Semiconductor light source having a reflector

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.

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 toFIG. 1, a laser diode10includes an active layer11sandwiched between p- and n-layers12and13, respectively. The n-layer13includes two sub-layers,13A and13B. A V-shaped groove14is etched into the p-layer12and the second n-sublayer13B from the p-layer12side. P- and n-electrodes15and16contact the p- and n-layers12and13, respectively. The p-electrode15can be made sufficiently thick to serve as a heat sink. Gaps17are cut into the p- and n-layers12and13and into the active layer11, to function as laser cavity mirrors. In operation, an electric current is applied between the p- and n-electrodes15and16, respectively, and generated light18is reflected from inside the faces of the grooves14, exiting through cut-outs19in the n-electrodes16.

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 layer11, 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.

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 toFIGS. 2A and 2B, a light source20of the invention includes a flat heat sink21and six edge-emitting laser diode chips22disposed flat on the heat sink21. A reflector23is affixed to an outer perimeter24of the heat sink21and optically coupled to emitting edges25of the laser diode chips22, for redirecting light26emitted from the emitting edges25to propagate substantially perpendicular to the semiconductor chips22. Herein, the term “substantially perpendicular” includes propagation of light away from the plane of the heat sink21and/or the plane of the laser diode chips22, and thus can include directions of propagation that are not geometrically perpendicular to the semiconductor chips22, as long as the light26emitted by the edge-emitting laser diode chips22can be gathered in a single direction, for example the vertical direction, as seen inFIG. 2B, for illuminating an object disposed out of plane of the heat sink21and/or the laser diode chips22.

The semiconductor chips22are disposed proximate the outer perimeter24of the heat sink21, the emitting edges25facing outwards. In the embodiment shown, the heat sink21has a round shape, and the reflector23is preferably injection-molded out of a thermoplastic material into a ring shape having inner and outer perimeters23A and23B, respectively, and a concave reflecting surface23C extending therebetween for reflecting the light26. The inner perimeter23A of the reflector23matches the outer perimeter24of the heat sink21. The reflector23is mounted, preferably overmolded, along its inner perimeter23A to the outer perimeter24of the heat sink21. The laser chips22are mounted in equiangular increments of 60 degrees, although other mounting geometries are of course possible. Drops of an index-matching gel27are placed between the emitting edges25of the laser diode chips22, on one hand, and the reflector23, 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 chips22and the reflector23. The closer the matching of the refractive indices is, the smaller the reflective loss at the interface between the emitting edges25and the reflector23. An optional coating, not shown, of the laser emitting edge25, would need to be modified to optimize the laser power performance, because the reflectivity at the emitting edge15is impacted by a surrounding medium, in this case the index-matching gel27. Driver circuits28for driving the laser diode chips22, connected to the laser diode chips22via wirebonds29, can be conveniently disposed on the heat sink21, as best seen inFIG. 2A. Wires37from the driver circuits28can be conveniently fed through an opening30in the center of the heat sink21.

In a preferred embodiment, the reflector23is configured for total internal reflection (TIR) of the emitted light26. To meet the condition for TIR, the angle of incidence of the light26emitted from the laser diode chips22needs to be greater than arcsin(1/n), where n is the relative index of refraction of the optical material making up the reflector23relative to the surrounding medium, in most cases air. In practical terms, that means that the index of refraction n of the reflector23needs to be sufficiently high for the TIR of the emitted light26to 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 reflector23. 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 surface23C of the reflector23can be optimized for capturing most of the laser beam26of the laser diode chips22and directing the beam26towards 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 chips22, 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 chips22, 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 surface23C and achievable divergence ranges meeting the TIR condition.FIG. 3illustrates the coordinates x, z, γ, and θ used to define the reflective surface23C. InFIG. 3, x is a lateral coordinate connecting foci31,32of the reflective surface23C; z is a linear coordinate perpendicular to x; γ is an angle of tilt of the x, z coordinate system relative to a perpendicular33to the semiconductor chip22; and θ is a polar angle measured from the x axis to a point34of the reflective surface23C. The emitting edge25is placed at the first focus31, and an object to be illuminated35is placed at the second focus32. A chief ray36is reflected at 45 degrees, turning by 90 degrees upon reflecting from the reflecting surface23C, although other reflection angles can be used. The tilt angle γ is selected according to a preferred angle of illumination of the object35. For the ellipsoidal surface presented in Table 1, the tilt angle γ is 135 degrees. The refractive index n of the reflector23is 1.55.

TABLE 1Angular rangeLocationfrom chief rayShape of theof the36 for totalreflectiveemittinginternalBeam characteristics after thesurface 23C.edges 25reflectionreflectorFlatanywhere+90° to −4.8°Diverging. The virtual point source isthe image of the emitting edge 25 of thelaser diode chip 22.ParabolicFocal+90° to −19.6°Collimated.pointEllipsoidal (x/a)2+ (z/b)2= 1Focal point+24.6° to −24.6°(b=a2)Converging to another focal point, then diverging.HyperbolicFocal+90° to −9.3°Diverging; the virtual source is at(x/a)2− (z/b)2= 1point(b = 0.3a)another (second) focal point.logarithmicOrigin ofAll.Diverging; some rays can hit thespiralthe spiralreflective surface 23C. more than once;x = eθfor this surface type, the x coordinatehas the origin at the emitting edge 25.

