FMCW heterodyne lidar imaging system with reduced space requirements

Description

DOMAINE TECHNIQUE

[0001] The field of the invention is that of LIDAR imaging systems with frequency modulated continuous wave (FMCW) heterodyne detection. Frequency Modulated Continuous Wave, in English). ÉTAT DE LA TECHNIQUE ANTÉRIEURE

[0002] FMCW heterodyne detection LIDAR imaging systems are used to determine the distance of a scene illuminated by a coherent optical signal. Such an imaging system is based on the principle of heterodyne detection, in the sense that it exploits the properties of a heterodyne signal formed by the interference between two coherent optical signals with a difference in optical path, usually a reference signal and a signal backscattered by the scene. These two optical signals are coherent with each other to the extent that they come from the same optical signal, called primary, emitted by an optical source. The scientific publication by Sandborn entitled FMCW Lidar: Scaling to the Chip-Level and Improving Phase-Noise-Limited Performance, EECS Department, University of California, Berkeley, Technical Report No.UCB / EECS-2019-148; December 1, 2019, as well as document US 2020 / 011994 A1 describe different examples of FMCW type LIDAR imaging systems.

[0003] In this respect, document WO2021 / 144357A1 describes an example of such an imaging system, called here flash type insofar as it is adapted to illuminate a plurality of points of the scene simultaneously and to determine a distance map (distance image).

[0004] There figure 1 is a schematic and partial view of such an imaging system 1. It comprises a minima : ∘ an optical source 10 of the so-called primary signal S p , coherent, continuous and frequency modulated; ∘ an optical device 20 for separation and recombination, comprising a separating optical element 21 adapted to divide the primary signal S p into an object signal S o directed towards the scene 2 and into a reference signal S r directed towards a photodetector 50; an optical element 22 for shaping the reference signal S r ; and an optical recombining element 23 adapted to direct towards the photodetector 50 along the same optical axis the reference signal S r as well as a part S or,c of the backscattered object signal S or ; ∘ an optical projection device 30, adapted to project the object signal S o to illuminate the entire scene 2 simultaneously; ∘ an optical imaging device 40, adapted to transmit the collected part S or,c of the backscattered object signal S or and to form the image of the illuminated scene 2 in the detection plane of the photodetector 50.It comprises an optical collection element which collects the light beams defining the part S or,c of the backscattered object signal S or; ∘ the photodetector 50, here a matrix photodetector, adapted to receive the collected part S or,c of the backscattered object signal S or and the reference signal S r , which interfere to form a heterodyne signal S h having a beat frequency fb; ∘ a processing unit 60, adapted to determine a distance z sc (and here a distance map) of the scene 2 from the beat frequency fb of the heterodyne signal S h .

[0005] The primary signal S p presents an instantaneous frequency variation, with for example a starting frequency f 0 and a variation of value B (called chirp, in English) over a period T. The primary signal S p says chirpé is a sinusoidal wave whose instantaneous frequency evolves linearly over time. It is divided to form the reference signal S r and the object signal S o , where the first does not pass through scene 2 and whose optical path is fixed and independent of the distance z sc to be determined. It is usually called the reference signal S r of a local oscillator.

[0006] The photodetector 50 thus receives the reference signal S r as well as the backscattered and collected object signal S or,c , which is an attenuated and delayed replica of the object signal S o with a delay τ. The delay results in a frequency difference fb between the two signals in the interval [τ ; T], with T>>τ, and τ = 2z sc / c approximately when the path of the reference signal S r is neglected, where c is the speed of light in a vacuum. This frequency fb , called the beat frequency, is equal to the difference between the frequency of the reference signal S r and the backscattered and collected object signal S or,c . Its value can be determined in the time domain by counting the number of oscillations of the heterodyne signal S h over the period T, or in the frequency domain by fast Fourier transform.

[0007] We can then determine, from the value of this beat frequency fb, the distance z sc between the illuminated scene 2 and the matrix photodetector 50. Indeed, knowing that fb / B = τ / T, and that τ = 2z sc / c, the distance z sc from scene 2 is expressed by the relation: z sc ≈ fb cT / 2B.

[0008] Note that the imaging system may have a Mach-Zehnder type architecture where the optical elements splitter 21 and recombiner 23 are physically distinct, or a Michelson type architecture where a single optical element performs the separation and recombination functions. As described in document WO2021 / 144357A1, the optical separation and recombination device may have a free space configuration and include in particular semi-reflecting plates and / or splitter cubes, or may have a guided optics configuration and include in particular waveguides and diffraction gratings. In any event, there is a need to reduce the size of such imaging systems. EXPOSÉ DE L'INVENTION

[0009] The invention aims to remedy at least in part the drawbacks of the prior art, and more particularly to propose a LIDAR imaging system with heterodyne detection of the FMCW type whose size is reduced, and in particular which does not include an optical separation / recombination device.

[0010] For this, the subject of the invention is a FMCW type LIDAR imaging system, suitable for determining a distance z sc separating it from a scene, comprising: an optical source suitable for emitting a coherent, continuous and frequency-modulated primary signal S p, to illuminate the scene; an optical collection element suitable for collecting a part called collected signal S ret,c of a backscattered signal S ret by the scene originating from the primary signal S p; a photodetector intended to receive a heterodyne signal S h associated with the collected signal S ret,c; a processing unit suitable for determining the distance z sc from the scene from a beat frequency of the heterodyne signal S h .

