Abstract:
A novel edge emitting laser diode assembly with adjustable mounting of the diodes is described. A laser diode submount carrier is adjustably mounted onto a base assembly in such a manner as to compensate for variations in thickness of each of the edge emitting laser diodes. The laser array assembly is usable for applications in either parallel optical modules or wavelength-division multiplexing modules.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of and priority from U.S. provisional application Ser. No. 60/730,219 filed Oct. 25, 2005. 
     
    
     BACKGROUND AND BRIEF SUMMARY  
       [0002]     The present invention pertains to optical communications. More particularly, this invention relates to the packaging of edge emitting laser diodes within an assembly that is usable in wavelength division multiplexing packages or parallel optical packages for fiber-optic data-communications and telecommunications systems. U.S. Pat. No. 6,201,908, incorporated by reference, describes a wavelength division multiplexer in which the present invention may be utilized.  
         [0003]     Edge emitting diodes have become widely used, but their use in optical communications systems has been complicated by inherent variations in the thickness of the diodes. These variations (in diodes that are available at reasonable cost) can be as great as plus or minus 5 to 15 microns. Variations in diode thickness directly affect the location of the light emitting facet of the diode and the location of the output beam. Optical systems, such as wavelength division multiplexers, for example, require a precise location of laser output beams of within 1 micron so that the output beams are aligned with single mode fibers included in the optical pathway of the multiplexer. Variations in location of edge emitting laser diode output beams in the range of plus or minus 5 to 15 microns are simply unacceptable in these devices.  
         [0004]     The present invention provides a cost effective, adjustable mounting system for edge emitting laser diodes that compensates for variations of thickness from diode to diode.  
         [0005]     The present invention is similar to aligning a series of diving boards of unequal lengths so that the tip ends of the diving boards extending over a pool are aligned precisely. The length of each diving board is analogous to the thickness of an edge emitting diode. Providing a horizontally adjustable mounting for each diving board is analogous to the present invention, which provides an adjustable mounting for each diode which adjusts to compensate for variations of thickness from diode to diode.  
         [0006]     A primary object of the invention is to provide a method and apparatus for adjustably mounting an array of edge emitting laser diodes to compensate for variations of thickness of the diodes.  
         [0007]     Another object of the present invention is to provide a means of isolating the light emitting z-axis from the x-axis and y-axis of an edge-emitting laser diode.  
         [0008]     Other objects and advantages will become apparent from the following description and drawings wherein: 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  shows a prior art single edge emitting laser diode mounted horizontally with respect to the carrier and an optical power monitor mounted perpendicular to the carrier;  
         [0010]      FIG. 2  shows a prior art for a single edge emitting laser diode mounted vertically with respect to the carrier and an optical power monitor mounted directly to the carrier;  
         [0011]      FIG. 3  is a schematic illustration of thickness variations of prior art edge emitting diode lasers;  
         [0012]      FIG. 4  is a schematic illustration showing how the present invention compensates for thickness Variations of edge emitting laser diodes;  
         [0013]      FIG. 5  illustrates an array of four edge emitting laser diodes according to the invention; and  
         [0014]      FIG. 6  illustrates a two dimensional, 2×2 array of four edge emitting laser diodes according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     Most prior art edge emitting laser diode assemblies, such as shown in  FIG. 1 , attach the laser diode chip  1  horizontally on a fixed support  3  within the carrier assembly  10 . A photo-monitor diode  2  is then typically mounted perpendicular to the carrier assembly to capture the light coming from the rear of the laser diode  1 . In order to create a laser array with this type of assembly construction, the variations of the laser diode chip thickness (plus or minus 5 to 15 microns) becomes unmanageable, causing unacceptable variations in the vertical positioning of the horizontal laser output beam in this application. Prior art edge emitting laser diode assemblies that are mounted vertically within the carrier assembly, such as shown in  FIG. 2 , use a side-flattened metal post  24  on which to mount the edge emitting laser diode  21 . Sometimes, a small conducting spacer chip  20  is used to increase the distance the facet of the laser diode extends out from the post. This allows one to place a photo-monitor diode  22  beneath the rear facet of the edge emitting laser diode  21  for monitoring of the optical power. Once again, in order to create a laser array, the tolerance on (or variations in) the laser diode chip thickness in this configuration becomes unmanageable. The invention described herein compensates for variations of thickness of edge emitting laser diodes by using a laser diode submount attached to the base of the assembly in such a manner as to extend along the y-axis of the assembly, like a diving board extends over a pool in the analogy described above. In creating an edge emitting laser diode array, the submounts of the laser diodes are positioned to align the light emitting facets of each laser diode with each other and with the optical system in which the diodes are working. The z-axis of the laser diode assembly is parallel to the laser output beam and is usually a coarse and non-critical alignment. The x-axis and y-axis are critical alignments and are controlled by the placement of the submount carrier relative to the base of the assembly. This isolation of the non-critical z-axis of the diodes relative to the critical x and y axes allows one to easily place multiple edge emitting lasers in a usable array with standard manufacturing tolerances. The cost of manufacture of the components and the arrays is significantly reduced.  
