PATENT DOCUMENT

Publication Number: US-10955531-B2
Application Number: US-201715849701-A
Country: US
Kind Code: B2

Title: Focal region optical elements for high-performance optical scanners

Abstract:
An optical device includes a light source, which is configured to emit a beam of light, and a sensor having a detection area. At least one scanning mirror is configured to scan the beam across a target scene. Light collection optics include a collection lens positioned to receive the light from the scene that is reflected from the at least one scanning mirror and to focus the collected light onto a focal plane, and a non-imaging optical element having a front surface positioned at the focal plane of the collection lens and a rear surface in proximity to the sensor and configured to spread the light focused by the collection lens over the detection area of the sensor.

Claims:
The invention claimed is: 
     
       1. An optical device, comprising:
 a light source, which is configured to emit a beam of light; 
 at least one scanning mirror configured to scan the beam across a target scene; 
 a sensor having a detection area; and 
 light collection optics comprising:
 a collection lens positioned to collect the light from the scene that is reflected from the at least one scanning mirror and to focus the collected light onto a focal plane; and 
 a non-imaging optical element comprising a ball lens having a front surface positioned at the focal plane of the collection lens and a rear surface in proximity to the sensor and configured to image a pupil of the collection lens onto the sensor so as to spread the light focused by the collection lens over the detection area of the sensor. 
 
 
     
     
       2. The optical device according to  claim 1 , wherein the rear surface of the non-imaging optical element is in contact with the sensor. 
     
     
       3. The optical device according to  claim 1 , wherein the beam of light comprises a beam of light pulses, and wherein the sensor is configured to output a signal indicative of a time of incidence of a single photon on the sensor. 
     
     
       4. The optical device according to  claim 3 , and comprising a controller, which is configured to find times of flight of the light pulses to and from points in the scene responsively to the signal. 
     
     
       5. A method of sensing, comprising:
 scanning a beam of light across a target scene using at least one scanning mirror; 
 collecting the light from the scene that is reflected from the at least one scanning mirror and focusing the collected light onto a focal plane using a collection lens; and 
 positioning a non-imaging optical element comprising a ball lens such that a front surface of the ball lens is positioned at the focal plane; and 
 sensing the collected light using a sensor positioned in proximity to a rear surface of the non-imaging optical element, wherein the ball lens images a pupil of the collection lens onto the sensor so as to spread the collected light over a detection area of the sensor. 
 
     
     
       6. The method according to  claim 5 , wherein the sensor is positioned in contact with the rear surface of the non-imaging optical element. 
     
     
       7. The method according to  claim 5 , wherein the beam of light comprises a beam of light pulses, and sensing the collected light comprises sensing the light pulses and emitting a signal indicative of a time of incidence of a single photon on the sensor. 
     
