Abstract:
A scanning light source system combines the outputs from multiple, spatially separated, sequentially pulsed light sources for travel in rapid succession along one or more common delivery paths to provide one or more collective outputs that are comparatively higher than would otherwise be possible with the light sources operating singly.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the benefit of priority from U.S. Provisional Patent Application No. 60/391,841 filed on Jun. 26, 2002 in the name of Thomas J. Brukilacchio, et al. with the title SCANNING LIGHT SOURCE SYSTEM, the entire contents of which are incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention generally relates to high brightness light sources and more particularly to apparatus by which the outputs of multiple light sources can be beneficially coupled into a single output whose brightness is greater than that of the individual sources from which it is derived.  
           [0003]    There are many uses for high brightness concentrated sources of light. Most of these applications require the use of a laser source, which are, to date, expensive and relatively complex. Therefore, laser sources are not applied to many problems needing a high brightness concentrated light source because they are too expensive and complex.  
           [0004]    Light emitting diodes (LEDs) represent another possible source for achieving high brightness. However, a single LED or single array of LEDs operated continuously (CW) is limited in the brightness that can be obtained when imaged into a spot focus with a given numerical aperture as, for example, that of an optical fiber. In addition, the optical power out of an LED is limited because of the build up of high levels of damaging heat when operated CW near peak power levels. Nevertheless, LEDs represent an attractive alternative to lasers since they are less complex and expensive, are available over a range of colors ranging from the ultraviolet through the mid infrared spectrum, and can be modulated at decreased duty cycles to provide increased peak power without experiencing heat damage. Other sources of optical radiation that can be modulated to provide increased peak power by operation at decreased duty cycles include laser diodes, xenon flash lamps, and the like.  
           [0005]    Hence, it is a primary object of the present invention to provide a low cost, high brightness concentrated source of light.  
           [0006]    It is another object of this invention to provide a low cost, high brightness source that uses a multiplicity of relatively lower cost sources whose outputs are serially coupled as a single output whose brightness is higher than would otherwise be available from a single source.  
           [0007]    It is another object of the invention to provide a low cost, high brightness source by serially coupling the outputs of a multiplicity of LEDs to provide a single source that is brighter than the brightness that otherwise could be made available from a single LED.  
           [0008]    It is another object of the invention to provide a low cost, high brightness source by serially coupling the outputs of a multiplicity of LEDs of different color to provide a single source that is brighter and of different color than would otherwise be available from a single LED.  
           [0009]    It is still another object of the present invention to provide a variety of different apparatus for serially coupling the outputs from a multiplicity of LEDs to provide a low cost, high brightness source.  
           [0010]    Another object of this invention is to provide relatively compact and lightweight apparatus for serially coupling the outputs from a multiplicity of LEDs to provide a low cost, high brightness source.  
           [0011]    Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter when the following description is read in connection with the drawings.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention relates to apparatus and methods for serially coupling multiple light sources into a single output. The multiple light sources are pulsed at a rate that allows them to output light at a much higher level than they otherwise could. By rapidly combining these higher output pulsed sources, a comparatively large amount of light can be coupled to a single output to provide a concentrated source of high brightness.  
           [0013]    Generally, the invention comprises a high brightness illumination system, comprising a plurality of modulatable light sources that are spatially separated in a prearranged pattern. Drive means are provided for exciting the light sources in a predetermined sequence to provide a plurality of light pulses that are separated in space and time. Scanning means serially receive and redirect the outputs of the plurality of light pulses for travel in rapid succession along one or more collection paths to provide a collective output that is comparatively higher in brightness than would otherwise be possible with the light sources operating individually.  
