Scene distance imaging system

Abstract

A scene imaging system (1) comprising: - emission means (2) configured to emit frequency-modulated continuous-wave laser radiation (RL) along a propagation direction (DP); - an image sensor (3) comprising an array of photodetectors; - an optical component (4) arranged to incorporate first and second beam splitters (S1, S2) such that the first and second beam splitters (S1, S2) intersect at right angles along a line of intersection perpendicular to the propagation direction (DP), and such that the first and second beam splitters (S1, S2) are rotationally fixed; - processing means (5) configured to process a heterodyne signal from each photodetector resulting from the recombination of an object beam with a reference beam (F1), so as to obtain a distance image of the scene (1). Figure 4

Description

Title of the invention: Scene distance imaging system technical field

[0001] The invention relates to the technical field of distance imaging systems of a scene, more particularly of a three-dimensional (3D) scene, from an emission of coherent waves sustained and frequency modulated (FMCW for "Frequency-Modulated Continuon Wave" in English) with heterodyne detection.

[0002] The invention finds its application in particular in facial recognition for mobile phones, augmented reality, robotics, drones, logistics, industrial control etc. State of the art

[0003] A prior art 3D scene distance imaging system uses light detection and ranging (LiDAR) technology with a frequency-modulated continuous wave (FMCW) laser source. This technology is generally referred to by the acronym LiDAR FMCW.

[0004] There are classically two types of interferometric setups for implementing this technology: (i) bistatic setup (e.g., Mach-Zehnder type) where the field of view (FOV) and the field of emission (FOE) are misaligned; (ii) Monostatic mounting (e.g., Michelson type) where the FOE and FOV are aligned.

[0005] An example of a bistatic Mach-Zehnder type mounting is illustrated in [Fig. 1a]. The distance imaging system for scene 1 comprises: - emission means 2, configured to emit a frequency-modulated sustained-wave laser RL radiation along a propagation direction DP; - of the first and second optical components Cl, C2 separators; - of the first and second detectors D1, D2. In case (i), the overlap between the FOE and the FOV is not total due to their misalignment which varies according to the distance between the laser source 2 and the first detector DL ​​II. It should be noted that a bi-static setup allows the use of a second detector D2 with two channels to analyze the interference signal.

[0006] An example of a single-static Michelson-type setup is illustrated in [Fig. 1b]. The distance imaging system for a scene 1 comprises: - emission means 2, configured to emit a frequency-modulated sustained-wave laser RL radiation along a propagation direction DP; - first and second optical components Cl, C2, the first optical component Cl being a separating component, the second optical component C2 being a mirror; - a detector Dl. In case (ii), the overlap between the FOE and the FOV is complete due to their alignment. However, a monostatic setup does not allow the use of a second detector. There is only one channel for analyzing the interference signal.

[0007] In the context of heterodyne detection, the alignment between the FOE and the FOV is a critical parameter in order to illuminate the entire scene seen in the FOV and to accurately measure the frequency of the beat signal of the oscillations produced by the interferences.

[0008] Such prior art interferometric setups are not entirely satisfactory insofar as it is necessary to adjust at least two optical components to obtain and maintain precise alignment between the FOE and the FOV. In case (i), the two optical components can be two beam splitters or two beam splitters. In case (ii), the two optical components can be a beam splitter (or a beam splitter) and the reflecting mirror. Description of the invention

[0009] The invention aims to remedy, in whole or in part, the aforementioned drawbacks. To this end, the invention relates to a distance imaging system for a scene, comprising: - emission means, configured to emit a laser beam with frequency-modulated sustained waves along a propagation direction; - an image sensor, comprising a set of photodetectors; - a first separating surface, arranged to separate the laser radiation into: a first beam, called the reference beam, reflected by the first separating surface towards the image sensor, and into a second beam, transmitted through the first separating surface towards the stage; - a second separating surface, arranged to separate the second beam reflected by the stage into: a third beam, called the object beam, reflected by the second separating surface towards the image sensors, and in a fourth beam, transmitted through the second separating surface; - an optical component, arranged to incorporate the first and second separating surfaces such that the first and second separating surfaces intersect at right angles along a line of intersection perpendicular to the direction of propagation, and so that the first and second separating surfaces are rotationally fixed; - processing means, configured to process a heterodyne signal from each photodetector resulting from a recombination of the object beam with the reference beam, in order to obtain a distance image of the scene.

