FMCW HETERODYNE-DETECTION LIDAR IMAGER SYSTEM WITH IMPROVED DISTANCE RESOLUTION

The invention relates to a FMCW lidar imager system with improved distance resolution. The imager system 1 comprises a reflector 42 configured to reflect, in the direction of the scene 2, a portion Sor,nc of the backscattered object signal Sor, which portion has not been collected by the collecting optical element 41. Thus, the collected portion Sor,c of the backscattered object signal Sor is formed from first light beams Sor,c(1) that have not been reflected by the reflector 42 and from light beams Sor,c(2) that have been reflected by the reflector 42. The heterodyne signal Sh therefore has a principal component Sh(1) associated with the light beams Sor,c(1), and a secondary component Sh(2) associated with the light beams Sor,c(2). The processing unit 60 is configured to determine the distance zsc of the scene 2 on the basis of a beat frequency fb(2) of the secondary component Sh(2) of the heterodyne signal Sh.

TECHNICAL FIELD

The field of the invention is that of frequency-modulated continuous wave (FMCW) heterodyne-detection lidar imager systems.

PRIOR ART

FMCW heterodyne-detection lidar imager systems allow a distance of a scene illuminated by a coherent optical signal to be determined. Such an imager system is based on the principle of heterodyne detection, in the sense that use is made of the properties of a heterodyne signal formed by the interference between a reference signal and a signal backscattered by the scene. These two optical signals are mutually coherent and originate from the same optical signal, called the primary signal, which is emitted by an optical source. Documents US 2020/300993 A1, US 2019/064358 A1, et US 2020/011994 A1 describe various examples of FMCW lidar imager systems.

In this regard, document WO2021/144357A1 describes an example of such an imager system, here called a flash imager system in that it is configured to illuminate a plurality of points of the scene simultaneously and to determine therefrom a distance map (distance image).

FIG.1is a schematic and partial view of such an imager system1. It comprises at least:an optical source10of what is called the primary signal Sp, which is coherent, frequency-modulated and continuous-wave;a splitting and recombining optical device20, which comprises a splitting optical element21configured to divide the primary signal Spinto an object signal Sodirected toward the scene2and into a reference signal Srdirected toward a photodetector50; an optical element22for shaping the reference signal Sr; and a recombining optical element23configured to direct toward the photodetector50, along the same optical axis, the reference signal Srand a portion Sor,cof the backscattered object signal Sor;a projecting optical device30, which is configured to project the object signal Sowith a view to illuminating the whole scene2simultaneously;an imaging optical device40, which is configured to transmit the collected portion Sor,cof the backscattered object signal Sorand to form the image of the illuminated scene2in the detection plane of the photodetector50. It comprises a collecting optical element that collects the light beams defining the portion Sor,cof the backscattered object signal Sor;the photodetector50, here a matrix-array photodetector, which is configured to receive the collected portion Sor,cof the backscattered object signal Sorand the reference signal Sr, which interfere to form a heterodyne signal Shhaving a beat frequency fb;a processing unit60, which is configured to determine a distance zsc(and here a distance map) of the scene2on the basis of the beat frequency fbof the heterodyne signal Sh.

The primary signal Spis chirped, i.e. exhibits an instantaneous frequency variation, with for example a starting frequency f0and a variation in value B over a period T. The chirped primary signal Spis a sinusoidal wave the instantaneous frequency of which varies over time, here linearly.

The photodetector50thus receives the collected portion Sor,cof the backscattered object signal Sor, which is an attenuated and delayed replica of the object signal So, with a delay τ. The delay results in a frequency difference fbbetween the two signals in the interval [τ; T], with T»T, that is between the reference signal Sorand the backscattered and collected object signal Sor,c. The delay T is equal to about 2zsc/c when the path travelled by the reference signal Sris neglected, where c is the speed of light in vacuum. This frequency fb, which is called the beat frequency, is equal to the difference between the frequency of the reference signal Srand the backscattered and collected object signal Sor,c. Its value may be determined in the time domain by counting the number of oscillations in the heterodyne signal Shover the period T, or in the frequency domain via fast Fourier transform.

It is then possible to determine, on the basis of the value of this beat frequency fb, the distance zscbetween the illuminated scene2and the matrix-array photodetector50. Specifically, given that fb/B=τ/T, and that τ=2zsc/c, the distance zscof the scene may then be determined using the relationship: zsc≈fbcT/2B, with a distance resolution Δzsc=c/2B. The distance resolution Δzscis defined as being the smallest difference in distance that the imager system is capable of measuring between two successive positions of a given object of the scene or between two laterally separate objects. By way of example, for a chirp B of 7.5 GHz, the distance resolution Δzscis equal to 2 cm.

There is however a need to provide an imager system having an improved distance resolution. To do so, one approach would be to increase the value of the chirp B, which is conventionally produced by moving one of the reflectors of the laser cavity, for example via the piezoelectric effect, or by modulating the injection current of the laser source. However, such an increase in the value of the chirp may notably lead to errors in the determined value of the beat frequency fband therefore in the determined value of the distance zsc, notably because of non-linearity in the chirp and/or of a modulation of the optical power of the laser source induced by the modulation of the injected current. One alternative, as described in the document Aflatouni et al. titled Nanophotonic coherent imager, Opt. Express 23 (4), 5117-5125 (2015), would be to increase the resolution of the measurement of the beat frequency fb, for example by counting a decimal number of oscillations in the heterodyne signal Shover the period T, but this would amount to complexifying the electronics of the processing unit.

