Patent Publication Number: US-9426435-B2

Title: Scanning laser projector

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This case is a continuation-in-part of co-pending U.S. patent application Ser. No. 13/208,806, entitled “Beam Combiner,” filed Aug. 12, 2011, which claims the benefit of European Patent Application EP10008424.3, filed Aug. 12, 2010, U.S. Provisional Application Ser. No. 61/344,553, filed Aug. 19, 2010, U.S. Provisional Application Ser. No. 61/376,483, filed Aug. 24, 2010, and U.S. Provisional Application Ser. No. 61/477,960, filed Apr. 21, 2011. All of these cases are incorporated herein by reference 
     If there are any contradictions or inconsistencies in language between this application and one or more of the cases that have been incorporated by reference that might affect the interpretation of the claims in this case, the claims in this case should be interpreted to be consistent with the language in this case. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to projectors in general, and, more particularly, to multi-wavelength laser projectors. 
     BACKGROUND OF THE INVENTION 
     The popularity of mobile digital devices, such as smart phones, personal data assistants, digital cameras, etc., has increased dramatically in recent years. As a result, a typical device user routinely carries vast amounts of digital information in their pockets. In addition, access to the Internet is rapidly becoming ubiquitous, increasing the amount of information at a user&#39;s fingertips dramatically. Much of this information is in the form of media, such as web pages, videos, live television, photographs, books, and the like, which is meant for display. Unfortunately, the small form-factor of many mobile digital devices, while enabling easy mobility, does not lend itself well to the display of the stored information. As a result, portable projectors (often referred to as “pico-projectors”) that attach to the mobile devices and project the information onto a convenient surface have become increasingly attractive. Pico-projectors have found use in many applications, including heads-up displays in cars and planes, business presentations, and entertainment. 
     One popular type of projector is based on the projection of laser light. Such projectors are enabled, in part, by the development of miniature solid-state lasers. In order to project color images, light signals from multiple lasers (typically emitting red, green, and blue light) are combined using free-space optics to form a composite light beam that is then scanned over the intended display region. Unfortunately, widespread adoption of such projectors remains slow due to their relatively large size. 
     Another available projector type is based on liquid crystals (LC&#39;s). LC-based projectors either project an image through a liquid-crystal display, or construct the image by reflection onto a liquid-crystal-on-silicon (LCoS) display using light bulbs or LEDs as light sources. 
     Unfortunately, currently available pico-projectors are still relatively large, complex, and expensive. Further, projectors based on free-space optical systems require labor-intensive assembly. As a result, it is difficult to manufacture such projectors in high volume at low cost. 
     SUMMARY OF THE INVENTION 
     The present invention enables a compact laser-beam projector that overcomes some of the costs and disadvantages of the prior art. Embodiments of the present invention are particularly well suited for use in applications such as entertainment, medical diagnostics, cancer treatment, insect control, thermal hot spot mapping for integrated circuits or printed-circuit boards, and virtual keyboards. 
     The present invention provides a platform for combining a plurality of light signals of different wavelengths into a single output beam that can be scanned to render an image on a surface. Embodiments of the present invention are based on planar lightwave circuit-based beam combiners that can combine a plurality of light signals of disparate wavelengths over a wide wavelength range. The beam combiner comprises planar lightwave circuit having a plurality of input ports, a mixing region, and an output port, wherein the mixing region includes a plurality of directional couplers that are arranged in a tree structure. The planar lightwave circuits are based on single-mode surface waveguides having a core comprising stoichiometric silicon nitride and cladding of stoichiometric silicon dioxide. 
     Beam combiners in accordance with the present invention enable low-cost, automated assembly, hybrid integration of the beam combiners and light sources that provide the constituent light signals. Embodiments of the present invention, therefore, can be less expensive and/or smaller than laser projectors of the prior art. Further, planar lightwave circuit-based beam combiners can be more robust and less sensitive to shock and vibration than free-space beam combiners, resulting in laser projectors that are more robust than typical prior-art laser projectors. 
     In some embodiments, at least one input port has a mode-matching region that enables direct, low-loss optical coupling of the output facet of a laser diode to the beam combiner. The mode-matching region includes a waveguide region that is tapered from an end facet to an interface so that the effective refractive-index contrast at the end facet is lower or higher than the effective refractive-index contrast at the interface. At the interface, the optical mode is mode-matched to the waveguide structure that forms the bulk of the planar lightwave circuit. 
     In some embodiments, the beam combiner comprises waveguide-based attenuators for controlling the intensity one or more of the light signals combined in the composite output signal. 
     In some embodiments, reflected light from each image point in the image region is used to determine one or more measurands of that image point. At least one light signal in the output beam is selected so that the measurand induces a detectable difference in its reflected light. In some embodiments, when a difference in the reflected light is detected, one or more additional light signals of different wavelengths are added to the output beam and are simultaneously projected onto the surface. 
     An embodiment of the present invention is a projector for forming a real-time image of the vein structure in a region of a patient&#39;s body on the surface of the skin. The illustrative embodiment comprises a first light source that emits a light signal of a first wavelength that is readily absorbed by the blood carried in the veins. This light signal is collimated to form an output beam that is scanned over a region of the skin, which reflects a portion of the output beam to a photodetector. When an image point comprises a portion of a vein, the intensity of the reflected light decreases, thereby indicating the presence of the vein. In response to a decrease in the reflected signal, the projector adds a second light signal of a second wavelength to the output beam, wherein the second wavelength is readily visible to a user—thereby enhancing visibility of that image point to the user. The projector scans the entire region at a rate sufficient to generate a real-time image of the sub-surface vein structure on the skin. 