[0010] Thus, such an imaging system according to the invention makes it possible to automatically guarantee (without adjustment of optical components as in the prior art) alignment between the reference beam and the object beam (i.e., between the FOE and the FOV), thanks to the optical component incorporating the first and second beam splitters such that the first and second beam splitters are rotationally fixed. Such automatic alignment between the FOE and the FOV improves performance in terms of heterodyne efficiency, and thereby the quality of the heterodyne signal of the distance (or depth) image.

[0011] Moreover, such an imaging system according to the invention allows alignment of the FOE and the FOV on the same optical axis (unlike a Mach-Zehnder type setup) and the optional use of a second detector (unlike a Michelson type setup).

[0012] The imaging system according to the invention may include one or more of the following features.

[0013] According to a feature of the invention: - laser radiation has a transverse extension to the direction of propagation; - the optical component has a projection onto the transverse extension covering the entire transverse extension.

[0014] Thus, one advantage provided is to improve the performance of the system by avoiding parasitic diffraction phenomena at the edges of the optical component.

[0015] According to one feature of the invention, the imaging system includes an optical isolator arranged to isolate the emission means of the fourth beam.

[0016] Thus, one advantage provided is to improve the quality of the heterodyne signal.

[0017] According to one feature of the invention, the imaging system comprises a dioptric device, preferably a lens or an objective, arranged between the optical component and the scene so as to diverge the second beam transmitted by the first separating surface towards the scene, the dioptric device having an image focal plane in which the image sensor is arranged.

[0018] Thus, an advantage provided by the dioptric device is to diverge the second beam transmitted by the first separating surface towards the scene to obtain a wide FOE and to converge the image of the scene on the image sensor.

[0019] According to one feature of the invention, the imaging system comprises an additional image sensor including an array of additional photodetectors; the second beam splitter surface being arranged to split the laser radiation into: - a fifth beam, called the additional reference beam, reflected by the second beam splitter surface towards the additional image sensor, and into - a sixth beam, transmitted by the second beam splitter towards the stage; the first beam splitter being arranged to split the sixth beam reflected by the stage into: - a seventh beam, called the additional object beam, reflected by the first separating surface towards the additional image sensor, and in - an eighth beam, transmitted through the first separating surface; the processing means being configured to process a heterodyne signal from each additional photodetector resulting from a recombination of the additional object beam with the additional reference beam, so as to obtain a distance image of the scene.

[0020] Thus, one advantage provided is to provide a second way to analyze the heterodyne signal.

[0021] According to one feature of the invention, the optical isolator is arranged to isolate the emission means of the eighth beam.

[0022] Thus, one advantage provided is to improve the quality of the heterodyne signal received by each additional photodetector.

[0023] According to one feature of the invention, the dioptric device is arranged between the optical component and the scene so as to diverge the sixth beam transmitted by the second separating surface towards the scene, the additional image sensor being arranged in the image focal plane of the dioptric device.

[0024] Thus, an advantage provided by the dioptric device is to diverge the sixth beam transmitted by the second separating surface towards the scene to obtain a wide FOE and to converge the image of the scene on the additional image sensor.

[0025] According to one feature of the invention, the optical component comprises a set of optical prisms arranged so that their interfaces form the first and second separating surfaces.

[0026] Thus, one advantage provided by optical prisms is that they confer high stability to the system in order to obtain a high-quality heterodyne signal that is easily usable with an optimized beat frequency signal. In particular, the optical prisms are not affected internally by mechanical vibrations that could alter the relative positioning of the first and second beam splitters.