SUMMARY OF THE INVENTION

The objective of the invention is to at least partially rectify the drawbacks of the prior art, and more particularly to provide an FMCW heterodyne-detection lidar imager system the distance resolution of which is improved without degrading the performance of the imager system or complexifying the processing electronics.

To do so, the subject of the invention is an FMCW lidar imager system, comprising:a coherent light source, which is configured to emit a frequency-modulated continuous-wave primary signal Sp;a splitting and recombining optical device, which is configured to split the primary signal Spinto a reference signal Srthat is directed toward a photodetector and into an object signal Sothat is directed toward the scene, which backscatters a portion of the object signal So, which portion is called the backscattered object signal Sor; and configured to direct toward the photodetector, along the same optical axis, the reference signal Srand a portion Sor,cof the backscattered object signal Sor, which portion is collected by a collecting optical element;the collecting optical element, which is configured to collect the portion Sor,cof the backscattered object signal Sor;the photodetector, which is intended to receive the reference signal Srand the collected portion Sor,cof the backscattered object signal Sor,c, which interfere to form a heterodyne signal Sh;a processing unit, which is configured to determine the distance zscof the scene on the basis of a beat frequency of the heterodyne signal Sh.

According to the invention, the imager system comprises a reflector configured to reflect, in the direction of the scene, a portion Sor,ncof the backscattered object signal Sor, which portion is not collected by the collecting optical element. Thus, the collected portion Sor,cof the backscattered object signal Soris formed from first light beams Sor,c(2)that have not been reflected by the reflector and from second light beams Sor,c(2)that have been reflected by the reflector and then by the scene. The heterodyne signal Shtherefore has a principal component Sh(1)associated with said first light beams Sor,c(1), and a secondary component Sh(2)associated with said second light beams Sor,c(2). The processing unit is configured to determine the distance zscof the scene on the basis of a beat frequency fb(2)of the secondary component Sh(2)of the heterodyne signal Sh.

Let us note here that the second light beams Sor,c(2), having been reflected by the reflector, belong to the collected part Sor,cof the backscattered object signal Sor, and that this backscattered object signal Soris a signal backscattered by the scene. It is then understood that the second light beams Sor,c(2)have been reflected by the scene before being collected.

The following are certain preferred but non-limiting aspects of this imager system.

The reflector may be retroreflective, so as to reflect incident light beams in the direction of the scene along an axis of reflection identical to their axis of incidence.

The reflector may have a lateral edge located at a maximum distance rmaxfrom an optical axis of the collecting optical element. It may be dimensioned so that the maximum distance rmaxis smaller than √(czsc/2B) when the reflector is retroreflective, where c is the speed of light in vacuum, and B is a variation in the frequency of the primary signal Spover one period T of the modulation. It may be dimensioned so that the maximum distance rmaxis smaller than √(czsc/6B) when the reflector is not retroreflective.

The reflector may be located in the plane of the collecting optical element.

The reflector may be located downstream of the collecting optical element, at the level of the photodetector.

In general, the reflector may be formed of a continuously reflective or retroreflective surface, or may be formed of non-contiguous reflective or retroreflective surfaces separated from each other by a surface transparent or reflective to the wavelength of the optical signals of interest.

The reflector may be located upstream of the collecting optical element and have a collection optical axis passing through it, the reflector then being formed from reflective or retroreflective areas that are separate from one another and that are encircled by an area that is transparent to the wavelength of the primary signal Sp.

The reflector may comprise a central area that is passed through by the collection optical axis, in which central area reflective or retroreflective areas that are separate from one another and that are encircled by a transparent area are formed, and a peripheral area that encircles the central area, in which peripheral area the reflective or retroreflective areas are joined to one another.

The imager system may be configured to illuminate only one point of the scene. As a variant, it may be configured to simultaneously illuminate a plurality of points of the scene and then to comprise an optical device for projecting the object signal Soonto the scene with a view to simultaneously illuminating the plurality of points of the scene, and an imaging optical device configured to form an image of the illuminated scene in the plane of the photodetector.

The processing unit may determine the beat frequency fb(2)by counting the oscillations in the second component Sor,c(2)of the heterodyne signal Sh.

The processing unit may be configured to determine a beat frequency fb(1)of the principal component Sh(1)of the heterodyne signal Sh, then to apply, to the heterodyne signal Sh, a bandpass filter that excludes the determined beat frequency fb(1)with a view to obtaining the secondary component Sh(2), and lastly to determine the beat frequency fb(2)of the secondary component Sh(2). The beat frequency fb(1)of the principal component Sh(1)of the heterodyne signal Shmay be determined by counting the oscillations in the heterodyne signal Sh.

The processing unit may determine the beat frequency fb(2)via Fourier transform applied to the heterodyne signal Sh.

The imager system may have what is called a mono-static configuration in which an optical axis of illumination of the scene by the object signal Sois identical to a collection optical axis of the collecting optical element. It may then comprise a mirror toward which the reference signal Sris directed by a splitting optical element of the splitting and recombining optical device.

The imager system may have what is called a bi-static configuration in which an optical axis of illumination of the scene by the object signal Sois different from a collection optical axis of the collecting optical element.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

In the figures and in the remainder of the description, the same references have been used to designate identical or similar elements. In addition, the various elements are not shown to scale for the sake of clarity of the figures. Moreover, the various embodiments and variants are not mutually exclusive and may be combined with one another. Unless indicated otherwise, the terms “substantially”, “about” and “of the order of” mean to within 10%, and preferably to within 5%. Moreover, the terms “comprised between . . . and . . . ” and equivalents mean inclusive of limits, unless indicated otherwise.

