Abstract:
A reflector is mounted to an outer perimeter of a heat sink holding at least one edge-emitting semiconductor chip, for example a laser diode. The reflector has a shape suitable for gathering light emitted by the laser diodes and redirecting the light in an upward direction away from the heat sink. The reflector can be overmolded onto the heat sink. The reflector can operate by total internal reflection, so that no additional reflector coating step is required. Injection molding of the reflectors onto the heat sink holding the laser diodes enables mass production of powerful yet inexpensive laser light sources.

Description:
TECHNICAL FIELD 
     The present invention relates to light sources, and in particular to semiconductor light sources for producing a directed beam of light suitable, for example, for range imaging. 
     BACKGROUND OF THE INVENTION 
     Range imaging is presently finding increasing use in gesture recognition applications. In range imaging, a pulsed light source illuminates an object, and a gated detector array is used to obtain an image of the object. The detector array is equipped with an electronic gate or shutter that makes the detector array responsive to light only during a narrow time window when the “gate” is open. The moment of opening the “gate” is delayed by a delay time with respect to the moment the light pulse is emitted. The emitted light pulse propagates a pre-defined distance corresponding to the delay time, reflects from an object located at that distance, and propagates back. Any light reflected from an object located before or after the pre-defined distance will be suppressed by the gated detector array. The time delay is varied to obtain 3D imagery slice-by-slice. 
     Another approach to range imaging consists in modulating the illuminating light at a high modulation frequency and detecting, for each pixel of a detector array, a modulation phase delay between the illuminating light and light detected by the pixel. The modulation phase delay in a pixel is proportional to a distance to the object, or more particularly, the distance to a point in the illuminated scenery imaged by the pixel. At least tens of megahertz modulation rates and 10 mW level output optical power are usually required for either type of range imaging. 
     The modulation speed and optical power requirements make edge-emitting laser diodes preferable light sources for range imaging. Directly modulated edge-emitting laser diode chips, generating hundreds of milliwatts of infrared light, can nowadays be mass produced at a reasonably low cost, however a reliable and efficient packaging of the laser diode chips into Watt-level light sources is still relatively expensive. Powerful laser diode chips require effective removal of heat generated during normal operation. The emitted light needs to be gathered with low optical loss, reshaped for optimal illumination of an object being imaged, and directed to the object. The edge-emitting geometry of the laser diode chips, which are usually mounted on a common flat heat sink, frequently requires a complex and costly combination of high-quality turning micromirrors to direct beams emitted by individual laser chips towards the imaged object. 
     To incorporate a range imaging system into a gesture recognition system, for example in a gaming and/or a mobile phone application, manufacturing costs need to be dropped considerably to make the range imaging system affordable by a mass consumer. At the same time, there is a strong market pressure to miniaturize the componentry for portable consumer devices. This necessitates miniaturization of range imaging light sources, while simultaneously dropping the manufacturing costs of these light sources. 
     Scifres et al. in U.S. Pat. No. 4,633,476 disclose a laser diode that can emit light perpendicular to the plane of the laser chip, allowing light from multiple lasers on a common heat sink to be combined into a single, more powerful beam. Referring to  FIG. 1 , a laser diode  10  includes an active layer  11  sandwiched between p- and n-layers  12  and  13 , respectively. The n-layer  13  includes two sub-layers,  13 A and  13 B. A V-shaped groove  14  is etched into the p-layer  12  and the second n-sublayer  13 B from the p-layer  12  side. P- and n-electrodes  15  and  16  contact the p- and n-layers  12  and  13 , respectively. The p-electrode  15  can be made sufficiently thick to serve as a heat sink. Gaps  17  are cut into the p- and n-layers  12  and  13  and into the active layer  11 , to function as laser cavity mirrors. In operation, an electric current is applied between the p- and n-electrodes  15  and  16 , respectively, and generated light  18  is reflected from inside the faces of the grooves  14 , exiting through cut-outs  19  in the n-electrodes  16 . 