     
       8. The method according to  claim 7 , and comprising finding times of flight of the light pulses to and from points in the target scene responsively to the signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application 62/522,725, filed Jun. 21, 2017, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to opto-electronic devices, and particularly to optical scanners. 
     BACKGROUND 
     Optical scanners are widely used for optically probing extended objects or target scenes. In a typical scanner a light source sends out a beam of light; the beam is scanned across the object by, for instance, a scanning mirror; then the light returned from the object is collected by collection optics and is directed to a sensor. The sensor emits a signal to a controller for further analysis. 
     High-performance optical scanners typically use both high-power light beams and high-speed scanners in order to sample and sense distant target scenes with high temporal and spatial resolution with a good signal-to-noise ratio. 
     SUMMARY 
     Embodiments of the present invention that are described hereinbelow provide improved optical scanners and methods for scanning. 
     There is therefore provided, in accordance with an embodiment of the invention, an optical device, including a light source, which is configured to emit a beam of light, at least one scanning mirror configured to scan the beam across a target scene, and a sensor having a detection area. Light collection optics include a collection lens positioned to receive the light from the scene that is reflected from the at least one scanning mirror and to focus the collected light onto a focal plane, and a non-imaging optical element having a front surface positioned at the focal plane of the collection lens and a rear surface in proximity to the sensor and configured to spread the light focused by the collection lens over the detection area of the sensor. 
     In a disclosed embodiment, the rear surface of the non-imaging optical element is in contact with the sensor. 
     In some embodiments, the non-imaging optical element includes a ball lens. In one embodiment, the ball lens is configured to image a pupil of the collection lens onto the sensor. 
     In other embodiments, the non-imaging optical element includes a compound parabolic concentrator (CPC) having an entrance oriented toward the collection lens and an exit oriented toward the sensor, and a transmitting diffuser extending across the entrance of the CPC. In one embodiment, the transmitting diffuser is in contact with the entrance of the CPC. 
     In some embodiments, the beam of light includes a beam of light pulses, and the sensor is configured to output a signal indicative of a time of incidence of a single photon on the sensor. Typically, the apparatus includes a controller, which is configured to find times of flight of the light pulses to and from points in the scene responsively to the signal. 
     There is also provided, in accordance with an embodiment of the invention, a method of sensing, which includes scanning a beam of light across a target scene using at least one scanning mirror. The light from the scene that is reflected from the at least one scanning mirror is collected and focused onto a focal plane. A front surface of a non-imaging optical element is positioned at the focal plane. The collected light is sensed using a sensor positioned in proximity to a rear surface of the non-imaging optical element, which spreads the collected light over a detection area of the sensor. 
     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 an optical scanner with a non-imaging optical element, in accordance with an embodiment of the invention; 
         FIG. 2  is a schematic side view of an optical scanner with a ball lens as a non-imaging optical element, in accordance with an embodiment of the invention; 
         FIGS. 3 a - b    are schematic optical ray trace diagrams, illustrating optical performance of the scanner of  FIG. 2  in accordance with an embodiment of the invention; and 
         FIG. 4  is a schematic side view of an optical scanner with a combination of a transmitting diffuser and a compound parabolic concentrator (CPC) as a non-imaging optical element, in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
     High-performance optical scanners, for instance light detection and ranging (LiDAR) sensors, typically use both high-intensity light beams and high-speed scanners in order to sample and sense distant target scenes with high spatial and temporal resolution with a good signal-to-noise ratio. The distance to the target scene may range from tens of centimeters to hundreds of meters. 
     The beam emitted by the light source is scanned across the target scene by a high-speed scanner, for instance by a rapidly oscillating scanning mirror. The light returned from the scene is reflected by a scanning mirror (either the scanning mirror described above or another mirror synchronized to the above mirror) towards a collection lens. An imaging collection lens focuses the returned light into a small spot on the sensor, and the signal emitted by the sensor is received by a controller for further analysis. 
     High-speed scanners are employed in this application to maximize the measurement rate. At very high angular speeds, the round-trip time for light can introduce a lag angle at the receiver. In order to compensate for deviations of the focused spot due to the scanner, a large sensing area is required. Furthermore, small focused spots of both returned light and scattered light have a very high irradiance that can damage the sensor and possibly degrade detection efficiency. 
     The embodiments of the present invention that are described herein address the above limitations so as to enable high-speed, high-resolution optical scanners utilizing a sensor with high bandwidth, low dark noise, and reduced potential for damage and saturation. The disclosed embodiments use a non-imaging optical element positioned between the focal plane of the collection optics and the sensor. This arrangement affords at least the following advantages:
         1) The beam returned from the target scene and focused by the collection optics is spread on the sensor over an area that is larger than the spot in the focal plane; and   2) The beam is stabilized on the sensor to a position that is essentially independent of the mirror speed and target distance.
 
The use of a non-imaging optical element enables reducing the detection area of the sensor relative to what would otherwise be required in the absence of the non-imaging optics, as well as lowering the peak irradiance on the sensor. Both advantages result in larger signal bandwidth and longer detection ranges than could otherwise be achieved without the extra-focal elements.
       