     An embodiment of the present invention is a scanning projector comprising: a first light source that provides a first light signal characterized by a first wavelength; a second light source that provides a second light signal characterized by a second wavelength; a beam combiner comprising a planar lightwave circuit including a plurality of waveguides that are arranged to define a first input port, a second input port, a mixing region, and an output port, the beam combiner: (1) receiving the first light signal at the first input port, (2) receiving the second light signal at the second input port, (3) combining the first light signal and the second light signal into a composite output signal, and (4) providing the composite output signal at the output port; and a scanner, the scanner receiving the composite output signal from the beam combiner and scanning the composite output signal over each of a plurality of image points in a first region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a schematic drawing of a scanning laser projector in accordance with the prior art. 
         FIG. 2  depicts a schematic drawing of a scanning projector in accordance with an illustrative embodiment of the present invention. 
         FIG. 3  depicts operations of a method for projecting an image in accordance with the illustrative embodiment of the present invention. 
         FIG. 4A  depicts a schematic drawing of a cross-sectional view of a waveguide in accordance with the illustrative embodiment of the present invention. 
         FIG. 4B  depicts a schematic drawing of a cross-sectional view of a waveguide in accordance with a first alternative embodiment of the present invention. 
         FIG. 5A  depicts a schematic drawing of a top view of an input port comprising a mode-matching region in accordance with the illustrative embodiment of the present invention. 
         FIG. 5B  depicts a schematic drawing of a cross-sectional view of end facet  504 - i.    
         FIG. 5C  depicts a schematic drawing of a cross-sectional view of interface  506 - i.    
         FIG. 6  depicts a schematic drawing of mixing region  214 . 
         FIG. 7  depicts a schematic drawing of a power controller in accordance with the illustrative embodiment of the present invention. 
         FIG. 8  depicts a schematic drawing of a scanning projector in accordance with a second alternative embodiment of the present invention. 
         FIG. 9  depicts operations of a method for enhancing the visibility of subcutaneous vein structure in accordance with the second alternative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts a schematic drawing of a scanning laser projector in accordance with the prior art. Projector  100  comprises light sources  102 - 1  through  102 - 3 , beam combiner  106 , scanner  108 , and processor  126 . Examples of conventional scanning laser projectors can be found in, for example, U.S. Pat. No. 7,978,189, issued Jul. 12, 2011, which is incorporated herein by reference. 
     Light sources  102 - 1  through  102 - 3  are laser diodes that provide narrow spectral-width light signals  104 - 1  through  104 - 3 , respectively (referred to, collectively, as light signals  104 ). Light source  102 - 1  provides light signal  104 - 1 , which is a light signal of a wavelength in the blue light region (e.g., 445 nm). Light source  102 - 2  provides light signal  104 - 2 , which is a light signal of a wavelength in the green light region (e.g., 532 nm). Light source  102 - 3  provides light signal  104 - 3 , which is a light signal of a wavelength in the red light region (e.g., 640 nm). Each of light sources  102 - 1  through  102 - 3  includes beam-shaping optic  110  that shapes each of light signals  104 - 1  through  104 - 3  into substantially collimated beams and directs them along axes  112 - 1 ,  112 - 2 , and  112 - 3 , respectively. 
     Beam combiner  106  comprises selective fold mirrors  114 - 1  and  114 - 2 . Selective fold mirror  114 - 1  is a dichroic mirror that is transparent for light signal  104 - 1  but substantially reflects light signal  104 - 2 . Selective fold mirror  114 - 1  is aligned at an angle of 45° with respect to each of axes  110 - 1  and  110 - 2 . 
     Selective fold mirror  114 - 2  is a dichroic mirror that is transparent for light signals  104 - 1  and  104 - 2  but reflects light signal  104 - 3 . Selective fold mirror  114 - 2  is aligned at an angle of 45° with respect to each of axes  110 - 1  and  110 - 3 . Typically, light sources  102 , beam-shaping optics  110 , and mirrors  114 - 1  and  110 - 2  are mounted in a rigid fixture, such as an optical sub-mount, which keeps them in their relative positions. 
     In operation, beam combiner  106  receives each of light signals  104 - 1  and  104 - 2  such that the light signals are coincident at the center of selective fold mirror  114 - 1 . Selective fold mirror  114 - 1  allows light signal  104 - 1  to pass through and continue along axis  110 - 1  but turns light signal  104 - 2  from axis  110 - 2  to axis  110 , where it combines with light signal  104 - 1  to form dual-wavelength light signal  116 . 
     In similar fashion, beam combiner  106  receives light signal  104 - 3  such that it is coincident with light signal  116  at the center of selective fold mirror  114 - 2 . Selective fold mirror  114 - 2  allows light signal  116  to pass through and continue along axis  110 - 1  but turns light signal  104 - 3  from axis  110 - 3  to optical axis  110 - 1 , where it combines with light signal  116  to collectively define composite output signal  118 . Composite output signal  118  exits beam combiner  106  as a collimated free-space beam  120 . 
     Scanner  108  receives beam  120  and steers it about a two-dimensional cone to render an image on image region  122  of surface  124 . Scanner  108  is a conventional scanner, such as a two-axis gimbal-mounted MEMS mirror, a galvo scanner, and the like. Processor  126  controls the angular position of scanner  108  and, therefore, the position of beam  120  in image region  122 . 
     Processor  126  is typically a conventional digital video processor suitable for interfacing with a video source (e.g., cell phone, PDA, etc.), modulating light sources  102 - 1  through  102 - 3  to control the relative intensities of light signals  104 - 1  through  104 - 3 , and controlling scanner  108  so that it traces beam  120  appropriately across image region  122 . 
     Processor  126  normally comprises a video controller that receives an input video signal from the video source and buffers the received video images in memory. To display a video frame, the controller reads the a stored video frame from the memory and drives light sources  102  so that they emit their respective light signals at the appropriate intensity for generating the desired color and brightness at each image pixel  128  as beam  120  is scanned across image region  122 . 
     Projectors based on free-space beam combiners, such as beam combiner  106 , are beset by several drawbacks, however. First, free-space beam combiners require optical elements that are relatively large and bulky, which results in a projector that is also relatively large and bulky. Typically, scanning laser projectors are intended for use with that is roughly pocketsize. A reliance on free-space beam combiners has resulted in conventional laser projectors that are much larger than the video sources, however. As a result, the relatively large size of conventional scanning projectors has, thus far, limited their widespread adoption. 
