Patent Publication Number: US-2021167240-A1

Title: Time of flight (tof) sensor with transmit optic providing for reduced parallax effect

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a divisional of United States application for patent Ser. No. 16/401,209, filed May 2, 2019, the disclosure of which is incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention generally relates to a time of flight (TOF) sensor and, in particular, to a transmit optic for use in a TOF sensor. 
     BACKGROUND 
     A time of flight (TOF) sensor is well known to those skilled in the art.  FIG. 1  presents a cross-sectional view of a typical prior art TOF sensor  10 . The sensor includes a support substrate  12  which may include interconnection wiring  14 ,  16 ,  18  that is embedded within the substrate  12  and further located on the front surface  20  and rear surface  22  of the substrate. The wiring  16  within the substrate serves to interconnect the wiring  14  on the front surface  20  to the wiring  18  on the rear surface  22 . A transmitter integrated circuit chip  30  is mounted to the front surface  20  of the substrate  12  and electrically connected to the wiring  14  (using bonding wires or other electrical connection means well known to those skilled in the art). The transmitter integrated circuit chip  30  includes a light source  32  (for example, a vertical-cavity surface-emitting laser (VCSEL)). A receiver integrated circuit chip  34  is also mounted to the front surface  20  of the substrate  12  and electrically connected to the wiring  14  (using bonding wires or other electrical connection means well known to those skilled in the art). The receiver integrated circuit chip  34  includes a first photosensor  36  and a second photosensor  38 . The photosensors  36 ,  38  may, for example, each comprise an array of single-photon avalanche diodes (SPADs). The first photosensor  36  functions as a reference signal detector and the second photosensor  38  functions as an object signal detector. The integrated circuit chips  30  and  34  are enclosed in an opaque housing  40  that is mounted to the front surface  20  of the substrate  12 . The housing  40  includes a transmit optic  42  (for example, a transparent glass plate) aligned with the light source  32  and a receive optic  44  (for example, a transparent glass plate) aligned with the second photosensor  38 . A central partition  46  of the housing  40  is positioned between the first photosensor  36  and the second photosensor  38  to function as a light isolation barrier. 
     Operation of the TOF sensor  10  involves triggering the emission of a pulse of light by the light source  32 . A first portion  50  of the emitted light passes through the transmit optic  42  and is directed toward an object  52 . A second portion  54  of the emitted light is reflected by an inner surface of the housing  40  and is detected by the first photosensor  36 . The first portion  50  of the emitted light reflects from the object  52 , and the reflected light  56  passes through the receive optic  44  and is detected by the second photosensor  38 . The difference in time between the detection of the second portion  54  by the first photosensor  36  and the detection of the reflected light  56  by the second photosensor  38  is indicative of the distance d between the TOF sensor  10  and the object  52 . 
     TOF sensors having the configuration as generally shown in  FIG. 1  suffer from a number of problems as illustrated by  FIG. 2 . The TOF sensor possesses a transmit field of view  60  for the light source  32  and the transmit optic  42  and a receive field of view  62  for the second photosensor  38  and the receive optic  44 . One problem relates to parallax. Parallax is introduced by the separation distance s between the transmit optic  42  and the receive optic  44 . As a result, there is a space  64  between the fields of view  60  and  62  where objects  52  cannot be seen and detected. Furthermore, problems with ranging spikes can be experienced with respect to region  66  just further than the nearest detectable distance d′. Also, the extreme edge areas  68  of the transmit field of view  60  are susceptible to concerns with poor mode mixing of a multi-modal VCSEL output light pulse. 
     There is a need in the art to address the forgoing problems. 
     SUMMARY 
     In an embodiment, a time of flight (TOF) sensor comprises: a transmit integrated circuit including a light source configured to generate a collimated beam of light; a receive integrated circuit including a first photosensor; and a transmit optic mounted over the transmit integrated circuit and the receive integrated circuit, said transmit optic formed by a prismatic light guide configured to receive the beam of light and having an annular body region surrounding a central opening which is aligned with the first photosensor, the annular body region including a first reflective surface defining the central opening and further including a ring-shaped light output surface surrounding the central opening and configured to output light in response to light that propagates within the prismatic light guide in response to the received beam of light and which reflects off the first reflective surface. 