[0011] According to the invention, it is adapted to entirely direct the primary signal S p to the scene. It therefore does not comprise an optical separation element adapted to divide the primary signal S p into a reference signal S r directed towards the photodetector without passing through the scene, and into an object signal S o directed towards the scene. This results in the fact that the primary signal incident on the scene has an optical power equal to that which it has at the output of the optical source. In addition, it comprises a reflector adapted to reflect towards the scene a part S pr,nc , called uncollected signal S ret,nc , of the backscattered signal S ret and not collected by the optical collection element.Also, the collected part S pr,c of the backscattered signal S pr being then formed of first light beams S ret,c(1) not having been reflected by the reflector and of second light beams S ret,c(2) having been reflected by the reflector then by the scene, and the heterodyne signal S h being then formed by the interference between the first light beams S ret,c(1) and the second light beams S ret,c(2).

[0012] Let us note here that the second light beams S ret,c(2) , having been reflected by the reflector, belong to the collected part S ret,c of the backscattered signal S ret , and that this backscattered signal S ret is a signal backscattered by the scene. We then understand that the second light beams S ret,c(2) were reflected by the scene before being collected.

[0013] Some preferred but non-limiting aspects of this imaging system are as follows.

[0014] The reflector can be retroreflective, to reflect incident light beams towards the scene along a reflection axis identical to their axis of incidence.

[0015] The reflector may have a lateral edge located at a maximum distance r max from an optical axis of the optical collection element, and be dimensioned so that the maximum distance r max is less than √(cz sc / B) when the reflector is retroreflective, where c is the speed of light in vacuum, and B is a variation of the frequency of the primary signal S p over a period T of the modulation, and so that the maximum distance r max is less than √(cz sc / 3B) when the reflector is non-retroreflective.

[0016] The reflector may be located in the plane of the optical collection element.

[0017] The reflector may be located downstream of the optical collection element at the photodetector.

[0018] Generally speaking, the reflector may be formed of a continuously reflective or retroreflective surface, or may be formed of non-joining reflective or retroreflective surfaces separated from each other by a transparent or reflective surface at the wavelength of the optical signals of interest.

[0019] The reflector may be located upstream of the optical collection element with a collection optical axis passing through it, the reflector then being formed of reflective or retroreflective surfaces separated from each other and surrounded by a surface transparent to the wavelength of the primary signal S p .

[0020] The reflector may comprise a central surface crossed by the optical collection axis, in which it is formed of reflective or retroreflective surfaces separated from each other and surrounded by a transparent surface, and a peripheral surface which surrounds the central surface, in which the reflective or retroreflective surfaces are placed next to each other.

[0021] The imaging system may be adapted to illuminate only one point of the scene. Alternatively, it may be adapted to simultaneously illuminate a plurality of points of the scene and then comprise an optical device for projecting the primary signal S p onto the scene to simultaneously illuminate the plurality of points of the scene and an optical imaging device adapted to form an image of the illuminated scene in the plane of the photodetector.

[0022] The imaging system may have a so-called mono-static configuration where an optical axis for illuminating the scene by the primary signal S p is identical to an optical axis for collecting the optical collection element, and comprise a semi-reflecting plate or a splitter cube transmitting the primary signal S p to the scene and reflecting the backscattered signal S ret to the photodetector.

[0023] The imaging system may have a so-called bi-static configuration where an optical axis of illumination of the scene by the primary signal S p is different from an optical axis of collection of the optical collection element. BRÈVE DESCRIPTION DES DESSINS

[0024] Other aspects, aims, advantages and characteristics of the invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the appended drawings in which: there figure 1 , already described, is a schematic and partial view of a flash-type imaging system according to an example of the prior art; figures 2A et 2B are schematic and partial views of imaging systems according to embodiments, one being of the single-point type ( fig.2A ) and the other flash type ( fig.2B ) ; there figure 3A is a schematic and partial view of an imaging system according to an embodiment similar to that of the fig.2A , where the different optical signals are highlighted, in particular the light beams S ret,c(1) which form a first echo, and the light beams S ret,c(2) which form a second echo; the figures 3B et 3C illustrate an evolution of the optical frequency of different signals, thus highlighting the beat frequency fb of the heterodyne signal S h , in the case of an imaging system according to an example of the prior art ( fig.3B ) and in the case of an imaging system according to an embodiment of the invention ( fig.3C ) ; THE figures 4A à 4C illustrate a numerical example of determining the beat frequency fb of the heterodyne signal S h , in the case of an imaging system according to an example of the prior art ( fig.4A ), in the case of an imaging system according to one embodiment ( fig.4B ), where the fig.4C represents the power spectral density highlighting the beat frequency fb in these two cases; figures 5A à 5D are schematic views of examples of the reflector of an imaging system according to one embodiment, where the reflector is a ring surrounding the optical collection element ( fig.5A ); is a square adjacent to or distant from the optical collection element ( fig.5B ); is located at the photodetector ( fig.5C ); is formed of retroreflective surfaces separated from each other by a transparent surface ( fig.5D ) ; there figure 6A is a schematic and partial view of an imaging system according to one embodiment, of the single-point type and where the reflector is retroreflective; and the fig.6B is a detailed view of the fig.6A highlighting the backscattered and uncollected signal S ret,nc , the reflected signal S ret,r , and the second echo S ret,c(2); the figure 7A is a schematic and partial view of an imaging system according to one embodiment, of the single-point type and where the reflector is non-retroreflective; and the fig.7B is a detailed view of the fig.7A highlighting the backscattered and uncollected signal S ret,nc , the reflected signal S ret,r , and the second echo S ret,c(2); the figures 8A et 8B are schematic and partial views of an imaging system according to embodiments, where the reflector is located upstream of the optical collection element and is crossed by the optical axis of the latter, in mono-static configuration ( fig.8A ) and in bi-static configuration ( fig.8B ). EXPOSÉ DÉTAILLÉ DE MODES DE RÉALISATION PARTICULIERS

[0025] In the figures and in the remainder of the description, the same references represent identical or similar elements. In addition, the different elements are not shown to scale so as to enhance the clarity of the figures. Furthermore, the different embodiments and variants are not mutually exclusive and may be combined with each other. Unless otherwise indicated, the terms "substantially", "approximately", "of the order of" mean to within 10%, and preferably to within 5%. Furthermore, the terms "between ... and ..." and equivalents mean that the limits are included, unless otherwise indicated.