     Second, free-space beam combiners convey light through a medium (e.g., air, glass, etc.) that provides no light-guiding capability. As a result, it is necessary to include beam-shaping optics, such as a collimator, for shaping each constituent light signal prior to its receipt by the beam combiner to ensure that each beam has substantially the same cross-sectional shape when combined. The need for beam-shaping optics adds significant system cost and complexity. 
     Third, the assembly of the optical elements of a free-space beam combiner is typically highly labor intensive. These optical elements must be carefully aligned in both position and angle to ensure that the constituent light signals are completely overlapping to avoid spectral non-uniformity through the cross-section of the composite output beam. In addition, angular misalignment of one or more of constituent light signals can lead to divergence of those light signals as they propagate through the beam combiner. The assembly of these optical elements becomes increasingly more difficult as additional light signals are included. Further, high-speed volume manufacture of free-space beam combiners is difficult in a cost-effective manner. Still further, the sources, mirrors, and lenses are normally aligned and positioned by mounting them in an optical fixture. Unfortunately, such fixtures are susceptible to temperature-induced misalignments (due to thermal expansion), as well as misalignments caused by shock and vibration that commonly occur through the lifetime of the projector. 
     Fourth, geometric distortion of the constituent light signals (e.g., light signals  102 - 2  and  102 - 3 ) occurs at each turning mirror due to the difference in the angle of incidence in the x- and y-directions between the beams and the mirrors (e.g., mirrors  114 - 1  and  114 - 2 ). As a result, it is often necessary to include beam-shape compensation optics, which further increases cost, complexity, and system size. 
     Fifth, conventional laser projectors rely on direct modulation of the laser sources to control the color of the composite output signal. Unfortunately, changing the drive current to a laser diode can lead to deleterious optical effects, such as wavelength chirping and mode hopping, that manifest as unwanted visible artifacts in the projected image. In addition, some laser sources, such as frequency-doubled lasers or diode-pumped solid-state lasers, cannot be controlled using direct current drive modulation and, therefore, rely upon external modulators, such as acousto-optic modulators, to control their output. External modulation adds significant complexity and expense to such systems. 
     In contrast to projectors of the prior art, laser projectors in accordance with the present invention employ beam combiners based on planar lightwave circuits (PLCs). For the purposes of this Specification, including the appended claims, a “planar lightwave circuit” is defined as an optical circuit that comprises one or more monolithically integrated surface waveguide structures that guide light in two dimensions, wherein the surface waveguides are arranged to provide at least one optical function. Beam combiners in accordance with the present invention comprise “high-contrast” surface waveguides whose cores comprise silicon nitride. Further, beam combiners in accordance with the present invention enable a single composite light signal to be formed by combining light signals of disparate, irregularly spaced wavelengths over a wide wavelength range. 
     It should be noted that PLC-based devices exist in the prior art that can be used to combine two or more light signals of different wavelengths—namely, array waveguide gratings (AWGs). An AWG, however, requires that the wavelengths being combined be closely spaced and be regularly spaced. As a result, an AWG does not have the diversity and flexibility required for laser projector applications. 
       FIG. 2  depicts a schematic drawing of a scanning projector in accordance with an illustrative embodiment of the present invention. Projector  200  comprises light sources  202 - 1  through  202 - 3 , beam combiner  206 , lens  220 , scanner  108 , and processor  126 . Projector  200  projects a full-color image in the visible light range. 
       FIG. 3  depicts operations of a method for projecting an image in accordance with the illustrative embodiment of the present invention. Method  300  begins with operation  301 , wherein light signals  204 - 1  through  204 - 3  (referred to, collectively, as light signals  204 ) are generated by light sources  202 - 1  through  202 - 3  (referred to, collectively, as light sources  202 ). Method  300  is described with continuing reference to  FIG. 2  and reference to  FIGS. 4-7 . 
     Light sources  202 - 1  through  202 - 3  are laser diodes that provide narrow spectral-width light signals  204 - 1  through  204 - 3 , respectively (referred to, collectively, as light signals  204 ). Light source  202 - 1  provides light signal  204 - 1 , which is a light signal of a wavelength in the blue region (e.g., 445 nm). Light source  202 - 2  provides light signal  204 - 2 , which is a light signal of a wavelength in the green light region (e.g., 532 nm). Light source  202 - 3  provides light signal  204 - 3 , which is a light signal of a wavelength in the red light region (e.g., 640 nm). In some embodiments, light signals  204  are characterized by different wavelengths within the visible light region. In some embodiments, at least one of light signals  204  is characterized by a wavelength that is outside of the visible light region, such as the ultraviolet region, near-infrared region, or mid-infrared region. 
     At operation  302 , light signals  204  are combined at beam combiner  206  to form composite output signal  218 . 
     Beam combiner  206  is a PLC-based beam combiner that comprises waveguides  208 - 1  through  208 - 3 , input ports  210 - 1  through  210 - 3 , power controllers  212 - 1  through  212 - 3 , mixing region  214 , and output port  216 . 
     Each of waveguides  208 - 1  through  208 - 3  (referred to, collectively, as waveguides  208 ) is a single-mode waveguide characterized by a large difference between the refractive index of its core material and cladding material (typically referred to as “a high-contrast waveguide”). As a result, each of waveguides  208  is characterized by strong optical mode confinement, can include curved sections that have small bend radii, and can include waveguides in a densely packed arrangement. Planar lightwave circuits based on waveguide  208 , therefore, can provide a high degree of functionality in a much smaller footprint than conventional low-index waveguide-based planar lightwave circuits. 
       FIG. 4A  depicts a schematic drawing of a cross-sectional view of a waveguide in accordance with the illustrative embodiment of the present invention. Waveguide  208  is a composite-core waveguide (referred to, herein, as a TriPleX™ waveguide), such as is described in U.S. Pat. No. 7,146,087, issued Dec. 5, 2006, which is incorporated herein by reference. Waveguide  208  comprises lower cladding  402 , core  404 , and upper cladding  406 . Waveguide  208  is representative of each of waveguides  208 - 1  through  208 - 3 . 