     The disclosed embodiments of the present invention provide optical scanners, wherein the light source can be either a non-laser source (such as a thermal source, a solid state source or a gas discharge source) or a laser source (either continuous-wave or pulsed). Such scanners can be used in depth-mapping systems, such as LIDARs, wherein the light source is a pulsed laser source, the sensor is a single-photon avalanche diode (SPAD), and a controller finds the distance to the target scene by time-of-flight analysis. 
     In alternative embodiments, other high-sensitivity sensors, such as an avalanche photodiode (APD), may be used. 
     System Description 
       FIG. 1  is a schematic side view of an optical scanner  20  with a non-imaging optical element  21 , in accordance with an embodiment of the invention. The beam from a light source  22 , such as, for example, a continuous-wave or pulsed laser, is directed to a target scene  24  by a scanning mirror  26 , forming and scanning an illumination spot  28  over the target scene. (The terms “light” and “illumination” are used herein to refer to any sort of optical radiation, including radiation in the visible, infrared, and ultraviolet ranges.) The light returned from illumination spot  28  is reflected by scanning mirror  26  and a beamsplitter  30  towards a collection lens  32 , which focuses the light onto its focal plane  34 . 
     Non-imaging optical element  21  has its front surface positioned at or near focal plane  34  and its rear surface in proximity to a sensor  38 , such as a photodiode, a SPAD, or an APD. Non-imaging optical element  21  is configured to spread the light focused by collection lens  32  over the detection area of sensor  38 . 
     A controller  40  is connected to light source  22 , scanning mirror  26 , and sensor  38 . Controller  40  is configured to drive light source  22  and scanning mirror  26 , as well as to receive and analyze signals emitted by sensor  38 . In a LIDAR used for mapping the depth of target scene  24 , for example, light source  22  comprises a pulsed laser, and sensor  38  comprises a SPAD. Controller  40  measures times of arrival of photons at sensor  38  relative to pulses emitted by light source  22  in order to derive time-of-flights across the target scene and thus to produce a depth map of the target scene. 
     As will be further detailed in  FIGS. 2-4 , proper choice of non-imaging optical element  21  enables the use of a sensor  38  with a detection area having smaller dimensions than would be required if the sensor were located directly in focal plane  34 . The use of a smaller detection area yields a higher bandwidth and a lower dark noise as compared to a larger sensor. Furthermore, non-imaging optical element  21  spreads the light focused at focal plane  34  over the detection area of sensor  38 , thus alleviating the problems that are associated with a high local irradiance. 
       FIG. 2  is a schematic side view of an optical scanner  50  with a ball lens  52  as a non-imaging optical element, in accordance with an embodiment of the invention. Optical scanner  50  is similar to optical scanner  20  of  FIG. 1 , with the exception of ball lens  52  replacing generic non-imaging optical element  21  of  FIG. 1 . Ball lens  52  is positioned with its front surface at focal plane  34  and with its rear surface in contact with or in proximity to sensor  38 . A rectangle  54  denotes a part of  FIG. 2  which is detailed in  FIGS. 3 a   - b.    
       FIGS. 3 a - b    are ray traces showing optical rays  56   a - b  propagating within rectangle  54  of  FIG. 2 , in accordance with an embodiment of the invention. In  FIG. 3 a   , optical rays  56   a  arrive from collection lens  32  to a center point  58   a  of focal plane  34 . As point  58   a  is located on the vertex of ball lens  52 , rays  56   a  change their direction according to Snell&#39;s law as if they had impinged on a plane surface at point  58   a , with the surface coinciding with focal plane  34 . Rays  56   a  spread over an area  60   a  on sensor  38 . 
     In  FIG. 3 b   , optical rays  56   b  arrive from collection lens  32  to an off-axis point  58   b  of focal plane  34 . Now rays  56   b  are both spread and refracted by ball lens  52 , and they impinge on sensor  38  over an area  60   b . Ball lens  52  images the pupil of collection lens  32  onto sensor  38 , and area  60   b  essentially coincides with area  60   a  of  FIG. 3   a.    
