Patent Publication Number: US-2006009749-A1

Title: Efficient diffuse light source assembly and method

Description:
This application is a continuation-in-part of U.S. application Ser. No 10/783,880, filed Feb. 19, 2004, which is hereby incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates to efficient, diffuse light sources, and more particularly to a laser diode assembly that efficiently produces a diffuse light output ideal for applications such as hair removal.  
     BACKGROUND OF THE INVENTION  
      Currently, laser diodes are used to supply optical output for many varying types of applications. One such application is hair removal, although the present invention is not so limited. For many applications, one or more laser diodes are used in a single assembly in order to supply the requisite amount of optical power. However, the optical delivery systems used with the such optical assemblies can be inefficient in delivering the optical output to its intended target. This is especially true for applications that rely on optical diffusion. For example, in the application of hair removal, it is important to sufficiently diffuse the light before it exits the device to enhance safety, and to produce an even distribution of the light on the target. However, diffusing the light typically decreases substantially the optical output power, and can reduce the system efficiency below acceptable levels.  
      An ideal disk diffuser is one that converts an input beam, without loss, to an output beam having a Lambertian divergence distribution. A Lambertian divergence distribution is one where the intensity measured in the far field has a cos(Ø) dependence where Ø is the angle to the normal of the output face of the diffuser. Here, a source of light having a Lambertian divergence distribution is considered ideal since the “brightness” of such a beam becomes independent of Ø when the beam intensity has a cos(Ø) dependence. This is because the apparent size of the beam (or of any two dimensional, flat surface) varies perfectly with cos(Ø). Thus the decrease in apparent size with increasing viewing angle is exactly proportional to the decrease in intensity reaching the viewer.  
      There are several reasons why a non-ideal (real) diffuser is less than ideal. Most diffusers are either bulk diffusers or surface diffusers. Bulk diffusers are diffusers in which the scattering of the input beam occurs mainly within the volume of the diffusing material. An example of a bulk diffuser is PTFE (“Teflon”). Surface diffusers are diffusers that scatter, refract, reflect and/or diffract the light as it enters, exits, or reflects off of the diffuser. Examples of surface diffusers are etched glass, ground glass, and substrates patterned with diffraction features. Unless care is taken to provide an anti-reflection coating on the input face of the bulk or surface diffuser, there will generally be Fresnel reflections due to the change in index of refraction upon entering the diffuser that will reflect some of the light back towards the source. When the input beam is not perfectly collimated, it is difficult to provide an efficient anti-reflection coating since the performance of anti-reflection coatings is generally dependent on the incident angle. Fresnel reflections will also occur at the output face of the diffuser due to the change in refractive index as the light passes from the diffuser. Providing an effective anti-reflection coating at this interface is even more challenging since the light exiting the diffuser has been made even more diffuse (i.e., even less collimated). Thus, light will be reflected back towards the source from the exit face of the diffuser as well.  
      Bulk diffusers present an additional challenge in that the light may be scattered back towards the source within the scattering material itself. The amount of back-scattered light generally increases with the thickness of the bulk diffuser. Unfortunately, an output distribution that most closely matches a Lambertian distribution is achieved by increasing the thickness of the bulk diffuser to a point that a significant amount of light is scattered backwards toward the light source. Additionally, most diffusers are limited in size or there is an output aperture through which the output from the diffuser must pass. In these cases light scattered laterally within the diffusing material to the edge of the diffuser or light that is emitted from the diffuser outside of the emission aperture may also be lost. Another source for loss of light is absorption of the light within the diffuser. However, for many wavelengths this problem is not significant since diffuser materials can be found for many wavelengths that have negligible absorption.  
      Surface diffusers present a somewhat similar problem. A single surface diffuser often does not provide adequate scattering. Therefore, to achieve a greater level of scattering, multiple scattering surfaces must be used. Unfortunately, employing more scattering surfaces decreases the amount of transmitted light.  