     Waveguide  208  is based on a material system of silicon nitride and silicon dioxide. Such a waveguide can be designed for operation at any wavelength from the ultraviolet range to the mid-infrared range. As a result, beam combiner  206  enables projector  200  to project light beams of a diversity of irregularly or regularly spaced wavelengths. One skilled in the art will recognize that conventional silica-based PLC technology is not well suited to applications wherein multiple light signals characterized by disparate wavelengths over a wide wavelength range must be combined. 
     Lower cladding  402  is a layer of silicon dioxide having a thickness typically within the range of approximately 1 micron to approximately 10 microns. Lower cladding  402  can be formed by any of a number of conventional methods, including thermal growth, LPCVD deposition, spin-on coating, and the like. It will be clear to one skilled in the art how to specify, make, and use lower cladding  402 . 
     Core  404  comprises inner core  408  and outer core  410 , which completely surrounds inner core  408 . 
     Inner core  408  comprises stoichiometric silicon dioxide. Inner core  408  has a substantially square cross-sectional shape having a size, w 1 , of approximately 1 micron on a side. In some embodiments, inner core  408  has a different shape and/or different dimensions. 
     Outer core  410  comprises stoichiometric silicon nitride having a thickness equal to w 2 . In the illustrative embodiment, w 2  is approximately equal to 200 nm; however, it will be clear to one skilled in the art, after reading this Specification, how to specify, make, and use alternative embodiments of the present invention wherein w 2  is any practical size. 
     In some embodiments, outer core  410  does not completely surround inner core  410 . In some embodiments, outer core  410  comprises a layer of stoichiometric silicon nitride disposed on inner core  408  and/or a layer of stoichiometric silicon nitride that interposes inner core  408  and lower cladding  402 . 
     Upper cladding  406  is a layer of silicon dioxide formed as a conformal coating over core  404 . Upper cladding  406  is formed using conventional conformal deposition techniques, such as plasma-enhanced chemical vapor deposition, low-pressure chemical vapor deposition using tetraethyl orthosilicate (TEOS) as a precursor gas, and the like. 
       FIG. 4B  depicts a schematic drawing of a cross-sectional view of a waveguide in accordance with a first alternative embodiment of the present invention. Waveguide  412  is a silicon-nitride-core waveguide that comprises lower cladding  402 , core  414 , upper cladding  406 , and barrier layer  420 . 
     Core  414  comprises lower core  416  and upper core  418 , which is disposed on lower core  416 . 
     Lower core  416  comprises stoichiometric silicon nitride. Lower core  416  has a substantially rectangular cross-sectional shape having a width within the range of approximately 0.8 microns to approximately 3 microns and a height within the range of approximately 3 nm to approximately 30 nm. In some embodiments, lower core  416  has a different shape and/or different dimensions. 
     Upper core  418  comprises silicon dioxide deposited using TEOS as a precursor gas. Upper core  418  has a width substantially equal to the width of lower core  416  and a thickness within the range of approximately three to approximately twenty times the thickness of lower core  416 . In some embodiments, upper core  418  enables a reduction in the internal stress of lower core  416  and/or reduces scattering in lower core  416 , as well as having other positive effects that lead to a reduction in propagation loss for light propagating through waveguide  412 . One skilled in the art will recognize, after reading this Specification, that the dimensions of lower core  416  and/or upper core  418  are based on several factors, including the wavelength of light that is expected to propagate through the waveguide. 
     Upper cladding  406  is covered with optional barrier layer  420 . Barrier layer  420  comprises silicon nitride and provides a barrier to moisture and contaminants that might otherwise be absorbed by waveguide  412 . In some embodiments, barrier layer  420  is not included. 
     One skilled in the art will recognize, after reading this Specification, that the waveguide structures depicted in  FIGS. 4A and 4B  represent only two examples of waveguide structures suitable for use in beam combiner  206 . It will be clear to one skilled in the art, after reading this Specification, that waveguide structures suitable for use in beam combiner  206  include, without limitation, ridge waveguides, channel waveguides, stripe waveguides, multi-layered waveguides, femto-second laser-written waveguides, graded-index waveguides, and the like. 
     At its input, each of waveguides  208  comprises an input port that comprises a mode-matching region, which enables the waveguide to optically couple light directly from the output facet of its corresponding light source  202  without large coupling losses. Specifically, waveguide  208 - 1  comprises input port  210 - 1 , which enables low-loss optical coupling with light source  202 - 1 , waveguide  208 - 2  comprises input port  210 - 2 , which enables low-loss optical coupling with light source  202 - 2 , and waveguide  208 - 3  comprises input port  210 - 3 , which enables low-loss optical coupling with light source  202 - 3 . 
       FIG. 5A  depicts a schematic drawing of a top view of an input port comprising a mode-matching region in accordance with the illustrative embodiment of the present invention. Input port  210 - i  is representative of each of input ports  210 - 1  through  210 - 3 . 
     Mode-matching region  502 - i  is a region of waveguide  208 - i  whose cross-sectional area is gently tapered such that it increases monotonically from end facet  504 - i  to interface  506 - i  along length L. Mode-matching region  402 - i , therefore, gently transitions from (1) an effective index-contrast that yields a mode-field diameter at end facet  504 - i  that substantially matches the mode-field diameter of the light source to (2) an effective index-contrast that yields a mode-field diameter at interface  506 - i  that substantially matches the mode-field diameter of waveguide  208 - i . As a result, the optical mode of light propagating through mode-matching region  402 - i  is matched to the optical mode of the output facet of light source  202 - i  at end facet  504 - i , while the optical mode of light propagating through mode-matching region  402 - i  is matched to the optical mode of waveguide  208 - i  at interface  506 - i.    