[0026] A frequency modulated continuous wave (FMCW) type heterodyne detection LIDAR imaging system is disclosed. Frequency-Modulated Continuous-Wave, in English), allowing the determination of a distance z sc of a scene, or even a distance map z sc(i,j) (distance image), having a reduced footprint insofar as it does not include an optical separation / recombination device such as that of the prior art.

[0027] As described in detail below, the imaging system is then adapted to direct the primary signal S p entirely to the scene to illuminate it. In addition, it comprises a reflector making it possible to collect, in addition to light beams S ret,c(1) which have not been reflected by the reflector and which are called the first echo, second light beams S ret,c(2) which have been reflected by the reflector and which are called the second echo. The distance z sc from the scene is then determined from a beat frequency fb of the heterodyne signal S h , the latter being formed by the interference of the first echo S ret,c(1) and the second echo S ret,c(2) , and no longer by interference between the reference signal S r of a local oscillator and the backscattered object signal S or .

[0028] The imaging system is called “LIDAR” (acronym for Light Detection and Ranging, in English) insofar as a coherent optical signal is used to determine a distance z sc of a point of the scene or a distance map z sc(i,j) of the scene. In the context of the invention, the imaging system can be either of the single-point type in the sense that the optical signal illuminates only one point of the scene, with possibly a spatial scan of the scene by the optical signal, or of the flash type in the sense that the optical signal simultaneously illuminates several points of the scene and where the imaging system acquires the image of the scene to determine a distance map. In the context of the invention, the optical signal which illuminates the scene is the primary signal S p , and no longer the object signal S o as in the prior art.

[0029] Furthermore, the imaging system is said to be heterodyne detection insofar as, to determine the distance of the illuminated scene, a so-called beat frequency of a heterodyne signal formed by the interference between two mutually coherent signals having a difference in optical path is determined. Unlike the prior art where the two optical signals are the reference signal S r (derived from the division of the primary signal S p to form the reference signal S r and the object signal S o ) and the backscattered object signal S or , in the context of the invention the two optical signals are both signals backscattered by the scene, namely the first echo S ret,c(1) not having been reflected by the reflector, and the second echo S ret,c(2) having been reflected by the reflector. These two optical signals S ret,c(1) and S ret,c(2) remain coherent with each other since they come from the same primary signal S p emitted by the optical source.Finally, heterodyne detection is of the FMCW type insofar as the primary signal S p is a continuous and frequency modulated signal.

[0030] THE figures 2A et 2B are schematic and partial views of a heterodyne detection LIDAR imaging system of the FMCW type, according to embodiments, where one corresponds to a single-point type imaging system, and the other to a flash type imaging system. The figures are very schematic: the scene here is a flat surface but in reality it may obviously not be.

[0031] Generally speaking, the imaging system 1 comprises a minima : ∘ an optical source 10, adapted to emit a primary signal S p , coherent, continuous and frequency modulated, to illuminate the scene 2; ∘ an optical collection element 41, adapted to collect a part called collected signal S ret,c of a backscattered signal S ret by the scene 2 coming from the primary signal S p ; ∘ a photodetector 50, intended to receive a heterodyne signal S h associated with the collected signal S ret,c ; ∘ a processing unit 60, adapted to determine a distance z sc from the scene 2 from a beat frequency of the heterodyne signal S h .

[0032] To reduce the size of the imaging system 1 and thus be able to do without the optical separation / recombination device, the imaging system 1 according to the invention is adapted to entirely direct the primary signal S p to the scene 2. In addition, it comprises a reflector 42 adapted to reflect in the direction of the scene 2 an uncollected part S ret,nc of the backscattered signal S ret . Also, the signal collected S ret,c by the optical collection element 41 is formed of first light beams S ret,c(1) not having been reflected by the reflector 42 (first echo), and second light beams S ret,c(2) having been reflected by the reflector 42 (second echo).

[0033] As a result, the photodetector 50 receives two optical signals S ret,c(1) and S ret,c(2), coherent with each other because they both come from the same primary signal S p , backscattered by the scene 2 but which have a difference in optical path between them. They interfere with each other and form the heterodyne signal S h which has a beat frequency fb .

[0034] The processing unit of the imaging system 1 is then able to determine the distance z sc of the scene 2 from the beat frequency fb of this heterodyne signal S h , without having to use the reference signal S r of the local oscillator as in the prior art. Consequently, it is not necessary to have an optical separation / recombination device, so that the imaging system 1 according to the invention has a reduced footprint.

[0035] Note that the fact of directing the primary signal S p entirely to the scene 2 results in the fact that the optical power of the primary signal S p incident on the scene (at the level of the illuminated surface of the scene 2) is equal to the optical power of the primary signal S p at the output of the optical source 10, obviously taking into account the possible low optical losses during the transmission of the primary signal S p to the scene 2. Here we consider the power of the primary signal S p at the output of the optical source 10, which may be slightly lower than that at the output of the laser source, in particular when the optical source 10 comprises optical elements (shaping of the light beam, optical filter, etc.) which may induce optical losses, as described later.