     Among advantages of the laser diode of Scifres et al. are low profile (height) and a possibility to combine light from multiple laser diode chips. Detrimentally, however, the light source of Scifres et al. is rather difficult to manufacture. Multiple grooves and gaps need to be etched or cut into the semiconductor chip across the active layer  11 , reducing yield, potentially impacting reliability, and increasing manufacturing costs. 
     The prior art is lacking an edge-emitting laser diode light source suitable for a range imaging system that would be inexpensive, compact, and reliable, while allowing light from many individual laser diode chips be easily combined to form a single powerful laser beam. Accordingly, it is an object of the present invention is to overcome the shortcomings of the prior art by providing an edge-emitting semiconductor light source suitable for a range imaging system. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, there is provided a method of manufacturing a light source, comprising: 
     (a) providing a heat sink and a first semiconductor chip having an edge for emitting light; 
     (b) mounting the first semiconductor chip flat on the heat sink proximate the outer perimeter of the heat sink, the edge facing outwards; 
     (c) mounting a reflector to an outer perimeter of the heat sink, the reflector comprising a reflecting surface for redirecting light emitted from the edge to propagate substantially perpendicular to the first semiconductor chip; and 
     (d) optically coupling the reflector to the edge of the first semiconductor chip. 
     In accordance with another aspect of the invention, there is further provided a light source comprising: 
     a heat sink; 
     a first semiconductor chip having an edge for emitting light, wherein the first semiconductor chip is disposed flat on the heat sink proximate the outer perimeter thereof, the edge facing outwards; and 
     a reflector affixed to an outer perimeter of the heat sink and optically coupled to the edge of the first semiconductor chip, the reflector comprising a reflecting surface for redirecting light emitted from the edge to propagate substantially perpendicular to the first semiconductor chip. 
     In a preferred embodiment of the invention, a plastic reflector is overmolded onto a round flat heat sink supporting multiple laser diode chips laying flat on the heat sink, emitting edges facing outwards and towards the plastic reflector. The reflector can be manufactured cheaply using injection molding or overmolding directly onto the heat sink. The overmolded reflector can encapsulate the laser diode chips for environmental protection. The reflector has a shape suitable for light gathering from the laser diode chips and redirecting the light in an upward direction away from the heat sink, approximately perpendicular to the heat sink. Preferably, the reflector operates by total internal reflection, such that no additional reflector coating step is required. This solution allows very simple, inexpensive, yet fast and powerful light sources for range imaging to be mass produced at low cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments will now be described in conjunction with the drawings, in which: 
         FIG. 1  is a side cross-sectional view of a prior-art light source; 
         FIG. 2A  is a plan view of a light source of the invention; 
         FIG. 2B  is a side cross-sectional view of  FIG. 2A  taken along lines B-B; 
         FIG. 3  is a side cross-sectional view of a reflector used in the light source of  FIGS. 2A and 2B , showing coordinates used to define the shape of its reflective surface; 
         FIG. 4  is a side cross-sectional view of a range imaging system of the invention; 
         FIG. 5  is a magnified cross-sectional view of  FIG. 3  showing a diffractive optical element mounted on the reflector; and 
         FIG. 6  is a block diagram of a method of manufacturing the light source of  FIGS. 2A and 2B . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives, modifications and equivalents, as will be appreciated by those of skill in the art. 
     Referring to  FIGS. 2A and 2B , a light source  20  of the invention includes a flat heat sink  21  and six edge-emitting laser diode chips  22  disposed flat on the heat sink  21 . A reflector  23  is affixed to an outer perimeter  24  of the heat sink  21  and optically coupled to emitting edges  25  of the laser diode chips  22 , for redirecting light  26  emitted from the emitting edges  25  to propagate substantially perpendicular to the semiconductor chips  22 . Herein, the term “substantially perpendicular” includes propagation of light away from the plane of the heat sink  21  and/or the plane of the laser diode chips  22 , and thus can include directions of propagation that are not geometrically perpendicular to the semiconductor chips  22 , as long as the light  26  emitted by the edge-emitting laser diode chips  22  can be gathered in a single direction, for example the vertical direction, as seen in  FIG. 2B , for illuminating an object disposed out of plane of the heat sink  21  and/or the laser diode chips  22 . 