The invention relates to a frequency-modulated continuous wave (FMCW) heterodyne-detection lidar imager system allowing a distance zscof a scene, or even a distance map zsc(i,j)(distance image), to be determined with an improved distance resolution Δzsc.

The imager system is called a lidar imager system (lidar being the acronym of light detection and ranging) in that a coherent optical signal is used to determine a distance zscof a point of the scene or a distance map zsc(i,j)of the scene. In the context of the invention, the imager system may be either of single-point type, in the sense that the object signal illuminates only one point of the scene, possibly with a spatial scan of the scene by the object signal, or of flash type, in the sense that the object signal simultaneously illuminates a plurality of points of the scene and in the sense that the imager system acquires the image of the scene with a view to determining a distance map therefrom.

In addition, the imager system is said to be a heterodyne-detection imager system in that, to determine the distance of the illuminated scene, a so-called beat frequency of a heterodyne signal formed by interference between a reference signal of a local oscillator and a signal backscattered by the illuminated scene is determined, these two optical signals being mutually coherent. Specifically, the reference signal and the signal projected onto the scene both originate from the same primary optical signal emitted by the optical source. Finally, the heterodyne detection is FMCW heterodyne detection in that the primary optical signal is a frequency-modulated, continuous-wave signal.

FIGS.2A and2Bare schematic and partial views of an FMCW heterodyne-detection lidar imager system according to some variant embodiments, in which one corresponds to a single-point imager system, and the other corresponds to a flash imager system. The figures are highly schematic: the scene is planar here, but in fact it may obviously not be.

Generally, the imager system1comprises at least:coherent light source10, which is configured to emit a coherent, frequency-modulated, continuous-wave primary signal Sp;splitting and recombining optical device20, comprising:at least one splitting optical element21configured to split the primary signal Spinto a reference signal Srdirected toward a photodetector50and into an object signal Sodirected toward the scene2, which backscatters a portion of the object signal So, which portion is called the backscattered object signal Sor;at least one recombining optical element23configured to direct toward the photodetector50, along the same optical axis, the reference signal Srand a portion Sor,cof the backscattered object signal Sor, which portion is collected or intended to be collected by a collecting optical element41;a collecting optical element41, which is configured to collect the portion Sor,cof the backscattered object signal Sor;the photodetector50, which is intended to receive the reference signal Srand the collected portion Sor,cof the backscattered object signal Sor, which interfere to form a heterodyne signal Sh;a processing unit60, which is configured to determine a distance zscof the scene2on the basis of a beat frequency of the heterodyne signal Sh.

To improve the distance resolution of the imager system1, the latter further comprises a reflector42configured to reflect in the direction of the scene2a portion Sor,ncof the backscattered object signal Sor, which portion is not collected by the collecting optical element41. Thus, the collected portion Sor,cof the backscattered object signal Soris formed from first light beams Sor,c(1)of the backscattered object signal Sorthat have not been reflected by the reflector42and from second light beams Sor,c(2)of the backscattered object signal Sorthat have been reflected by the reflector42. As a result, the heterodyne signal Shhas a principal component Sh(1)associated with the first light beams Sor,c(1), and a secondary component Sh(2)associated with the second light beams Sor,c(2). The principal component Sh(1)has a beat frequency fb(1), and the secondary component Sh(2)has a beat frequency fb(2)different from fb(1).

In addition, the processing unit60is configured to determine the distance zscof the scene2on the basis of the beat frequency fb(2)of the secondary component Sh(2)of the heterodyne signal Sh. On so doing, as described in detail below, the distance zscis determined with a distance resolution Δzscthat is improved by a factor of 2 with respect to the conventional situation in which the distance zscis determined on the basis of the beat frequency fb(1)alone.

FIG.2Aillustrates an imager system1according to a single-point embodiment. Thus, the object signal Soilluminates only one point of the scene2at a time, and the photodetector50may be a photodiode. The imager system1is therefore configured to determine the distance zscof the illuminated point of the scene2. It should be noted that the imager system1may be configured to spatially scan the scene2with the object signal So, point by point.

The imager system1comprises an optical source10of what is called a primary signal Sp, which is coherent, frequency-modulated and continuous-wave. The optical source10is preferably a laser source that emits the primary signal Sp. By way of example, the primary signal Spmay have an optical frequency located in the infrared. In addition, the primary signal Spis frequency-modulated, for example linearly here, from a starting frequency f0over a repetition period T with a bandwidth B (chirp). The signal is here a chirped signal, that is to say a sinusoidal wave the instantaneous frequency of which varies linearly over time.

The optical source10has a coherence length that is typically longer than the optical path difference between the reference channel and the object channel. The reference channel is the path followed by the reference signal Srbetween the optical source10and the photodetector50. The object channel is the path followed by the object signal Sofrom the optical source10to the scene2, and the path followed by the object signal Sorbackscattered by the scene to the photodetector50. This optical path difference may correspond, to the first order, to twice the maximum distance between the imager system1and the scene2.

The optical source10may thus comprise, in the case of emission in the near-infrared region (between 0.7 and 2 μm), a VCSEL source11(VCSEL standing for vertical-cavity surface-emitting laser), VCSELs generally having a coherence length of the order of one meter, or even an EEL source11(EEL standing for edge-emitting laser), which may have a coherence length of the order of around ten or even one hundred meters.

The imager system1comprises a splitting/recombining optical device20. The latter comprises at least one splitting optical element21configured to split the primary signal Spinto an object signal Soon the one hand and into a reference signal Sron the other hand. The reference signal Srcorresponds, in the context of heterodyne detection, to the signal of a local oscillator (LO). It also comprises at least one recombining optical element23configured to direct toward the photodetector50, along the same optical axis, in such a way as to spatially superpose them at least partially, the reference signal Srand a collected portion Sor,cof the backscattered object signal Sor.

