PATENT DOCUMENT

Publication Number: US-11480685-B2
Application Number: US-202016805888-A
Country: US
Kind Code: B2

Title: Compact optical packaging of LiDAR systems using diffractive structures behind angled interfaces

Abstract:
Sensing apparatus includes a transparent window and a LiDAR assembly, including a beam source, which is configured to emit one or more beams of optical radiation along a beam axis, and which is configured to scan the one or more beams over an angular range about the beam axis. A diffractive structure is mounted approximately parallel to the transparent window and positioned to intercept the one or more beams emitted by the LiDAR assembly and turn the beam axis to pass through the transparent window at an angle greater than 30° relative to a normal to a surface of the transparent window.

Claims:
The invention claimed is: 
     
       1. Sensing apparatus, comprising:
 a transparent window; 
 a LiDAR assembly, comprising a beam source, which is configured to emit one or more beams of optical radiation along a beam axis, and which is configured to scan the one or more beams over an angular range about the beam axis; and 
 a diffractive structure mounted approximately parallel to the transparent window and positioned to intercept the one or more beams emitted by the LiDAR assembly and turn the beam axis to pass through the transparent window at an angle greater than 30° relative to a normal to a surface of the transparent window. 
 
     
     
       2. The apparatus according to  claim 1 , wherein the LiDAR assembly comprises at least one sensor, which is configured to detect the optical radiation that is reflected from a target and returned to the at least one sensor through the transparent window. 
     
     
       3. The apparatus according to  claim 2 , wherein the at least one sensor is positioned on the beam axis in a monostatic configuration with the beam source. 
     
     
       4. The apparatus according to  claim 1 , wherein the beam source comprises a substrate and an array of emitters, which are disposed on the substrate and are configured to emit respective beams. 
     
     
       5. The apparatus according to  claim 1 , wherein the LiDAR assembly comprises a folding optic, which is positioned between the beam source and the one or more diffractive structures configured to intercept and turn the beam axis toward the transparent window. 
     
     
       6. The apparatus according to  claim 5 , wherein the folding optic comprises a grating. 
     
     
       7. The apparatus according to  claim 5 , wherein the beam source is configured to tune a wavelength of the one or more beams over a predefined tuning range, and wherein the folding optic comprises a dispersive element, which is configured to deflect the one or more beams at a turning angle that varies as a function of the wavelength. 
     
     
       8. The apparatus according to  claim 5 , wherein the folding optic comprises a mirror, which is configured to rotate so as to deflect the one or more beams over a range of turning angles. 
     
     
       9. The apparatus according to  claim 1 , wherein the diffractive structure is configured to rotate relative to the beam axis so as to scan the one or more beams passing through the transparent window over a range of beam angles. 
     
     
       10. The apparatus according to  claim 9 , wherein the LiDAR assembly is configured to scan the one or more beams in a first scan direction, and the diffractive structure is configured to rotate so as to scan the one or more beams in a second scan direction, transverse to the first scan direction. 
     
     
       11. The apparatus according to  claim 1 , wherein the diffractive structure is mounted in proximity to the transparent window. 
     
     
       12. The apparatus according to  claim 11 , wherein the LiDAR assembly is packed into a support structure of a windshield of a vehicle, and the transparent window is the windshield or is placed in the support structure approximately parallel to the windshield. 
     
     
       13. A method for sensing, comprising:
 providing a LiDAR assembly, which is configured to emit one or more beams of optical radiation along a beam axis and to scan the one or more beams over an angular range about the beam axis; and 
 mounting a diffractive structure approximately parallel to a transparent window so as to intercept the one or more beams emitted by the LiDAR assembly and turn the beam axis to pass through the transparent window at an angle greater than 30° relative to a normal to a surface of the transparent window. 
 
     
     
       14. The method according to  claim 13 , wherein providing the LiDAR assembly comprises positioning at least one sensor to detect the optical radiation that is reflected from a target and returned to the at least one sensor through the transparent window. 
     
     
       15. The method according to  claim 13 , wherein providing the LiDAR assembly comprises positioning a folding optic, between a beam source in the LiDAR assembly and the one or more diffractive structures so as to intercept and turn the beam axis toward the transparent window. 
     
     
       16. The method according to  claim 15 , wherein providing the folding optic comprises tuning a wavelength of the one or more beams over a predefined tuning range, wherein the folding optic comprises a dispersive element, which is configured to deflect the one or more beams at a turning angle that varies as a function of the wavelength. 
     
