Abstract:
A new optical image rotating device, used in combination with laser diode bars or laser diode bar arrays, cylindrical lens elements or cylindrical lens element arrays, and an afocal image relay system, to achieve a highly collimated output laser beam suitable for use in many applications. Due to the simplicity of the optical image rotating device, fabrication of such a device is relatively easy and straightforward. The optical image rotation device is suitable for use with high-density laser diode bars, as well as continuous linear and 2D sources. Performance of the system is relatively insensitive to alignment of the optical image rotation device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the general field of semiconductor laser systems using diode bars and stacked diode bars, providing high-brightness beams, used in applications such as pumping of solid-state lasers, materials processing and medical applications. Specifically, this invention relates to an optical image rotation device, and the associated laser and optical systems. 
     2. Description of the Prior Art 
     Various systems previously designed to modify the bi-lateral divergence of laser diodes and laser diode arrays use sequential image rotation devices, which depend on a specific and predetermined order of reflective surfaces. Specifically, each laser diode image is rotated by an individual and specific series of optical elements. Examples of these optical elements include prisms, external reflectors and lenses. Due to the relatively small spacing between individual diode emitters, and the subsequent relatively small spacing between subsequent adjacent beampaths, the aforementioned optical elements are relatively small, and generally assembled in an array of size on the scale of the laser diode array. Fabrication and assembly of such arrays of optical elements is relatively complex, as compared to the use of a single lens element. Careful alignment of such an optical element array must be performed to achieve optimal performance of the system as a whole. In U.S. Pat. No. 5,513,201, “Optical Path Rotating Device Used With Linear Array Laser Diode And Laser Apparatus Applied Therewith”, several externally and internally reflective array designs are presented, all of which require careful one-to-one alignment with the laser diode array. 
     SUMMARY OF THE INVENTION 
     The present invention provides a new optical image rotating device, used in combination with laser diode bars or laser diode bar arrays, cylindrical lens elements or cylindrical lens element arrays, and an afocal image relay system, to achieve a highly collimated output laser beam suitable for use in many applications. 
     The optical image rotating device comprises a plurality of pairs of perpendicular reflective surfaces. This device may be a mirror array, prism array, or a plurality of prisms. The working surfaces of the optical image rotating device are perpendicular to a common plane, so that fabrication of the optical image rotation device is relatively easy and straightforward. The optical image rotation device is suitable for use with high density laser diode bars, as well as continuous linear and 2D sources. Performance of the system is relatively insensitive to alignment of the optical image rotation device. 
     In the first embodiment, semiconductor laser sources, comprising a laser diode bar having arranged linearly therein 10 to 100 active layer stripes, are collimated in the fast axis by a single cylindrical lens element. An afocal image relaying optical system then reimages the laser diode bar through a polarizing beamsplitter onto the optical image rotating device. The optical image rotating device rotates the image, and therefore the associated polarization, 90 degrees around the Z-axis. The optical image rotating device reflects and redirects the radiation back towards the polarizing beamsplitter. The polarizing beamsplitter reflects and redirects the beam in a direction substantially different from the incident beam. A single cylindrical lens subsequently collimates the beam in the slow axis. The laser diode bar radiation is therefore brought to a high degree of collimation, and can be readily used in many applications. 
     In the second embodiment, semiconductor laser sources, comprising a stack of laser diode bars having arranged linearly therein 10 to 100 active layer stripes, are collimated in the fact axis by a stack of cylindrical lens elements. An afocal image relaying optical system then reimages the stack of laser diode bars, through a polarizing beamsplitter, onto the optical image rotating device. The optical image rotating device rotates the image, and therefore the associated polarization, 90 degrees around the Z-axis. The optical image rotating device reflects and redirects the radiation back towards the polarizing beamsplitter. The polarizing beamsplitter reflects and redirects the beam in a direction substantially different from the incident beam. A stack of cylindrical lens elements subsequently collimates the beam in the slow axis. The radiation from the stack of laser diode bars is, therefore, brought to a high degree of collimation, and can be readily used in many applications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects of the present invention and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, in which like reference numerals designate like parts throughout the figures thereof and wherein: 