The splitting optical element21may be, for example, a semi-reflective plate or a splitter cube. Here, a semi-reflective plate21transmits a portion of the primary signal Sp, which becomes the object signal So, and reflects a portion of the primary signal Sp, which becomes the reference signal Sr. The distribution of intensity between the object signal Soand the reference signal Sris preferably not equal, and may thus be 90% to the object signal Soand 10% to the reference signal Sp.

The recombining optical element23is therefore configured to direct in the direction of the photodetector50, along the same optical axis, the backscattered optical signal Sorand the reference signal Sr. It may be a question of a semi-reflective plate or of a combiner cube. Here, a semi-reflective plate23reflects the reference signal Srin the direction of the photodetector50, along an optical axis passing through the center of the semi-reflective plate23and through the center of the photodetector50, and transmits the backscattered object signal Sor,calong the same optical axis. The two optical signals therefore propagate in the direction of the matrix-array photodetector50over a common channel, along the same optical axis.

The optical device20is configured to ensure an (at least partial) spatial superposition of the two optical signals Srand Soralong the same optical axis, thus improving the combination of the two optical signals via interference, this allowing the amplitude of the heterodyne signal to be improved Sh. To do so, optical elements for shaping the optical signals (which elements are not shown here) may be provided, as is described in the aforementioned document WO2021/144357A1.

The imager system1comprises at least one optical element41for collecting a portion Sor,cof the backscattered object signal Sor, this portion Sor,cthen being received by the photodetector50. It may be a free-space optical element, and it may be an aperture diaphragm that defines the physical pupil. The aperture diaphragm may be defined by the outline of a focusing lens. The collecting optical element may, moreover, be formed from a plurality of lenses between which the aperture diaphragm is placed. The collecting optical element41may be located upstream or downstream of the recombining optical element23. As a variant, the collecting optical element41may not be a dedicated optical object, but instead be defined by the sensing area of the photodetector50, notably in the case of a single-point imager system1.

According to the invention, the imager system1comprises a reflector42configured to reflect, in the direction of the scene2, a portion, denoted Sor,nc, of the backscattered object signal Sor, which portion has not been collected by the collecting optical element41. The reflector42is here a specular reflector, in the sense that it reflects the light beams non-diffusely or almost non-diffusely. The reflector42may be located at the collecting optical element41, for example by being coplanar therewith, or be located upstream or downstream. As described below, it may also be located by the photodetector50.

As described below, the reflector42may be simply reflective, i.e. reflect incident light beams in accordance with Snell's law of reflection, or be retroreflective, i.e. the light beams are reflected along an axis of reflection identical to the axis of incidence. In this regard, the reflector42may be a corner-cube retroreflector or a layer of microbeads, as notably described in document WO2015/158999A1.

The recombining optical element23may be located upstream or downstream of the collecting optical element41and of the reflector42. In the case where it is located upstream, the reference signal Sris preferably oriented so as to pass through only the collecting optical element41and not to be incident on the reflector42. It is thus not reflected by the reflector42, as if it were it could induce additional interference with a backscattered object signal Sorat beat frequencies different from the frequencies fb(1)and fb(2). Moreover, placing the recombining optical element23upstream allows the collecting optical element41and the reflector42to be placed as close as possible to the photodetector50, this increasing the field of view of the imager system1.

The imager system1further comprises a photodetector50, which is here a photodiode (or for example a pair of balanced photodiodes) insofar as the imager system1is of ‘single-point’ type. It receives the reference signal Srand the collected portion Sor,cof the backscattered object signal Sor, which signals interfere with each other to form a heterodyne signal Shthat has a principal beat frequency fb(1)and, as described in detail below, a secondary beat frequency fb(2).

The imager system1comprises a processing unit60configured to determine the distance zscof the illuminated point of the scene2on the basis of a beat frequency of the heterodyne signal Shreceived by the photodetector50, and more precisely on the basis of the beat frequency fb(2).

Before detailing the operation of the imager system1and expounding the improvement in distance resolution Δzsc, it will be noted that the invention also covers the flash-mode configuration of the imager system1.

In this regard,FIG.2Billustrates such an imager system1, this imager system being similar to that ofFIG.1, but differing therefrom in that it comprises a reflector42configured to reflect, in the direction of the scene2, a portion Sor,ncof the backscattered object signal Sor, which portion is not collected by the collecting optical element41, and in that the processing unit60is configured to determine the distance zscof the illuminated scene2on the basis of the secondary beat frequency fb(2)of the heterodyne signal Sh.

Unlike the imager system1ofFIG.2A, the imager system1according to this variant comprises an optical device30for projecting the object signal Soin the direction of the scene so as to illuminate all of it simultaneously. Moreover, the optical device20also comprises at least one shaping optical element22, which is located on the optical path of the reference signal Srbetween the splitting optical element21and the recombining optical element23. This allows the light beam of the reference signal Srto be shaped with a view to improving its spatial superposition with the light beam of the collected portion Sor,cof the backscattered object signal Sor. It also comprises an imaging optical device40configured to transmit the portion Sor,cof the backscattered object signal Sorand to form the image of the illuminated scene2in the detection plane of the photodetector50. These optical elements and devices are similar to those described in document WO2021/144357A1 and are therefore not described in detail here.

