermined distance zmax between the scene to be observed Sc and the lidar 3.

[0020] The lidar 3 also comprises an optical device DS, called a separator corresponding to the first optical device DO1 mentioned above.

[0021] The second optical device DO2 comprises an optical recombination device DR adapted to spatially superimpose the reference beam Lref on the beam reflected by the scene Lo,r, so as to form the recombined beam. The second optical device DO2 also comprises an optical imaging system Im with optical axis AO (diaphragm Diaph) which produces an image of the scene by imaging the beam reflected by the scene Lo,r on the detector 4L. The scene is typically at a distance much greater than the focal length of the optics Im, the detector 41 is placed substantially in the focal plane of the optics Im.

[0022] The optical devices DO1 and DO2 are configured so that each pixel Pij of the detector receives a portion of the image beam from the scene called Lo,r / pix and a portion of the reference beam called Lref / pix, and that the portions are spatially superimposed on each pixel. Preferably, these devices are configured, for example with the addition of an additional optic SI (not shown), to transport the reference beam from the laser source to an image plane. intermediate plane PI, perpendicular to the optical axis AO of the imaging optical system Im, so as to produce a coherent reference source (virtual or real) with the reflected beam. The intermediate plane PI is located near the imaging optical system, so as to generate flat-hue fringes, obtained by interference between the portion of the reflected beam detected Lo,r / pix and the portion of the reference beam detected Lref / pix, on each illuminated pixel Pij. For simplicity, the portion of beam illuminating a pixel is considered to be the portion of beam detected by the photodetector of this pixel. This flat-hue condition implies that, on each pixel Pij, a propagation axis of the portion Lo,r / pix of the reflected beam is collinear or substantially collinear with a propagation axis of the portion Lref / pix of the reference beam.A virtual or real intermediate image PS of the reference beam is formed in the intermediate image plane PI, the plane PI being arranged so as to generate flat-toned fringes, obtained by interference between the portions, on each illuminated pixel.

[0023] The coherent lidar imaging system 3 further comprising at least one electronic processing circuit configured to calculate, for each pixel Pij, a frequency F(i,j) of the beat of the portion of the image beam with the portion of the reference beam illuminating the pixel.

[0024] Finally, the lidar 3 comprises a processing unit UT connected to the laser source and to the detector 41, and configured to determine a distance of points of the scene imaged on the pixels, from the beat frequency calculated and associated with each pixel and from the modulated optical frequency of the laser radiation. The processing circuit can be located in each pixel, along a row or a column, or in the processing unit UT.

[0025] Having a local oscillator for each pixel makes it possible to have an image of each point in the scene simultaneously. Thus, the architecture of lidar 3 integrating a matrix detector is compatible with a large number of pixels (no scanning) making it possible to produce a high-resolution lidar image. The heterodyne mixing is carried out here in each pixel.

[0026] In lidar 3, B and T are constant. To detect a point at a distance z between zmin and zmax, the frequency fR must be able to be measured over the entire range AfR from fRmin defined by zmin to fRmax defined by zmax. AfR is therefore the frequency range of the signal to be measured and:

[0027] AfR = 2.B.(zmax-zmin) / cT (2)

[0028] As the distance range to be measured increases, AfR increases, which implies increasing the bandwidth of the iPD amplification circuit(s), which increases the noise and consumption of these circuits. This also increases the photonic noise of the DC component of iPD and decreases the signal-to-noise ratio. In effect in a known manner the signal to noise ratio is inversely proportional to ■ A decrease in the signal to noise ratio can lead to poor counting due to noise.

[0029] Furthermore, in general, a difficulty persists concerning the measurement of the frequency of the beat signal. Whether for the FFT method or the counting method, it is necessary to integrate complicated and fairly dense electronics into each pixel. This has the consequence of not being able to access “global shutter” type imagers given the number of MOS transistors to be fitted into the surface of a single pixel. Even in the case of a “rolling shutter” type imager where part of the electronics is placed at the bottom of the column, it is also difficult to achieve the video rate with this type of solution.

[0030] An aim of the present invention is to remedy the aforementioned drawbacks by proposing a coherent lidar imaging system allowing a reduction in the detection electronics necessary for measuring the frequency of the heterodyne signal and an improvement in the signal-to-noise ratio. DESCRIPTION OF THE INVENTION

[0031] The present invention relates to a coherent lidar system comprising: • a laser source configured to generate laser radiation with a laser optical frequency fopt_i varying linearly over a plurality of N successive frequency ranges indexed i, with N greater than or equal to 2, a frequency range having a width Bi and a duration Ti, a sum of all said frequency ranges corresponding to an overall duration, an absolute value of a ratio Bi / Ti being different for each frequency range, • a first optical device configured to spatially separate the laser radiation into a reference beam and an object beam directed towards a scene to be observed, • a detection device comprising at least one pixel comprising a photo-detector component, • a second optical device configured to simultaneously provide said pixel with a recombined beam (Lrecomb) corresponding to a superposition of the reference beam and a beam reflected by the scene when it is illuminated by the object beam, • a frequency shift device arranged on the path of the reference beam and configured to shift the laser optical frequency by an offset frequency included in the interval [fRmax, fRmin], with fRmax and fRmin corresponding respectively to a heterodyne beat frequency associated with a maximum measurement distance and a minimum measurement distance in the absence of said frequency shift device, • a processing unit configured to control the laser source and connected to the detection device, • the coherent lidar imaging system being further configured to determine distance information from a signal detected by said pixel, the distance information being determined from the Bi0 and Ti0 values ​​of the laser optical frequency range applied at the time when a beat frequency of the detected signal is canceled or minimized.