[0036] There fig.2A illustrates an imaging system 1 according to a single-point type embodiment. Also, the primary signal S p only illuminates one point of the scene 2 at each instant, and the photodetector 50 can be a simple photodiode (or a pair of balanced photodiodes). The imaging system 1 is therefore adapted to determine the distance z sc of the illuminated point of the scene 2. Note that the imaging system 1 can be adapted to spatially scan the scene 2 by the object signal S o , point by point. According to the invention, it comprises a reflector 42, but does not comprise a separation / recombination device like that illustrated in the fig.1 .

[0037] The imaging system 1 comprises an optical source 10 of a so-called primary signal S p , continuous coherent and frequency modulated. The optical source 10 comprises a laser source and may comprise additional optical elements (not shown) located downstream of the laser source.

[0038] The primary signal S p is frequency modulated, for example here linearly, from a starting frequency f 0 over a repetition period T with a bandwidth B ( chirp ). The signal here is a signal chirpé, that is to say a sinusoidal wave whose instantaneous frequency evolves linearly over time. For example, the primary signal S p can have an optical frequency located in the infrared.

[0039] In the case of emission in the near infrared range (between 0.7 and 2µm), the laser source can be a vertical cavity surface emitting laser diode (VCSEL, for Vertical-Cavity Surface Emitting Laser, in English) which generally has a coherence length of the order of a meter, or even an edge-emitting laser diode (EEL, for Edge Emitting Laser, in English) which can have a coherence length of the order of ten or even a hundred meters.

[0040] The optical source 10 has a coherence length typically greater than the optical path difference between the first echo S ret,c(1) and the second echo S ret,c(2). This optical path difference corresponds, to the first order, to twice the maximum distance between the imaging system 1 and the scene 2.

[0041] According to the invention, unlike the prior art, the imaging system 1 does not include an optical separation / recombination device identical or similar to that described with reference to the fig.1 . It therefore does not include an optical separating element suitable for separating the primary signal S p into an object signal S o on the one hand and a reference signal S r on the other hand. There is therefore no reference signal S r from a local oscillator (LO, for Local Oscillator in English) which would be directed towards the photodetector without passing through scene 2, to interfere in fine with the backscattered signal S ret . Furthermore, it also does not include an optical recombiner element adapted to direct towards the photodetector 50 along the same optical axis, by spatially superimposing them at least in part, the reference signal S r and the collected part S ret,c of the backscattered signal S ret .

[0042] As a result, the primary signal 10 is the signal which illuminates the scene 2, without it having been previously divided into a reference signal S r and an object signal S o . Also, as indicated previously, the power of the primary signal S p at the output of the optical source 10 is identical to that of the same primary signal S p which illuminates the scene 2.

[0043] The imaging system 1 comprises at least one optical element 41 for collecting a portion denoted S ret,c of the backscattered signal S ret by the scene 2, originating from the primary signal S p , this collected signal S ret,c then being received by the photodetector 50. It may be a free-space optical element, and it may be an aperture diaphragm which defines the physical pupil. The aperture diaphragm may be defined by the outline of a focusing lens, in particular in the case of a flash-type imaging system 1. The optical collection element may, moreover, be formed of several lenses between which the aperture diaphragm is arranged. The optical collection element 41 may also be defined by the sensitive surface of the photodetector 50, in particular in the case of a single-point type imaging system 1, in which case it is not a dedicated optical object.

[0044] According to the invention also, the imaging system 1 comprises a reflector 42 adapted to reflect in the direction of the scene 2 a part denoted S ret,nc of the backscattered signal S ret which has not been collected by the optical collection element 41. The reflector 42 is here a specular reflector, in the sense that it reflects the light beams in a non-diffuse or almost non-diffuse manner. The reflector 42 can be located at the level of the optical collection element 41, for example by being coplanar thereto, or be located upstream or downstream. As described later, it can also be located at the level of the photodetector 50.

[0045] As described further, the reflector 42 may be simply reflective, that is to say it reflects the incident light beams according to the Snell-Descartes law of reflection, or be retroreflective, that is to say the light beams are reflected along a reflection axis identical to the axis of incidence. In this respect, the reflector 42 may be a cube corner mirror or a layer of microbeads, as described in particular in document WO2015 / 158999A1.

[0046] The optical collection element 41 preferably has small lateral dimensions relative to the distance of this assembly from the scene 2, so that the first and second echoes S ret,c(1) and S ret,c(2) physically pass through almost the same optical path. They are therefore collected along almost the same optical axis and with good spatial superposition, thus improving the combination of the two optical signals by interference, which makes it possible to improve the intensity of the heterodyne signal S h .

[0047] The imaging system 1 further comprises a photodetector 50, which is here a photodiode (or for example a pair of balanced photodiodes) insofar as the imaging system 1 is of the single-point type. It receives, not the reference signal S r from a local oscillator, but a first echo S ret,c(1) and a second echo S ret,c(2) of the backscattered signal S ret by the scene 2 and collected by the optical collection element 41, which interfere with each other to form the heterodyne signal S h which has a beat frequency fb.

[0048] The imaging system 1 comprises a processing unit 60 adapted to determine the distance z sc of the illuminated point of the scene 2 from the beat frequency fb of the heterodyne signal S h received by the photodetector 50. The distance z sc is here the distance separating the scene 2 from the reflector 42.