     
       17. The method according to  claim 15 , wherein the folding optic comprises a mirror, which is configured to rotate so as to deflect the one or more beams over a range of turning angles. 
     
     
       18. The method according to  claim 13 , wherein mounting the diffractive structure comprises rotating the diffractive structure relative to the beam axis so as to scan the one or more beams passing through the transparent window over a range of beam angles. 
     
     
       19. The method according to  claim 18 , wherein the LiDAR assembly is configured to scan the one or more beams in a first scan direction, and rotating the diffractive structure scans the one or more beams in a second scan direction, transverse to the first scan direction. 
     
     
       20. The method according to  claim 13 , wherein the diffractive structure is mounted in proximity to the transparent window, and wherein providing the LiDAR assembly comprises packing the LiDAR assembly into a support structure of the windshield.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application 62/843,464, filed May 5, 2019, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to systems and methods for optical sensing, and particularly to depth mapping. 
     BACKGROUND 
     Time-of-flight (ToF) imaging techniques are used in many LiDAR systems (also referred to as depth mapping, 3D mapping or 3D imaging systems). In direct ToF techniques, a light source, such as a pulsed laser, directs pulses of optical radiation toward the scene that is to be mapped, and a high-speed detector senses the time of arrival of the radiation reflected from the scene. The depth value at each pixel in the depth map is derived from the difference between the emission time of the outgoing pulse and the arrival time of the reflected radiation from the corresponding point in the scene, which is referred to as the “time of flight” of the optical pulses. The radiation pulses that are reflected back and received by the detector are also referred to as “echoes.” 
     SUMMARY 
     Embodiments of the present invention that are described hereinbelow provide improved apparatus and methods for optical sensing, such as LiDAR-based depth sensing. 
     There is therefore provided, in accordance with an embodiment of the invention, sensing apparatus, including a transparent window and a LiDAR assembly, including a beam source, which is configured to emit one or more beams of optical radiation along a beam axis, and which is configured to scan the one or more beams over an angular range about the beam axis. A diffractive structure is mounted approximately parallel to the transparent window and positioned to intercept the one or more beams emitted by the LiDAR assembly and turn the beam axis to pass through the transparent window at an angle greater than 30° relative to a normal to a surface of the transparent window. 
     In some embodiments, the LiDAR assembly includes at least one sensor, which is configured to detect the optical radiation that is reflected from a target and returned to the at least one sensor through the transparent window. In a disclosed embodiment, the at least one sensor is positioned on the beam axis in a monostatic configuration with the beam source. 
     Additionally or alternatively, the beam source includes a substrate and an array of emitters, which are disposed on the substrate and are configured to emit respective beams. 
     In some embodiments, the LiDAR assembly includes a folding optic, such as a grating, which is positioned between the beam source and the one or more diffractive structures configured to intercept and turn the beam axis toward the transparent window. In a disclosed embodiment, the beam source is configured to tune a wavelength of the one or more beams over a predefined tuning range, and the folding optic includes a dispersive element, which is configured to deflect the one or more beams at a turning angle that varies as a function of the wavelength. Alternatively, the folding optic includes a mirror, which is configured to rotate so as to deflect the one or more beams over a range of turning angles. 
     In some embodiments, the diffractive structure is configured to rotate relative to the beam axis so as to scan the one or more beams passing through the transparent window over a range of beam angles. In one embodiment, the LiDAR assembly is configured to scan the one or more beams in a first scan direction, and the diffractive structure is configured to rotate so as to scan the one or more beams in a second scan direction, transverse to the first scan direction. 
     Typically, the diffractive structure is mounted in proximity to the transparent window. In some embodiments, the LiDAR assembly is packed into a support structure of a windshield of a vehicle, and the transparent window is the windshield or is placed in the support structure approximately parallel to the windshield. 
     There is also provided, in accordance with an embodiment of the invention, a method for sensing, which includes providing a LiDAR assembly, which is configured to emit one or more beams of optical radiation along a beam axis and to scan the one or more beams over an angular range about the beam axis. A diffractive structure is mounted approximately parallel to a transparent window so as to intercept the one or more beams emitted by the LiDAR assembly and turn the beam axis to pass through the transparent window at an angle greater than 30° relative to a normal to a surface of the transparent window. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic side view of apparatus for depth sensing, in accordance with an embodiment of the invention; 