[0032] According to one embodiment, each frequency range indexed i corresponds to a distance range from the sensor to the scene Azi ranging from zmini to zmaxi with zmini < zmaxi, and in which the ratios Bi / Ki are determined so that for i ranging from 1 to N-1, we have:

[0033] Zmax^ ~ ZminM

[0034] with:

[0035] fK =Ki.f LJ Rmaxi Rmim

[0036] r _ B. J Rmint cT; J Rmax, cT,

[0037] According to one embodiment, the durations Ti are all identical.

[0038] According to one embodiment, the widths Bi verify the relationship:

[0039] B; = K. Bi+i K real greater than 1

[0040] According to one embodiment, the coherent lidar system according to the invention further comprises a scanning device configured to illuminate the scene point by point or line by line with the object beam.

[0041] According to another embodiment, the laser source and / or the first optical device are configured to illuminate the entire scene, the detector comprises a plurality of pixels distributed in a matrix, and the second optical device is configured to superimpose, at the level of the photo-detector of each pixel, the reference beam and the beam reflected by the scene in a substantially identical propagation direction.

[0042] According to one embodiment, the frequency shift device comprises at least one acousto-optic modulator (AOM) operating in order -1.

[0043] According to one embodiment, a modulation over the frequency range i is called sub-phase i, and each pixel comprises a reading circuit (CL) coupled to the photodetector (PD), the reading circuit comprising: • an integrator (INTEG) which integrates, at each sub-phase, a reference signal (REF) detected by the pixel when the laser beam is not frequency modulated and the pixel signal detected during said sub-phase, • a comparator (COMP) which compares the integrated reference signal (REF') and the integrated pixel signal (Spix'), and switches to the high state when the pixel signal integrated is different from the integrated reference signal, and • a logic block (CLOG) which provides the address (Xadd, Yadd) of said pixel when the comparator is in the high state.

[0044] According to another aspect, the invention relates to a method for acquiring distance from a coherent lidar system to a scene, comprising the steps of: • To generate laser radiation with a laser optical frequency fopt_i varying linearly over a plurality of N successive frequency ranges indexed i, a frequency range having a width Bi and a duration Ti, a sum of all said frequency ranges corresponding to an overall duration, an absolute value of a ratio Bi / Ti being different for each frequency range, • B spatially separate the laser radiation into a reference beam (Lref) and an object beam directed towards a scene to be observed, • C illuminate the scene with the object beam, • D shift the laser optical frequency of the reference beam by an offset frequency within the interval [fRmax, fRmin], with fRmax and fRmin corresponding respectively to a heterodyne beat frequency associated with a maximum measurement distance and a minimum measurement distance in the absence of said shift of the laser optical frequency of said reference beam, • E simultaneously supply to at least one pixel of a detection device, a recombined beam corresponding to a superposition of the reference beam and a beam reflected by the scene, detect said recombined beam and generate a detected signal, • F determine, from the detected signal, a time when a beat frequency of the detected signal is canceled or minimized, • G determine the Bi0 and Ti0 values ​​of the range of the laser optical frequency applied at said time, • H determine distance information from said values ​​Bi0 and Ti0.

[0045] According to one embodiment, a modulation over the frequency range i is called sub-phase i, and in step A the optical frequency of the generated laser radiation is not modulated for a duration T0 prior to the durations Ti, and in step E a so-called reference signal is detected during the duration T0, and step F comprises a comparison sub-step in which, for each sub-phase, the integrated reference signal and the detected signal integrated during said sub-phase are compared.

[0046] The following description presents several exemplary embodiments of the device of the invention: these examples are not limiting of the scope of the invention. These exemplary embodiments present both the essential characteristics of the invention as well as that additional characteristics related to the embodiments considered.

[0047] The invention will be better understood and other characteristics, aims and advantages thereof will appear during the detailed description which follows and with reference to the appended drawings given as non-limiting examples and in which:

[0048] The [Fig. 1] already cited illustrates the principle of a known coherent lidar.

[0049] The [Fig.2] already cited illustrates the known principle of frequency modulation lidar.

[0050] [Fig.3] illustrates a known variant of frequency modulation lidar in in which the entire scene is illuminated simultaneously and the signal is detected on an imager comprising several pixels.

[0051] [Fig.4] illustrates a first example of frequency modulation for a so-called “multichirp” lidar.

[0052] [Fig.4bis] illustrates a second example of frequency modulation for a “multichirp” lidar.

[0053] [Fig.5] illustrates an intuitive way to get rid of the heterodyne signal frequency measurement for a "monochript" laser of parameters (B, T) which is not feasible.

[0054] [Fig.6] another way to flatten the heterodyne signal consists of shifting the optical frequency of the reference beam with a frequency fd.

[0055] [Fig.7] illustrates the concept of the frequency shift of the wave from the laser applied to a monochirp lidar of parameter (B, T) of a frequency fd varying in the interval [fRmin; fRmax], so that the difference between the reference frequency and the shifted reference frequency covers a set of frequencies including the frequency fR to be measured.