[0027] According to one feature of the invention, the optical component is a cube comprising four triangular optical prisms arranged to obtain an "X" shaped configuration.

[0028] Thus, one advantage of such a component is its ease of manufacture. It should be noted that the dichroic property of X-cubes (trade name) is not useful in the context of the invention. The four triangular optical prisms, arranged to obtain an "X"-shaped configuration, are advantageously free of dichroic coatings.

[0029] According to one feature of the invention, the first and second separating surfaces each have a reflection coefficient of 50% and a transmission coefficient of 50%.

[0030] Thus, one advantage provided is to maximize the intensity of the heterodyne signal.

[0031] Definitions

[0032] By "scene" we mean the set of objects, surfaces, textures etc. arranged in a three-dimensional space, which are captured by the imaging system.

[0033] By “separating surface”, we mean a surface designed to spatially separate an incident beam into two distinct beams.

[0034] By "incorporate," it is meant that the first and second separating surfaces are integrated within the optical component so as to obtain mechanical rigidity, particularly in rotation, between the optical component and the first and second separating surfaces. In other words, the first and second separating surfaces cannot be moved independently of the optical component.

[0035] By "intersect at right angles", it is meant that the planes defining the first and second separating surfaces form a dihedral angle of 90°.

[0036] By "line of intersection," we mean the line (more precisely, the line segment) shared by the two planes defining the first and second separating surfaces. In other words, the "line of intersection" is the common edge along which the two planes defining the first and second separating surfaces meet.

[0037] By "heterodyne signal" is meant a signal resulting from the superposition (interference) of two light waves of different frequencies, used to measure the frequency shifts between the reference beam and the object beam, and to deduce distance and velocity information from the scene. More precisely, the interference generates beats whose frequency corresponds to the difference between the frequencies of the two superimposed waves.

[0038] By "optical isolator" is meant an optical component designed to allow light to propagate in only one specific direction, while blocking its return in the opposite direction.

[0039] By “dioptric device”, we mean a device comprising a set of diopters enabling the propagation of light to be modified by refraction.

[0040] By "reflection coefficient", we mean a reflection coefficient in intensity at the wavelength of the laser radiation.

[0041] By "transmission coefficient" is meant a transmission coefficient in intensity at the wavelength of the laser radiation. Brief description of the drawings

[0042] Other features and advantages will become apparent in the detailed description of different embodiments of the invention, the description being accompanied by examples and references to the accompanying drawings.

[0043] [Fig. la] (already commented) is a synoptic diagram of an imaging system according to the prior art, using a Mach-Zehnder type bi-static setup.

[0044] [Fig.lb] (already commented on) is a synoptic diagram of an imaging system according to the prior art, using a monostatic Michelson type setup.

[0045] [Fig.2] is a synoptic diagram of an imaging system according to the invention, illustrating the forward path (i.e. upstream of the scene) of the laser radiation, according to an embodiment involving a single detector (one channel).

[0046] [Fig.3] is a synoptic diagram of an imaging system according to the invention, illustrating the return path (i.e. downstream of the scene) of the laser radiation, according to an embodiment involving a single detector (one channel).

[0047] [Fig.4] is a synoptic diagram of an imaging system according to the invention, illustrating the forward path (i.e. upstream of the scene) of the laser radiation, according to an embodiment involving two detectors (two paths).

[0048] [Fig.5] is a synoptic diagram of an imaging system according to the invention, illustrating the return path (i.e. upstream of the scene) of the laser radiation, according to an embodiment involving two detectors (two paths).

[0049] [Fig.6] is a schematic view illustrating an optical component used in an imaging system according to the invention.

[0050] It should be noted that the drawings described above are schematic, and are not necessarily to scale for the sake of readability and to simplify their understanding. Detailed description of the implementation methods

[0051] Identical elements or elements performing the same function shall bear the same references for the different embodiments, for the sake of simplification.