     In an embodiment, a prismatic light guide receives a beam of light and includes an annular body region surrounding a central opening. The annular body region of the prismatic light guide includes a first reflective surface defining the central opening and further includes a ring-shaped light output surface surrounding the central opening and configured to output light in response to light that propagates within the prismatic light guide in response to the received beam of light and which reflects off the first reflective surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the embodiments, reference will now be made by way of example only to the accompanying figures in which: 
         FIG. 1  is a cross-sectional view of a typical prior art TOF sensor; 
         FIG. 2  illustrates parallax concerns with the sensor of  FIG. 1 ; 
         FIGS. 3A-3B  show isometric and partially transparent views of a transmit optic; 
         FIG. 3C  is a cross-section of a microlens structure; 
         FIGS. 4A-4B  are top and bottom perspective views of the transmit optic; 
         FIGS. 4C-4D  are cross-sectional views of the transmit optic; 
         FIG. 4E  is a side view of the transmit optic; 
         FIG. 5  is a cross-sectional view of a TOF sensor incorporating the transmit optic. 
     
    
    
     DETAILED DESCRIPTION 
     Reference is now made to  FIG. 3A  which shows an isometric and partially transparent view of a transmit optic  100  for use in a time of flight (TOF) sensor.  FIG. 3B  shows a cross-section of the isometric and partially transparent view of  FIG. 3A . The transmit optic  100  is a prismatic light guide formed by a unitary body of highly optically transparent material such as polycarbonate or poly methyl methylacrylate (PMMA). The unitary body of the transmit optic  100  is preferably made using an injection molding process followed by optical finishing of external surfaces in a manner well known to those skilled in the art. Furthermore, certain external surfaces of the molded unitary body which are desired to be reflective may be treated with a mirror coating in manner well known to those skilled in the art. 
     The unitary body of the transmit optic  100  includes an annular body region  102  and a radial projection region  104 . The annular body region  102  is in the form of a ring which encircles a central opening  106  and has a radial cross-section in the general shape of a trapezoid where the longer side of the two parallel sides of the trapezoidal cross-section defines a light outlet surface  110  of the transmit optic  100 . The light outlet surface  110  is ring-shaped (in a plane perpendicular to an axis of the central opening and is preferably textured and/or patterned to include a plurality of microlens  158  structures (for example, convex in cross-section as shown in  FIG. 3C ), wherein each microlens  158  may have a size and shape as needed in order to produce a desired beam divergence (reference  160 ). With this annual body region configuration, the central opening  106  takes the shape of a truncated cone (i.e., it is frusto-conical in shape). The outer and inner non-parallel sides of the trapezoidal cross-section respectively define angled light reflecting surfaces  112  and  114  (more specifically, internally light reflecting surfaces) which may, for example, have a mirror coating for reflection or be configured as total internal reflection surfaces. The shorter side of the two parallel sides of the trapezoidal cross-section defines part of a base surface  116  of the transmit optic  100 , and as will be described in more detail herein a portion of this base surface provides the light input surface of the transmit optic  100 . The base surface  116  may further be treated to be reflective (for example, by use of a mirror coating layer). 
     The radial projection region  104  extends in a radial direction out from the annular body region  102 . The radial projection region  104  may have a cross-section perpendicular to the radial direction in the general shape of a rectangle or square. A first pair of opposed parallel sides  120  of the rectangular or square cross-section are extensions of the outer non-parallel side of the trapezoidal cross-section for the annular body region  102  associated with the reflecting surface  112 . A second pair of opposed parallel sides  122  of the rectangular or square cross-section are extensions of the light outlet surface  110  and base surface  116 . The distal end of the radial projection region  104  includes an angled light reflecting surface  128  (more specifically, an internally light reflecting surface) which may, for example, have a mirror coating for reflection or be configured as a total internal reflection surface. The portion of the base surface  116  associated with the parallel side  122  in the radial projection region  104  is shaped to include a collimating optical lens  132  whose optical axis is aligned to intersect at the angled light reflecting surface  128 . 