[0056] [Fig.8] illustrates an example of the fdO frequency shift for modulation of the laser source of a “multichirp” lidar as illustrated in [Fig.4].

[0057] [Fig.9] illustrates a coherent lidar system 10 according to the invention.

[0058] [Fig. 10] illustrates a first variant of the coherent system according to the invention further comprising a scanning device configured to illuminate the scene with the object beam point by point.

[0059] [Fig. 11] illustrates a second variant of the coherent lidar system according to the invention which has an architecture as described in [Fig. 3]. The laser source and / or the first optical device DI are configured to illuminate the entire scene, and the detector comprises a plurality of pixels distributed in a matrix.

[0060] [Fig. 12] illustrates the use of an acousto-optic modulator as a frequency shifting device.

[0061] [Fig. 13] illustrates an embodiment of the frequency modulation of the laser source integrating a calibration phase

[0062] [Fig. 14] illustrates an embodiment of the detection of the zero frequency at pixel level. DETAILED DESCRIPTION OF THE INVENTION

[0063] Document FR 2207829 (not published on the date of filing of this application) proposes improving the signal-to-noise ratio while maintaining the same Zmin-Zmax dynamics, by reducing the AfR bandwidth.

[0064] For this, the overall acquisition duration T is divided into N consecutive intervals of duration Ti, i ranging from 1 to N, N > 2. Each capture interval or sub-phase Ci corresponds to the provision of a beam having its optical frequency f modulated continuously and linearly over a frequency range of width Bi during the duration Ti of this sub-phase. Furthermore, it is provided that each sub-phase Ci corresponds to a ratio Bi / Ti different from that of the ratios Bi / Ti of the N1 other sub-phases.

[0065] Each sub-phase Ci (Bi, Ti) then corresponds to a bandwidth AfRi reduced compared to AfR and makes it possible to detect distances z included in a range Azi going from zmini to zmaxi with zmini < zmaxi. The bandwidth of each of the sub-phases Ci extends from a minimum frequency fRmini to a frequency fRmaxi:

[0066] AfRi fRmaxi fRmini With :

[0067] f (3) 7 RmciXj cTf And

[0068] r _ (4) Rrnbij cTj And so:

[0069] Af _2A^(5) J “ < T, With Azi — zmax; - zmmi

[0070] For a given sub-phase Ci and for a given pixel there corresponds a beat frequency fRi measured by the heterodyne signal between fRmini and fRmaxi if the point associated with the pixel is at a distance z from the pixel between zmini and zmaxi and the distance z can be calculated with the formula:

[0071] _ '■ 2.Bi

[0072] A lidar exhibiting such frequency modulation is called a “multichirp lidar”.

[0073] The Bi / Ti ratios are determined so that for i ranging from 1 to Nl zmini+isoit is substantially equal to zmaxi without being strictly greater than zmaxi.

[0074] According to a preferred embodiment, so that the different measurement phases maintain the continuity of the distances (this avoids overlapping of the ranges), we have preferentially:

[0075] Zmax, — zmmM (6)

[0076] For each sub-phase Ci (or frequency range), we define the coefficient Ki such that that :

[0077] (7)

[0078] According to one embodiment, Ki is identical for all i with Ki = K, K being a real number strictly greater than 1. However, in other examples, the values ​​of Ki of at least two sub-phases are different.

[0079] With K constant this implies:

[0080] / „ —Kf . . for all i: LJ u Rmaxï Rmmi r

[0081] And

[0082] (10) Either

[0083] Zmaxi ~ (11)

[0084] According to one embodiment, the durations Ti are all chosen to be identical, and therefore:

[0085] Ti = T / N.

[0086] According to one embodiment, fRmini is chosen to have the same value for each sub-phase Ci, called fRmax, and fRmaxi to have the same value for each sub-phase Ci, called fRmin. This minimizes the bandwidth of the pixel, equal to (fRmax-fRmin). The determination of z is carried out by noting the instant for which the pixel measures a frequency included in the interval [fRmin, fRmax]. This instant is located in only one of the sub-phases. Knowing the Bi and Ti of this sub-phase, z is determined.

[0087] For constant K this implies that the sub-phases all have the same frequency fRmin and the same frequency fRmax and therefore all have the same bandwidth AfR.

[0088] In this case, from (11) and choosing ^min “ ^mint and — Zm^x. we have:

[0089] >^^^(12) Snm

[0090] With this set of equations, knowing zmin and zmax and the value of K, we can determine N with (12) (taking for example N as a rounded integer greater than the value obtained by applying the logarithm to base K of (zmax / zmin). Then knowing the value of T we deduce Ti = T / N.