     As shown in  FIGS. 3 a - b   , rays arriving from collection lens  32  are spread over area  60   a  or  60   b , respectively, on sensor  38 , with the position of areas  60   a  and  60   b  being essentially stationary with respect to the varying position of the rays in focal plane  34 . This configuration enables the use of a sensor whose dimensions are independent of target distance and scan speed, and typically smaller than would be otherwise required if positioned at the focal plane  34 . The spreading of the rays lowers the irradiance on sensor  38  by typically 20 dB, or a factor of 100, relative to what would otherwise be received if the sensor were positioned directly in focal plane  34 , alleviating the problems associated with high local irradiance. 
       FIG. 4  is a schematic side view of an optical scanner  70  with a combination of a transmitting diffuser  72  and a compound parabolic concentrator (CPC)  74  as a non-imaging optical element, in accordance with another embodiment of the invention. Optical scanner  70  is similar to optical scanner  20  of  FIG. 1 , with the exception of the combination of transmitting diffuser  72  and CPC  74  replacing generic non-imaging optical element  21  of  FIG. 1 . 
     A CPC is an optical element with a reflective, concave, rotationally symmetrical surface. The shape of the reflective surface is defined by rotating a section of a parabola (parabolic section) around an axis. This axis is defined as the perpendicular bisector of the line connecting the focal point of the parabola to the point of the parabolic section closest to the focal point. The aperture defined by the rotation of this connecting line forms the exit aperture of the CPC. The entrance aperture of the CPC is determined by a suitable choice of the end-point of the parabolic section away from the exit aperture. The maximal acceptance angle of the CPC, typically tens of degrees, is equal to the tilt angle between the axis of the parabola and the axis of rotation. A CPC is manufactured typically of glass or metal, and its inside is coated with a suitable reflective material, such as aluminum. 
     Diffuser  72  is positioned at focal plane  34  of collection lens  32 . CPC  74  is positioned with its entrance either in contact with or in proximity to diffuser  72 , and with its exit in contact with or in proximity to sensor  38 . 
     Diffuser  72  transmits and diffuses the rays focused by collection lens  32  onto the diffuser. The transmitted rays are spread and reflected towards sensor  38  by CPC  74 , filling nearly uniformly the exit aperture of the CPC. Thus, independently of the position of the focused rays in focal plane  34 , all the rays transmitted by diffuser  72  arrive within the detection area of sensor  38 , enabling the use of a sensor whose dimensions are independent of target distance and scan speed. As the exit aperture of CPC  74  is substantially smaller than its entrance aperture, the detection area of sensor  38  can be made substantially smaller than would be required if the sensor were positioned at the focal plane. Due to the nearly uniform filling of the exit aperture of CPC  74 , the irradiance of the radiation received by sensor  38  is typically 20 dB lower than the irradiance that a sensor positioned in focal plane  34  would receive, alleviating the problems that are associated with a high local irradiance. 
     Although the embodiments of non-imaging optical element  21  of  FIG. 1  have been described in the embodiments in  FIGS. 2 and 4  as ball lens  52  and a combination of transmitting diffuser  72  and CPC  74 , respectively, other embodiments of the present invention can use other sorts of non-imaging optical elements that are known in the art, comprising both reflective and refractive optics and diffusers. 
     It will thus 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: 20171221
Publication Date: 20210323
Grant Date: 20210323
Priority Date: 20170621
Inventors: UPTON, ROBERT S
SUTTON, ANDREW J.
LIPSON, ARIEL
AXELROD, NOEL
Assignee: APPLE INC
CPC Classifications: [{"code": "G01S17/42", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01S7/4816", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01S17/42", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01S7/4817", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01S7/4816", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B26/105", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B26/105", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01S17/08", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01S7/4816", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01S7/4817", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01S17/42", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B26/105", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01S17/08", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 64693103