Lastly, the photodetector50is a matrix-array photodetector, and comprises a matrix array of detection pixels lying in a reception plane. It may be a CMOS photodetector (or even a CCD photodetector). The reception plane of the matrix-array photodetector50is located in a conjugate plane of the scene by the imaging optical device40(to within the depth of field). In other words, the image of the scene2is formed in the reception plane of the matrix-array photodetector50. Each detection pixel is intended to receive the heterodyne signal Sh.

The operation of the imager system1will now be described with reference toFIGS.3A and3B, whereFIG.3Ais a schematic and partial view of an imager system1similar to that ofFIG.2A, allowing the various optical signals that are present to be seen, and whereFIG.3Billustrates the variation as a function of time in the frequency of various optical signals, allowing the principal beat frequency fb(1)and secondary beat frequency fb(2)to be seen.

The optical source10emits the coherent, frequency-modulated and continuous-wave primary signal Sp, a portion (object signal So) of which is transmitted by the splitting optical element21in the direction of the scene2with a view to illuminating one point. A portion of the primary signal Spis directed in the direction of the photodetector50and forms the reference signal Sr.

The scene2backscatters a portion of the object signal Sowhich then forms the backscattered object signal Sor. The latter comprises a portion Sor,cthat is collected by the collecting optical element41, and a portion Sor,ncthat is not collected by the collecting optical element41. The not collected portion Sor,ncmay however be reflected by the reflector42in the direction of the scene2, thereby forming, in return, the reflected signal Sor,r, a portion of which is then backscattered again by the scene2and then collected by the collecting optical element41.

As a result, the collecting optical element41collects a portion Sor,cof the backscattered object signal Sor, which portion is formed from light beams Sor,c(1)that have been collected directly without having been reflected by the reflector42(and which is called the ‘first echo’ below), and from light beams Sor,c(2)that have been reflected by the reflector42before subsequently being collected (and which is called the ‘second echo’ below).

The recombining optical element23thus receives the collected portion Sor,cof the backscattered object signal Sorand the reference signal Sr. It will be noted that the light beams that have been or will be collected by the collecting optical element41are called the collected portion Sor,c. The signals Sor,cand Srare directed in the direction of the photodetector50along the same optical axis and in such a way as to be at least partially superposed with each other. They interfere with each other and form the heterodyne signal Sh.

As a result thereof the received heterodyne signal Shcomprises a principal component Sh(1)associated with the first echo Sor,c(1)and having a beat frequency fb(1), and a secondary component Sh(2)associated with the second echo Sor,c(2)and having a beat frequency fb(2)different from fb(1). The secondary component Sh(2)has an amplitude that is generally smaller than that of the principal component Sh(1), insofar as the diffuse reflectance of the scene is lower than 1, and insofar as the reflector42does not collect all of the light backscattered by the scene and not collected by the collecting optical element41.

The beat frequency fb(1)between the reference signal Srand the first echo foc,r(1)is equal to: fb(1)=τ(1)B/T=((2zsc−zr)/c)×(B/T). The beat frequency fb(2)between the reference signal Srand the second echo foc,r(2)is equal to: fb(2)=τ(2)B/T=((4zsc−zr)/c)×(B/T). Thus, if the distance zrof the reference channel is neglected, the frequency fb(2)is equal to 2×fb(1). As for the distance resolution Δzsc, it is independent of the distance zr, and is equal to c/2B when it is defined using the beat frequency fb(1), but is equal to c/4B when it is defined using the beat frequency fb(2). Thus, the distance resolution Δzscin the context of the invention is improved by a factor of 2 with respect to the conventional situation in which the distance resolution is based only on use of the beat frequency fb(1).

The processing unit60then determines the beat frequency fb(2)of the secondary component Sh(2)of the detected heterodyne signal Sh, and then deduces therefrom the distance zscof the scene2. As indicated above, the beat frequency fb(2)may be determined in the time domain by counting the number of oscillations in the heterodyne signal over the period T, or in the frequency domain via fast Fourier transform.

In the case of a method for determining beat frequency by counting oscillations, one approach consists in firstly determining the beat frequency fb(1)of the principal component Sh(1)of the heterodyne signal, by counting the oscillations in the received heterodyne signal Sh. Subsequently, electronic bandpass filtering is applied to the heterodyne signal Shin a spectral band excluding the beat frequency fb(1)and containing the beat frequency fb(2)of the secondary component Sh(2)of the heterodyne signal. It is then possible to determine the beat frequency fb(2)by counting the oscillations in the filtered heterodyne signal. It will be noted that the electronic filter may be fixed or adjustable depending on the distance range of the scene. When the spectral band is fixed and predefined, it is then not necessary to determine the value of the beat frequency fb(1). In contrast, determining the value of the beat frequency fb(1)beforehand allows a narrower filter bandwidth and therefore a more precise measurement of the frequency fb(2)to be achieved, and allows a broader spectral range to be measured.

It will be noted that the received heterodyne signal Shmay also comprise an additional component Sh(3)associated with interference between the first echo Sor,c(1)and the second echo Sor,c(2), with a beat frequency fb(3). The amplitude of this component Sh(3)may be comparable to that of the component Sh(1)in particular when the amplitude of the reference signal Srand the amplitude of the first echo Sor,c(1)are similar. The beat frequency fb(3)is proportional to 2zsc, this being close to fb(1)which is proportional to 2zsc−zr. Thus, the electronic filtering preferably blocks at least the two beat frequencies fb(1)and fb(3), for example by blocking a continuous spectral band including the values fb(1)and fb(3).

