Abstract:
A method of increasing the spatial uniformity of the detected intensity of a beam of light from a laser in a system including the laser and a light detector. In one embodiment the method includes the steps of generating a beam of light with the laser; and moving the beam of light and the light detector relative to each other, such that the detector averages the spatial intensity of the beam of light over time. In another embodiment the invention relates to a system for increasing the detected spatial uniformity of the intensity of a beam of light. In one embodiment the system comprises a light detector; a laser source for generating the beam of light; and a means for moving the beam of light and the detector relative to one another such that the detector averages the intensity of the light beam over time

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to lasers, and specifically to improved methods for reducing detected intensity non-uniformity in a laser beam.  
       BACKGROUND OF THE INVENTION  
       [0002]     Many measurements require the use of a laser as a source of light. For example, lasers are used in systems ranging from imaging to the detection of gene sequences. In such systems various types of lasers are used, including gas lasers, chemical lasers, excimer lasers, solid-state lasers, semiconductor lasers (including diode lasers), dye lasers and hollow cathode sputtering metal ion lasers. Each type of laser has its own set of advantages and disadvantages when used for a specific application. For example, the characteristics of the different types of lasers including power output, wavelength, cost, size, tunability and uniformity of intensity over a cross-section of the beam are either advantages or disadvantages depending on the application in which the laser is used.  
         [0003]     Diode lasers are low cost, have a relatively high power output and are small in size. A diode laser includes a radiating area or facet that has a very low aspect (height to width) ratio. That is, the height of the radiating facet is much smaller than its width. The beam produced by the laser diode facet is not uniform in intensity across the radiating facet. Because of the lack of uniformity in beam intensity, laser diodes cannot be used in applications where beam intensity uniformity is required. Therefore, in applications in which high power and uniformity are required other lasers such as gas lasers, which are more expensive and quite large, are generally used.  
         [0004]     In many applications, decreasing the cost and size of the laser greatly decreases overall equipment and hence research costs. Thus, a method or system to decrease the detected non-uniformity in the intensity of a diode laser beam could increase the efficiency and decrease the size of the equipment used in selected applications.  
         [0005]     The present invention addresses the problem of detected beam intensity non-uniformity in a laser beam of light.  
       SUMMARY OF THE INVENTION  
       [0006]     This invention relates to systems and methods of reducing detected intensity non-uniformity of a laser diode beam so that laser diodes can be implemented in circumstances requiring a detection of a uniform beam of laser light.  
         [0007]     In one aspect the invention relates to a method of increasing the spatial uniformity of the detected intensity of a beam of light from a laser in a system including the laser and a light detector. In one embodiment the method includes the steps of generating a beam of light with the laser; and moving the beam of light and the light detector relative to each other, such that the detector averages the spatial intensity of the beam of light over time. In another embodiment the step of moving the beam comprises the step of passing the beam of light into a two-dimensional retroreflector and moving a reflective wall of the retroreflector. In yet another embodiment the step of moving the beam comprises passing the beam into a rotating polygon, which may be a transparent polygon. In yet another embodiment, the step of moving the beam comprises physically moving the light source relative to the detector.  
         [0008]     In another aspect the invention relates to a system for increasing the detected spatial uniformity of the intensity of a beam of light. In one embodiment, the system comprises a light detector, a laser source for generating the beam of light, and a means for moving the beam of light and the detector relative to one another such that the detector averages the intensity of the light beam over time. In another embodiment the, system includes a retroreflector, having a moving reflective wall, into which is passed the beam of light. In yet another embodiment the system includes a rotating transparent polygon through which is passed the beam of light.  
         [0009]     In another aspect, the invention relates to a system for increasing the detected spatial uniformity of the intensity of a beam of light. In one embodiment, the system includes a light detector; a laser source for generating the beam of light; and a translator for moving the beam of light and the detector relative to each other such that the detector averages the intensity of the light beam over time. In another embodiment, the laser source is a laser diode. In another embodiment, the translator comprises a corner cube reflector having a moving reflective wall into which is passed the beam of light. In yet another embodiment, the translator comprises a rotating transparent polygon through which is passed the beam of light. In another embodiment the translator comprises a translator physically moving the light source relative to the detector.  