[0091] The Bi, zmini and zmaxi are determined with the equations:

[0092] o = A (13) 'K1'1

[0093] 7 . = ■ (14) '-min / “'-min

[0094] z =^(15) HAS

[0095] We also choose the frequency fRmin (identical for all the sub-phases) and we deduced:

[0096]

[0097]

[0098]

[0099]

[0100]

[0101]

[0102]

[0103] [Fig.4] illustrates a first example of frequency modulation with constant K and identical Ti and N=4. In this example, the frequency of the laser beam is modulated from the same frequency fstart, which implies that at the end of each sub-phase the frequency must be instantly brought back to the frequency fstart. This requires the response of the laser source and its CCL control circuit. [Fig.4bis] illustrates a second example in which there is no instantaneous return of the frequency, with as before K constant, Ti identical and N=4 and. In this example, we arrange for the last frequency of the sub-phase Ci fendi to be equal to the first frequency of the following sub-phase fstarti+l. Moreover in this example the sign of Bi, or in other words the direction of travel of the frequency excursion Bi, changes at each new sub-phase. A particular numerical example will be described which illustrates the determination of the different parameters. We take T= 200ps; zmin = 0.3 m (zmin = zminO and zmax = 10m (zmax = zmaxN); K=2; fRmin = 75kHz for all sub-phases; Ti all identical. The dynamics zmax / zmin = 33.33. We deduce from this: N=5 or Ti=40 ps. fRmax = 150 KHz and AfR = 75 kHz for each sub-phase. BI = 1.5 109 Hz and the sub-phase Cl allows to detect the distance z between Zmini 0.3 m and zmaxj 0.6 m. B2 = 750.106 Hz and the sub-phase C2 allows to detect the distance z between zmin2=0.6 m and zmax2=1.2 m. B3 = 375.106 Hz and the sub-phase C3 allows to detect the distance z between zmin3=1.2 m and zmax3=2.4 m. B4 = 187.5.106 Hz and the sub-phase C4 allows to detect the distance z between zmin4=2.4 m and zmax4=4.8 m. B5 = 93.75.106 Hz and the sub-phase C5 allows to detect the distance z between zmin5=4.8 m and zmax5=9.6 m. For a monochirp laser with parameters (B, T) an intuitive way to get rid of the heterodyne signal frequency measurement would be to have the same time delay AtL0 on both the LO arm (reference signal Lref) and the stage as shown in Figure 5. This would consequently produce a zero-frequency beat signal. The distance would then be encoded in the added delay on the LO arm as A tLO = 2z / c . Obviously, this way is almost impossible to achieve.

[0104] A second way to flatten the heterodyne signal is to work on the optical frequency of the beam Lref. For a monochirp lidar with parameters (B, T) this concept, illustrated [Fig.6], consists of shifting the optical frequency of the beam Lref with a frequency fd in such a way as to have fopt-LOd= fopt Lo-fd =fopt sc with fopt.Lo frequency emitted by the laser corresponding to the frequency of Lref as described in the state of the art. In this way the Lref signal with shifted frequency fopt_LOd interfering with the scene signal with frequency fopt sc produces a constant heterodyne signal with a zero beat frequency which is detected on the imager with more simplified electronics.

[0105] Thus, Figures 5 and 6 explain the difference between adding a time delay and the optical frequency shift of Lref. Both solutions produce a heterodyne signal with zero beat frequency.

[0106] To clarify the notations we now call: • fopt-i the optical frequency emitted by the laser (which corresponds to the “classic” reference / local oscillator frequency fopt Lo), • fR a classical type beat frequency as described previously, • fd the offset frequency, • fopt LOdla offset frequency of the local oscillator beam Lref according to the invention, called offset reference frequency, • fs the frequency of the beat signal resulting from the interference of Lref of shifted frequency fopt LOd with the signal Lo,r resulting from the scene of frequency fopt sCj called the beat signal frequency:

[0107] fs = I fopt LOd - fopt sc I

[0108] The signal beat frequency fs is determined from the signal detected Spix by the photodetector.

[0109] The concept of the shift applied to a monochirp lidar of parameter (B, T) consists in shifting (decreasing) the frequency of the wave coming from the laser fopti (the "classical" reference frequency) by a frequency fd varying in the interval [fRmin; fRmax], so that the difference between the reference frequency and the shifted reference frequency covers a set of frequencies including the frequency fR to be measured. The frequency fopt.LOd is thus between [fopt.i- fRmax; fopt i- fRmin]. With an offset frequency fd varying in the interval [fRmin; fRmax] the signal beat frequency fs is canceled or minimized at a given "instant" (in fact a time range to be able to measure a frequency).

[0110] For the record, in the monochirp case the minimum beat frequencies fRmin and maximum and fRmax are given by: [YES] = Rmir&nwno} cT

[0112] f Rma^niono) c?

[0113] The distance information z is then encoded in the particular fd value fda corresponding to this “instant”, such that:

[0114] \fdc\ = ^t-

[0115] This principle is synthesized [Fig.7] on an example for which B = 10 GHz; T=800 ms; z=2m. We calculate with the formula (1) fR = 166.67 Hz.

[0116] On the left is illustrated the classic operation of an FMCW lidar, and on the right the operation of a lidar according to the invention in which the frequency of Lref has been reduced until its value coincides with the frequency from the scene fopt_sc.

[0117] The lidar according to the invention is a multichirp lidar for which the aforementioned shift concept is applied, in order to improve its performance. In this case, we seek to cancel the beat frequency fs in one or more sub-phases depending on the distances of the objects in the scene.

[0118] We wish to measure the distance z with a distance resolution ôz.

[0119] The coefficient K is then chosen so that the range of distances to be measured corresponds to the theoretical resolution of the FMCW system which is Zmax; - Zmin. = A z = c / 2B, in each measurement phase (see formula Ibis). 1 i

[0120] A Zj changes according to the phases, and therefore the resolution also. This is not a problem, in practice we will have a distance resolution which degrades with the distance in absolute value, but this is generally acceptable. For example: resolution of 1mm at 1m, and of 1cm at 10m.