In the case of a method for determining beat frequency via fast Fourier transform (FFT), the processing unit performs an FFT of the received heterodyne signal Sh, then identifies the peak of the beat frequency fb(1)and the peak of the beat frequency fb(2). This spectral analysis may be performed by an algorithm or an electrical circuit. As indicated above, a portion of the electrical circuit of the processing unit may be located in the detection pixels, or be located in a unit separate from these pixels, which unit may be integrated into a structure comprising the matrix-array photodetector or be remotely located in a computer.

Next, knowing the value of the beat frequency fb(2), the processing unit60determines the distance zscof the scene using the relationship zsc=cTfb(2)/4B, with a distance resolution Δzsc=c/4B, which distance resolution is improved by a factor of 2 with respect to that associated with the beat frequency fb(1). Specifically, the measurement is here based on a double there-and-back trip of the object signal and not on a single there-and-back trip.

Thus, the imager system1according to the invention has the advantage of determining the distance zscof the scene2with a distance resolution Δzscimproved by a factor of 2, both in single-point mode and in flash mode. This is obtained by collecting the second echo Sor,c(2)of the backscattered object signal Sor, which was previously reflected by the reflector42then backscattered again by the scene2, and by exploiting the beat frequency fb(2)of the component Sh(2)of the heterodyne signal Sh. Thus, the chirp B of the primary signal Spis not modified, this avoiding any increase in the consumption of the imager system1and any degradation in its performance (because of the non-linearity of the chirp or of a modulation of the optical power emitted by the optical source10). In addition, the complexity of the electronics that the processing unit60employs to detect the beat frequencies fb(1)and fb(2)is not significantly increased.

A concrete example of determination of the distance zscwith an improved resolution Δzsc, on the basis of measurement of the beat frequency fb(2)of the secondary component Sh(2)of the heterodyne signal Sh, will now be described with reference toFIGS.4A to4C. In this example, the primary signal Sphas a chirp B equal to 7.5 GHz, a period T equal to 30 ms, a wavelength λ of 633 nm, and the scene is located at a distance zscof 50 cm, the path length zrof the reference signal Srbeing negligible with respect to the distance zsc. The amplitude of the first echo Sor,c(1)is considered to be equal to 1% of the amplitude of the reference signal Sr, and the amplitude of the second echo Sor,c(2)is considered to be equal to 0.09% of the amplitude of the reference signal Sr. Lastly, the various sources of noise in the imager system are not taken into account, insofar as the objective of this example is to illustrate the improvement in the distance resolution Δzsc.FIG.4Aillustrates the intensity Ihof the heterodyne signal Sh(t) received and detected by the photodetector50. It will be noted that the signal is not a perfect sinusoid since it is the result of interference between 3 waves and not of interference between 2 waves.

In the case where the method of counting oscillations is used to determine the beat frequency fb(2), the number N(1)of oscillations in the heterodyne signal Sh(t) is counted inasmuch as it is equal to the number of oscillations in the principal component Sh(1). In this example, N(1)is counted equal to 24 over a period T, and it is deduced therefrom that the principal beat frequency fb(1)=N(1)/T=800 Hz. Next, an electronic bandpass filter that excludes the frequency fb(1)is applied to the heterodyne signal Sh(t), and the filtered signal is obtained.FIG.4Billustrates the intensity Ihfof the obtained filtered signal. This corresponds to the secondary component Sh(2)(t) associated with the second echo Sor,c(2). The number N(2)of oscillations in the component Sh(2)(t) is then counted, it here being equal to 49 over a period T, this corresponding to a secondary beat frequency of about fb(2)=N(2)/T=1633 Hz. It is then possible to determine the distance of the scene, which is here equal to 49 cm, using the relationship zsc=N(2)c/4B, this distance zscbeing computed with a resolution Δzsc=c/4B equal to only 1 cm.

In the case where the FFT method is used to determine the beat frequency fb(2), an FFT is performed on the detected heterodyne signal Sh(t) illustrated inFIG.4A, the obtained spectrum of which is illustrated inFIG.4C. The latter indeed shows the principal frequency fb(1)and the secondary frequency fb(2). Here the secondary frequency fb(2)is equal to about 1633 Hz, this allowing the distance of the scene, which is equal to 49 cm, to be determined, using the relationship zsc=fb(2)cT/4B, with a distance resolution Δzsc=c/4B also equal to 1 cm.

A comparison of the determination of the distance zscwith a resolution Δzscin the case of an approach according to the prior art (i.e. based on the beat frequency fb(1)) and in the case of the invention (i.e. based on the beat frequency fb(2)) will now be described, with reference toFIGS.5A to5D, for two objects, one of which is located at a distance of 50 cm and the other of which is located at a distance of 51.5 cm.

In the case of an imager system according to the prior art, the imaging optical device comprises no reflector according to the invention, and hence the backscattered and collected object signal Sorcomprises only beams (first echo) that have made a single there-and-back trip with respect to the scene2. Thus, the detected heterodyne signals (cf.FIG.5Afor the object located at 50 cm, andFIG.5Bfor the object located at 51.5 cm), are perfect sinusoids generated by interference between 2 waves (the reference signal Srand the backscattered object signal Sor). In the case of the object at 50 cm, a number N of 24 oscillations is counted, i.e. a distance zsc=Nc/2B of 48 cm with a resolution Δzsc=c/2B of 2 cm. In the case of the object at 51.5 cm, a number N of 24 oscillations is counted, i.e. a distance zscof 48 cm with a resolution Δzscof 2 cm. Hence, unsurprisingly, the imager system according to this prior-art example is not able to discriminate between two objects separated by a distance smaller than its distance resolution.