     The semiconductor chips  22  are disposed proximate the outer perimeter  24  of the heat sink  21 , the emitting edges  25  facing outwards. In the embodiment shown, the heat sink  21  has a round shape, and the reflector  23  is preferably injection-molded out of a thermoplastic material into a ring shape having inner and outer perimeters  23 A and  23 B, respectively, and a concave reflecting surface  23 C extending therebetween for reflecting the light  26 . The inner perimeter  23 A of the reflector  23  matches the outer perimeter  24  of the heat sink  21 . The reflector  23  is mounted, preferably overmolded, along its inner perimeter  23 A to the outer perimeter  24  of the heat sink  21 . The laser chips  22  are mounted in equiangular increments of 60 degrees, although other mounting geometries are of course possible. Drops of an index-matching gel  27  are placed between the emitting edges  25  of the laser diode chips  22 , on one hand, and the reflector  23 , on the other, for optical coupling therebetween. As is known to a person skilled in the art, the refractive index of the index-matching gel is selected to be between the refractive indices of the semiconductor chips  22  and the reflector  23 . The closer the matching of the refractive indices is, the smaller the reflective loss at the interface between the emitting edges  25  and the reflector  23 . An optional coating, not shown, of the laser emitting edge  25 , would need to be modified to optimize the laser power performance, because the reflectivity at the emitting edge  15  is impacted by a surrounding medium, in this case the index-matching gel  27 . Driver circuits  28  for driving the laser diode chips  22 , connected to the laser diode chips  22  via wirebonds  29 , can be conveniently disposed on the heat sink  21 , as best seen in  FIG. 2A . Wires  37  from the driver circuits  28  can be conveniently fed through an opening  30  in the center of the heat sink  21 . 
     In a preferred embodiment, the reflector  23  is configured for total internal reflection (TIR) of the emitted light  26 . To meet the condition for TIR, the angle of incidence of the light  26  emitted from the laser diode chips  22  needs to be greater than arcsin(1/n), where n is the relative index of refraction of the optical material making up the reflector  23  relative to the surrounding medium, in most cases air. In practical terms, that means that the index of refraction n of the reflector  23  needs to be sufficiently high for the TIR of the emitted light  26  to occur. In practice, index of refraction of 1.45 or higher is sufficient for most cases. 
     For any light ray meeting the TIR condition, the reflectivity is 100%, as compared to about 80% reflectivity of a typical metallic reflective overcoating of a plastic. Thus, TIR can considerably improve the light throughput of the reflector  23 . Furthermore, the optical damage threshold of an uncoated optical material is generally much higher than of its coated counterpart. 
     For a given optical material and a given direction of reflection, shape of the reflecting surface  23 C of the reflector  23  can be optimized for capturing most of the laser beam  26  of the laser diode chips  22  and directing the beam  26  towards the target. For example, when the index of refraction is 1.55, and the reflection is strictly perpendicular to the plane of the laser diode chips  22 , rays within ±24.6° from the chief ray meet the total internal reflection condition in an ellipsoid with a 0.707 major axis to minor axis ratio when the light source is placed at one of the foci of the ellipsoid. This angular range covers most of the beam divergence of the laser diode chips  22 , capturing at least 99% of the emitted optical power at full width at half maximum (FWHM) of 18 degrees. In practice, capturing at least 90% of light can be targeted. 
     Table 1 below summarizes various possible types of the reflective surface  23 C and achievable divergence ranges meeting the TIR condition.  FIG. 3  illustrates the coordinates x, z, γ, and θ used to define the reflective surface  23 C. In  FIG. 3 , x is a lateral coordinate connecting foci  31 ,  32  of the reflective surface  23 C; z is a linear coordinate perpendicular to x; γ is an angle of tilt of the x, z coordinate system relative to a perpendicular  33  to the semiconductor chip  22 ; and θ is a polar angle measured from the x axis to a point  34  of the reflective surface  23 C. The emitting edge  25  is placed at the first focus  31 , and an object to be illuminated  35  is placed at the second focus  32 . A chief ray  36  is reflected at 45 degrees, turning by 90 degrees upon reflecting from the reflecting surface  23 C, although other reflection angles can be used. The tilt angle γ is selected according to a preferred angle of illumination of the object  35 . For the ellipsoidal surface presented in Table 1, the tilt angle γ is 135 degrees. The refractive index n of the reflector  23  is 1.55. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Angular range 
                   