      The efficiency of the diffuser is the fraction of the incident light that is transmitted by the diffuser. The designer of an optical system requiring a diffuse beam of light must often sacrifice the degree to which the output beam is Lambertian with the need for an efficient diffuser, or must employ the use of a more intense light source that may add size, expense and power consumption. This is especially true in optical systems that use a laser for the source of the light since laser outputs are generally fairly well collimated and must be diffused significantly in order to achieve a Lambertian divergence distribution. It is therefore imperative that the light reaching, and eventually transmitted beyond, the diffuser is maximized while maintaining the requisite degree to which the transmitted output is Lambertian.  
     SUMMARY OF THE INVENTION  
      The present invention is a diffuse light source assembly that includes a light source for generating forward propagating light, a solid lightguide disposed adjacent the light source and having an input face for receiving the forward propagating light, a sidewall for conveying the forward propagating light via total internal reflection, and an output face for transmitting the forward propagating light, a diffuser disposed for diffusing the forward propagating light, and a back reflecting surface. A portion of the forward propagating light is transformed into reverse propagating light by the output face, which is conveyed by the sidewall via total internal reflection and transmitted by the input face. The back reflecting surface is disposed adjacent the light source for reflecting the reverse propagating light back into the lightguide via the input face.  
      In another aspect of the present invention, a diffuse light source assembly includes a light source for generating forward propagating light, a solid lightguide disposed adjacent the light source and having an input face for receiving the forward propagating light, a sidewall for conveying the forward propagating light via total internal reflection, and an output face for transmitting the forward propagating light, a diffuser disposed adjacent the output face for diffusing the transmitted forward propagating light, wherein a portion of the forward propagating light is transformed into reverse propagating light by the diffuser that is conveyed by the sidewall via total internal reflection and transmitted by the input face, and a back reflecting surface disposed adjacent the light source for reflecting the reverse propagating light back into the lightguide via the input face.  
      In yet another aspect of the present invention, a method of generating diffuse light that includes generating forward propagating light, conveying the forward propagating light using a solid lightguide having an input face for receiving the forward propagating light, a sidewall for conveying the forward propagating light via total internal reflection, and an output face for transmitting the forward propagating light, diffusing the forward propagating light using a diffuser wherein a portion of the forward propagating light is transformed into reverse propagating light by the output face which is conveyed by the sidewall via total internal reflection and transmitted by the input face, and reflecting the reverse propagating light back into the lightguide through the input face using a back reflecting surface disposed adjacent the light source.  
      In yet one more aspect of the present invention, a method of generating diffuse light includes generating forward propagating light, conveying the forward propagating light using a solid lightguide having an input face for receiving the forward propagating light, a sidewall for conveying the forward propagating light via total internal reflection, and an output face for transmitting the forward propagating light, diffusing the forward propagating light using a diffuser, wherein a portion of the forward propagating light is transformed into reverse propagating light by the diffuser which is conveyed by the sidewall via total internal reflection and transmitted by the input face, and reflecting the reverse propagating light back into the lightguide through the input face using a back reflecting surface disposed adjacent the light source.  
      Other objects and features of the present invention will become apparent by a review of the specification, claims and appended figures.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1A  is an exploded perspective view of the diffuse light source assembly of the present invention.  
       FIG. 1B  is an exploded side view of the diffuse light source assembly of the present invention.  
       FIG. 2  is a cross-sectional side view of the diffuse light source assembly of the present invention.  
       FIG. 3  is a cross-sectional side view of the optical cavity of the present invention, illustrating forward propagating light, reverse propagating light, and light reflected back toward the forward propagating direction.  
       FIG. 4  is a top view of the mask member of the present invention.  
       FIG. 5  is a system schematic view illustrating an optical fiber delivery system used with the present invention.  
       FIGS. 6A and 6B  are side views illustrating embodiments with reflective-type diffusers adjacent the laser diodes or optical fiber delivery system, respectively.  