         FIG. 2  is a schematic side view of apparatus for depth sensing, in accordance with another embodiment of the invention; 
         FIG. 3  is a schematic pictorial illustration of rays output by the apparatus of  FIG. 2 , in accordance with an embodiment of the invention; 
         FIG. 4  is a schematic side view of apparatus for depth sensing, in accordance with an alternative embodiment of the invention; and 
         FIG. 5  is a schematic side view of apparatus for depth sensing, in accordance with yet another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
     In some applications, a depth mapping device is mounted behind a steeply-inclined transparent window, with the central axis of the field of view of the device angled steeply relative to the normal to the surface of the window. For example, when a forward-looking LiDAR device is mounted behind the windshield of a car or other vehicle, the optical axis of the field of view of the device may be angled by 45° or more relative to the normal to the windshield. 
     Embodiments of the present invention that are described herein use diffractive structures to fold the optical axis of an optical sensing device, such as a LiDAR device, and thus facilitate compact packaging of such devices behind a steeply-inclined transparent surface. The diffractive structure in these embodiments is mounted approximately parallel to the transparent surface. (The term “approximately parallel” in the present context means that the plane of the diffractive structure is within 200 of the plane of the transparent surface; but for compact packaging, it may be more advantageous that the planes be within 100 or even 5°. For compactness, the diffractive structure is typically mounted in proximity to the transparent window, meaning that the distance between the diffractive structure and the window is less that a transverse dimension of the diffractive structure (meaning a dimension measured in a direction parallel to the window); but alternatively, the diffractive structure may be positioned at a greater distance from the window. Any suitable type of diffractive structure may be used in this context, such as a surface grating, polarization grating, or volume holographic grating. 
     The LiDAR assembly, comprising a beam source and optics mounted behind the transparent window, directs one or more beams of optical radiation through the diffractive structure, which then turns the beams to pass through the transparent window in the desired direction. In other words, the diffractive structure turns the beam axis along which the beams are propagating so that the axis passes through the window at a high angle, for example at an angle more than 30° from a normal to the surface of the window, and possibly more than 45°. 
     In some of the embodiments that are described below, the LiDAR beam source scans the beam or beams in a first direction, while the diffractive structure is rotated to scan the beam or beams in a second direction, transverse to the first direction, across the field of view. In one such embodiment, the scan in the first direction is accomplished by scanning the wavelength of a laser beam (or beams), and using dispersive elements, such as a grating and/or prism, to convert the wavelength scan into an angular scan. In another embodiment, an opto-mechanical scanner, such as a scanning mirror, is used to scan a laser beam in at least the first direction. 
     Grating-Based Scanning 
       FIG. 1  is a schematic side view of apparatus  20  for depth sensing, in accordance with an embodiment of the invention. Apparatus  20  is particularly (although not exclusively) adapted for mounting behind a transparent window  52 , such as the windshield of an automobile or other vehicle. 
     Apparatus  20  comprises a LiDAR assembly  21 , comprising a beam source  22 , which emits beams  24  of optical radiation along a beam axis  26 . The beam source may in general comprise one or more emitters. In the embodiment shown in  FIG. 1 , beam source  22  comprises an array of LiDAR sensors  28 , disposed on a substrate  30 , such as a silicon substrate. Each sensor  28  comprises an emitter  32 , such as a pulsed laser diode, and a detector  38 , such as a single-photon avalanche diode (SPAD) or other type of ToF sensing device. In this embodiment, emitters  32  and detectors  38  are mounted in a monostatic configuration, sharing the same aperture and beam axis. A beamsplitter  40  separates the outgoing (transmitted) and incoming (received) light, for example on the basis of polarization or optical non-reciprocity, as is known in the art. 
     In the pictured embodiment, LiDAR sensors  28  comprise photonic integrated circuits, including respective emitters  32  and detectors  38 , with shared input/output couplers  34 . This coupler  34  may comprise, for example, a single-mode or double-clad optical fiber, which may be aligned in a V-groove on substrate  30 , thus precisely defining the aperture of sensor  28 . Additionally or alternatively, coupler  34  may comprise a waveguide configured as a spot-size converter with an edge or vertical coupling structure, as described, for example, in U.S. patent application Ser. No. 16/752,773, filed Jan. 27, 2020, whose disclosure is incorporated herein by reference. The spot size and divergence of the output beams may be controlled, for example, by tapering the waveguides, as well as by means of a microlens  36  positioned in alignment with each of the beams. 