[0121] As an example, we can choose to work with the smallest resolution:

[0122] Az = c / (2.B 1) for all phases.

[0123] This therefore implies zmaxi - zmini < Az whatever i.

[0124] Knowing that zmaxi = K.zmin; we can write:

[0125] zmini.(Kl) < Az whatever i.

[0126] therefore K < Az / zmini + 1 whatever i.

[0127] The worst case is for the largest zmini so i=N:

[0128] K < Az / zminN + 1.

[0129] Knowing that zminN = KN '.zmin this implies:

[0130] K <Az / (KN '.zmin) +1.

[0131] An interesting case is when zmin = Az

[0132] This allows us to choose K such that KN < KN 1 +1

[0133] This formula is always true for K>1.

[0134] More generally, if we choose a resolution Az it is necessary that K < Az / zminN + 1 so that all the following phases respect this resolution. This goes in the direction where K must tend towards 1 if we want better resolutions.

[0135] In the case of the invention of a multichirp modulated optical source frequency, a single offset frequency fdO, chosen in the interval [fRmin; fRmax], makes it possible to cancel the beat frequency fs for a sub-phase Ci associated with the distance interval [zmin; zmax] included in the distance excursion of the desired lidar [zmin; zmax]. In other words, for each sub-phase Ci fs will be canceled for the distances included in zmin and zmax of said sub-phase.

[0136] The frequencies fRmin and fRmax are respectively the frequencies of the minimum and maximum heterodyne beat signal in the multichirp case in the absence of a frequency shifter (corresponding to zmin and zmax).

[0137] An example of the applied frequency shift fdO is illustrated [Fig.8] for a modulation of the laser source as illustrated in [Fig.4].

[0138] Then, at the detection level, the moment at which the beat frequency becomes zero or minimized is identified (the moment is in fact a time range for measuring a frequency). To identify this moment, according to one embodiment, it is detected that the continuous level of the measured signal is different from the continuous level of the signal without modulation (ambient signal), because it is added to the latter (see below).

[0139] Once this moment is identified, we determine the pair of parameters (Bi0,Ti0) corresponding to the instantaneous frequency of the applied laser source, which contains the information on the distance z to be measured: [°14°] f tl- fo ' opt-LOd Either

[0141] _ _ c-TiyfM (|9) 2.3¾

[0142] The offset frequency fdO is chosen between fRmax and fRmin.

[0143] Thus, with a multichirp lidar according to the invention, a heterodyne frequency is no longer measured but a moment is determined for which the frequency of the detected signal is zero or minimal (or for which the detected signal is different from the ambient continuous signal). This determination is carried out with a simplified local CL reading circuit at the pixel level. We also benefit from the increase in the signal-to-noise ratio inherent in the multichirp lidar.

[0144] [Fig.9] illustrates a coherent lidar system 10 according to the invention. It comprises a laser source SL configured to generate laser radiation L with a laser optical frequency fopt.i varying linearly over a plurality of N successive frequency ranges indexed i, with N greater than or equal to 2. A frequency range having a width Bi and a duration Ti, a sum of all said frequency ranges cor corresponding to an overall duration T, an absolute value of a ratio Bi / Ti being different for each frequency range, as described previously.

[0145] The lidar 10 also comprises a first optical device DI configured to spatially separate the laser radiation L into a reference beam Lref and an object beam Lo directed towards the scene to be observed Sc, a detection device Det comprising at least one pixel P comprising a photo-detector component PD. The lidar 10 also comprises a second optical device D2 configured to simultaneously provide said pixel with a recombined beam Lrecomb corresponding to a superposition of the reference beam Lref and a beam reflected by the scene Lo,r when it is illuminated by the object beam.

[0146] The lidar 10 also comprises a frequency shift device FSD disposed in the path of the reference beam and configured to shift the laser optical frequency by an offset frequency fdO comprised in the interval [fRmin, fRmax].

[0147] We have fRmax and fRmin corresponding respectively to the heterodyne beat frequency associated with a maximum measurement distance zmax and a minimum measurement distance zmin in the absence of the FSD shift device. The frequency of the reference beam at the output of the modulation device is called the offset reference frequency fopt_Lod.

[0148] The lidar 10 also comprises a processing unit UT configured to drive the laser source (via its control circuit CCL) and which is connected to the detection device Det.

[0149] The coherent lidar system 10 according to the invention is further configured to determine distance information from a signal detected Spix by the pixel P. The distance information z associated with P (distance from the lidar 10 to the point of the scene whose reflected signal is detected by P) is determined from the values ​​Bi0 and Ti0 of the laser optical frequency range applied at the moment when the beat frequency fs of the detected signal is cancelled or minimized, as explained previously.

[0150] The processing unit UT, connected to the control circuit of the laser source CCL, allows the identification of the frequency range of parameters (Bi0, Ti0), once the moment of cancellation of fs has been determined by the reading circuit CL.

[0151] According to a first illustrated variant [Fig. 10], the coherent system according to the invention further comprises an SD scanning device configured to illuminate the scene with the object beam point by point (first embodiment). According to a second embodiment, the SD device illuminates the scene line by line.