           [0014]    The light sources can comprise LEDs, laser diodes (LDs), or xenon flash tubes, although LEDs are preferred. In one aspect, arrays of LEDs are used in conjunction with arrays of compound parabolic concentrators and collimator optics to provide sequential collimated beams that are subsequently directed into the output of a downstream collector in the form of an optical fiber cable. Scanning preferably is done by focusing lenses that form an image of the collimated beams on a scanning element after which it is reimaged onto the entrance pupil of the collecting fiber such that the product of a sources emission solid angle and its emitting area is substantially equal to the collection area and solid angle of acceptance of the fiber cable to assure optimal coupling.  
           [0015]    In variant alternative embodiments, acousto-optical modulators (AOMs) and diffractive arrangements can be configured to provide the scanning function and color mixing with diffractive elements is shown. Phosphor layers may be used to provide desired colors.  
           [0016]    A method provides a high brightness source of illumination by steps comprising mounting a plurality of modulatable light sources so that they are spatially separated in a prearranged pattern and then exciting the light sources in a predetermined sequence to provide a plurality of light pulses that are separated in space and time and higher in intensity than would otherwise be produced by the sources operating continuously. The outputs of the light sources are scanned to serially receive and redirect them for travel in rapid succession along one or more collection paths to provide a collective output that is comparatively higher in brightness than would otherwise be possible with the light sources operating individually. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    The structure, operation, and methodology of the invention, together with other objects and advantages thereof, may best be understood by reading the detailed description in connection with the drawings in which each part has an assigned a descriptive label or numeral that identifies it wherever it appears in the various drawings and wherein:  
         [0018]    [0018]FIG. 1 is a diagrammatic high-level block diagram for the general architecture of a scanning light system of the invention;  
         [0019]    [0019]FIG. 2 is a diagrammatic system timing diagram of the invention showing the relative sequence of pulses for its individual light sources, the position of the scanner of the invention in relation to the light pulses provided by the individual light sources, and the coupled outputs of the individual light sources as they appear as a single source of high brightness;  
         [0020]    [0020]FIG. 3 is a graph of the continuous optical power output of an individual source used in the invention as a function of electrical input power;  
         [0021]    [0021]FIG. 4 is a graph illustrating peak optical power as a function of duty cycle for individual light sources used in the invention;  
         [0022]    [0022]FIG. 5 is a diagrammatic perspective exterior view of a preferred embodiment of the invention;  
         [0023]    [0023]FIG. 6 is a cross-sectional elevational view of the embodiment of FIG. 5 taken generally along line  6 - 6  of FIG. 5;  
         [0024]    [0024]FIG. 7 is an enlarged cross-sectional elevational view of a portion of FIG. 6 showing in greater detail collection optics and LED sources used in practicing the invention;  
         [0025]    [0025]FIG. 8 a  is a diagrammatic elevational view of a portion of the optical system of the invention illustrating the path taken by certain rays emerging from an axially located LED as they course their way to the input of an optical fiber that serves as the at the output of the system;  
         [0026]    [0026]FIG. 8 b  is a diagrammatic elevational view of a portion of the optical system of the invention illustrating the path taken by certain rays emerging from an LED located at an off-axis position as they course their way to the input of an optical fiber that serves as the output of the system;  
         [0027]    [0027]FIG. 8 c  is a diagrammatic elevational view of a portion of the optical system of the invention illustrating the path taken by certain rays emerging from an LED located at an off-axis position other than that shown in FIG. 8 b  as they course their way to the input of an optical fiber that serves as the output of the system;  
         [0028]    [0028]FIG. 8 d  is a table presenting lens prescription data for a design for practicing the invention;  
         [0029]    [0029]FIG. 9 is a tree diagram showing a variety of possibilities for practicing the invention by combining different light sources, actuators, and scanning elements;  
         [0030]    [0030]FIG. 10 is a diagrammatic elevational view of a rotary embodiment of the invention in which a prism and DC servo motor are employed to serially couple the outputs from radially located individual sources to provide a high brightness single source;  
         [0031]    [0031]FIG. 11 is a diagrammatic elevational view of another embodiment of the invention in which an AOM is used to serially couple the outputs from off axis located individual sources to provide a high brightness single source;  
         [0032]    [0032]FIG. 12 is a diagrammatic elevational view of yet another embodiment of the invention in which rotating and fold mirrors are employed to serially couple the outputs from off axis located individual sources to provide a high brightness single source;  
         [0033]    [0033]FIG. 13 is a diagrammatic elevational view illustrating another embodiment of the invention in which a rotating diffraction grating is used to serially couple the outputs from off axis located individual multiple wavelength sources to provide a high brightness single source;  
         [0034]    [0034]FIG. 14 is a diagrammatic elevational view of a multiple wavelength division multiplexer embodiment of the invention; and  
         [0035]    [0035]FIG. 15 is a diagrammatic elevational view of an LED with a phosphor layer to provide a color-converted source that may be used with the invention. 