InFIG. 3, the reflector23is completely overmolded over the semiconductor chips22for environmental protection of the latter. When the semiconductor chips22are overmolded with the reflector23, drops of the index matching gel27is not required. Alternatively, a thermoplastic material can be overmolded over the semiconductor chips22to optically couple the emitting edges25of the semiconductor chips22to the pre-installed reflector23, and to encapsulate the semiconductor chips22for environmental protection of the latter.

Other types of the reflecting surface23C are possible, including non-rotationally-symmetric surfaces, ellipsoidal/hyperbolic or otherwise, and/or convex surfaces for better spreading of the illuminating light26. The number of the laser diode chips22can vary from a single chip22to three or more chips22and even sixteen or more chips22. The maximum number of semiconductor chips22can 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 chips22can be disposed on the heat sink21, but the outer portion of the light beam26from each semiconductor chip22would hit the surface23C that is optimized for an adjacent semiconductor chip22. In other words, only the inner portions of the beams26would be optimally reflected.

Furthermore, not only laser diode chips22, but in principle, other semiconductor chips22capable of emitting light, such as light emitting diodes, can be used as well. The semiconductor chips22can be positioned anywhere proximate the outer perimeter24of the heat sink21, as long as the light26can be coupled to the reflector23mounted to the outer perimeter24of the heat sink21. A reflector coating, not shown, can be used on the reflecting surface23C, although TIR reflecting surface23C is preferable for cost, power handling, and optical throughput reasons.

Turning now toFIG. 4, a range imaging system40of the invention includes the light source20mounted on a base41, and a camera42mounted to the base41proximate the light source20. The base41can include, or be mounted on, a computer display, a television set cover, a cell phone cover, etc. In operation, the light source20emits pulsed or modulated light46to illuminate an object, for example a user43. The camera42obtains three-dimensional images of the user43, which are then processed by a gesture recognition system, not shown, to determine gestures of the user43in real time.

Additional optics can be used to reshape and direct the pulsed or modulated light46towards the user43. Referring now toFIG. 5, a diffractive optic50is disposed on an outer surface51of the reflector23for redirecting the beams46. Alternatively, the outer surface51can be concave, convex, etc., or include a refractive and/or a diffractive element for modifying the angular distribution of the beams46to propagate substantially perpendicular to the semiconductor chips22, or to create an angular distribution of the light beams46, appropriate for the illumination task at hand. The refractive surface can include a Fresnel refractive surface, a binary diffractive pattern, and the like. The optic50allows one to select the shape of the reflecting surface23C that captures the emitted light26most efficiently through TIR, and then to select the optic50to redirect the beams46onto a target in a most efficient manner, thus decoupling the TIR and the target illumination requirements from each other.

Turning toFIG. 6, a method60of manufacturing the light source20includes a step61of providing the heat sink21and at least one of the laser diode chips22. In a step62, the semiconductor chip22is mounted flat on the heat sink. In a step63, the reflector23is mounted, for example molded or overmolded, to the outer perimeter24of the heat sink21. Finally, in a step64, the reflector23is optically coupled to the edge25of the semiconductor chip22, preferably using the index matching gel27or another index-matching material. The laser diode chip22is preferably mounted proximate the outer perimeter24of the heat sink21, for example flash with the outer perimeter24, the emitting edge25facing outwards, towards to the reflector23. The reflector23is preferably injection molded out of a thermoplastic material, as is well known to a person skilled in the art. In one embodiment, the reflector23is injection-molded or overmolded directly to the heat sink21in step63. In one embodiment, the reflector23is overmolded over the semiconductor chips22, thus uniting the two last steps63and64into a single step. Alternatively, the reflector23can be pre-installed, and an additional thermoplastic can be overmolded between the reflector23and the semiconductor chips22, preferably to completely encapsulate the semiconductor chips22for the environmental protection of the latter. Although a reflector coating, not shown, can be applied to the reflecting surface23C of the reflector23, it is preferable that the molded reflector23have an index of refraction sufficiently high for the TIR of the emitted light26by the uncoated reflecting surface23C. In this way, the reflector23can be inexpensively mass produced in a single injection molding operation.

Although the heat sink21can have many different shapes, a round shape is generally preferable; for the round heat sink21, the reflector23can be injection molded into a ring shape having the inner23A and outer23B perimeters, the reflecting surface23C extending therebetween, as shown inFIGS. 2A,2B,FIG. 3, andFIG. 5. For round reflectors23, a plurality of the laser diode chips22can be disposed, for example, three, six, twelve laser diode chips22, etc., preferably in equiangular increments to save space, around the outer perimeter24of the heat sink21, and optically coupled to the same reflector23.