[0049] Before detailing the operation of the imaging system 1, note that the invention also covers the configuration of the flash-type imaging system 1 where several points of the scene 2 are illuminated simultaneously by the same primary signal S p .

[0050] In this respect, the fig.2B illustrates such an imaging system 1, similar to that of the fig.1 but which is distinguished from it in that it does not include an optical separation / recombination device, and in that it includes a reflector 42 adapted to reflect in the direction of scene 2 a part S ret,nc of the backscattered signal S ret not collected by the optical collection element 41.

[0051] For example, the optical source 10 may comprise passive optical elements (not shown) located downstream of the laser source. Thus, a shaping lens may be provided, which makes it possible to collimate the optical beam while widening its lateral dimension, for example to a diameter of a few millimeters. In addition, a spatial filtering device may be present to suppress high spatial frequencies. Thus, the primary signal S p propagates in a collimated manner, with a Gaussian profile and a diameter of the order of a few millimeters, for example 5 mm. It then has an optical power which is substantially the same at the illuminated scene.

[0052] Unlike the imaging system 1 of the fig.2A , the imaging system 1 according to this variant comprises an optical device 30 for projecting the primary signal S p towards the scene 2 so as to illuminate it simultaneously. It also comprises an optical imaging device 40 adapted to transmit the part S ret,c of the backscattered signal S ret and to form the image of the illuminated scene 2 in the detection plane of the photodetector 50. These optical devices are similar to those described in document WO2021 / 144357A1 and are therefore not described in detail here. Note that the imaging system 1 also does not comprise the optical shaping element 22 of the reference signal S r described in the fig.1 .

[0053] Finally, the photodetector 50 is of the matrix type, and comprises a matrix of detection pixels extending in a reception plane. It may be a CMOS type photodetector (or even a CCD type). The reception plane of the matrix photodetector 50 is located in a plane conjugated to the scene by the optical imaging device 40 (to the extent that the scene is not necessarily a flat surface). In other words, the image of the scene 2 is formed in the reception plane of the matrix photodetector 50. Each detection pixel is intended to receive the heterodyne signal S h .

[0054] The operation of the imaging system 1 is now described with reference to figures 3A, 3B et 3C , where the fig.3A is a schematic and partial view of an imaging system 1 similar to that of the fig.2A highlighting the different optical signals present, and where the fig.3B et 3C illustrate the temporal evolution of the frequency of different optical signals, highlighting the beat frequency fb, in the case of the prior art ( fig.3B ) and in the case of the invention ( fig.3C ).

[0055] The optical source 10 emits the coherent, continuous and frequency-modulated primary signal S p, which is entirely directed towards the scene 2. Also, the power of the optical signal illuminating the scene 2 is equal to that at the output of the optical source 10.

[0056] Scene 2 backscatters a portion of the primary signal S p which then forms the backscattered signal S ret . This comprises a portion S ret,c which is collected by the optical collection element 41, and a portion S ret,nc which is not collected by this optical collection element 41. Indeed, scene 2 usually has a component that is at least partially diffuse in reflection, that is to say that the light is reflected according to a relatively wide angular indicatrix. Also, the light backscattered by scene 2 illuminates the optical collection element 41 but also a portion of the space surrounding this collection optic. Also, the uncollected portion S ret,nc can be reflected by the reflector 42 in the direction of scene 2, which in return forms the reflected signal S ret,r , a portion of which is then backscattered again by scene 2 and then collected by the optical collection element 41.

[0057] This results in the optical collection element 41 collecting a part S ret,c of the backscattered signal S ret , which is then formed from the light beams S ret,c(1) having been directly collected without having been reflected by the reflector 42 (first echo), and from the light beams S ret,c(2) having been reflected by the reflector 42 before being subsequently collected (second echo).

[0058] Note that the collected part S ret,c is the light beams which have been or which will be collected by the optical collection element 41. The signals S ret,c(1) and S ret,c(2) are directed towards the photodetector 50 along the same optical axis and at least partly superimposed on each other, all the more so when the lateral dimensions of the optical collection element 41 are small compared to the distance between this element 41 and the scene 2. The signals S ret,c(1) and S ret,c(2) then interfere with each other and form the heterodyne signal S h .

[0059] The second echo S ret,c(2) has an amplitude which is generally lower than that of the first echo S ret,c(1), especially since the diffuse reflectance factor of the scene is small, and where the reflector 42 does not collect all of the light backscattered by the scene 2 and not collected by the optical collection element 41.

[0060] In reference to the fig.3B , in the case of a prior art imaging system such as that of the fig.1 , the beat frequency fb comes from the optical path difference between the reference signal S r and the backscattered and collected object signal S or,c (which corresponds to the first echo S ret,c(1) ), which is to the first order equal to a round trip between scene 2 and the imaging system 1. We can write fb / B = τ / T. Moreover, knowing that τ = 2z sc / c when we neglect the distance zr of the reference path in front of z sc , we can then determine the distance z sc such that z sc = fb cT / 2B.

[0061] In reference to the fig.3C , in the case of an imaging system 1 according to an embodiment of the invention, the beat frequency fb comes from the optical path difference between the first echo S ret,c(1) and the second echo S ret,c(2), which is equal to the first order, here too, to a round trip between the scene 2 and the imaging system 1. We can therefore write fb / B = τ / T with τ = 2z sc / c, and therefore determine the distance z sc such that z sc = fb cT / 2B.

[0062] The processing unit 60 then determines the beat frequency fb of the detected heterodyne signal S h, then deduces the distance z sc from scene 2. The beat frequency fb(2) can be determined, in a known manner, in the time domain by counting the number of oscillations of the heterodyne signal over the period T, or in the frequency domain by fast Fourier transform.