       FIG. 5B  depicts a schematic drawing of a cross-sectional view of end facet  504 - i . End facet  504 - i  comprises lower cladding  402 , inner core  510 , outer core  512 , and upper cladding  406 . 
     Inner core  510  is analogous to inner core  408 ; however, inner core  510  has a substantially square cross-sectional shape having a size of w 1 , where w 1  is smaller than the size of inner core  408  (1 micron in the illustrative embodiment). 
     Outer core  512  is analogous to outer core  410 ; however, outer core  512  has a thickness of w 2 , where w 2  is smaller than the thickness of outer core  410  (200 nm in the illustrative embodiment). 
       FIG. 5C  depicts a schematic drawing of a cross-sectional view of interface  506 - i . Interface  506 - i  comprises lower cladding  402 , inner core  408 , outer core  410 , and upper cladding  406 . In other words, interface  506 - i  has the same dimensions and layer structure as waveguide  208 . As a result, light propagating through mode-matching region  503 - i  experiences substantially lossless (i.e., adiabatic) transition into waveguide  208 . 
     It should be noted that, in some embodiments, the inclusion of mode-matching regions in the input ports of a beam combiner mitigates many of the assembly issues that impact prior-art projectors by enabling light sources  202  to be easily integrated with beam combiner  206  by mounting the laser diodes directly on the beam combiner substrate itself. This enables each light signal output at the output facets of the laser diodes to be directly coupled into its corresponding input port  210 . As a result, the integrated light sources and beam combiner collectively define a substantially solid “light engine” that is more robust and can be significantly more compact than possible with prior-art approaches. 
     In some embodiments, however, light sources  202  are not directly integrated with beam combiner  206 . In some embodiments, light sources  202  are fiber-coupled laser diodes that provide light signals  204  to beam combiner  206  via optical fiber pigtails that are attached directly to appropriately mode-matched input ports of a beam combiner. 
     One skilled in the art will recognize, after reading this specification, that since waveguides  208  provide waveguiding in two dimensions, light signals  204  need not be collimated prior to their coupling into the beam combiner. This eliminates the need for beam-shaping optics at each light source. It also mitigates the effects of angular misalignment, since spectral non-uniformity through the cross-section of the composite output beam and beam divergence issues are eliminated. Further, the elimination of the beam-shaping optics and the integration of the light sources and beam combiner enable a significant reduction in the overall size of the optical system. In fact, in some embodiments, the size of the optical system is small enough to enable a pen-sized projector  200 , or enable projector  200  to be integrated directly into a mobile digital device, such as a cell phone or personal digital assistant (PDA). 
       FIG. 6  depicts a schematic drawing of mixing region  214 . Mixing region  214  comprises portions of waveguides  208 - 1  through  208 - 3 , which have been arranged to form directional couplers  602 - 1  and  602 - 2 . 
     Directional coupler  602 - 1  comprises first portions of waveguides  208 - 1  and  208 - 2 , which are separated by gap g 1  along interaction length L 1 . Directional coupler  602 - 1  is a symmetric coupler (i.e., the portions of waveguides  208 - 1  and  208 - 2  have substantially the same width) that is characterized by wavelength-dependent power coupling that varies slowly with wavelength. The values of g 1  and L 1  are carefully chosen to enable substantially all of the optical energy in light signal  204 - 1  to optically couple from waveguide  208 - 1  into waveguide  208 - 2  along interaction length L 1 , but substantially none of the optical energy in light signal  204 - 2  optically couples from waveguide  208 - 2  to waveguide  208 - 1 . As a result, directional coupler  602 - 1  provides both light signals  204 - 1  and  204 - 2 , combined as dual-wavelength signal  604 , on waveguide  208 - 2 . 
     Directional coupler  602 - 2  comprises a second portion of waveguide  208 - 2  and a portion of  208 - 3 , which are separated by gap g 2  along interaction length L 2 . Directional coupler  602 - 2  is also a symmetric coupler that is characterized by wavelength-dependent power coupling that varies slowly with wavelength. The values of g 2  and L 2  are carefully chosen to enable substantially all of the optical energy in light signal  204 - 3  to optically couple from waveguide  208 - 3  into waveguide  208 - 2  along interaction length L 2 , but substantially none of the optical energy in light signal  604  optically couples from waveguide  208 - 2  to waveguide  208 - 3 . As a result, directional coupler  602 - 2  provides all three of light signals  204 - 1  through  204 - 3 , combined as composite output signal  218 , on waveguide  208 - 2 . 
     Although the illustrative embodiment comprises a beam combiner having three waveguides and three input ports, it will be clear to one skilled in the art, after reading this Specification, how to specify, make, and use alternative embodiments that comprise beam combiners having any practical number of waveguides and input ports. As discussed in U.S. patent application Ser. No. 13/208,806, beam combiners in accordance with the present invention can include additional branches or hierarchy stages that enable the addition of additional light signals to the system. The ability to readily include more waveguides and ports enables, for example, the inclusion of redundant light sources for each light signal without significantly increasing the overall size of projector  200 . 
     At operation  303 , processor  126  controls the ratio of optical power from light signals  204  in composite output signal  218  via power controllers  212 - 1  through  212 - 3 . 
       FIG. 7  depicts a schematic drawing of a power controller in accordance with the illustrative embodiment of the present invention. Controller  212 - i  comprises attenuator  702 - i  and power monitor  704 - i . In response to control signals from processor  126  (not shown for clarity) and the output of power monitor  704 - i , controller  212 - i  diverts optical power (via attenuator  702 - i ) from waveguide  208 - i  to a light dump in order to control the amount of optical power of light signal  204 - i  that reaches mixing region  214 . 
     Attenuator  702 - i  comprises waveguide  208 - i  and waveguide  708 - i , which are arranged to define directional couplers  708 - 1  and  708 - 2 . 