               
               
                   
                 Location 
                 from chief ray 
                   
               
               
                 Shape of the 
                 of the 
                 36 for total 
                   
               
               
                 reflective 
                 emitting 
                 internal 
                 Beam characteristics after the 
               
               
                 surface 23C. 
                 edges 25 
                 reflection 
                 reflector 
               
               
                   
               
             
             
               
                 Flat 
                 anywhere 
                 +90° to −4.8° 
                 Diverging. The virtual point source is 
               
               
                   
                   
                   
                 the image of the emitting edge 25 of the 
               
               
                   
                   
                   
                 laser diode chip 22. 
               
               
                 Parabolic 
                 Focal 
                 +90° to −19.6° 
                 Collimated. 
               
               
                   
                 point 
                   
                   
               
               
                   
               
               
                 Ellipsoidal (x/a) 2  + (z/b) 2  = 1 
                 Focal point 
                 +24.6° to −24.6° 
       (     b   =     a     2         )       
 
                 Converging to another focal point, then diverging. 
               
               
                   
               
               
                 Hyperbolic 
                 Focal 
                 +90° to −9.3° 
                 Diverging; the virtual source is at 
               
               
                 (x/a) 2  − (z/b) 2  = 1 
                 point 
                 (b = 0.3a) 
                 another (second) focal point. 
               
               
                 logarithmic 
                 Origin of 
                 All. 
                 Diverging; some rays can hit the 
               
               
                 spiral 
                 the spiral 
                   
                 reflective surface 23C. more than once; 
               
               
                 x = e θ   
                   
                   
                 for this surface type, the x coordinate 
               
               
                   
                   
                   
                 has the origin at the emitting edge 25. 
               
               
                   
               
             
          
         
       
     
     In  FIG. 3 , the reflector  23  is completely overmolded over the semiconductor chips  22  for environmental protection of the latter. When the semiconductor chips  22  are overmolded with the reflector  23 , drops of the index matching gel  27  is not required. Alternatively, a thermoplastic material can be overmolded over the semiconductor chips  22  to optically couple the emitting edges  25  of the semiconductor chips  22  to the pre-installed reflector  23 , and to encapsulate the semiconductor chips  22  for environmental protection of the latter. 
     Other types of the reflecting surface  23 C are possible, including non-rotationally-symmetric surfaces, ellipsoidal/hyperbolic or otherwise, and/or convex surfaces for better spreading of the illuminating light  26 . The number of the laser diode chips  22  can vary from a single chip  22  to three or more chips  22  and even sixteen or more chips  22 . The maximum number of semiconductor chips  22  can be estimated from the formula
 
Max. Number of Chips=135°/(Full Width at Half Maximum of beam divergence in a lateral direction)  (1)
 