[0063] Also, the imaging system 1 according to the invention has a reduced bulk insofar as, thanks to the presence of the reflector 42, it does not include an optical separation / recombination device present in the imaging systems of the prior art. Also, it does not include an optical separator element ensuring the division of the primary signal S p into the reference signal S r and the object signal S o , nor an optical recombining element ensuring the recombination of the reference signal S r with the backscattered object signal S or . In the case of a flash type imaging system (or even a single-point type), it also does not include the optical element for shaping the reference signal S r . This eliminates the photometric losses associated with these optical elements.

[0064] This advantage is obtained by collecting, in addition to the first echo S ret,c(1), the second echo S ret,c(2) of the backscattered signal S ret, which was reflected by the reflector 42 then backscattered again by the scene 2. Furthermore, the beat frequency fb remains substantially equal to that measured in the prior art, so that the imaging system 1 according to the invention uses a processing unit 60 which can remain unchanged compared to that of the prior art.

[0065] Note that the imaging system 1 according to the invention therefore does not include a reference signal S r of a local oscillator, insofar as the two signals S ret,c(1) and S ret,c(2) are backscattered by the scene 2, and that the optical paths of these signals vary when the distance z sc varies, which is not the case in an imaging system according to the prior art where the optical path of the reference signal S r does not pass through the scene and remains independent of the distance z sc .

[0066] We now describe, with reference to the figures 4A à 4C , a concrete example of determining the beat frequency fb of the heterodyne signal S h , here by FFT, in the case of a prior art imaging system of single-point type ( fig.4A ), and in the case of an imaging system according to the invention, here also of the single-point type ( fig.4B ). There fig.4C represents the power spectral density highlighting the beat frequency fb in these two cases.

[0067] In this example, the primary signal S p has a wavelength λ of 633nm, a chirp B equal to 12GHz for a period T equal to 1.6ms, and the scene is located at a distance z sc of 50cm. Scene 2 is formed of a frosted silicon plate, exhibiting semi-diffusing behavior in reflection.

[0068] There fig.4A represents the imaging system according to the prior art. It therefore comprises an optical separation / recombination device 20 formed of a separating optical element 21 and a recombining optical element 23. The primary signal S p is therefore divided by the separating optical element 21 into a reference signal S r and into an object signal S o . Then the backscattered object signal S or,c is transmitted and spatially superimposed on the reference signal S r by the recombining optical element 23, and collected by the collection optical element 41. In this example, the collection optical element 41 is located between the recombining optical element 23 and the photodetector. fig.4B represents the imaging system according to the invention, here similar to that described with reference to the fig.2A . It is therefore not described again.

[0069] In reference to the fig.4C , the continuous line C1 curve represents the power spectral density associated with the imaging system of the prior art ( fig.4A ). The measured beat frequency fb is here equal to 25kHz, in accordance with the theoretical relation fb = 2Bz sc / cT.

[0070] Furthermore, the C2 curve in short dotted lines represents the power spectral density associated with the imaging system according to the invention ( fig.4B ). The measured beat frequency fb is here equal to 26kHz, which also conforms to the theoretical value = 2Bz sc / cT. As expected, the amplitude of this signal is lower than that obtained with the prior art imaging system.

[0071] Finally, the C2' curve in long dotted lines represents the power spectral density associated with the imaging system according to the invention ( fig.4B ) but for a distance z sc = 25cm. The measured beat frequency fb is here equal to 13kHz, which confirms that the signal at 26GHz is not an experimental artifact.

[0072] THE figures 5A à 5D are schematic views of the reflector 42 according to different embodiment variants.

[0073] The reflector 42 may be located at the level of the optical collection element 41, for example coplanar with it, and have a ring shape (crown) continuously surrounding the latter (cf. fig.5A ). It can be formed from annular segments partially surrounding the optical collection element 41. Alternatively, it can have any shape, here square, attached or not to the optical collection element 41 ( fig.5B ).

[0074] The reflector 42 may not be located in the plane of the optical collection element 41, and may be located upstream or downstream of the latter. For example, it may be located at the photodetector, i.e. in the plane or close to the detection plane, whether the photodetector 50 is a photodiode or of the matrix type. As such, the fig.5C illustrates the situation where the photodetector is matrix-based (flash-type imaging system 1) and comprises a matrix of detection pixels 51 with a fill factor less than unity, the reflector 42 can be located in the non-photosensitive zones of the matrix of detection pixels 51. In this case and when the reflector 42 is retroreflective, the cube corners preferably have a lateral dimension much greater than the wavelength, for example greater than 10 times the wavelength of the primary signal, so that the cube corners do not diffract.

[0075] Furthermore, the reflector 42 may be formed from a continuously reflective or retroreflective surface, or be formed from non-joining reflective or retroreflective surfaces 42.2 separated from each other by a transparent or reflective surface 42.1 at the wavelength of the optical signals of interest, as illustrated in fig.5D . In the case where the surface 42.1 is transparent, this embodiment is particularly interesting when the reflector 42 is located upstream of the optical collection element 41 and is crossed by the optical collection axis (cf. fig.8A et 8B ). In this respect, the reflector 42 may be formed of a central surface crossed by the optical collection axis, comprising the reflective or retroreflective surfaces 42.2 separated from each other and surrounded by a transparent surface 42.1, and of a peripheral surface which surrounds the central surface, in which the reflective or retroreflective surfaces 42.2 are placed next to each other.