     FIG. 1 is a plan view of the preferred embodiment of the invention; 
     FIGS. 2 a  and  2   b  are, respectively, schematic diagrams of the laser diode bar and stacked laser diode bars; 
     FIGS. 3 a ,  3   b ,  3   c  and  3   d  show side and top views, respectively, of the cylindrical lens and cylindrical lens array which collimate the radiation in the fast axis for the laser diode and stacked laser diode bars; 
     FIGS. 4 a ,  4   b  and  4   c  show three examples of afocal doubly telocentric imaging systems; 
     FIG. 5 shows the relationship of orientation between the laser diode bar and the optical image rotating device; 
     FIG. 6 shows the combined action of the collimating lenses of the polarizing beamsplitter element and the optical image rotation device; 
     FIGS. 7 a ,  7   b ,  7   c  and  7   d  show top and side views, respectively, of two embodiments of the polarizing beamsplitter element; 
     FIGS. 8 a ,  8   b  and  8   c  shown the cross section (in the X-Z plane) of an alternate optical image rotating device for use in the present invention; 
     FIG. 9 a  and  9   b  shows, in literal form, the image rotation performed by the image rotating device; and, 
     FIGS. 10 a ,  10   b ,  10   c  and  10   d  show, respectively, the cylindrical lens element and cylindrical lens element array which collimate the radiation in the slow axis. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a plan view of the semiconductor laser system of the present invention. The light source  5  is a laser diode bar or stack of laser diode bars  10 . Each bar  10 , for example, is 10 mm long, comprising 10 to 100 individual laser diode emitters  12   a - 12   n , as also shown in FIG. 2 a . The cross section of each emitter  12   a - 12   n  is 0.1 to 1 um in the Y-axis, and 50 to 200 um in the X-axis. The angular divergence in the Y-Z plane (also referred to as the “fast axis”) is 30 to 50 degrees FWHM. The angular divergence in the X-Y plane (also referred to as the “slow axis”) is approximately 10 degrees FWHM. 
     The cylindrical lens element  20  collimates the radiation from the laser diode bar(s)  10  in the fast axis. 
     An image relay system  30  projects an image of the laser diode bar(s)  10  onto the optical image rotation device  50 . The image relay system  30  is telocentric in both object and image space, and is afocal. 
     A polarizing beamsplitter element  40  is located following the image relay system  30 . Polarizing beamsplitter element  40  is oriented such that the radiation emanating from the image relay system  30  transmits through the polarizing beamsplitter element  40 . 
     The optical image rotation device  50  rotates the images of individual diode emitters  12   a - 12   n  90 degrees around multiple axes which are parallel to the Z-axis. Taking as an example, the image of one of specific diode emitter  12   a - 12   n  is rotated 90 degrees around a Z′-axis; the Z′-axis being parallel to the Z-axis, and located approximately at the location of the specific diode emitter  12   a - 12   n . Thus, the 90 degree image rotations of the individual diode emitters  12   a - 12   n  occur around approximately their individual locations, while the relative arrangement of the laser diode emitters  12   a - 12   n  within the array of laser diodes  10  remains unchanged. The images of each emitter  12   a - 12   n , and therefore the polarization, are rotated 90 degrees and redirected back towards the polarizing beamsplitter element  40 . 
     The polarizing beamsplitter element  40  is oriented such that the radiation emanating from the optical image rotation device  50  reflects from the polarizing beamsplitter element  40 , and is redirected in direction substantially different from the image relay system  30 . 
     The cylindrical lens  60  element collimates the radiation from the laser diode bar(s)  10  in the slow axis. 
     The radiation in the XY plane at location  70  is collimated in both fast and slow axis, and can be subsequently used in a number of applications. 
     FIGS. 2 a  and  2   b  show, respectively, the laser diode bar  10  and stacked laser diode bars  10 ′. For simplicity, only eight emitters  12   a - 12   n  on laser diode bar(s)  10  and  10 ′ are shown and only eight emitters  12   a - 12   n  are shown on each of the stacked laser diode bars  10 ′. 
     FIG. 3 a  illustrates an end view of cylindrical lens element  20  and FIG. 3 b  illustrates a top view of the cylindrical lens element  20 , which collimate the radiation in the fast axis, where all numerals correspond to those elements previously or otherwise described. 
     FIG. 3 c  illustrates an end view of a cylindrical lens array element  20 ′ and FIG. 3 d  illustrates a top view of the cylindrical lens array element  20 ′, which collimate the radiation in the fast axis. 
     Shown in FIGS. 4 a  and  4   b  are two examples of afocal doubly telocentric imaging systems, where all numerals correspond to those elements previously or otherwise described. FIG. 4 a  is one embodiment, using tow positive lens group  22  and  32  (see, for example, Kingslake, Lens Design Fundamentals, Academic Press, 1978). Radiation from the object plane  13  is imaged by two positive lens groups  22  and  32 , respectively, and focused on the image plane  36 . 