     The apertures defined by couplers  34  may be located in a common plane, or they may alternatively be disposed along a curve, as part of the overall optical design of LiDAR assembly  21 . Couplers  34  and microlenses  36  may be oriented so as to emit beams  24  in a direction normal to the edge of substrate  30 , or alternatively in non-normal directions, for example at respective angles that converge toward a focal plane. These sorts of arrangements of the apertures and beam directions add flexibility in the design of the LiDAR optics, as explained further on pages 9-24 in the above-mentioned U.S. patent application Ser. No. 16/722,773. 
     Collimating optics  42  in LiDAR assembly  21  map the positions of the apertures of sensors  28  to angles, as illustrated in  FIG. 1 . These angles are typically sparsely distributed, meaning that there are angular gaps between the collimated beams that are not covered by the areas of the beams. Although collimating optics  42  are shown in the figure, for the sake of simplicity, as a single lens, in practice the collimating optics typically include multiple lens elements, as well as microlenses  36 . 
     A folding optic  44  is positioned between beam source  22  and window  52 , and serves to intercept and turn beam axis  26  toward the window. In the present embodiment, folding optic  44  comprises a diffractive element, such as a grating, which is blazed so as to direct beams  24  into a specific, target diffraction order relative to a normal  46  to the grating surface. Folding optic  44  is useful in enabling compact packaging of LiDAR assembly  21  by reducing the physical dimension of the LiDAR assembly in the direction normal to window  52 . 
     Furthermore, in addition to redirecting beam axis  26 , the grating in folding optic  44  can also be used in scanning the angles of beams  24  over a certain range about the beam axis. Specifically, the grating introduces wavelength-dependent angular dispersion (initial dispersion). To scan the beams, beam source  22  tunes the wavelengths of the beams over a predefined tuning range, for example over a range of 40-60 nm, using techniques of micromechanical or thermal wavelength tuning that are known in the art. The turning angle of the grating inherently varies as a function of the wavelength, so that scanning the wavelengths causes folding optic  44  to scan the deflection angles of beams  24 . Alternatively or additionally, folding optic  44  may comprise other sorts of dispersive optical elements, such as a prism. The range of angular scanning by folding optic  44  may or may not be sufficient to fill in the angular gaps between beams  24 , meaning that the area covered by the beams may or may not remain sparse. 
     Another diffractive structure, such as a grating  48 , is mounted approximately parallel to window  52  (and possibly precisely parallel, as shown in  FIG. 1 ). As noted earlier, grating  48  may comprise any suitable sort of diffractive structure that is known in the art, such as a surface grating, polarization grating, or volume holographic grating. Grating  48  is positioned to receive and intercept beams  24  output from folding optic  44  and turn beam axis  26  to pass through transparent window  52  at an angle greater than 30° relative to a normal to the surface of the window. Grating  48  is designed and patterned so as to have a range of acceptance angles large enough to accommodate all of beams  24 , while diffracting beams  24  preferentially into a diffraction order that is directed toward the desired field of view. Thus, in the vehicular example noted above, grating  48  will typically deflect the beams in a forward direction, notwithstanding the slant of the windshield. 
     In addition, in the present embodiment, grating  48  is mounted and driven to rotate relative to an axis  50  (which is parallel to beam axis  26  in the present case). As grating  48  rotates, rotation of the grating lines cause the diffraction angle at which beams  24  are deflected to scan in an azimuthal direction about axis  50 . The scanned beams thus pass through window  52  over a range of beam angles and scan across the field of view of apparatus  20 . By combining the angular scanning induced by rotation of grating  48  with the wavelength-tuned scanning of folding optic  44 , the overall scan range of apparatus  20  is able to densely cover the field of view. In other words, every output angle within the field of view of apparatus  20  can be addressed by an appropriate combination of beam selection, wavelength tuning and grating rotation. This sort of scanning performance can be achieved, for example, by arranging LiDAR assembly  21  to scan beams  24  in a first scan direction (such as a scan direction within the plane of the page in  FIG. 1 ), while the rotation of grating  48  scans the beams in a second scan direction, transverse to the first scan direction (such as the X-direction, perpendicular to the page in  FIG. 1 ). 