     More semiconductor chips  22  can be disposed on the heat sink  21 , but the outer portion of the light beam  26  from each semiconductor chip  22  would hit the surface  23 C that is optimized for an adjacent semiconductor chip  22 . In other words, only the inner portions of the beams  26  would be optimally reflected. 
     Furthermore, not only laser diode chips  22 , but in principle, other semiconductor chips  22  capable of emitting light, such as light emitting diodes, can be used as well. The semiconductor chips  22  can be positioned anywhere proximate the outer perimeter  24  of the heat sink  21 , as long as the light  26  can be coupled to the reflector  23  mounted to the outer perimeter  24  of the heat sink  21 . A reflector coating, not shown, can be used on the reflecting surface  23 C, although TIR reflecting surface  23 C is preferable for cost, power handling, and optical throughput reasons. 
     Turning now to  FIG. 4 , a range imaging system  40  of the invention includes the light source  20  mounted on a base  41 , and a camera  42  mounted to the base  41  proximate the light source  20 . The base  41  can include, or be mounted on, a computer display, a television set cover, a cell phone cover, etc. In operation, the light source  20  emits pulsed or modulated light  46  to illuminate an object, for example a user  43 . The camera  42  obtains three-dimensional images of the user  43 , which are then processed by a gesture recognition system, not shown, to determine gestures of the user  43  in real time. 
     Additional optics can be used to reshape and direct the pulsed or modulated light  46  towards the user  43 . Referring now to  FIG. 5 , a diffractive optic  50  is disposed on an outer surface  51  of the reflector  23  for redirecting the beams  46 . Alternatively, the outer surface  51  can be concave, convex, etc., or include a refractive and/or a diffractive element for modifying the angular distribution of the beams  46  to propagate substantially perpendicular to the semiconductor chips  22 , or to create an angular distribution of the light beams  46 , appropriate for the illumination task at hand. The refractive surface can include a Fresnel refractive surface, a binary diffractive pattern, and the like. The optic  50  allows one to select the shape of the reflecting surface  23 C that captures the emitted light  26  most efficiently through TIR, and then to select the optic  50  to redirect the beams  46  onto a target in a most efficient manner, thus decoupling the TIR and the target illumination requirements from each other. 
     Turning to  FIG. 6 , a method  60  of manufacturing the light source  20  includes a step  61  of providing the heat sink  21  and at least one of the laser diode chips  22 . In a step  62 , the semiconductor chip  22  is mounted flat on the heat sink. In a step  63 , the reflector  23  is mounted, for example molded or overmolded, to the outer perimeter  24  of the heat sink  21 . Finally, in a step  64 , the reflector  23  is optically coupled to the edge  25  of the semiconductor chip  22 , preferably using the index matching gel  27  or another index-matching material. The laser diode chip  22  is preferably mounted proximate the outer perimeter  24  of the heat sink  21 , for example flash with the outer perimeter  24 , the emitting edge  25  facing outwards, towards to the reflector  23 . The reflector  23  is preferably injection molded out of a thermoplastic material, as is well known to a person skilled in the art. In one embodiment, the reflector  23  is injection-molded or overmolded directly to the heat sink  21  in step  63 . In one embodiment, the reflector  23  is overmolded over the semiconductor chips  22 , thus uniting the two last steps  63  and  64  into a single step. Alternatively, the reflector  23  can be pre-installed, and an additional thermoplastic can be overmolded between the reflector  23  and the semiconductor chips  22 , preferably to completely encapsulate the semiconductor chips  22  for the environmental protection of the latter. Although a reflector coating, not shown, can be applied to the reflecting surface  23 C of the reflector  23 , it is preferable that the molded reflector  23  have an index of refraction sufficiently high for the TIR of the emitted light  26  by the uncoated reflecting surface  23 C. In this way, the reflector  23  can be inexpensively mass produced in a single injection molding operation. 
     Although the heat sink  21  can have many different shapes, a round shape is generally preferable; for the round heat sink  21 , the reflector  23  can be injection molded into a ring shape having the inner  23 A and outer  23 B perimeters, the reflecting surface  23 C extending therebetween, as shown in  FIGS. 2A ,  2 B,  FIG. 3 , and  FIG. 5 . For round reflectors  23 , a plurality of the laser diode chips  22  can be disposed, for example, three, six, twelve laser diode chips  22 , etc., preferably in equiangular increments to save space, around the outer perimeter  24  of the heat sink  21 , and optically coupled to the same reflector  23 . 
     The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.