Patent Publication Number: US-2016223809-A1

Title: Correction of laser diode bar error

Description:
FIELD OF THE INVENTION 
     The present invention relates to methods and apparatus for correcting of cross-array error of plurality of laser bar emission to emit in aligned straight line. 
     BACKGROUND OF THE INVENTION 
     Laser diode bars consists of individual emitters arranged in a line. The manufacturing process of the bars leads to emitters not being aligned in a straight line. This intrinsic property of the bars is referred to as “cross-array errors” or “bar smile.” Typical bar smile  12  is illustrated in  FIG. 1 . Typical bar smile can vary from sub-micron to several microns values. The net result of the bar smile is that it decreases the laser brightness in the fast-axis direction, which otherwise is only diffraction limited. The reason for this is that the smile prevents the fast axis collimator (FAC) to be positioned correctly for each emitter in the array. Usually the bar smile is defined as peak-to-valley (P-V) position variation in cross-array direction of the individual array members (emitters) from a straight line obtained by linear least squares fitting of their geometrical positions. 
     One of the properties of the laser bar smile is that it generally depends in the laser current. Typical example is shown on  FIG. 2  illustrating measurement results of individual emitters&#39; positions deviation from a straight line at four different currents 20 A, 35 A, 50 A, and 65 A. As shown in this particular example, the bar smile (P-V position variation) can be as low as 0.91 μm at 20 A and as high as 1.2 g. at 50 A laser current. It is important to notice that as though in most cases the lower smile corresponds to lower currents there are cases, as the illustrated on  FIG. 2 , in which the dependence is not monotonic and lower currents can have higher smile value. 
     The smile effect is illustrated on  FIG. 3 . Emitters  100  are positioned in the plane constituted by planes  201  but are not arranged in a straight line. After passing through the FAC  301 , which vertex  303  lies in the main optical plane  202 , only beams originating from emitters  102  that are positioned in the main optical plane  203  will propagate parallel to this plane. In the focal plane  202  of the FAC all beams will form spots arranged in a straight line. In any subsequent plane  203  downstream though, the spots produced by beams originating from emitters  101  that are not positioned in the main optical plane will be deviated. The further the beams propagate downstream, the bigger this deviation from a straight line will be as illustrated by the broken lines  204 . 
     Consider a typical application of an optical system imaging the emitters&#39; fast-axis onto a target with magnification for example ×20 and overlapping their slow-axes. If the focal length of the FAC is 1 mm, which is a typical value, a smile of 2 microns will lead to a beam with divergence increased only by about 1 mRad (depending on the particular cross-array distribution of the emitters), but a spot with spatial distribution very different to the one compared to the case of zero smile. For a parabolic shaped smile of P−V=2 μm these spatial and angular distributions are illustrated on  FIGS. 4 a  and 4 b    respectively. For comparison the same figures present the spot characteristics of the same optical system incorporating a laser bar with zero smile. 
     Prior art shown in U.S. Pat. No. 5,854,651 uses one-time adjustable mirrors or parallel plane plates to correct the beams propagation directions. U.S. Pat. No. 5,861,992 uses individual FAC associated to each emitter and displaced in fast-axis direction to compensate for the corresponding emitter displacement. Another smile correction approach using phaseplates with continuously-varying wedge surfaces correcting the pointing of each emitter after the FAC has been collimated is illustrated in an industrial product reported in an article by Roy McBride, Natalia Trela, Jozef J. Wendland, Howard J. Baker, Proc. SPIE 8039, Laser Technology for Defense and Security VII, 80390F, 2011 and shown at: http://www.powerphotonic.com/wpcontent/uploads/2014/01/Smile-Corrector-Data-Sheet.pdf, which presents methods to correct smile of single laser bar at a single particular current. 
     SUMMARY OF THE INVENTION 
     Briefly, according to one aspect of the present invention a method for dynamic correction of cross-array errors in an optical systems incorporating laser diode bar including emitting laser beams from a plurality of emitters on the laser diode bar; analyzing the beams produced from each of the plurality of emitters; measuring a deviation of each emitter relative to a straight line; transferring the deviation for each of the emitters relative to the straight line to a controller; and driving a plurality of actuators connected to micro-mirrors to correct for the deviation from the straight line of each of the emitters. 
     These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  represents an image of a typical bar smile (prior art); 
         FIG. 2  shows a diagram of fast axis deviation of plurality of emitters operated at different currents; 
         FIG. 3  shows a smile bar effect generated by an arrangement of plurality of emitters and fast axis collimator; 
         FIGS. 4 a  and 4 b    show a spot with spatial and angular distributions generated by smile of two microns; 
         FIG. 5 a    shows an apparatus arrangement applying a dynamic correction of cross-array errors of laser diode bars with a mirror for each emitter; 