     
    
     DETAILED DESCRIPTION  
       [0036]    The present invention provides a variety of embodiments for simple, low-cost, high brightness concentrated sources of light that may be used in place of lasers in a range of applications.  
         [0037]    Examples of white light applications for the present invention include, but are not limited to, LCD projection systems (Metal Halide or High Pressure Mercury arc lamp replacement), surgical headlights, endoscope illumination, video system illumination, major surgical auxiliary lighting, high brightness industrial illumination, remote light delivery, automotive interior light engine, and/or architectural lighting, etc.  
         [0038]    Examples of single color applications for the present invention include, but are not limited to, photodynamic therapy (PDT), adhesive curing systems, and/or medical or dental curing, etc.  
         [0039]    The present invention applies to any source of optical radiation that can be modulated to provide increased peak power with decreased duty cycle including, but not limited to, LED&#39;s, laser diodes, xenon flash lamps, etc.  
         [0040]    The use of Light Emitting Diodes (LED&#39;s), however, is a preferred source because LEDs are available in multiplicity of colors or wavebands ranging from the ultraviolet through the mid infrared spectrum and can be mixed to synthesize other colors.  
         [0041]    A preferred embodiment of the present invention is illustrated in FIG. 1. As seen in FIG. 1, the invention is a lighting system  10  comprising two major components the light sources  12 , comprised of a multiplicity of individual light sources and the scanning system or collection and directing system  16 . The function of the individual light sources  12  is to provide a high power light for a relatively brief period of time. The purpose of the scanning system  16  is to optically couple the outputs from individual light sources into a single high brightness output, which is designated at  18 .  
         [0042]    [0042]FIG. 2 illustrates a system timing diagram of a preferred embodiment of the present invention. As shown, each light source  12  (labeled LS- 1 , LS- 2 , etc.) is turned on at a different time, stays on for a brief period of time, and then turns off. The scanner  16  (SP) operates in synchronization with the timing of the individual pulses to optically couple each in turn into output  18 . Thus, the scanner serially operates with one pulse at a time to sequentially couple each pulse in turn into output  18 . This process repeats in rapid succession, producing a nearly continuous output of the form shown at the bottom of FIG. 2 labeled “System Output”. As seen in FIG. 2, the System Output is substantially higher than the CW output that can be provided by the individual sources. For visual applications, the pulse rate should be higher than the human visual system flicker rate and for other applications can be higher as needed.  
         [0043]    The reason why the peak of the System Output is significantly higher that is otherwise possible operating the sources CW has to do with the operating properties of the individual sources. When light sources such as light emitting diodes (LEDs) or laser diodes are run with continuous power, they have a finite limit to the amount of light that can be extracted from them as illustrated in FIG. 3. As seen there, optical power increases with electrical input power until a limit is approached. When more electrical power is applied beyond this limit, no more, or even a decreasing amount of light is extracted. One of the prime mechanisms for the limit to optical power output is the build-up of heat within the device. When the same device is pulsed for a short period of time, then turned off for a period of time before being pulsed again, it can be operated at a much higher peak optical power while maintaining the same average electrical power and heat load.  