[0052] An object of the invention is a distance imaging system of a scene 1: - emission means 2, configured to emit a frequency-modulated sustained-wave laser RL radiation along a propagation direction DP; - an image sensor 3, comprising a set of photodetectors; - a first separating surface SI, arranged to separate the laser radiation RL into: a first beam Fl, called the reference beam, reflected by the first separating surface SI towards the image sensor 3, and in a second beam F2, transmitted by the first separating surface SI towards scene 1; - a second separating surface S2, arranged to separate the second beam reflected F2r by scene 1 into: a third beam F3, called the object beam, reflected by the second separating surface S2 towards the image sensors 3, and in a fourth beam F4, transmitted by the second separating surface S2; - an optical component 4, arranged to incorporate the first and second separating surfaces SI, S2 such that the first and second separating surfaces SI, S2 intersect at right angles along a line of intersection perpendicular to the propagation direction DP, and such that the first and second separating surfaces SI, S2 are rotationally fixed; - processing means 5, configured to process a heterodyne signal from each photodetector resulting from a recombination of the object beam F3 with the reference beam Fl, so as to obtain a distance image of the scene 1.

[0053] Emission means

[0054] The emission means 2 are configured to emit a frequency-modulated continuous-wave laser RL radiation along a propagation direction DP. The optical frequency of the laser RL radiation is preferably modulated with a periodic linear ramp.

[0055] The RL laser radiation has a transverse extension to the propagation direction DP.

[0056] By way of non-limiting examples, the emission means 2 comprise a laser source selected from an edge-emitting laser, a vertical cavity laser diode emitting from the surface, a quantum cascade laser.

[0057] By way of non-limiting examples, the wavelength of the RL laser radiation can be 850 nm (GaAs), 940 nm (InP), within the range 1.3 pm - 1.55 pm, within the range 3 pm - 5 pm, within the range 8 pm - 14 pm.

[0058] Image sensor(s)

[0059] The image sensor 3 comprises a set of photodetectors.

[0060] The imaging system may include an additional image sensor 3' comprising a set of additional photodetectors.

[0061] By way of non-limiting examples, the photodetectors can be chosen from photodiodes (possibly avalanche), micro-bolometers (for infrared).

[0062] Optical component

[0063] The optical component 4 incorporates the first and second separating surfaces SI, S2. In other words, the first and second separating surfaces SI, S2 are integrated within the optical component 4 so as to obtain mechanical solidarity, in particular rotational solidarity.

[0064] The first and second beam splitters SI, S2 are incorporated into the optical component 4 so as to intersect at right angles along a line of intersection perpendicular to the propagation direction DP of the laser beam RL. The optical component 4 is advantageously arranged so as to have a projection onto the transverse extent of the laser beam RL that completely covers said transverse extent. By way of non-limiting example, the projection of the optical component 4 onto the transverse extent of the laser beam RL may be between 3 mm and 1 cm.

[0065] The first separating surface SI is arranged to separate the laser radiation RL into: - a first beam Fl, called the reference beam, reflected by the first separating surface SI towards the image sensor 3, and in - a second beam F2, transmitted by the first separating surface SI towards scene 1.

[0066] The second separating surface S2 is arranged to separate the second beam reflected F2r by scene 1 into: - a third beam F3, called the object beam, reflected by the second separating surface S2 towards the image sensors 3, and in - a fourth beam F4, transmitted by the second separating surface S2.

[0067] If an additional image sensor 3' is present, the second separating surface S2 is arranged to separate the laser radiation RL into: - a fifth beam F5, called the additional reference beam, reflected by the second separating surface S2 towards the additional image sensor 3', and - a sixth beam F6, transmitted by the second separating surface S2 towards scene 1. If necessary, the first beam splitter SI is arranged to separate the sixth beam reflected F6r by scene 1 into: - a seventh beam F7, called the additional object beam, reflected by the first separating surface SI towards the additional image sensor 3', and in - an eighth beam F8, transmitted by the first separating surface SL

[0068] The optical component 4 advantageously comprises a set of optical prisms PI, P2, P3, P4 arranged such that their interfaces 112, 123, 134, 114 form the first and second beam splitters SI, S2. As illustrated in [Fig. 6], the optical component 4 may comprise four optical prisms, denoted PI, P2, P3, P4 in a clockwise direction. The first beam splitter SI is formed by the interface 112 between the first optical prism PI and the second optical prism P2 and by the interface 134 between the third optical prism P3 and the fourth optical prism P4. The second beam splitter S2 is formed by the interface 123 between the second optical prism P2 and the third optical prism P3 and by the interface 114 between the first optical prism PI and the fourth optical prism P4.