[0152] In the first embodiment, it is appropriate to illuminate each point of the scene for a duration at least equal to T (so that the identification of zero / minimal fs is possible) and the detector comprises a single pixel.

[0153] In the second embodiment, the scene should be illuminated according to a line for a duration at least equal to T, and the detector comprises a pixel bar.

[0154] According to a second illustrated variant [Fig. 11], the coherent lidar system 10 according to the invention has an architecture as described in document WO2021144357 and recalled [Fig.3]. The laser source and / or the first optical device DI are configured to illuminate the entire scene, and the detector comprises a plurality of pixels Pij distributed in a matrix. The second optical device D2 is configured to superimpose, at the photo-detector PDij of each pixel, the reference beam Lref / pix and the beam reflected by the scene Lo,r / pix in a substantially identical propagation direction. The device D2 typically comprises for this purpose an optical recombination device DR and an optical imaging system Im as described previously.

[0155] According to one embodiment, the frequency shift device FSD arranged on the path of the reference beam (local oscillator) is an acousto-optic modulator AOM illustrated in figure 12. An acousto-optic modulator is based on the principle of the photo-elastic effect. An acoustic wave of frequency f^ propagates in a material which slightly disturbs the optical parameters of this material and creates a grating of pitch A, acoustic Fonde wavelength. The incident optical beam is deflected by the grating (diffraction) proportionally to the frequency of the acoustic wave which propagates in the material. In addition the optical frequency of the deflected beam is shifted by the value of the acoustic Fonde frequency fRF.

[0156] The regime most often used in AOM type modulators is the Bragg regime where only the first order ± 1 of diffraction exists. This regime is obtained when the angular extension of the acoustic Fonde is very small (a plane acoustic wave). To obtain maximum diffraction the angle of incidence of the optical Fonde must respect the Bragg formula:

[0158] ^<>pt the wavelength of the optical beam, A the wavelength Acoustic foundation and dB the Bragg angle of incidence of the optical beam.

[0159] Furthermore, in an isotropic medium the first order diffraction angle (+ 1 or -1) a = 20b.

[0160] In a known manner we have:

[0161] .... . LZ — y

[0162] V speed of the acoustic wave in the material.

[0163] We can thus shift the Fonde frequency fopt-1 by a value equal to fj^. When the optical wave is diffracted according to order +1 the frequency f^ is added to the optical frequency and when the optical wave is diffracted according to order -1 the frequency fRF is subtracts from the optical frequency. As a reduction of the laser reference frequency fopt-1 is desired here, the AOM is used in its -1 diffraction order as illustrated [Fig. 12], and the frequency fRF is chosen to be equal to the frequency fdO.

[0164] When the frequency fRF is modulated the diffraction angle varies according to Aa and the frequency of the optical wave is also modulated. This property is not used here.

[0165] It is possible that obtaining the desired shift frequency cannot be achieved with a single acousto-optic modulator. According to one embodiment, the frequency shifting device comprises two or more acousto-optic modulators, operating on orders -1 or +1 depending on the desired shift frequency.

[0166] An acousto-optic deflector can also be used in silicon photonics. Other types of frequency shift devices can also be used.

[0167] In free space, it is possible to make an optical frequency shift with more or less X / 2 or X / 4 plates arranged on the path of an optical wave and which are rotating with known angular frequencies. The frequency shift of the optical wave passing through these plates is proportional to the angular frequencies of rotation.

[0168] In integrated photonics, there are mainly two ways to achieve frequency shifting. Serrodyne-type modulation or single sideband modulation (SSB for “Single Side Band”).

[0169] For serrodyne modulation, a "phase-shifter" (device that shifts the phase) The electro-optical phase shifter is driven by a sawtooth signal with a peak-to-peak drive amplitude that leads to exactly 2ir phase shift of the optical carrier. This results in a phase shift that is piecewise linear in time and thus leads to a frequency shift that corresponds to the fundamental frequency of the sawtooth drive signal. It is also possible to use thermo-optical phase shifters instead of electro-optical phase shifters.

[0170] In the case of SSB modulation, two Mach-Zehnder modulators are combined with a phase shift of ir / 2 to form an IQ optical modulator. The in-phase (I) and quadrature (Q) Mach-Zehnder modulators are driven with a sine signal and a cosine signal, respectively, which, in the small-signal regime, leads to a frequency shift that corresponds to the frequency of the sinusoidal driving signals.

[0171] The processing for determining the moment for which the beat frequency fs of the detected Spix signal is minimal or zero is preferably carried out, at least in part, at the pixel level in a dedicated reading circuit CL. The pixel P comprises a photodetector PD configured to generate a Spix electrical signal, typically at from the heterodyne photocurrent iPD (see [Fig.2]), the amplitude of which depends on the intensity of the detected optical beam.

[0172] As illustrated in figures 9 and 10 the dedicated reading circuit CL is coupled to the photodetector PD which delivers a Spix signal.

[0173] According to an embodiment of the circuit CL adapted for a matrix detector, a reference signal REF is detected before the Spix signal, which corresponds to the signal from the scene without modulation of the optical frequency. For this purpose, the optical frequency of the laser radiation generated is not modulated for a duration T0 prior to the durations Ti, as illustrated [Fig. 13]. The non-modulation of the laser beam for a duration T0 just before the application of the modulations during the sub-phases Ci is called the calibration phase. The signal detected by the photodetector during this duration T0 is called the reference signal REF.