         [0010]     Another aspect the invention relates to an apparatus for moving a beam of light in a direction perpendicular to its direction of propagation. In one embodiment the apparatus includes a first reflective surface; a second reflective surface oriented orthogonal to the first reflective surface; and a translator moving the first and second reflective surfaces relative to each other. In another embodiment the translator is a piezoelectric transducer driven by a waveform generator. In yet another embodiment the translator is a speaker cone driven by a waveform generator. In still yet another embodiment the speaker cone is a subwoofer. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     These embodiments and other aspects of this invention will be readily apparent from the detailed description below and the appended drawings, which are meant to illustrate and not to limit the invention, and in which:  
         [0012]      FIG. 1  is a diagram of a light ray being reflected by two dimensional depiction of a two-dimensional retroreflector;  
         [0013]     FIGS.  2 ( a,b ) are two dimensional depictions of a light ray entering a two-dimensional retroreflector with the reflective surface at various positions;  
         [0014]     FIGS.  3 ( a,b ) are two dimensional depictions of a light beam comprising many rays entering a two-dimensional retroreflector with the reflective surface at various positions;  
         [0015]      FIG. 4   a  is a plot of the intensity of light across the detector face as seen by a detector viewing a diode source;  
         [0016]      FIG. 4   b  is a plot of the intensity of light across the detector face as seen by a detector viewing a diode source as averaged by an embodiment of the invention;  
         [0017]      FIG. 5  is a two-dimensional depiction of two light rays entering a transparent polygon at normal (90 degree) incidence;  
         [0018]      FIG. 6  is a two-dimensional depiction of two light rays entering a transparent polygon at an angle of incidence other than 90 degrees;  
         [0019]      FIG. 6   a  is an enlargement of a portion of  FIG. 6 ; and  
         [0020]      FIG. 7 ( a,b ) is a two-dimensional depiction of a light beam comprising many rays entering a transparent polygon, at an angle of incidence other than 90 degrees. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]     The present invention will be more completely understood through the following detailed description, which should be read in conjunction with the attached drawings. In this description, like numbers refer to similar elements within various embodiments of the present invention. Within this detailed description, the claimed invention will be explained with respect to preferred embodiments. However, the skilled artisan will readily appreciate that the methods and systems described herein are merely exemplary and that variations can be made without departing from the spirit and scope of the invention.  
         [0022]     In general, the solution to the problem of spatial non-uniformity in the detected intensity of the light beam from a laser is to move the light beam, which typically is larger than the detector, and the detector relative to each other such that the detector “sees” the spatial variations in the intensity of the beam as it crosses the detector and averages them. One way to produce such relative movement is by use of a retroreflector.  
         [0023]     Retroreflectors, usually consisting of three mutually perpendicular intersecting flat reflecting surfaces, return a reflected incident light beam in the direction of its point of origin. An example of a two dimensional retroreflector, in the form of a corner cube reflector  5 , is shown in  FIG. 1 . The corner cube reflector  5  includes two reflective surfaces  10  and  20  oriented 90 degrees to each other. A light ray  30 , from a source  34 , incident at 45 degrees with respect to the first reflective surface  10  is reflected toward the second reflective surface  20 . This second reflecting surface  20  reflects the light ray  30  back in the direction of the source  34 . The reflected portion  42  of the light ray  30  is reflected parallel to the incident portion  38  of the light ray  30 . The distance (D) between the incident portion  38  of the light ray  30  and the reflected portion  43  is determined by the distance between the reflecting surfaces  10 ,  20 .  
         [0024]     Referring to  FIG. 2   a , by adjusting the positions of the reflective surfaces  10 ,  20  relative to each other, the outgoing ray  43  may be translated, such that the distance (D) between the incident portion  38  and the outgoing portion  42  of the light ray  30  varies. In one embodiment of the invention, the first reflective surface  10  is movable, in a direction perpendicular to the reflective surface  10 . By moving the first reflective surface  10  while maintaining its orientation with respect to the second reflective surface  20  the distance (D) between the incident portion  38  of the ray and the outgoing  42  portion of the light ray  30  may be altered.  