[0069] The optical component 4 is advantageously a cube C comprising four triangular optical prisms PI, P2, P3, P4 arranged in an "X" configuration. The first and second separating surfaces SI, S2 advantageously each have a reflection coefficient of 50% and a transmission coefficient of 50%.

[0070] Means of processing

[0071] The processing means 5 are configured to process a heterodyne signal from each photodetector resulting from a recombination of the object beam F3 with the reference beam Fl, so as to obtain a distance image of the scene 1. The processing means 5 are electrically connected to the image sensors 3, 3'. The processing means 5 can be electrically connected to the emitting means 2. However, it is possible to detect ramps without necessarily electrically connecting the processing means 5 to the emitting means 2.

[0072] The interference between the object beam F3 and the reference beam Fl produces beats whose frequency corresponds to the difference between the frequencies of the object beam F3 and the reference beam FL. More precisely, when the optical frequency of the laser radiation RL is modulated with a periodic linear ramp, the beat frequency of the oscillations, denoted fR, satisfies the following relationship: * R~ cT - "T" is the duration of the ramp, - "c" is the speed of light in a vacuum, - "B" is the excursion of the optical frequency ("chirp" in English) of the laser RL radiation during the duration "T" of the ramp, - “z” is a distance (depth) information on scene 1.

[0073] An approximation of "z" for the number of periods, denoted "N", measured during the duration "T" of the ramp can be deduced by the following formula (denoted Frm 1): Afc 2B The distance resolution, denoted oz, can be approximated using the following formula: Ox, ~ 25 It is therefore possible to obtain a distance map z=f(x, y) where "x" and "y" denote coordinates of scene 1 and "f" denotes a mathematical function.

[0074] In the event of the presence of an additional image sensor 3', the processing means 5 are advantageously configured to process a heterodyne signal from each additional photodetector resulting from a recombination of the additional object beam F7 with the additional reference beam F5, so as to obtain a distance image of scene 1.

[0075] By way of non-limiting example, the processing means include a processor configured to calculate "z" from the formula "Frm 1" explained above.

[0076] Such means of treatment 5 are known to a person skilled in the art.

[0077] Dioptric device

[0078] The imaging system advantageously includes a dioptric device 6 arranged between the optical component 4 and the scene 1 so as to diverge the second beam F2 transmitted by the first separating surface SI towards the scene 1.

[0079] The dioptric device 6 has an image focal plane in which the image sensor 3 is arranged. The focal length is chosen so as to be less than the distance between the optical center of the dioptric device 6 and the scene 1.

[0080] The dioptric device 6 is preferably a lens or an objective.

[0081] In the presence of an additional image sensor 3', the dioptric device 6 is advantageously arranged between the optical component 4 and the scene 1 so as to diverge the sixth beam F6 transmitted by the second separating surface S2 towards the scene 1. The additional image sensor 3' is then advantageously arranged in the image focal plane of the dioptric device 6.

[0082] Optical isolator

[0083] The imaging system advantageously comprises an optical isolator 20 arranged to isolate the emission means 2 from the fourth beam F4. In other words, the fourth beam F4, transmitted by the second separating surface S2, is isolated from the laser radiation RL by the optical isolator 20.

[0084] In the presence of an additional image sensor 3', the optical isolator 20 is advantageously arranged to isolate the emission means 2 from the eighth beam F8. In other words, the eighth beam F8, transmitted by the first separating surface SI, is isolated from the laser radiation RL by the optical isolator 20.