In the case of the imaging system according to the invention, the imaging optical device40comprises the reflector44, and hence the collected backscattered object signal Sorcomprises the first echo Sor,c(1)formed from beams that have made a single there-and-back trip with respect to the scene, and the second echo Sor,c(2)formed from beams that have made a double there-and-back trip. Thus, the detected heterodyne signal Sh(t) is filtered and the second component Sh(2)(t) obtained (cf.FIG.5Cfor the object located at 50 cm, andFIG.5Dfor the object located at 51.5 cm). In the case of the object at 50 cm, a number N(2)of 49 oscillations is counted, i.e. a distance zsc=N(2)c/4B of 49 cm with a resolution Δzsc=c/4B of 1 cm. In the case of the object at 51.5 cm, a number N(2)of 50 oscillations is counted, i.e. a distance zscof 50 cm again with a resolution Δzscof 1 cm. Thus, the improvement in the distance resolution Δzscachieved by collecting backscattered-object-signal beams that have made a double there-and-back trip with respect to the scene (second echo) indeed allows two objects separated by a small distance to be discriminated between (or a small variation in the distance of the same object between two successive measurement times to be detected).

According to one embodiment, the amplitude of the reference signal Srmay be adjusted to facilitate the determination of the frequency fb(2). Thus, in the case where the frequency fb(2)is determined using the method of counting oscillations, it is advantageous to optimize the contrast of the oscillations in the component Sh(2)of the heterodyne signal Sh, and therefore to adjust the amplitude of the reference signal Srto a value substantially equal to the amplitude of the second echo Sor,c(2). It is also possible to maximize the amplitude of the oscillations in the component Sh(2)by increasing the amplitude of the reference signal Sr, for example via a servocontrol loop that acts on the amplitude of the reference signal Sr. Care will be taken not to saturate the heterodyne signal Shreceived and detected by the photodetector50(i.e. the unfiltered detected total signal comprising all the frequency components).

FIGS.6A to6Dare schematic views of the reflector42according to various variant embodiments.

The reflector42may be located at the collecting optical element41, for example in a manner coplanar therewith, and have a ring (crown) shape that continuously encircles it (cf.FIG.6A). It may be formed from annular segments partially encircling the collecting optical element41. As a variant, it may have any shape (here a square shape) and be joined or not joined to the collecting optical element41(FIG.6B). In these examples, the reflector42is preferably formed of a continuously reflective or retroreflective surface.

The reflector42may not be located in the plane of the collecting optical element41, and may be located upstream or downstream thereof. For example, it may be located by the photodetector, i.e. in the detection plane or close to the detection plane, irrespectively of whether the photodetector50is a photodiode or a matrix array. In this regard,FIG.6Cillustrates the situation in which the photodetector is a matrix-array photodetector (flash imager system1) and comprises a matrix array of detection pixels51with a fill factor lower than one—the reflector42may be located in the non-photosensitive regions of the matrix-array of detection pixels51. In this particular case and when the reflector42is retroreflective, the corner-cube retroreflectors of the reflector42preferably have a lateral dimension very much greater than the wavelength, for example 10 times greater than the wavelength of the primary signal, so that the corner-cube retroreflectors do not diffract.

Moreover, the reflector42may be formed from a continuously reflective or retroreflective area, or be formed from reflective or retroreflective areas42.2that are not contiguous and that are separated from one another by an area42.1that is transparent or reflective to the wavelength of the optical signals of interest, as illustrated inFIG.6D. In the case where the area42.1is transparent, this embodiment is particularly advantageous when the reflector42is located upstream of the collecting optical element41and is passed through by the collection optical axis (cf.FIGS.10A and10B). In this regard, the reflector42may be formed from a central area that is passed through by the collection optical axis, this central area comprising the reflective or retroreflective areas42.2that are separated from one another and encircled by a transparent area42.1, and from a peripheral area that encircles the central area and in which the reflective or retroreflective areas42.2are joined to one another.

FIG.7Ais a schematic and partial view of an imager system1according to a single-point embodiment, in which the reflector42is retroreflective.FIG.7Billustrates a detail ofFIG.7Aallowing a condition on the maximum distance rmaxof the lateral edge of the reflector42with respect to the collection optical axis to be seen.

According to one embodiment, the reflector42is retroreflective, in the sense that incident light beams are reflected with an axis of reflection identical to the axis of incidence. Thus, the light beam backscattered by a point of the scene2and reflected by the retroreflective reflector42is returned to the same point of the scene2or to within its immediate proximity. Therefore, such a reflector42makes it possible not to mix, among the light beams of the collected signal Sor,c, light beams coming from a plurality of different points of the scene2, and therefore not to worsen the lateral resolution of the imager system1. In the case of a single-point imager system1, this allows the determination of the distance zscnot to be disrupted by light beams coming from objects located at other distances. Furthermore, in the case of a flash imager system1, this thus avoids worsening the quality or the spatial resolution of the determined distance map.

The reflector42is arranged facing the optical axis of the collecting optical element41, such that it has an outer lateral edge located at a maximum distance rmaxfrom this optical axis. Preferably, this maximum distance rmaxis smaller than √(czsc/2B), so as not to worsen the distance resolution Δzscof the imager system1. Specifically, as illustrated inFIG.7B, should a light beam of the not collected portion Sor,ncbe retroreflected at the edge of the reflector42, and then backscattered along the optical axis of the collecting optical element41, it would then cover a distance of zsc+√/(zsc2+rmax2), and not of 2zsc, which would cause an error of the order of rmax2/2zscunder the assumption that rmax»zsc. Therefore, this error rmax2/2zscshould advantageously be less than the distance resolution Δzsc, which is equal to c/4B, this leading to the condition rmax<√(czsc/2B). It will be noted that this condition is not very restrictive, insofar as rmax<10 cm is obtained for zsc=50 cm and B=7 GHz.