         [0044]    [0044]FIG. 4 illustrates the relationship between peak optical power and duty cycle (the ratio between device on time and on time plus off time). As the FIG. 4 shows, with lower duty cycle, higher peak optical power can be extracted from each individual device. Thus, the average power of the output signal shown in FIG. 2 is many times higher than the continuous output of a single light source. As FIG. 4 illustrates, peak relative power can change from approximately 2 to in excess of 90 for a change in duty cycle of 100 to 1, thus permitting the concentrated brightness of the output  18  to be significantly brighter than any of the individual source inputs.  
         [0045]    [0045]FIG. 5 illustrates a preferred embodiment of the present invention where it is designated generally as a system  20  having a body  21  for its major components. System  20  includes an LED die or chip  28  as light sources and a voice-coil actuated 2-axis scanner  24 . As shown in FIG. 5, a heat sink  22  is provided to cool the LED chip  28 . An optical fiber cable  26  optically and mechanically couples to complementary configured structure on or in body  21  and serves as the output for the collective system light output  18 . Light emerging from the distal end of optical fiber cable  26  is thus a highly bright and concentrated source that may be used for any sensible downstream application requiring its properties.  
         [0046]    [0046]FIG. 6 shows a cross-sectional view of the system  20  illustrated in FIG. 5, and FIG. 7 illustrates a more detailed view of heat sink  22  and LED die  28  illustrated in FIG. 5 and FIG. 6.  
         [0047]    In the system  20  as illustrated in FIGS. 5, 6 and  7 , the LEDs  28  are mounted on well-known headers that efficiently pass the heat to the heat sink  22 . In an alternative embodiment, heat sink  22  can also use a fan, liquid cooled heat exchanger, or TE coolers to more effectively remove heat.  
         [0048]    As best seen in FIGS. 6 and 7, the LEDs may take on the form of a one or two dimensional array composed of individual emitting areas each of which is coupled into a corresponding optical element of an array of optical elements  30 . An array of collimating lenses  32  operates on the outputs from the optical element array  30  to provide a series of collimated beams when an individual LED is turned on. The collimated beams each pass through a focusing lens subsystem comprising a field lens  34  and two converging lenses  36  and  38  all of which are constructed to image each collimated beam onto a mirror  42  that is mounted for two-dimensional displacement and actuated by a voice coil  40  or the like so that it can be made to tip and tilt with respect to the optical axis.  
         [0049]    The images formed on mirror  42  are, in turn, re-imaged into the core of the optical fiber cable  26  by a re-imaging lens subsystem comprising three elements  44 ,  46  and  48 . A quadrant detector  58  is included to provide signals for controlling the position of mirror  42 . Well-known system electronics are provided to control the various system functions and drive the LEDs and mirror  42  in concert with one another as required by the timing diagram of FIG. 2. System electronics are located in an electronics area  56  provided for this purpose.  
         [0050]    As Illustrated in FIG. 7, the output of each LED in die  28  is efficiently out-coupled by a non-imaging concentrator such as, but not limited to, a compound parabolic concentrator (CPC)  60  integrally formed as an element of collection array  30 . Each CPC, which is structured to operate by total internal reflection (TIR) or may be provided with reflective surfaces as needed with. e.g. UV light, effectively collects all of the light from its corresponding LED in chip  28  and emits substantially 100 percent of it through an exit pupil that is preferably 1.5 mm in diameter. The collimating lens array  32  is comprised of an array of collimating lens elements  64  (typical) that form individual collimated beams of the light emerging form the exit pupil of a corresponding CPC. Thus, each collimating lens  64 , illustrated in FIG. 7 collects the light exiting points on the exit pupil of a corresponding optical concentrator and focuses it to infinity as is illustrated in FIGS. 8 a ,  8   b , and  8   c . which are ray traces corresponding to an on-axis LED emitting, an off-axis upper LED emitting, and an off-axis lower LED emitting.  