     Directional couplers  708 - 1  and  708 - 2  are substantially identical directional couplers arranged in series and interposed by waveguide portions  710 - i  and  712 - i . Waveguide portion  710 - i  is a first portion of waveguide  208 - i . Waveguide portion  712 - i  is a waveguide having structure analogous to that of waveguide  208 . Waveguide portion  712 - i  is optically coupled with light dump  716 - i . It will be clear to one skilled in the art how to specify, make, and use light dump  716 - i.    
     Modulator  714 - i  comprises a heater strip for thermally inducing a phase shift in the light that propagates through waveguide portion  710 - i . This phase shift determines the amount of optical coupling occurs between waveguide portions  710 - i  and  712 - i . This optical coupling, in turn, dictates how much of the optical power of light signal  204 - i  is diverted into waveguide portion  712 - i  and lost at light dump  716 - i . The remainder of light signal  204 - i  continues propagating in waveguide  208 - i  to output  724 - i.    
     Although the illustrative embodiment comprises modulators that operate on a thermo-optic effect, it will be clear to one skilled in the art, after reading this Specification, how to specify, make, and use alternative embodiments of the present invention that comprise modulators that induce a phase shift in light propagating in a waveguide based on a different effect, such as electro-optic, opto-mechanical, etc. 
     Power monitor  704 - i  comprises waveguide  208 - i , waveguide  718 - i , and photodetector  722 - i . Waveguide  208 - i  and waveguide  718 - i  are arranged to define directional coupler  720 - i.    
     Directional coupler  720 - i  enables a small percentage of its optical power to be coupled from waveguide  208 - i  into waveguide portion  718 - i.    
     Waveguide portion  718 - i  is optically coupled with conventional photodetector  722 - i  (e.g., photodiode, avalanche photodiode, CCD sensor element, etc.), which is electrically coupled with controller  132  to provide the controller with a feedback signal suitable for controlling attenuator  702 - i.    
     It is an aspect of the present invention that the use of power controllers  212 - 1  through  212 - 3  obviates the need to vary the drive current to each of light sources  202  to control the color of composite output signal  218 . As a result, embodiments of the present invention can avoid undesirable optical effects in the composite output signal, such as wavelength chirping, accelerated degradation, mode hopping, and the like. The use of power controllers  212  also enables the use of laser sources, such as frequency doubled lasers or diode-pumped solid-state lasers, without a need for their customary external modulators. Further, since the attenuators are easily integrated in the PLC design, their inclusion adds little or no additional size to the overall system. It should be noted, however, that in some embodiments, power controllers  212  are not included and the intensity of light signals  204  is controlled conventionally by controlling the drive current to each of light sources  202 . 
     At operation  304 , composite output signal  218  is emitted into free space at output port  216 . Composite output signal  218  is received by lens  220 , which substantially collimates the signal to form output beam  222 . Lens  220  is a conventional bulk optic lens suitable for collimating the optical emission at output port  216 . In some embodiments, lens  220  is an integrated lens formed directly on the exit facet of output port  216 . 
     At operation  305 , scanner  108  receives output beam  222  and scans it over region  122  to form an image on sample  124 . 
     It should be noted that projector  200  provides a collimated light beam that remains in focus, even when projected onto a surface that is not flat. In addition, image resolution is determined only by the spot size of output beam  222 , rather than the resolution of a CCD camera, video processor, or liquid-crystal element, which enables a simpler, cheaper projection system. 
     The advantages afforded embodiments of the present invention enable a projector platform that is adaptable for use in many applications beyond that of simple image projection. It should be noted, therefore, that the illustrative embodiment represents only one example of a projector in accordance with the present invention. Alternative embodiments include projectors adapted for use in applications that include, without limitation, visualization and treatment of cancer tissue, pest control, insect control, visual enhancement of hot spots in semiconductor material or integrated circuits, virtual keyboards, smart targeting systems for laser-guided munitions, laser-based weapons, confocal microscopy, laser light based quality control in the medical or food industry or laser based sorting in waste and recycling processes, etc. As a result, details of the projector and the method for its use, such as the number of light signals, wavelengths selected, component arrangement, and the like, are for example only and one skilled in the art will recognize that embodiments of the present invention suited for some or all of these applications will differ from the illustrative embodiment described herein. 
     Vein Imaging 
     An application for which the present invention is particularly well suited for is the enhancement of the visibility of subcutaneous vein structure in bodily tissue. Enhanced vein visibility facilitates the performance of medical procedures by a medical practitioner, such as insertion of catheters, syringes, intravenous tubes, identification of blood clots or internal hemorrhaging, minimally invasive surgery, and the like. 
     Vein imaging systems are known in the prior art, such as systems described in U.S. Patent Application Publication 2008/0004525, published Jan. 3, 2008 and U.S. Patent Application Publication 2008/027317, published Jan. 31, 2008, each of which is incorporated herein by reference. 
     In U.S. Patent Application Publication 2008/0004525 (hereinafter referred to as the “525 application”), a scanning projector scans a first light signal over a image points in a sample region, wherein the first light signal of a wavelength that is readily absorbed by blood (i.e., 740 nm). The first light signal is reflected from the image points and detected at a photodetector. The output of the photodetector is used to create a digital map of the absorption of the first light signal in the sample region, wherein the absorption map represents the vein structure within the scanned area. Once this absorption map has been created, it is projected, using the same projection system, over the sample region using a second light signal of a wavelength visible to the eye (e.g., 630 nm). 
     Unfortunately, the system disclosed in the 525 application has several drawbacks. First, the absorption map is generated over the entire sample region (during a period of 60 frames of the video cycle) prior to rendering the visible image of the vein structure (also during a period of 60 frames of the video cycle). This leads to a significant delay between the projection of each successive visible image on the sample. As a result, this system is prone to errors due to practitioner hand jitter or patient motion. In addition the system is also prone to the development of image flicker, which, in addition to creating an annoyance, can also lead to more serious issues such as epileptic seizure. 
     In addition, the system disclosed in the 525 application employs a free-space beam combiner into which the outputs of the 740 nm and 630 nm lasers are “projected.” Further, the 740 nm and 630 nm lasers are directly driven to control their output. As a result, this system is characterized by most, if not all, of the same disadvantages as projector  100 , described above and with respect to  FIG. 1 . 