       FIG. 7  is a side cross-sectional view illustrating diffusers that are integrally formed as part of the input and output faces of the solid lightguide. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The present invention is a diffuse light source assembly and method that efficiently generates and delivers sufficiently diffuse optical output to intended targets. The diffuse light source assembly  10  of the present invention is illustrated in  FIGS. 1 and 2 , and includes a laser diode assembly  12  and a delivery assembly  14 .  
      Laser diode assembly  12  includes one or more laser diodes  16  mounted onto a heat sink  18 . In the embodiment illustrated in the figures, a pair of laser diode bars  16  mounted between three mounting blocks  20  are used to create an optical output. The mounting blocks  20  are used to position the laser diode bars  16  during manufacture, and are made of electrically conductive metal to complete the electrical circuit that operates the laser diodes  16 . Various techniques of mounting of laser diodes, such as laser diodes separated by mounting blocks or laser diodes placed in grooves formed in a monolithic substrate, are well known in the art, and are not further described herein. However, according to the present invention, the top surfaces of mounting blocks  20  are made of, or plated with, a highly reflective material such as gold, as explained further below.  
      The delivery assembly  14  includes a barrel shaped housing  22  with an elongated cavity  24  therein. A lightguide  26  (sometimes termed a “mixer”) made of an elongated block of transparent material (e.g. boro-silica glass, acrylic, sapphire, etc.) is disposed in the elongated cavity  24 . In this embodiment, lightguide  26  includes a rectangular shaped input face  28  positioned adjacent the laser diodes  16  and a circular shaped output face  30 , with a sidewall  32  extending therebetween having a cross-sectional shape that gradually changes from rectangular to circular. The gradual change is such that light traveling down the lightguide  26  will be reflected with little or no loss by total internal reflection (TIR), as explained further below. A diffuser  34  is disposed over the output face  30 , and is preferably made of PTFE, opal glass, or similar diffusive material. In a preferred embodiment, diffuser  34  is displaced from output face  30  by a small air gap. A protective transparent window  36  is disposed over the diffuser  34 , and is preferably made of sapphire. Preferably, the cavity  24  includes a stepped shoulder  38  in which the window  36  is held by adhesive and/or friction fit. The housing  22  is preferably bolted onto the heat sink  18  to secure the delivery assembly  14  to the laser diode assembly  12 . Optionally the barrel may be maintained at a different temperature than the heatsink, by, for example, placing a thermoelectric module  46  between the housing  22  and the heatsink  18  as shown in  FIG. 2 .  
      The operation of the diffuse light source assembly  10  is illustrated in  FIG. 3 , where light output  40  emitted by the laser diodes  16  enters the lightguide  26  through input face  28 . The lightguide  26  conveys the light output  40  to the output face  30  via TIR from sidewall  32 . The light output  40  exits the lightguide through output face  30 , where it is subjected to diffusion by diffuser  34 . The diffused light output  40  then exits the delivery assembly  14  after passing through window  36 .  
      There are several sources of light loss in the optical configuration of  FIG. 3 . Specifically, some of the forward propagating light (i.e. propagating away from the laser diodes  16 ) is reflected back toward the laser diodes (i.e. in a reverse propagating direction) by the input face  28 , by the output face  30 , by the surfaces and within diffuser  34 , and by the surfaces of window  36 . Much of this reverse propagating light is conveyed by the lightguide  26  (via TIR) back to the laser diode assembly  12 . Therefore, to re-use the reverse propagating light  42  according the present invention, that portion of the laser diode assembly  12  receiving this reverse propagating light is formed of or coated with a highly reflective material, essentially creating a back reflective surface  44  around the laser diodes  16 . The term “reflective” is used herein to refer not only to specular reflection but also diffuse reflection or remission of light. Thus, an optical cavity is formed by back reflective surface  44 , lightguide  26 , diffuser  34  and window  36 . For the embodiment described above, this means that mounting blocks  20  are made of or coated with a highly reflective material, to reflect the reverse propagating light  42  back in the forward propagating direction, whereby much of this light will be diffused by diffuser  34  and emitted by window  36 , thus increasing the efficiency of the system. This process of reflecting reverse propagating light back towards the diffuser is repeated until all of the light is transmitted or lost to parasitic absorption within the lightguide  26 , diffuser  34  or back reflective surface  44 . There may also be a slight loss of light due to lateral scattering within the diffuser where light may be lost to the edge of the diffuser  34  or scattered backwards outside of the lightguide  26 ; or lost due to the gap between the input face  28  of the lightguide  26  and the back reflective surface  44 . (Some gap may be necessary to reduce the intensity of the laser diode light on the input face  28  of the lightguide  26 , although in general this gap should be minimized to reduce lateral loss of light.) Another source for loss of light may be re-absorption by the laser diodes  16  when reverse propagating light is incident directly on the laser diodes.  