The reflector42has an area configured to maximize the number of photons of the signal Sor,ncthat are intercepted and then reflected, this accordingly increasing the amplitude of the second echo Sor,c(2)and therefore the distance range of the imager system1, without however decreasing the compactness of the reception module of the imager system1. Whatever the case may be, it is advantageous for the reflector42to have dimensions that meet the abovementioned condition on the maximum distance rmaxof the lateral edge.

FIG.8Ais a schematic and partial view of an imager system1according to one embodiment, in which the reflector42is not retroreflective, i.e. it is solely reflective.FIG.8Billustrates a detail ofFIG.8Aallowing a condition on the maximum distance rmaxof the lateral edge of the reflector42with respect to the collection optical axis to be seen.

A light beam incident on the reflector42is therefore not reflected along an axis of reflection identical to the axis of incidence, but obeys Snell's law of reflection. It is therefore reflected in the direction of a point different from the one from which the light beam of the portion Sor,ncof the backscattered signal Sororiginated. For a light beam that is reflected from the reflector42at the distance rmax, it will be noted that the distance between these two points of the scene is of the order of 2rmax(for a scene substantially orthogonal to the optical axis). As a result, the preceding condition on the distance rmaxof the outer lateral edge of the reflector42is modified and becomes: rmax<√(czsc/6B). This condition is still not very restrictive since rmax<6 cm is obtained in the case where zsc=50 cm and B=7 GHz. Whatever the case may be, it is preferable to reserve the use of a non-retroreflective reflector42for a single-point imager system1. Moreover, care will be taken to ensure that the angular separation between the two illuminated points of the scene2remains less than the individual-field-of-view (iFOV) angular resolution of the collecting optical element41, with the condition 2 atan (rmax/zsc)<iFOV, so as not to worsen lateral resolution. It will be noted that the field of view (FOV) of the collecting optical element41is the angle in which the photodetector50is sensitive to the portion Sret,cof the object signal Sretbackscattered through the collecting optical element41.

According to one embodiment, the imager system1may have what is called a mono-static configuration, in the sense that the optical axis of the illumination of the scene2and the collection optical axis are collinear. In this regard,FIG.9is a schematic and partial view of such an imager system1, here of single-point type, but this configuration may also be implemented for a flash imager system1. Moreover, the reflector42is here retroreflective.

This configuration is for example based on a Michelson-interferometer architecture. The imager system1comprises an additional mirror24placed on the optical path of the reference signal Sr, and the collecting optical element41(and the reflector42) is (are) located between the scene2and the splitting optical element21(and therefore upstream of the recombining optical element23).

Thus, the splitting optical element21splits the primary signal Spinto the object signal Sothat is transmitted in the direction of the scene, and into the reference signal Srthat is reflected in the direction of the additional mirror24. The latter reflects the reference signal Srin the direction of the recombining optical element23. A portion Sor,cof the backscattered object signal Soris collected by the collecting optical element41, then is reflected by the splitting optical element21in the direction of a steering mirror25. Thus, the splitting optical element21performs the function of recombining the reference signal Srand the collected portion Sor,cof the backscattered object signal Sor, and is denoted “21,23” in the figure.

The imager system1here has the advantage of facilitating the detection and the determination of the distance zscof objects having a high specular component, that is to say that light is backscattered by the object in question from the scene in a backscatter cone (angular distribution of the backscattered luminous intensity) centered on a principal direction, this direction possibly being close to the specular reflection direction. Specifically, in the case of a bi-static imager system1such as presented above (FIGS.2A-2BandFIG.6A-7A), the second echo Sor,c(2)could be backscattered mainly in the direction of the optical source10, and slightly in the direction of the collecting optical element41, thus reducing the amplitude of the collected signal Sor,c(2), and therefore the ability of the imager system1to detect this type of object in the scene2. This is not the case for a mono-static imager system1, insofar as the second echo is mainly returned toward the collecting optical element41and the reflector42(the angular indicatrix is narrow). Moreover, this mono-static configuration of an imager system1has the advantage of reducing or even eliminating shadowing effects in the determined distance map, by the very fact that these two optical axes are coincident.

According to one embodiment, the reflector42is reflective or retroreflective, and is formed from reflective or retroreflective areas that are not contiguous and that are separated from one another by a transparent area. The reflector42may then be located upstream of the collecting optical element41, in the sense that the collection optical axis passes through the reflector42. In this regard,FIGS.10A and10Bare schematic and partial views of such an imager system1, in a bi-static configuration (FIG.10A) and in a mono-static configuration (FIG.10B).

In the example ofFIG.10A, the imager system1is similar to that ofFIG.6Aand differs therefrom essentially in that the reflector42is not coplanar with the collecting optical element41but is located upstream thereof. Moreover, the collecting optical element41and the reflector42are here located downstream of the recombining optical element23but could be located upstream.

In the example ofFIG.10B, the imager system1is similar to that ofFIG.9and differs therefrom essentially in that the reflector42is also located upstream of the collecting optical element41. It will be noted that the optical element21(denoted “21, 23” in the figure) is both the splitting optical element21and the recombining optical element23.

An imager system1according to this embodiment has the advantage of decreasing the lateral bulk of the receiving module with respect to the examples ofFIGS.2A and2B.

Particular embodiments have just been described. Various variants and modifications will seem obvious to anyone skilled in the art. The imager system1may thus have a free-space configuration or a guided-optic configuration, as described in document WO2021/144357A1 mentioned above.