[0076] There figure 6A is a schematic and partial view of an imaging system 1 according to a single-point type embodiment, in which the reflector 42 is retroreflective. figure 6B illustrates a detail of the fig.6A to highlight a condition on the maximum distance r max of the lateral edge of the reflector 42 with respect to the optical collection axis.

[0077] According to one embodiment, the reflector 42 is retroreflective, in the sense that the incident light beams are reflected with a reflection axis identical to the incidence axis. Thus, the light beam backscattered by a point of the scene 2 and reflected by the retroreflecting reflector 42 is returned to this same point of the scene 2 or in the immediate vicinity. Also, such a reflector 42 makes it possible not to mix, among the light beams of the collected signal S ret,c , the light beams coming from several different points of the scene 2, and therefore not to degrade the lateral resolution of the imaging system 1. In the case of a single-point type imaging system 1, this allows the determination of the distance z sc not to be interfered with by light beams coming from objects located at other distances.And in the case of a flash-type imaging system 1, this avoids degrading the quality or spatial resolution of the determined distance mapping.

[0078] The reflector 42 is arranged with respect to the optical axis of the optical collection element 41, so that it has an external lateral edge located at a maximum distance r max from this optical axis. Preferably, this maximum distance r max is less than √(cz sc / B), this so as not to degrade the distance resolution Δz sc of the imaging system 1. Indeed, as illustrated in figure 6B , a light beam from the uncollected part S ret,nc which would be retroreflected at the edge of the reflector 42, then backscattered along the optical axis of the optical collection element 41, would then travel a distance z sc + √(z sc 2< +r max 2< ), and not 2z sc , which would induce an error of the order of r max 2< / 2z sc in the hypothesis where r max « z sc . Also, this error r max 2< / 2z sc should advantageously be less than the distance resolution Δz sc equal to c / 2B, which leads to the condition r max < √(cz sc / B). Note that this condition is not very restrictive, since we obtain r max < 15cm for z sc =50cm and B=7GHz.

[0079] The reflector 42 has a surface area adapted to maximize the number of photons of the signal S ret,nc intercepted then reflected, which consequently increases the amplitude of the second echo S ret,c(2) and therefore the distance range of the imaging system 1, without however harming the compactness of the reception module of the imaging system 1. In any case, it is advantageous for the reflector 42 to have dimensions which respect the condition mentioned above on the maximum distance r max from the lateral edge.

[0080] There figure 7A is a schematic and partial view of an imaging system 1 according to one embodiment, in which the reflector 42 is non-retroreflective, i.e. only reflective. The figure 7B illustrates a detail of the fig.7A to highlight a condition on the maximum distance r max of the lateral edge of the reflector 42 with respect to the optical collection axis.

[0081] A light beam incident on the reflector 42 is therefore not reflected along a reflection axis identical to the incidence axis, but follows the Snell-Descartes reflection law. It is therefore reflected in the direction of a different point than that from which the light beam of the part S ret,nc of the backscattered signal S ret comes. For a light beam which is reflected on the reflector 42 at the distance r max , we note that the distance between these two points of the scene is of the order of 2r max (for a scene substantially perpendicular to the optical axis). As a result, the previous condition on the distance r max from the external lateral edge of the reflector 42 is modified and becomes: r max < √(cz sc / 3B). This condition still remains not very restrictive since we obtain r max < 8.5cm in the case where z sc = 50cm and B=7GHz. In any case, it is preferable to reserve the use of a non-retroreflective 42 reflector for a single-point type 1 imaging system.Furthermore, care will be taken to ensure that the angular separation between the two illuminated points of scene 2 remains lower than the iFOV angular resolution (for . individual Field Of View, in English) of the optical collection element 41, with the condition 2atan(r max / z sc ) <iFOV, de manière à ne pas dégrader la résolution latérale. Notons que le champ de vue FOV ( Field of View, in English) of the optical collection element 41 is the angle in which the photodetector 50 is sensitive to the part S ret,c of the backscattered signal S ret through the optical collection element 41.

[0082] According to one embodiment, the imaging system 1 may have a so-called mono-static configuration, in the sense that the optical axis of illumination of the scene 2 and the optical axis of collection are collinear. As such, the figure 8A is a schematic and partial view of such an imaging system 1, here of the single-point type, but this configuration can also be implemented for a flash-type imaging system 1. Furthermore, the reflector 42 is here retroreflective and is located upstream of the optical collection element 41, in the direction where the optical collection axis passes through the reflector 42. Also, the latter is at least partially reflective or retroreflective, and is formed of non-joining reflective or retroreflective surfaces separated from each other by a transparent surface (see the example of the fig.5D ).

[0083] The imaging system 1 here comprises a semi-reflecting plate 43 (or a splitter cube) placed on the optical path of the primary signal S p , and the optical collection element 41 (and the reflector 42) is located between the scene 2 and the plate 43. It is also possible to use, instead of the semi-reflecting plate 43, a polarization splitter cube associated with a quarter-wave plate, which makes it possible to reduce optical losses. This optical assembly forms an optical isolator making it possible to prevent light from being backscattered onto the laser source (and therefore to avoid any overheating of the laser source).

[0084] Thus, the primary signal S p is transmitted towards the scene 2 by the semi-reflecting plate 43. A part S ret,c(1) (first echo) of the backscattered signal S ret is collected by the optical collection element 41 and then reflected by the semi-reflecting plate 43 towards the photodetector 50. A part S ret,nc of the backscattered signal S ret is not collected by the optical collection element 41, but is retroreflected by the reflector 42, then is collected by the optical collection element 41 (second echo), then is reflected by the semi-reflecting plate 43 towards the photodetector 50. The two echoes S ret,c(1) and S ret,c(2) interfere to form the heterodyne signal S h detected by the photodetector 50.