      With the present invention, a nearly Lambertian output beam is realized with very little loss of light. To optimize the design of this highly efficient Lambertian diffuser, the thickness of the diffuser (or in the case of a surface diffuser, the number of surfaces) can be reduced so as to minimize the amount of light that is scattered laterally. The diffuser thickness (or number of surfaces) that is required for adequate scattering may be less than what would be required for a single pass of light since light that has be redirected back to the diffuser by reflections off the mounting blocks in subsequent passes will likely be more diffuse than the initial beam of light.  
      It is also important for an optimal design to minimize the amount of absorption within the diffuser material and the absorption at the lightguide sidewall  32 . Since the light returned from the diffuser into the cavity may be nearly Lambertian (and therefore very divergent), the reflected light will impinge upon the sidewall  32  many times if the length of the lightguide  26  is large. Multiple reflections from imperfectly reflecting cavity walls will absorb some of the back-scattered light. This is why a solid lightguide  26  using TIR to reflect the light along the lightguide is ideal and preferred over a hollow lightguide relying on surface reflections. So long as the angle of incidence is high enough (given the refractive index of material), losses are minimized or even essentially eliminated, even though the diffuser  34  creates high angle reverse propagating light. In order to collect and return as much of the light as possible, any gaps between the lightguide  26 , diffuser  34  and back reflective surface  44  should be minimized. Further, it is desirable to minimize the size of the laser diodes  16  relative to the back reflective surface  44  so that the maximum amount of light is reflected and the minimum amount of light is absorbed by the laser diodes. Further, the reflective surface  44  should extend over an area at least as large (and preferably somewhat larger) than the area of the input face  28  of the lightguide  26 . It is also important for the lightguide  26  to have sufficient length to spatially mix the light from the laser diodes, a length of several centimeters being typically sufficient.  
      An embodiment of the present invention has been reduced to practice, using a gold reflective coating on the mounting blocks  20  to form back reflecting surface  44  (which is 94% reflective at 800 nm), a 0.015″ thick PTFE disk for the diffuser  34 , an acrylic solid lightguide  26 , and a pair of laser diode bars having a total area of about 2×1 cm×0.03 cm. The output window  36  is about 1 cm in diameter. The sidewall  32  is not perfectly orthogonal to the back reflective surface or diffuser disk. However, no portions of the sidewall  32  exceed about 7 degrees away from a perfect orthogonal orientation relative to the back reflective surface or diffuser disk, so that light will not leak out sidewall  32 .  
      The shape of lightguide is such that a generally rectangular distribution of the light output  40  is transformed to a generally circular distribution. It should be noted, however, that the lightguide  26  need not have a rectangular input face  28  and a circular output face  30 . Such a configuration is preferred, however, because a round optical output cross-section can be achieved at the window  36  (permitting use of a conventional round output window  36 ) while using a back reflective surface  44  that is not any longer than the laser diodes  16 . That is, the reflective mounting blocks  20  can have the same length as the laser diodes  16  (a desirable feature for manufacturability) and yet completely fill the input face  28  for reflecting all of the reverse propagating light. Alternatively, a back reflective surface of greater dimension than input face  28 , or a mask as described below, can be used so that lightguide  26  can have a uniform cross sectional shape. In addition and/or alternately, the lightguide sidewall  32  can be tapered, so that input face  28  can have a different desired cross-sectional area compared to output face  30 . The higher the refractive index of material used to form lightguide  26 , the greater the amount of taper that can implemented before significant amounts of light leakage out of lightguide  26  occur (due to light rays striking the sidewall  32  below the critical angle for TIR).  