     In U.S. Patent Application Publication 2008/027317 (hereinafter referred to as the “317 application”), a scanning projector scans a first light signal over a image points in a sample region, wherein the first light signal is also characterized by a wavelength that is long enough to be absorbed by blood (e.g., 740 nm). The first light signal is reflected from the image points and detected at a photodetector that is selectively sensitive to the wavelength of the first light signal. When the output of the photodetector indicates the presence of a vein at an image point, a second light signal of a visible wavelength is added to the first light signal. The two light signals are combined using a free-space optical system based on “dielectric mirrors.” 
     Unfortunately, the system disclosed in the 317 application also has several drawbacks. First, in contrast to the present invention, free-space optics are used to combine the light signals. Second, this system also relies on direct modulation of the intensities of the laser sources. As a result, the system disclosed in the 317 application also has all the same disadvantages of projector  100 , as described above and with respect to  FIG. 1 . 
     In contrast to the prior art, however, embodiments of the present invention are suitable for enhancing the visibility of vein structure without some of the disadvantages inherent in projectors that rely on free-space optics-based beam combiners and directly modulated laser sources. 
       FIG. 8  depicts a schematic drawing of a scanning projector in accordance with a second alternative embodiment of the present invention. Projector  800  is a projector for enhancing the visibility of the subcutaneous vein structure of a region of bodily tissue. Projector  800  comprises light sources  202 - 4  and  202 - 5 , beam combiner  802 , scanner  108 , photodetector  818 , and processor  822 . 
     Projector  800  is analogous to projector  200  in that it scans an output beam having multiple light signals over an image region. Projector  800 , however, scans a beam including a first signal of a first wavelength over the image region and controls the presence of a second light signal in the beam based a property of each image point in the image region. The property of each image point is determined based on reflected light of the first light signal. As a result, projector  800  projects real-time data about the image points in the image region, wherein projector  200  projects only predetermined information (e.g., video frames, still images, etc.). 
     Specifically, projector  800  interrogates image points in the region with a light beam comprising a first light signal of a first wavelength, which is readily absorbed by blood. The projector monitors how much of the first light signal is reflected from each image point to determine the presence of a vein at that image point. When a drop in the reflected light from an image point is detected (indicating the presence of a vein), the projector adds a second light signal of a visible wavelength to the light beam to illuminate that image point for the user. By scanning the light beams over all of the image points in the region at a sufficient rate, a real-time video image of the subcutaneous vein structure is rendered directly on the sample region. 
       FIG. 9  depicts operations of a method for enhancing the visibility of subcutaneous vein structure in accordance with the second alternative embodiment of the present invention. Method  900  begins with operation  901 , wherein light signal  204 - 4  is coupled into output beam  808 . 
     Light signal  204 - 4  is provided to beam combiner  802  by light source  202 - 4 , which is a laser diode that emits narrow-spectral-width light at a wavelength of approximately 740 nm. In some embodiments, light signal  204 - 4  is characterized by a different wavelength that is readily absorbed by blood (e.g., 850 nm, etc.). Light source  202 - 4  is mounted such that it is directly optically coupled with input port  210 - 4  of beam combiner  802 . 
     Beam combiner  802  is analogous to beam combiner  206 , described above and with respect to  FIG. 2 ; however, beam combiner  802  is dimensioned and arranged to combine only two light signals into a composite output beam. Beam combiner  802  comprises input ports  210 - 4  and  210 - 5 , attenuators  702 - 4  and  702 - 5 , mixing region  804 , and output port  216 . 
     Each of input ports  210 - 4  and  210 - 5  includes a mode-matching region for enabling direct, low-loss optical coupling of light signals  204 - 4  and  204 - 5  from the output facets of light sources  202 - 4  and  202 - 4 , respectively, to beam combiner  802 . Input ports  210 - 4  and  210 - 5  are analogous to input ports  210 - i , described above and with respect to  FIGS. 5A-C . 
     Light signal  204 - 4  is conveyed to mixing region  804  via attenuator  702 - 4 , which controls the intensity of light signal  204 - 4  in output beam  808  as described above and with respect to  FIG. 7 . 
     Mixing region  804  is analogous to mixing region  206 ; however, since projector  800  utilizes only two light signals, mixing region  804  comprises only directional coupler  602 - 3 . Directional coupler  602 - 3  is dimensioned and arranged to combine light signals  204 - 4  and  204 - 5  into composite output signal  806 , as described above and with respect to  FIG. 6 . 
     Beam combiner  802  provides composite output signal  806  at output port  216 , where it is collimated by lens  220  to form output beam  808 . 
     At operation  902 , scanner  108  directs output beam  808  to image point  810 - i - j , wherein i=1 to M, j=1 to N, M is the number of rows of image points in region  812 , and N is the number of columns of image points in region  812 . 
     At operation  903 , photodetector  818  receives reflected signal  816  from image point  810 - i - j . Reflected signal  816  is a portion of output beam  808  that is reflected from the image point toward photodetector  818 . 
     Photodetector  818  is a conventional photodetector that generates electrical signal  820 , whose instantaneous magnitude is based on the instantaneous intensity of reflected signal  816 . In some embodiments, photodetector  818  comprises a filter that enables photodetector  818  to selectively detect light of the wavelength of light signal  204 - 4 . In such embodiments, the overall system signal-to-noise ratio is improved since background noise can be reduced significantly. In some embodiments, photodetector  818  comprises a filter that enables photodetector  818  to selectively detect light of a wavelength other than the wavelength of light signal  204 - 4 . In some embodiments, photodetector  818  comprises a filter that enables photodetector  818  to detect light of only one polarization. 
     At operation  904 , processor  822  tests reflected signal  816  for absorption of light signal  204 - 4  at image point  810 - i - j . Due to the choice of wavelength for light signal  204 - 4 , the presence of a vein at image point  810 - i - j  induces a detectable drop in the intensity of reflected signal  816 . 