[0174] From the interference equation, we have:

[0175] -For a modulated signal from the laser source and a detected signal corresponding to a distance which is not correct, a signal I is detected such that:

[0176] ] — / 1 + / 2 + 2^1 / 2 cos + ) With : It intensity from the scene, 12 intensity of the reference beam / shifted local oscillator, q heterodyne efficiency, fs frequency of the beat signal, q> phase shift between the two channels.

[0177] The DC part of this signal is thus:

[0178] DC=I[+I2

[0179] This is true assuming that the integration of the AC part will give zero, which assumes that Ti corresponds to an integer number of oscillations.

[0180] -For a modulated signal from the laser source and a desired distance z (formula 19), a signal is detected such that:

[0181] Z = / i + / 2 + 2?77Â4cos( <p) (fs=O)

[0182] The continuous part of this signal is thus: DC = I[+I2 + 2^1 J 2 COs(^)

[0183] Note that q> is the phase shift between the two channels, which can be anywhere between 0 and 2ir, so the additional DC component compared to the previous case can be positive, negative or zero.

[0184] -For an unmodulated laser source signal, I is detected —and the DC component is DC = +12.

[0185] In conclusion, it is therefore necessary to detect the Cig= part cos ( ^ ) is superimposed on DC = / j +12-

[0186] In other words, if the pixel detects a distance equal to z of the expression (19), its signal is equal to the reference signal REF plus a “delta” corresponding to the additional signal Isig.

[0187] The detection of the instant for which for a pixel the detected Spix signal is continuous is thus carried out using the REF signal, in the manner illustrated [Fig.14], by detecting a “delta” as defined above.

[0188] We use the fact that since the signal to be measured is continuous, a simple integration of duration equal to Ti recovers the signal to be detected at each sub-period.

[0189] An integrator INTEG performs an integration of the signals REF and SIG, respectively called REF' and SIG' for each sub-period Ti (sub-phase Ci). The current sub-phase Ci during the integration is identified by the circuit CL, that is to say that the current values ​​(Bi, Ti) are memorized.

[0190] At each integration, if the integrated signal Spix' is different from the integrated reference REF', REF' and SIG' at the input of the comparator COMP triggers it and its output outc gives the order to the logic block CLOG to provide the address of the pixel (Xadd, Yadd) (by dialoguing with the reading system). Indeed when the modulation frequency is zero the detected integrated signal Spix' is different from the signal REF', as explained above.

[0191] This pixel is assigned the value of z calculated with the current values ​​(Bi, Ti), and this value is stored in a memory. Once the pixel is read, it is deactivated for the rest of the frame. In this way, at each sub-period Ti, all the pixels which are at this same distance are read and deactivated. At the end of the multi-chirp signal, a depth image is available on the entire matrix. It is thus possible to obtain a depth map at the video rate.

[0192] This architecture allows the generation of scene distance histograms in real time. By counting the number of pixels at each distance value in a memory, the distribution of the scene depth values ​​is obtained. This can be useful for adjusting and optimizing the "multichirp" signal to target the range of distances to be detected.

[0193] Similarly, it is possible to target a single distance or a reduced range in order to obtain a faster cadence.

[0194] According to this embodiment, the circuit CL thus comprises an integrator INTEG which integrates REF and Spix, the result of which is the integrated signals REF' and Spix', a comparator COMP which switches to the high state when Sipx'>REF' and a logic block CLOG which provides the address (Xadd, Yadd) of the pixel in question when the comparator is in the high state.

[0195] According to another aspect, the invention relates to a method for acquiring a distance z from a coherent lidar system 10 to a scene.

[0196] In a first step, laser radiation L is generated with a laser optical frequency fopt_i varying linearly over a plurality of N successive frequency ranges indexed i as described previously.

[0197] Then, the laser radiation L is spatially separated into a reference beam Lref and an object beam Lo directed towards a scene to be observed (Sc) and the scene is illuminated with the object beam.

[0198] At the same time, the laser optical frequency of the reference beam is shifted by an offset frequency fdO included in the interval [fRmax, fRmin], with fRmax and f Rmin corresponding respectively to a heterodyne beat frequency associated with a maximum measurement distance zmax and a minimum measurement distance zmin in the absence of the shift of the laser optical frequency of the reference beam.

[0199] At least one pixel P of the detection device Det is simultaneously supplied with a recombined beam Lrecomb corresponding to a superposition of the reference beam Lref and a beam reflected by the scene Lo,r), the recombined beam is detected and a detected signal Spix is ​​generated.

[0200] In a processing step, firstly, a time is determined from the detected signal at which the beat frequency fs of the detected signal is cancelled or minimized. Then, values ​​Bi0 and Ti0 of the range of the laser optical frequency applied at said time are determined and finally, distance information z is determined from the values ​​Bi0 and Ti0.

[0201] According to an embodiment in step A, the optical frequency of the generated laser radiation is not modulated for a duration T0 prior to the durations Ti (calibration phase). In step E, a so-called reference signal REF is detected during the duration T0 which is stored. And step F comprises a comparison sub-step in which, for each sub-phase, the integrated reference signal REF' and the integrated detected signal Spix' are compared during said sub-phase.