         [0025]     Thus, as the first reflective surface  10  moves, the outgoing portion  42  of the ray  30  translates across an aperture  70  located in the path of the outgoing portion  42  of the ray  30 . This change in displacement of the outgoing portion of  42  of the ray  30  determines whether the outgoing portion  42  of the ray  30  will be able to pass through the aperture  70  and reach a detector  74 .  
         [0026]     In more detail,  FIG. 2   a  shows one light ray  30 , reflecting off of the first reflective surface  10 , then reflecting off of the second reflective surface  20  to produce the outgoing portion  42  of the ray  30 , parallel to and reversed 180 degrees with respect to the incident portion  38  of ray  30 . At this location  80  of the first reflecting surface  10 , the outgoing portion  42  of the ray  30  then travels through an aperture  70  to reach detector  74 .  
         [0027]     Referring to  FIG. 2   b , by shifting (arrow T) the location  82  of the first reflective surface  10  the distance (D′) between the incoming portion  38  of the ray  30  and the outgoing ray  42 ′ changes. This displacement in the outgoing portion  42 ′ of the ray  30  causes the ray  30  not to go through the aperture  70  and is thus not seen by the detector  74 .  
         [0028]     Referring to  FIG. 3   a , when a beam made up of many rays of light  30 ′ is considered, for example from an extended source  34 ′, again at the first location  80 ′ of the reflective surface  10  multiple light rays will be reflected toward the aperture  70  and only a few  72  will pass through to the detector  74  as in the case of the single ray  30  in  FIG. 2   a . Referring also to  FIG. 3   b , as the first reflective surface  10  is moved, (arrow T) those rays  72 ′ which previously passed through the aperture  70  to reach the detector  74  will not translate across the aperture  70  and no longer reach the detector  74 , while some other rays  72 ′ originally blocked by the aperture  70  will now pass through the aperture  70  and reach the detector  74  again as occurred with the single ray  30  in  FIG. 2   b.    
         [0029]     As the first reflective surface  10  moves back and forth, the different rays that make up the outgoing portion  42  of beam  30  will pass through the aperture  70  and reach the detector  74  at a different point on the detector surface. Thus, although the beam  42  may vary in intensity across its width, the movable reflective surface  10  in conjunction with the aperture  70  creates an averaged resultant image on the detector  74  with greater uniformity of intensity. Every pixel of a multi-pixel detector, will see over time, approximately the same average amount of light as every other pixel if the first reflective surface  10  moves through several cycles over the course of an exposure. Therefore, the retroreflector  5  creates conditions that promote detection of a more uniform light beam when averaged over time.  
         [0030]      FIG. 4   a  shows the measured value of the intensity of light as seen across the detector face when the detector is viewing a diode laser source. The peaks in the intensity plot are caused by “hot spots” in the facet of the diode laser.  FIG. 4   b  shows the measured value of the intensity of light as seen across the detector face when the detector is viewing a diode laser source that has been averaged using the invention. The peaks and valleys in the intensity plot, caused by mode structure in the laser, are averaged out as the image is moved across the detector by the movement of the first reflector.  
         [0031]     In various embodiments the reflective surfaces  10 ,  20  are preferably silvered mirrors, but may be composed of any reflective material. Although the invention has been described in terms of moving the first reflective surface  10  in a direction perpendicular to the surface, in another embodiment, the second reflective surface  20  is movable. In another embodiment, both reflective surfaces are movable in opposite directions at the same time.  
         [0032]     The reflective surfaces may be translated by any reciprocating means, such that the orientation of the reflective surfaces relative to the beam and each other remains constant. Additionally, in order to make the beam uniform in across both dimensions of the surface of the detector, the first reflective surface  10  may be rotated so a normal to the surface points in a direction that is at 45 degrees to both axes of the incident beam and moves at a 45 degree angle to both the incident beam and the beam reflected to the second surface reflector.  