     A second embodiment, shown in FIG. 4 b , is a catadioptric Dyson system described in the articles: J. Dyson, “Unit magnification optical system without Seidel aberrations”, Journal of the Optical Society of America, Vol. 49(7), 713-716, 1959 and C. G. Wynne, “A unit-power telescope for projection copying”, Optical Instruments and Techniques, Oriel Press, 429-434, 1970. The Dyson system includes a concave mirror  38  of radius of curvature R and a lens  34  with one surface convex of radius r and the other flat which lies in the center of curvature of the convex surface. The radii are connected by the relationship R/r=n/(n−1), where n is the index of the lens  34 . If the object is  13 ′, the image is  36 ′ at the same distance from the axis as the object. 
     The third embodiment, shown in FIG. 4 c , is a reflective Offner system described in U.S. Pat. No. 3,748,015, 07/1973. The Offner system includes a primary mirror  42  and a secondary mirror  44 . The radius of curvature of the secondary mirror  44  is half that of the primary mirror  42 . The object plane  13 ″ and the image plane  36 ″ are in a vertical plane through the concentric point of the two mirrors  42  and  44 . 
     FIG. 5 shows the relationship of orientation between the laser diode bar  10  and the optical image rotation device  50 , where all numerals correspond to those elements previously or otherwise described. The laser diode bar  10  is aligned with the emitter height and width parallel to the X- and Y-axes. The optical image rotation device  50  has its fold axes parallel to the X-axis, which is rotated 45 degrees from the X-axis. 
     FIG. 6 illustrates the combined action of the polarizing beamsplitter element  40  and the optical image rotation device  50 , where all numerals correspond to those elements previously or otherwise described. Shown for example is an image and polarization parallel to the X-axis which enters and transmits through the polarizing beamsplitter element  40 . The optical image rotation device  50  rotates the image and polarization by 90 degrees, such that both image and polarization are parallel to the Y-axis. The radiation subsequently reflects off the polarizing beamsplitter element  40 , and is redirected in a different direction. 
     FIGS. 7 a  and  7   b  illustrate a top view and a side view of the cube beamsplitter element  40 , respectively. This element may be a cube beamsplitter, shown in FIG. 6, or, in the alternative, can be a polarizing plate beamsplitter. 
     FIGS. 7 c  and  7   d  illustrate a top view and a side view of a polarizing plate beamsplitter  41 . 
     FIGS. 8 a ,  8   b  and  8   c  show the cross section (in the X-Y plane) of alternative optical image rotation devices  50  in the present invention. The optical image rotation device  50  may be a mirror array  52 , comprising plurality of a pair of reflective surfaces  56  and  58 , which are perpendicular to each other. Alternatively, the optical image rotation device  50 ′ may be a prism array  54 , comprising a plurality of a pair of total internal reflection surfaces  61  and  63 , which are perpendicular to each other. An additional alternative embodiment includes a prism array  65  which can be an optical image rotation device  50 ″ formed by a substrate  66  and a plurality of prisms  64 , comprising a plurality of a pair of total internal reflection surfaces  61  and  63 , which are perpendicular to each other. 
     FIGS. 9 a  and  9   b  show, in literal form, the image rotation performed by the image rotation device(s)  50 . The image is incident on surface  90  at a compound angle of incidence. The image reflected by surface  90  is directed towards surface  92 , also being incident at a compound angle of incidence. The two reflections, both at a compound angle of incidence, work to rotate the image by 90 degrees around an axis parallel to the Z-axis. 
     FIG. 10 a  illustrates an end view of the cylindrical lens  72  and FIG. 10 b  illustrates a top view of the cylindrical lens  72  which collimates the radiation of the slow axis, where all numerals correspond to those elements previously or otherwise described. 
     FIG. 10 c  illustrates an end view of a cylindrical lens array  72 ′ and FIG. 10 d  illustrates a top view of the cylindrical lens array  72 ′ which collimates the radiation of the slow axis. 
     MODE OF OPERATION 
     With reference to FIG. 6, radiation from an array  10  or stacked array  10 ′ of laser diode emitters  12   a - 12   n  is collimated in the fast axis by a single cylindrical lens element  20  or by a cylindrical lens array  20 . An afocal image relaying optical system  30  then reimages the laser diode emitters  12   a - 12   n , through a polarizing beamsplitter, onto the optical image rotation device  50 . The optical image rotation device  50  rotates the image, and therefore the associated polarization, 90 degrees around the Z-axis. The optical image rotation device  50  also reflects and redirects the radiation back towards the polarizing beamsplitter element  40 . The polarizing beamsplitter element  40  reflects and redirects the beam in a direction substantially different from the incident beam. A single cylindrical lens element  80  or a cylindrical lens array  20  collimates the beam in the slow axis. Radiation from the laser diode array  10  is then collimated to a high degree of collimation, and can be readily used in many applications. 
     Various modifications can be made to the present invention without departing from the apparent scope hereof.