[0085] By way of non-limiting example, the optical isolator 20 may be a Faraday type isolator.

[0086] The invention is not limited to the embodiments described. A person skilled in the art is able to consider their technically operative combinations, and to substitute equivalents for them.

Claims

Demands

1. A scene (1) distance imaging system comprising: - emission means (2), configured to emit frequency-modulated continuous wave laser (RL) radiation along a propagation direction (DP); - an image sensor (3), comprising an array of photodetectors; - a first beam splitter (SI), arranged to split the laser (RL) radiation into: a first beam (F1), called the reference beam, reflected by the first beam splitter (SI) towards the image sensor (3), and a second beam (F2), transmitted by the first beam splitter (SI) towards the scene (1); - a second separating surface (S2), arranged to separate the second beam reflected (F2r) by the scene (1) into: a third beam (F3), called the object beam, reflected by the second separating surface (S2) towards the image sensors (3), and into a fourth beam (F4), transmitted by the second separating surface (S2);- an optical component (4), arranged to incorporate the first and second separating surfaces (SI, S2) such that the first and second separating surfaces (SI, S2) intersect at right angles along a line of intersection perpendicular to the direction of propagation (DP), and such that the first and second separating surfaces (SI, S2) are rotationally fixed; - processing means (5), configured to process a heterodyne signal from each photodetector resulting from a recombination of the object beam (F3) with the reference beam (F1), so as to obtain a distance image of the scene (1).

2. System according to claim 1, wherein: - the laser beam (RL) has a transverse extension to the direction of propagation (DP); - the optical component (4) has a projection onto the transverse extension completely covering the transverse extension.

3. System according to claim 1 or 2, comprising an optical isolator (20) arranged to isolate the emission means (2) from the fourth beam (F4).

4. System according to any one of claims 1 to 3, comprising a dioptric device (6), preferably a lens or objective, arranged between the optical component (4) and the scene (1) so as to diverge the second beam (F2) transmitted by the first separating surface (SI) towards the scene (1), the dioptric device (6) having an image focal plane in which the image sensor (3) is arranged.

5. A system according to any one of claims 1 to 4, comprising an additional image sensor (3') including an array of additional photodetectors; the second beam splitter (S2) being arranged to split the laser radiation (RL) into: - a fifth beam (F5), called the additional reference beam, reflected by the second beam splitter (S2) towards the additional image sensor (3'), and into - a sixth beam (F6), transmitted by the second beam splitter (S2) towards the scene (1); the first beam splitter (SI) being arranged to split the sixth beam reflected (F6r) by the scene (1) into: - a seventh beam (F7), called the additional object beam, reflected by the first beam splitter (SI) towards the additional image sensor (3'), and into - an eighth beam (F8), transmitted by the first beam splitter (SI);the processing means (5) being configured to process a heterodyne signal from each additional photodetector resulting from a recombination of the additional object beam (F7) with the additional reference beam (F5), so as to obtain a distance image of the scene (1).;

6. System according to claim 5 in combination with claim 3, wherein the optical isolator (20) is arranged to isolate the emission means (2) from the eighth beam (F8).

7. A system according to claim 5 or 6 in combination with claim 4, wherein the dioptric device (6) is arranged between the optical component (4) and the stage (1) so as to diverge the sixth beam (F6) transmitted by the second separating surface (S2) towards the scene (1), the additional image sensor (3') being arranged in the image focal plane of the dioptric device (6).

8. System according to any one of claims 1 to 7, wherein the optical component (4) comprises a set of optical prisms (PI, P2, P3, P4) arranged so that their interfaces (112,123,134, 114) form the first and second separating surfaces (SI, S2).

9. System according to claim 8, wherein the optical component (4) is a cube (C) comprising four triangular optical prisms (PI, P2, P3, P4) arranged to obtain an "X" shaped configuration.

10. System according to any one of claims 1 to 9, wherein the first and second separating surfaces (SI, S2) each have a reflection coefficient of 50% and a transmission coefficient of 50%.

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