         [0051]    To summarize, the focusing lenses illustrated in FIG. 6 form an image on the mirror in the 2-axis scanner  24 . The scanner  24  acts to direct each of the images of the output face of the optical concentrators through the re-imaging lenses and into the core of output fiber, which is illustrated in FIG. 8.  
         [0052]    In a preferred embodiment of the present invention, the re-imaging lens system is generally telecentric in image space to effectively couple into the numerical aperture of the optical fiber. In other words, the solid angle over which light emerges from a CPC, call it Ω e , times its emission area, A e , is substantially equal to the solid angle of collection of the optical fiber core, call it Ω c , times its collection area A c . Preferably A e =A c , i.e., A c =1.5 mm also, for one to one imaging. Alternatively, the re-imaging lenses may be replaced with a CPC.  
         [0053]    In a preferred embodiment, the focal length of the optical system is 1.78 mm, the semi-field angle of the output of a CPC is 30 degrees and the NA of the optical fiber cable is 0.5 thus providing it with a semi-field acceptance angle of 30 also.  
         [0054]    Constructional or prescription data for a preferred optical system is given in convention form Table 1 of FIG. 8 d , where it has been scaled to a focal length of 100 mm. Those skilled in the art will appreciate that this constructional data may be rescaled keeping in mind that aberrations do not scale linearly.  
         [0055]    Illustrated in FIG. 6, the quadrant detector  58  is positioned at an angle relative to the scanner mirror  42 , but outside of the effective ray bundle (See FIGS. 8 a - c ). A collimated light source (not shown) is positioned at an equal, but opposite angle relative to the mirror  42 . As the mirror  42  moves in space, the collimated light from the light source also moves and falls on the quadrant detector in a changed manner. The new position is interpreted by electronics to represent the position of mirror  42 . This mirror position signal is then used by well-known scanner controller electronics to control the motion of the mirror  42 . The control and LED drive electronics (not shown) as mentioned are contained in housing  21 .  
         [0056]    In an alternative embodiment of the present invention, the mirror control optics are incorporated into the scan mirror system by reflecting off the backside of the mirror  42 .  
         [0057]    As mentioned earlier, the present invention generally comprises a plurality of light sources  12  and a scanner  16 . Within the scanner  16 , there is an actuator and a scanning element. Examples of the light source include, but are not limited to light emitting diodes, laser diodes and/or xenon flash lamps, etc. Examples of the actuator include, but are not limited to, a stepping motor, a DC servomotor, a voice coil, a galvanometer, and/or a micro electromechanical device (MEMS), etc. Examples of the scanning element (and/or scanning system) include, but are not limited to, a mirror, a prism, diffraction grating, an acousto optical modulator, a wavelength division multiplexer, dichroic elements, a Fresnel mirror, and/or a Pellicle type mirror, etc. These examples are illustrated in FIG. 9 which also provides a tree arrangement illustrating various possibilities by which the foregoing elements may be beneficially combined to practice the invention.  
         [0058]    [0058]FIG. 10 illustrates a cross-sectional view of an alternative embodiment of the present invention where it is designated generally as a system  100 . System  100  uses a single axis scanning actuator (DC servomotor with encoder)  108  and a prism  106  and holder therefor as a scanning element. The light sources are LED die  102  arranged radially around the center of rotation, facing radially inward or perpendicular to the system optical axis or path. Each LED  102  has a concentrating optical component  103  and a collimating lens  104  that directs collimated light to the prism  106 , which reflects the light into focusing lenses  110 . The focusing lenses  110  focus the light into the core of an output optical fiber  112 .  
         [0059]    [0059]FIG. 11 illustrates an alternative embodiment of the present invention designated here as a system  120 . With system  120  multiple light sources  122  and  124 , similar to those in the system shown in FIG. 10, are arranged so that collimated light beams  126  and  128 , respectively, are directed into an acousto-optic modulator or AOM  130 . AOM  130 , which may be KDP or lithium niobate (LiNbO 3 ) is modulated in a well-known manner so that it directs the light from each light source into a focusing lens  134  and subsequently into an output element  136 , which can be a fiber as before. It may be possible to arrange two AOM units at 90 degrees orientation and collect light from a 2-dimensional arrangement of light sources.  