[0085] The imaging system 1 here has the advantage of facilitating the detection and determination of the distance z sc of objects having a strong specular component, that is to say that the light is backscattered by the object in question in the scene in a backscattering cone (angular distribution of the backscattered light intensity) centered on a main direction, this direction being able to be close to the direction of specular reflection. Indeed, in the case of a bi-static type imaging system 1 presented previously ( fig.2A-2B And fig.6A-7A ), the second echo S ret,c(2) could be backscattered mainly in the direction of the optical source 10, and little in the direction of the optical collection element 41, thus reducing the intensity of the signal S ret,c(2) collected, and therefore the capacity of the imaging system 1 to detect this type of object of the scene 2. This is not the case for an imaging system 1 of the mono-static type, insofar as the second echo S ret,c(2) is mainly returned towards the optical collection element 41 and the reflector 42 (the angular indicator is narrow). Furthermore, this mono-static configuration for an imaging system 1 has the advantage of reducing or even eliminating shadowing phenomena in the determined distance mapping, by the very fact that these two optical axes are merged.

[0086] There figure 8Bis a schematic and partial view of an imaging system 1 according to another embodiment, of single-point type and bi-static configuration, similar to that of the fig.6A , where the reflector 42 is also located upstream of the optical collection element 41.

[0087] Particular embodiments have just been described. Different variants and modifications will appear to those skilled in the art. Thus, the imaging system 1 can have a free space configuration as well as a guided optics configuration, as described in the document WO2021 / 144357A1 mentioned above.

Claims

1. An FMCW type LIDAR imaging system (1), adapted to determine a distance zsc separating it from a scene (2), comprising: ∘ an optical source (10), adapted to emit a coherent, continuous and frequency-modulated primary signal Sp, to illuminate the scene (2); ∘ an optical collection element (41), adapted to collect a portion referred to as collected signal Sret,c of a backscattered signal Sret by the scene (2) from the primary signal Sp; ∘ a photodetector (50), intended to receive a heterodyne signal Sh associated with the collected signal Sret.c; ∘ a processing unit (60), adapted to determine the distance zsc of the scene based on a beat frequency of the heterodyne signal Sh; ∘ characterised in that it is adapted to direct the primary signal Sp entirely to the scene (2); ∘ and in that it comprises a reflector (42) adapted to reflect towards the scene (2) a portion Spr,nc, referred to as uncollected signal Sret,nc, of the backscattered signal Sret and not collected by the optical collection element (41), • the collected portion S pr,c of the backscattered signal Spr then being formed of: first light beams Sret,c(1) that have not been reflected by the reflector (42); and second light beams Sret,c(2) that have been reflected by the reflector (42) then by the scene (2); • the heterodyne signal Sh then being formed by the interference between the first light beams Sret,c(1) and the second light beams Sret,c(2).

2. The imaging system (1) according to claim 1, wherein the reflector (42) is retroreflective, to reflect incident light beams in the direction of the scene (2) along an axis of reflection identical to their axis of incidence.

3. The imaging system (1) according to claim 1 or 2, wherein the reflector (42) has a lateral boundary located at a maximum distance rmax from an optical axis of the optical collection element (41), and is dimensioned so that the maximum distance rmax is less than √(czsc / B) when the reflector (42) is retroreflective, where c is the speed of light in vacuum, and B is a variation of the frequency of the primary signal Sp over a modulation period T, and so that the maximum distance rmax is less than √(czsc / 3B) when the reflector (42) is non-retroreflective.

4. The imaging system (1) according to any one of claims 1 to 3, wherein the reflector (42) is located in the plane of the optical collection element (41).

5. The imaging system (1) according to any one of claims 1 to 3, wherein the reflector (42) is located downstream of the optical collection element (41) at the photodetector (50).

6. The imaging system (1) according to any one of claims 1 to 3, wherein the reflector (42) is located upstream of the optical collection element (41) with an optical axis of collection passing therethrough, the reflector (42) then being formed of reflective or retroreflective surfaces (42.2) separated from one another and surrounded by a surface (42.1) that is transparent at the wavelength of the primary signal Sp.

7. The imaging system (1) according to claim 6, wherein the reflector (42) comprises a central surface through which the optical axis of collection passes, wherein it is formed of reflective or retroreflective surfaces (42.2) separated from one another and surrounded by a transparent surface (42.1), and a peripheral surface which surrounds the central surface, wherein the reflective or retroreflective surfaces (42.2) are joined to one another.

8. The imaging system (1) according to any one of claims 1 to 7, adapted to illuminate only one point of the scene (2), or adapted to simultaneously illuminate a plurality of points of the scene (2) and then comprising an optical device (30) for projecting the primary signal Sp onto the scene (2) to simultaneously illuminate the plurality of points of the scene (2) and an optical imaging device (40) adapted to form an image of the illuminated scene in the plane of the photodetector (50).

9. The imaging system (1) according to any one of claims 1 to 8, having a configuration referred to as monostatic where an optical axis of illumination of the scene by the primary signal Sp is identical to an optical axis of collection of the optical collection element (41), and comprising a semi-reflective plate (43) or a splitting cube transmitting the primary signal Sp towards the scene and reflecting the backscattered signal Sret towards the photodetector (50).

10. The imaging system (1) according to any one of claims 1 to 8, having a configuration referred to as bistatic where an optical axis of illumination of the scene by the primary signal Sp is different from an optical axis of collection of the optical collection element (41).