     The collimating optical lens  132  receives divergent light  150  emitted from an external light source (not shown) and collimates the received external light to produce a beam  152  directed towards the angled light reflecting surface  128 . The beam  152  is reflected by the angled light reflecting surface  128  to produce a beam  154  which propagates through the radial projection region  104  generally in a radial direction towards the central opening  106 . The beam  154  is reflected by the reflecting surface  114  to produce a beam  156  directed towards the light outlet surface  110  and the plurality of microlens  158  structures. The microlens  158  structures refract the beam  156  to produce a spread of beams  160 . It will be understood, even though not explicitly illustrated in  FIG. 3B , that portions of the beam  154  will in effect spread when propagating through the radial projection region  104  and could bounce of other surfaces of the prismatic light guide before reaching the microlens  158  structures. The illustrated paths for beams  154 ,  156  and  160  is just one example of light propagation within the prismatic light guide of the transmit optic  100 . Illumination from the received collimated light  150  will be output across the light outlet surface  110  at locations which surround the central opening  106 . 
       FIGS. 4A-4B  are top and bottom perspective views of the transmit optic  100 ,  FIGS. 4C-4D  are cross-sectional views of the transmit optic  100  taken along lines  4 C and  4 D, respectively, of  FIG. 4A , and  FIG. 4E  is a side view of the transmit optic  100 . 
     Reference is now made to  FIG. 5  which presents a cross-sectional view of a TOF sensor  200  that utilizes the transmit optic  100 . The sensor includes a support substrate  212  which may include interconnection wiring  214 ,  216 ,  218  that is embedded within the substrate  212  and further located on the front surface  220  and rear surface  222  of the substrate. The wiring  216  within the substrate serves to interconnect the wiring  214  on the front surface  220  to the wiring  218  on the rear surface  222 . A transmitter integrated circuit chip  230  is mounted to the front surface  220  of the substrate  212  and electrically connected to the wiring  214  (using bonding wires or other electrical connection means well known to those skilled in the art). The transmitter integrated circuit chip  230  includes a light source  232  (for example, a vertical-cavity surface-emitting laser (VCSEL)). A receiver integrated circuit chip  234  is also mounted to the front surface  220  of the substrate  212  and electrically connected to the wiring  214  (using bonding wires or other electrical connection means well known to those skilled in the art). The receiver integrated circuit chip  234  includes a first photosensor  236  and a second photosensor  238 . The photosensors  236 ,  238  may, for example, each comprise an array of single-photon avalanche diodes (SPADs). The first photosensor  236  functions as a reference signal detector and the second photosensor  238  functions as an object signal detector. The integrated circuit chips  230  and  234  are enclosed in an opaque housing  240  that is mounted to the front surface  220  of the substrate  212 . The housing  240  supports the transmit optic  100  with the collimating lens  132  aligned with the light source  232  and the central opening  106  aligned with the second photosensor  238 . An adhesive may be used to mount the transmit optic  100  to the housing  240 . A central partition  246  of the housing  240  is positioned between the first photosensor  236  and the second photosensor  238  to function as a light isolation barrier. 
     A light pipe  260  with a receive optic  262  (for example, a transparent plate) is mounted within the central opening  106 . The light pipe has the shape of a truncated cone (i.e., frusto-conical) with a central bore within which the receive optic  262  is installed. The outer conical surface of the light pipe  260  may be adhesively bonded to the inner conical surface  114  transmit optic  100 . The light pipe may be made of an optically opaque molded material. 
     Operation of the TOF sensor  200  involves triggering the emission of a pulse of light by the light source  232 . A first portion  250  of the emitted light forms the divergent light  150  which is directed towards the collimating lens  132  and passes through the transmit optic  100  to be emitted from the light outlet surface  110  as the spread of beams  160  which are directed toward an object  252 . A second portion  254  of the emitted light is reflected by the base surface  116  of the transmit optic  100  and is detected by the first photosensor  236 . The first portion  250  of the emitted light reflects from the object  252 , and the reflected light  256  passes through the light pipe  260  and receive optic  262  and is detected by the second photosensor  238 . The difference in time between the detection of the second portion  254  by the first photosensor  236  and the detection of the reflected light  256  by the second photosensor  238  is indicative of the distance d between the TOF sensor  200  and the object  252 . 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.