         [0060]    [0060]FIG. 12 illustrates an alternative embodiment of the present invention where it is designated generally as a system  140 . In some respects, system  140  is similar to system  100  as illustrated in FIG. 10. However, it differs by the addition of a stationary mirror  148 , preferably torroidal in form, and surrounding the optical axis of system  140 . The use of stationary mirror  148  allows the overall system package to be smaller. In this arrangement, a collimated light beam  146  exits a light source assembly  142  or  144 , each of which consist of some light source with lenses to collimate the light. This collimated ray bundle  146  reflects off of the stationary mirror  148 , then off of a rotating mirror  150  and into an output optical fiber system  156 . This output system  156  consists of lenses that refocus the light into the optical fiber for output. A rotational servo  152  connects to mirror  150  via a shaft  154  to selectively control the position of mirror  150  in concert with the timing and duty cycle of sources  142  and  144 .  
         [0061]    While the invention so far has been discussed with light sources emitting over one wavelength or wave band, the present invention may also mix wave band or wavelengths to synthesize colors. FIG. 13 illustrates an alternative embodiment of the present of invention, designated generally as a system  160  that uses optically dispersive elements to couple multiple wavelengths of light into a single source.  
         [0062]    System  160  illustrated in FIG. 13 is a variation of system  140  illustrated in FIG. 12, only each light source is replaced by a group of multiple light sources  162  and  164 , and the rotating mirror is replaced by a rotating diffraction grating  168 . These multiple groups of light sources are placed radially around the axis of rotation. Each light source  162  and  164  is of a different wavelength to provide a multiplicity of output beams  164 , each of different spectral content or color. The light from each source  162  travels in a diverging ray bundle from the source, reflects off of a stationary mirror  166 , substantially with 100 percent reflection, then reflects off of a diffraction grating  168 . The diffraction grating  168  serves to collimate the light and reflect it toward an output optical fiber system  172 , approximately matching the NA of the fiber in the process. The light is then focused down into an output fiber forming part of optical fiber system  172 . In an alternative embodiment of the present invention, all of the light sources within each group are on simultaneously on. In yet another alternative embodiment of the present invention, all of the light sources within each group are pulsed, and/or separately turned on.  
         [0063]    In yet another alternative embodiment of the present invention illustrated in FIG. 14, where it is designated generally as system  180 , uses optically dispersive elements. The system  180  illustrated in FIG. 14 is similar to the system  160  illustrated in FIG. 13, only the diffraction grating is replaced by a wavelength division multiplexer or WDM. The WDM, which comprises a Cooke triplet  184  and grating  186 , combines multiple, differing wavelength sources  188  into a single output ray bundle  198  that reflects off of a rotating mirror  192 , with a common focus point. The output ray bundle  198  from the WDM is reflected off of a rotating mirror whose position is controlled by a servo  194  via shaft  196  and into fiber output  200 .  
         [0064]    In a variant of this alternative embodiment of the present invention, a multiple wavelength system is configured such that colors are combined to form any mixture of these chosen wavelengths. For example, the mixture could form a white light source.  
         [0065]    In an alternative embodiment of the present invention a single wavelength system  210  includes a phosphor-filled epoxy layer  214  placed over the face of an LED  212  to form a white light source, as illustrated in FIG. 15. Again, a heat sink-header  216  is provided for cooling purposes and a CPC  218  serves to collect and redirect the light emitted by the exit facet of the LED  212 .  
         [0066]    In an alternative embodiment, the light source  210  illustrated in FIG. 15 creates a scanning white light system.  
         [0067]    Based on the teachings of the invention, other changes to the invention will occur to those skilled in the art and such changes are intended to be within the scope of the invention.