         FIG. 5 b    shows an apparatus arrangement applying a dynamic correction of cross-array errors of laser diode bars with a mirror common to two emitters; 
         FIGS. 6 a  and 6 b    show results from angle correction of the system from diagrams shown in  FIGS. 4 a  and 4 b    to corrected results in diagrams illustrated by  FIGS. 6 a  and 6 b   ; and 
         FIG. 7  shows a closed loop system for dynamic correction of the array cross-errors using beam analyzing and angle correction. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the disclosure. However, it will be understood by those skilled in the art that the teachings of the present disclosure may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the teachings of the present disclosure. 
     While the present invention is described in connection with one of the embodiments, it will be understood that it is not intended to limit the invention to this embodiment. On the contrary, it is intended to cover alternatives, modifications, and equivalents as covered by the appended claims. The object of this invention is to provide a method for dynamic correction of cross-array errors (bar smile) of laser diode bars including current dependent cross-array errors. 
       FIGS. 5 a , 5 b   , and  6  illustrate the invention. The laser bar emitters  101  are positioned in the plane  201  and deviate from a straight line  22 . The beams emanating from the emitters  101  are being collimated by the FAC  301  and the slow axis collimator (SAC)  302 . Due to the cross-array positioning errors after the FAC the beams from the individual emitters do not propagate parallel to the main optical plane  204 , but at different angles depending on the cross-array positioning error of each emitter. The correcting element is an array of micro-mirrors  400  with each micro-mirror  401  dedicated to a single emitter from emitters  101  as shown in  FIG. 5 a   . Each micro-mirror  401  is dedicated to a single emitter  101  by selectively choosing the angle of this mirror relative to the optical axis  210 . The beams from all emitters can be made to propagate parallel or to intersect any chosen line  209  downstream the optical system, as illustrated. 
       FIG. 5 b    shows that micro-mirror  504  of the micro-mirrors array  400  is dedicated to a group of neighboring emitters such as  508  and  512 , the correction applied will be the averaged for this group of emitters. The angle of the array members is set by means of actuators  500 . The actuators can be of electrostatic, magnetic, or any other type used in micromechanical technology. 
     The result of such angle correction of the system from  FIGS. 4 a  and 4 b    is illustrated on  FIGS. 6 a  and 6 b   . Array  400  can be successfully replaced by an adaptive mirror, undisrupted reflecting surface locally deformed to provide for different angles of reflection. In less demanding applications, where some residual error can be permitted, one correcting mirror can serve two of more members of the laser array as illustrated on  FIG. 5   b.    
     To take full advantage of the proposed dynamic correction of the array cross-errors, a closed-loop beam analyzing and angle correction is preferable. An exemplary system is illustrated on  FIG. 7 . The beams  105  emitted from the individual emitters of the laser diode bar  100  are collimated by the FAC and SAC, depicted for simplicity as a single entity  300 , and reflected by the individual members of the array  400 . After being shaped, partially or finally by any optical elements  600  in the system, a small amount, in terms of energy, of the beam  704  is being sampled by a beam sampler  700  and deviated to a beam analyzer  800 . The beam analyzer is designed to sense spatial and/or angular position of the beams. The beam analyzer can be based on a multi-pixel light sensor like charge-coupled device (CCD) or complementary metal-oxide-semiconductor (CMOS) or any means suitable for the purpose. Alternatively single-pixel position sensitive detectors known as PSD can be used. These same devices, single or multi pixel, when positioned in the focal plane of simple lens added in front of them, will become beam angle sensors. 
     The results from the beam analyzing are being transferred to a controller  810  which drives the actuators  500  setting angles of the individual members of the array  400  in a manner to provide parallel propagation of the beams or their intersection on a single line (illustrated) on the target  900 , depending on the application. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. 
     PARTS LIST 
     
         
           12  bar smile 
           22  straight line 
           100  emitter bar (laser diode bar) 
           101  emitters 
           102  emitters 
           105  beams 
           201  plane 
           202  plane 
           203  plane 
           204  deviation from focal plane 
           209  line downstream the optical system 
           210  optical axis 
           300  FAC and SAC 
           301  fast axis collimator (FAC) 
           302  slow axis collimator (SAC) 
           303  fast axis collimator vertex 
           400  array of micro-mirrors 
           401  micro-mirror 
           500  actuators for each micro-mirrors 
           504  micro mirror that handle more than one emitter 
           508  first emitter that emits towards micro-mirror  504   
           512  second emitter that emits towards micro-mirror  504   
           600  optical element 
           700  beam sampler 
           704  deviated beam energy 
           800  beam analyzer 
           810  controller 
           900  imaging target