       FIG. 4  illustrates an alternative embodiment of the present invention, which includes a mask  50  placed over the laser diodes  16  and mounting blocks  20 , and having an upper surface that serves as the back reflective surface  44 . The mask includes apertures  52  through which the light output  40  from the laser diodes  16  passes. To maximize the efficiency of the diffuser assembly, apertures  52  are preferably as narrow as possible without blocking significant light from the laser diodes, which will require careful alignment of the mask apertures with the laser diodes. The mask can be sandwiched between assemblies  10  and  12 , attached to the lightguide input face  28 , and/or attached to the laser diode mounting blocks  20 . It should be noted that the use of reflective mounting blocks  20 , rather than mask  50 , eliminates this alignment task and thus is a key advantage of using reflective mounting blocks.  
      It should be noted that other light sources can be used instead of one or more laser diodes. For example, other solid state lasers (e.g. Nd:YAG, etc.), gas lasers (e.g. argon, krypton, etc.) or dye laser lasers, or even a flash lamp, can be used to generate light output  40 . Because these types of light sources tend to be less compact than laser diodes, any light source  54  used as part of the present invention (including laser diodes, solid state lasers, gas lasers, flash lamps, etc.) can include a delivery system such as an optical fiber  56  as shown in  FIG. 5 . In that case, the back reflective surface  44  would be disposed at the output of the delivery system.  
       FIGS. 6A and 6B  illustrate another alternative embodiment of the present invention, which utilizes a reflective-type diffuser  58  instead of a transmissive-type diffuser. In this embodiment, the output of the laser diodes  16  (as shown in  FIG. 6A ), or an optical fiber (as shown in  FIG. 6B ), is directed to a reflective-type diffuser  58 , which either has an irregular reflecting surface or includes a diffusive material through which the light passes before and/or after reflection, that both reflects and diffuses the light, and directs the diffused light output to the lightguide  26 . Any light directed back toward the laser diodes  16  or optical fiber  56  will be reflected by back reflective surface  44  disposed adjacent the laser diode output facets or the optical fiber&#39;s delivery end. Reflective diffusers can be made of a highly scattering material such as PTFE, or a commercially available material termed Spectralon (LabSphere, Inc., North Sutton, N.H.); or a scattering material applied to a reflecting surface, such as Duraflect (also available from LabSphere, Inc.).  
       FIG. 7  illustrates yet another alternative embodiment, where the lightguide input and/or output faces  28 / 30  integrally include diffusers, by including a diffusive material on these faces. If the input and/or output faces  28 / 30  produce a sufficient amount of diffusion for the light, separate diffuser  34  may be eliminated.  
      It should be noted that, as used herein, the terms “over” and “on” both inclusively include “directly on” (no intermediate materials, elements or space disposed therebetween) and “indirectly on” (intermediate materials, elements or space disposed therebetween). Likewise, the term “adjacent” includes “directly adjacent” (no intermediate materials, elements or space disposed therebetween) and “indirectly adjacent” (intermediate materials, elements or space disposed therebetween).  
      It is to be understood that the present invention is not limited to the embodiment(s) described above and illustrated herein, but encompasses any and all variations falling within the scope of the appended claims. For example, materials, and numerical examples described above are exemplary only, and should not be deemed to limit the claims. The back reflective surface is adjacent the laser diodes, meaning that the output facets of the laser diodes  16  can be flush with, be recessed relative to, or extend slightly beyond, the back reflective surface  44  (i.e. laser diodes can be even with, disposed outside of, or extend into, the optical cavity formed by back reflective surface  44 , lightguide  26 , diffuser  34  and window  36 ). Back reflective surface  44  can be a spectral reflective surface (e.g., polished or polished and plated) or simply coated without polishing, creating a diffuse reflective surface that efficiently remits light.