         [0033]     The reflective surface can be driven by any reciprocating means; for example a motor driven cam. In another embodiment, the first reflective surface is mounted to the speaker cone of a subwoofer. In other embodiments the reflective surface is moved using a piezoelectric transducer. In one embodiment the subwoofer or piezoelectric transducer is driven by a sinusoidal wave. In other embodiments, the reflective surface is driven by other types of waves. In another embodiment, the second reflective surface is driven by a second subwoofer. In still another embodiment, both reflective surfaces are driven by respective subwoofers.  
         [0034]     In another embodiment, a rotating polygon may be used to create a beam of uniform intensity from a laser beam. As shown in  FIG. 5 , two incident light rays  80 ,  84  enter the polygon  90 , perpendicular to the surface  92  of the polygon  90 . Using one incident ray  80  as an example; the incident ray  80  is at normal incidence (90 degrees) to the air/polygon interface  92 , and thereby results in the transmitted ray  96  being also perpendicular to that interface  92 . When the transmitted ray  96 , now the incident ray at the polygon/air interface  100  exits the polygon  90 , the incident ray  96  and transmitted ray  104  are both perpendicular to the interface  100 . Therefore, the ray  80  incident to the polygon  90  and the ray  104  transmitted out through the polygon  90  have the same orientation, i.e., they are both perpendicular to the surface of the polygon. Thus, at normal incidence, an incoming ray passes straight through the polygon. The same process affects the other incident ray  92  depicted in  FIG. 5 .  
         [0035]     Referring to  FIG. 6 , as the polygon  90  is rotated, the angle of incidence  106  to the air-polygon interface  92  changes, thus changing the angle of refraction  108  inside the polygon  90 . ( FIG. 6   a  is an enlargement to clearly show these angles.) For example, incident ray  80  meets the interface  92  of the polygon at an angle  106  that is not normal to the polygon surface  92  and #, the beam  96 ′ within the polygon  90  is refracted toward the normal  110  of the interface  92  at an angle  108  as dictated by Snell&#39;s Law. The ray  96 ′ passes through the polygon  90  and becomes the incident ray at the polygon-air interface  110 . Because the light is passing from the material into air the incident ray  96 ′ is refracted away from the normal to the surface resulting in the transmitted ray  104 ′. The same process affects the other incident ray  84  depicted in  FIG. 6 . As the polygon  90  is rotated, the beams are deflected less and eventually when the interface  92  is again perpendicular to the beam the light passes through the polygon as described above with regard to  FIG. 5 . The result is such that the transmitted rays  104  and  110  walk across each other as the polygon is rotated.  
         [0036]     Referring to  FIG. 7   a , when a beam  120  made up of many rays of light is considered, for example from an extended source  34 , is incident to the surface  92  at an angle of 90°, the beam  80  will pass straight through the polygon  90  to a detector  74 . Referring also to  7   b , as the polygon  90  is rotated the angle of incidence of the rays with respect to the interface  92  changes, changing the angle of refraction and the point on the opposite surface of the polygon  90 , where the beam  128  will exit and reach the detector  74 . As the polygon  32 ′ rotates, the rays of the beam will walk across each other such that each part of the detector  74  will detect substantially the same intensity of light over time. Thus, the rotating polygon  90  provides a way to spatially average the intensity of the beam over the width of the beam  120 .  
         [0037]     The polygon may be composed of any material able to transmit light rays. In the embodiment shown, the polygon is an octagon, but any polygon can be used. The preferred embodiment uses a glass polygon, with an index of refraction greater than about 1.9. The polygon may be rotated at various speeds to obtain the correct level of uniformity of intensity. In the preferred embodiment the polygon is rotated at a speed about twice the exposure time.  
         [0038]     Variations, modification, and other implementations of what is described herein will occur to those of ordinary skill in the art without departing from the spirit and scope of the invention as claimed. Accordingly, the invention is to be defined not by the preceding illustrative description but instead by the spirit and scope of the following claims.