     Processor  822  is a conventional processor suitable for executing computer programs, storing data, receiving electrical signal  820  from photodetector  818 , and providing drive signals  824 - 4  and  824 - 5  to light sources  204 - 4  and  204 - 5 , control signals  826 - 4  and  826 - 5  to attenuators  702 - 4  and  702 - 5 , and drive signal  828  to scanner  108 . 
     If absorption is detected at image point  810 - i - j , method  200  continues with operation  905 , wherein light signal  204 - 5  is added to output beam  808 . 
     Light signal  204 - 5  is provided to beam combiner  802  by light source  202 - 5 , which is a laser diode that emits narrow-spectral-width green light having a wavelength of approximately 532 nm. In some embodiments, light signal  204 - 5  is characterized by a different wavelength in the visible wavelength range. Although the human eye is most sensitive to light within the green wavelength range (i.e., the range from approximately 520 nm to approximately 570 nm), one skilled in the art will recognize that many other wavelengths of light are suitable for use in light signal  204 - 5 , such as 633 nm, 650 nm, 670 nm, 593.5 nm, 473 nm, and 405 nm, among others. The addition of light signal  204 - 5  to output beam  808  while it is directed at image point  812 - i - j  enhances the visibility of this image point for a user. 
     In some embodiments, light signal  204 - 5  is characterized by a wavelength that is suitable for exciting a phosphor used in vision enhancement systems (e.g., night vision goggles, etc.). Such embodiments are suitable, for example, to enable medical procedures to be carried out when stealth is desired, such as on a battlefield, or in other applications wherein a medical practitioner might be wearing such vision enhancement equipment. 
     In some embodiments, at least one of light signals  204 - 4  and  204 - 5  is characterized by a wavelength suitable for exciting a fluorescent material or phosphor, such as a fluorescent biomarker, that is located in or on the target sample. 
     The presence of light signal  204 - 5  in output beam  808  is controlled via control signal  826 - 5  provided by processor  822  to modulator  714 - 5  of attenuator  702 - 5 . To add light signal  204 - 5  to output beam  808 , modulator  714 - 5  is driven such that substantially all of the optical energy of light signal  204 - 5  remains in waveguide  208 - 5 , as discussed above and with respect to  FIG. 7 . To eliminate light signal  204 - 5  from output beam  808 , modulator  714 - 5  is driven such that substantially all of the optical energy of light signal  204 - 5  couples into waveguide portion  712 - 5  to be lost at light dump  716 - 5 . In some embodiments, light signal  204 - 5  is added to output beam  808  by directly controlling drive signal  824 - 5 . 
     At operation  906 , light signal  204 - 5  is removed from output beam  808 . 
     At operation  907 , scanner  108  indexes output beam  808  to the next image point in the scanning pattern used to interrogate region  812 . In some embodiments, scanner  108  raster scans output beam  808  over the image points in region  812 . In some embodiments, scanner  108  uses a non-raster scanning pattern to interrogate the image points. 
     Returning to operation  905 , if absorption is not detected at image point  810 - i - j  during operation  205 , method  900  skips operation  206  and moves directly to operation  907 . As a result, image point  810 - i - j  is not illuminated by light signal  108 . 
     Operations  902  through  907  are repeated for each i=1 to M and j=1 to N such that projector  800  repeatedly scans the entirety of region  812  at a rate that enables a visible image of the subcutaneous vein pattern in region  812  to be rendered on surface  814 . 
     One skilled in the art will recognize, after reading this Specification, that more than two light signals can be combined via beam combiners in accordance with the present invention, to enable, for example, interrogation of region  812  with more than one wavelength of light. In some embodiments, interrogating region  812  with a plurality of wavelengths provides advantages that include: improved measurand selectivity, the ability to determine more than one measurand, and improved measurement resolution, among others. In some embodiments, the use of multiple wavelengths for interrogation enables detection of specific chemicals, nuclear material, explosive materials, surface structure in an image region, and/or sub-surface structure in an image region. Further, additional wavelengths can be included for enhancing the projected image to render additional images of, for example, surface structure in the image region, sub-surface structure in the image region, graphics, text, and the like. 
     Vein-imaging systems in accordance with the present invention have many advantages over vein-imaging systems known in the prior art. By directing a single, collimated light beam that both interrogates and illuminates a scan region, all optical power can be provided to a single image point at the same time and the light beam is always in focus. In addition, by using one high-sensitivity photodetector, the need to spread the reflected signal over a large array of detector pixels is obviated. As a result, the collector optics can be simpler and less expensive, no alignment is needed between the detector array and the projection system, and a higher signal-to-noise ratio can be achieved. Further, since no detector array is included, no video processing is necessary. This reduces cost and increases system speed. 
     High-Power Laser Projection 
     In some applications, such as laser-guided munitions, laser-weaponry, and the like, it is desirable to scan and/or track an object with an output beam that comprises a very high power laser beam. For example, in some cases, once a projector has identified the presence of a target material at an image point, a user might want to add a high-power laser signal to the output beam to induce an effect at that image point, such as: illuminating the image point with a targeting-laser signal to direct a laser-guided munition; inducing combustion, or other chemical change, of material at the image point to burn the wings off an insect (e.g., a mosquito, wasp, etc.), ablate cancerous tissue, or add an identifying mark to the image point. 
     Conventional PLC-based systems, however, are unable to handle high-power light signals, due to the fact that they require doping with impurities to affect a refractive index difference between the core and cladding of their constituent waveguides. 
     In contrast to the prior art, waveguides in accordance with the present invention are not doped and, therefore, can handle higher power laser signals. Beam combiners in accordance with the present invention are based on waveguide structures exhibit guiding capability based on the difference of the refractive indices of stoichiometric silicon nitride and stoichiometric silicon dioxide. As a result, projectors comprising PLC-based beam combiners in accordance with the present invention enable the inclusion of high-power laser signals in their output beams. 
     It is to be understood that the disclosure teaches just one example of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.