Claims

Claims

1. A coherent lidar system (10) comprising: a laser source (SL) configured to generate laser radiation (L) with a laser optical frequency fopt_i varying linearly over a plurality of N successive frequency ranges indexed i, with N greater than or equal to 2, a frequency range having a width Bi and a duration Ti, a sum of all said frequency ranges corresponding to an overall duration (T), an absolute value of a ratio Bi / Ti being different for each frequency range, a first optical device (DO1) configured to spatially separate the laser radiation (L) into a reference beam (Lref) and an object beam (Lo) directed towards a scene to be observed (Sc), a detection device (Det) comprising at least one pixel (P) comprising a photo-detector component (PD), a second optical device (DO2) configured to simultaneously provide said pixel with a recombined beam (Lrecomb) corresponding to a superposition of the reference beam (Lref) and a beam reflected by the scene (Lo,r) when it is illuminated by the object beam, a frequency shift device (FSD) arranged on the path of the reference beam and configured to shift the laser optical frequency by an offset frequency (fdO) comprised in the interval [fRmax, fRmin], with fRmax and fRmin corresponding respectively to a heterodyne beat frequency associated with a maximum measurement distance (zmax) and a minimum measurement distance (zmin) in the absence of said frequency shift device, a processing unit (UT) configured to control the laser source and connected to the detection device, the coherent lidar imaging system being further configured to determine distance information from a signal detected (Spix) by said pixel (P), the distance information being determined from the values ​​Bi0 and Ti0 of the laser optical frequency range applied at the time when a beat frequency (fs) of the detected signal is canceled or minimized.

2. Coherent lidar system according to the preceding claim in which each indexed frequency range i corresponds to a distance range from the sensor to the scene Azi ranging from zmini to zmaxi with zmini < zmaxi, and in which the ratios Bi / Ki are determined so that for i ranging from 1 to Nl, we have: Zmaxi = Zmm!+l with: Rmaxi Rnum and r ... Rmiiij cT, J Rmax; cTt

3. Coherent lidar system according to one of the preceding claims in which the durations Ti are all identical

4. Coherent lidar system according to one of the preceding claims in which the widths Bi verify the relation: B; = K. Bi+i K real greater than 1

5. Coherent lidar system according to the preceding claim further comprising a scanning device (SD) configured to illuminate point by point or line by line the scene with the object beam.

6. Coherent lidar system according to one of claims 1 to 4 in which the laser source and / or the first optical device are configured to illuminate the entire scene, in which the detector comprises a plurality of pixels (Pij) distributed in a matrix, in which the second optical device is configured to superimpose, at the level of the photodetector (PDij) of each pixel, the reference beam (Lref / pix) and the beam reflected by the scene (Lo,r / pix) in a substantially identical propagation direction.

7. Coherent lidar system according to one of the preceding claims wherein the frequency shift device (FSD) comprises at least one acousto-optic modulator (AOM) operating in order -1.

8. Coherent lidar system according to one of the preceding claims in which a modulation on the frequency range i is called sub-phase i, and in which each pixel comprises a reading circuit (CL) coupled to the photodetector (PD), the reading circuit comprising: • an integrator (INTEG) which integrates, at each sub-phase, a reference signal (REF) detected by the pixel when the

9. laser beam is not frequency modulated and the pixel signal detected during said sub-phase, • a comparator (COMP) which compares the integrated reference signal (REF') and the integrated pixel signal (Spix'), and switches to the high state when the integrated pixel signal is different from the integrated reference signal, and • a logic block (CLOG) which provides the address (Xadd, Yadd) of said pixel when the comparator is in the high state. A method of acquiring distance (z) from a coherent lidar system (10) to a scene, comprising the steps of: • To generate laser radiation (L) with a laser optical frequency fopt_i varying linearly over a plurality of N successive frequency ranges indexed i, a frequency range having a width Bi and a duration Ti, a sum of all said frequency ranges corresponding to an overall duration (T), an absolute value of a ratio Bi / Ti being different for each frequency range, • B spatially separate the laser radiation (L) into a reference beam (Lref) and an object beam (Lo) directed towards a scene to be observed (Sc), • C illuminate the scene with the object beam, • D shift the laser optical frequency of the reference beam by an offset frequency (fdO) included in the interval [f Rmax, fRmin], with fRmax and fRmin corresponding respectively to a heterodyne beat frequency associated with a maximum measurement distance (zmax) and a minimum measurement distance (zmin) in the absence of said shift of the laser optical frequency of said reference beam, • E simultaneously supply to at least one pixel (P) of a detection device, a recombined beam (Lrecomb) corresponding to a superposition of the reference beam (Lref) and a beam reflected by the scene (Lo,r), detect said recombined beam and generate a detected signal (Spix), • F determine, from the detected signal (Spix), a moment when a beat frequency (fs) of the detected signal is cancelled or minimized, • G determine the Bi0 and Ti0 values ​​of the range of the laser optical frequency applied at said time, • H determine distance information (z) from said values ​​Bi0 and Ti0.

10. Distance acquisition method according to the preceding claim in which a modulation on the frequency range i is called sub-phase i, in which in step A the optical frequency of the generated laser radiation is not modulated for a duration T0 prior to the durations Ti, in which in step E a so-called reference signal is detected during the duration T0, and in which step F comprises a comparison sub-step in which, for each sub-phase, the integrated reference signal and the detected signal integrated during said sub-phase are compared.

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