         [0016]      FIGS. 3 and 4  are schematic representations of how the present invention ( FIG. 4 ) differs from the prior art ( FIG. 3 ).  FIGS. 3 and 4  are not to scale and are intended to illustrate the operation of the invention.  
         [0017]      FIG. 3  illustrates a prior art array of three edge emitting laser diodes  31 ,  32  and  33  rigidly mounted to bases  41 ,  42  and  43 , respectively. Each of the laser diodes  31 ,  32  and  33  has a thickness t 1 , t 2  and t 3 , respectively, wherein the thickness of each laser diode varies widely as described above. Each laser diode  31 - 33  has a light emitting facet  31   a ,  32   a  and  33   a  that emits an output beam upwardly and perpendicularly to the x and y plane as shown in  FIG. 3 . A series of three “alignment targets”  51 ,  52  and  53  are shown, which represent the described spots to be aligned with light emitting facets  31   a - 33   a . The alignment targets  51 - 53  represent, for example, the centers of three single mode fibers used in conjunction with a wavelength division multiplexer, as shown and described in U.S. Pat. No. 6,201,908, referenced above. A noted above, each of the alignment targets  51 - 53  is commonly about 1 micron in size. As shown in  FIG. 3 , light emitting facet  31   a  is approximately 5 microns above target  51  on the y-axis; facet  32   a  is approximately 4 microns below target  52  on the y-axis, and facet  33   a  is approximately 10 microns above (on the y-axis) and 5 microns to the left (on the x-axis) of target  53 . The misalignment of facets  31   a - 33   a  with targets  51 - 53  shown in  FIG. 3  represents the prior art difficulty of using low cost, edge emitting laser diodes  31 - 33  in wavelength division multiplexers; the misalignment is unacceptable and a wavelength division multiplexer with this misalignment would not function.  
         [0018]      FIG. 4  illustrates schematically how the present invention will operate successfully with the same variable thickness edge emitting diode lasers  31 - 33  shown in  FIG. 3 . In  FIG. 4 , each diode laser  31 - 33  is attached to an adjustable submount  61 - 63 . Each adjustable submount  61 ,  62  and  63  is movable on the x and y axes in order to align each light emitting facet  31   a ,  32   a  and  33   a  with targets  51 - 53 . The desired alignment is shown in  FIG. 4 . Facet  31   a  has been aligned with target  51  by moving adjustable submount  61   a  downwardly on the y-axis as shown by arrow  61   a . Facet  32   a  has been aligned with target  52  by moving adjustable submount  62  upwardly on the y-axis as shown by arrow  62   a . Facet  33   a  has been aligned with target  53  by moving adjustable submount  63  downwardly on the y-axis as shown by arrow  63   a  and to the right on the x-axis as shown by arrow  63   b.    
         [0019]      FIG. 5  is a perspective view of an array of four edge emitting laser diodes  71 - 74 . As shown in  FIG. 5 , the “array” is a one dimensional array of four diodes spaced apart along the x-axis. The term “array,” as used herein and in the claims, is used broadly to include n edge emitting laser diodes, and one and two dimensional arrays (such as a 2×2 array) of edge emitting diode lasers. The x, y and z axes described herein are Cartesian coordinate axes. Each diode laser  71 - 74  is mounted on one of adjustable or movable submounts  81 - 84 . Submounts  81 - 84  are adjustable along both the x and y axes to align the output beams emitted from light emitting facets  71   a - 74   a  with alignment targets not shown in  FIG. 5 . A base  90  lies in a plane including x and y axes, and light emitted from facets  71   a - 74   a  is emitted parallel to the z-axis around the periphery of base  90 . Submounts  81 - 84  are adjustable relative to spacer  95  to allow movement of submounts  81 - 84  on the x and y axes. When submounts  81 - 84  have been moved or adjusted to align the light emitting facets  71   a - 74   a  with their respective “targets” (not shown in  FIG. 5 ), submounts  81 - 84  are attached to spacer  95  by soldering, ultrasonic welding, or other means known in the art. An array of monitoring photodiodes  101 - 104  are mounted on base  90  to monitor the output of each edge emitting laser  71 - 74 . The monitoring photodiodes  101 - 104  are aligned with the light emitting facets  71   a - 74   a  and receive light emitted from the rear faces  71   b - 74   b  of facets  71   a - 74   a.    
         [0020]      FIG. 6  is a schematic illustration showing how an array of four edge emitting diode lasers  111 - 114  may be positioned to form a two dimensional, 2×2 array with two diodes extending along the x-axis and two diodes extending along the y-axis.  
         [0021]     The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teaching. The embodiments were chosen and described to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best use the invention in various embodiments and with various modifications suited to the particular use contemplated. The scope of the invention is to be defined by the following claims.