       FIG. 2  is a schematic side view of apparatus  60  for depth sensing, in accordance with another embodiment of the invention. The principles of operation of apparatus  60  are very similar to those of apparatus  20 , and the same indicator numbers are used to refer to corresponding components in the two embodiments. 
     As shown in  FIG. 2 , drive circuits  62  control the operation of beam source  22 , including tuning the wavelengths of beams  24  and sensing the times of flight of photons received by detectors  38 . Typically, drive circuits  62  comprise current pulse generators and voltage sources suitable for actuating and tuning emitters  32 , along with amplification and time/digital conversion circuits for processing the outputs of detectors  38 . Collimating optics  42  direct the beams through a pupil  64 , toward folding optic  44 , which in this example comprises a prism  66  with a surface grating on its entrance surface. A scan drive  68 , typically comprising a suitable electric motor, rotates grating  48  so as to scan the beams of apparatus  60  across the field of view. A controller  70 , such as a programmable microprocessor or microcontroller with suitable interfaces, regulates the operation of drive circuits  62  and scan drive  68  in order to scan beams  24  over the field of view, and processes signals output by the drive circuits in order to generate a depth map of the field of view. 
     All the elements of apparatus  60  are contained inside a compact housing  72 , which extends mainly along a direction parallel to window  52 . Assuming window  52  to be the windshield of a vehicle, LiDAR assembly  21  can be packed in this manner into a support structure of the windshield. Alternatively, window  52  may be separate from the windshield and may be placed, for example, in a suitable opening in the support structure, typically in an orientation that is approximately parallel to the windshield of the vehicle. 
       FIG. 3  is a schematic pictorial illustration of rays  74 ,  76 ,  78 ,  80 , . . . , which are output by apparatus  60 , in accordance with an embodiment of the invention. The different elevation angles of rays  74 ,  76 ,  78 ,  80 , . . . , relative to window  52  can be generated, for example, by scanning the wavelength of emitters  32 . Each vertical column of rays  74 ,  76 ,  78 ,  80 , . . . , is thus produced by the beam  24  that is output from a corresponding emitter  32  in beam source  22 . Rotation of grating  48  about its axis shifts the columns of rays in the azimuthal direction, i.e., a direction transverse to the vertical axes of the columns, and thus fills in the gaps between the columns to give dense coverage of the field of view of the apparatus. 
     Mirror-Based Scanning 
       FIG. 4  is a schematic side view of apparatus  90  for depth sensing, in accordance with an alternative embodiment of the invention. The elements of apparatus  90  are similar to the preceding embodiments, except that the folding optic in the present embodiment comprises a mirror  94 , which is configured to rotate so as to deflect the beams over a range of turning angles, in either one or two dimensions. 
     Apparatus  90  comprises a beam source  92 , which is generally similar in construction and operation to beam source  22  and collimation optics  42  in  FIG. 1 . In the present embodiment, however, mirror  94  both turns and scans the beams output by beam source  92 , so that wavelength scanning of the beam source is not required. Therefore, beam source  92  typically comprises one or more laser diodes operating at a constant wavelength (for example 1545 nm), and a narrow linewidth, generally less than 1 nm. 
     Mirror  94  is attached to a scanning mechanism (not shown in the figures), such as a galvanometer drive or a micro-electro-mechanical systems (MEMS) drive, or any other suitable sort of scanning mechanism known in the art, which drives the mirror to scan in one or two dimensions. In the pictured example, mirror  94  rotates about an axis perpendicular to the page of  FIG. 4 , between orientations  94 A and  94 B, and thus scans the elevation of the beams exiting through window  52 . As in the preceding embodiment, a grating  96  turns beam axis  26  to pass through window  52  at a high angle, and also rotates so as to scan the beams over a desired range in the azimuthal direction. 
     In an alternative embodiment, in which mirror  94  is able to scan in two dimensions, grating  96  may be static, so as to turn the beam axis without any additional scanning function. 
       FIG. 5  is a schematic side view of apparatus  100  for depth sensing, in accordance with yet another embodiment of the invention. Apparatus  100  comprises a monostatic beam source  102 , comprising emitter  32  and detector  38  as in the embodiment of  FIG. 1 . Collimating optics  104  direct the beam or beams output by beam source  102  toward a scanning mirror  106 , which rotates so as to scan the elevation of the beams. Transverse scanning is carried out by rotation of grating  96 . 
     It will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.

Metadata:
Filing Date: 20200302
Publication Date: 20221025
Grant Date: 20221025
Priority Date: 20190505
Inventors: SUTTON, ANDREW J.
WHARTON, MICHAEL C.
TEIL, ROMAIN A.
Assignee: APPLE INC
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Family ID: 73016385