Abstract:
Multiple broad area lasers are coupled to a planar lightwave circuit, where the waveguides come together to form a single wide emitting aperture. A tapered lens is used at the output of the planar lightwave circuit to transform the highly asymmetric mode into a conventional round mode. This configuration allows much higher “brightness”, allowing 10 or more 100 um wide broad area lasers to be coupled into a single 100 um core multimode fiber. This is considerably more efficient than the standard method of combining a single 100 um wide broad area laser to a 100 um core multimode fiber.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of the filing date of U.S. Provisional Patent Application No. 61/371,616, filed Aug. 6, 2010, the disclosure of which is incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Single mode semiconductor lasers with emitting apertures on the order of 2 um×2 um are generally limited to roughly a watt of output power. Higher power lasers can be realized by increasing the output aperture much wider to a 100 um or so. The laser becomes multimode in the wide lateral direction, but stays single mode in the vertical direction. Such lasers can provide on the order of 5 to 10 Watts of optical power reliability and are used in various applications such as materials processing and industrial processes. 
     In addition to the total power output, another key performance metric of these lasers is the “brightness”, which is inversely proportional to the product of the emitting area and the numeric aperture—the brighter the source, the smaller the aperture or the smaller the divergence of the beam. A brighter source is generally preferred as it can be focused tighter to provide higher energy density. 
     A recent commercially interesting application of these broad area lasers is for pumping double clad fiber lasers. Generally multiple broad area lasers are individually coupled to multimode fibers. These fibers are in turn combined into even larger core diameter multimode fibers, which subsequently pump the double clad fiber. Hundreds of watts of pump power can be obtained in this manner. However, since “brightness” must be preserved, the more power one desires, the larger the core diameter or the NA of the final merged fiber. For example, if one desires 90 Watts of pump power using nine elements of low individual sources that are coupled to 100 um core diameter fibers, the final fiber in the assembly should have an area that is 9 times as much or a diameter of 300 um microns. As the diameter of the final combined fiber increases, the overlap with the single mode core of the double clad fiber decreases. This large core size in turn lowers efficiency and makes the fiber harder to manufacture and handle. Thus it is usually desirable to use the brightest sources possible in the first place. 
     Considerable brightness is wasted in the first step of the process, where a 100 micron broad area laser with a vertical aperture of 2-3 um is coupled to a 100 micron diameter round fiber. Though the laser fills the fiber laterally, vertically a lot of the fiber area is wasted. In theory multiple lasers of such kind can be coupled to a single multimode core if appropriate packaging can be realized. 
     BRIEF SUMMARY OF THE INVENTION 
     Aspects of the present invention relate to broad area lasers. In one aspect the invention provides A broad area laser device, comprising: a plurality of broad area lasers; a planar lightwave circuit (PLC) to receive light from the lasers; and a flattened tapered fiber to receive light from the PLC. 
     In another aspect the invention provides A broad area laser device, comprising: a plurality of broad area laser assemblies, each including a broad area laser, a first lens to collimate light of the broad area laser in a first axis, and a second lens to collimate light of the broad area laser in a second axis; a coupling lens to couple light of the broad area lasers into an output: and a mirror structure to direct light of the broad area lasers to the coupling lens. 
     These and other aspects of the invention are more fully comprehended upon review of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present patent are illustrated by way of examples. 
         FIG. 1   a  (prior art) illustrates a broad area laser optically coupled to a multimode fiber. 
         FIG. 1   b  (prior art) illustrates illumination area of a fiber illuminated by a broad area laser. 
         FIG. 2   a  (prior art) illustrates a configuration where three lasers are coupled to a multimode fiber simultaneously. 
         FIG. 2   b  (prior art) illustrates illumination of a fiber by three lasers. 
         FIG. 3   a  illustrates a configuration in which light from multiple broad area emitters is coupled into a multimode fiber using a Planar Lightwave Circuit (PLC) and a flattened tapered fiber. 
         FIG. 3   b  illustrates a portion of the PLC of  FIG. 3   a.    
         FIG. 4   a  illustrates a view of a flattened tapered fiber useful in aspects of the invention. 
         FIG. 4   b  illustrates a top view of the fiber of  FIG. 4   a.    
         FIG. 4   c  illustrates a side view of the fiber of  FIG. 4   a.    
         FIG. 4   d  illustrates a sectional view of the fiber of  FIG. 4   c.    
         FIG. 4   e  illustrates a further sectional view of the fiber of  FIG. 4   c.    
         FIG. 4   f  illustrates a sectional view of the fiber of  FIG. 4   b.    
         FIGS. 5   a  and  5   b  illustrate a MEMs alignment unit in accordance with aspects of the invention. 
         FIG. 6  illustrates an assembly including the MEMs alignment unit of  FIGS. 5   a  and  5   b  and broad area laser. 
         FIG. 7  illustrates a configuration where fold mirrors are positioned closer to an output fiber. 
         FIG. 8  illustrates a configuration of a stack of four mirrors. 
         FIG. 9  illustrates the adjustability of the mirror angles by using a 1-D array of MEMs mirrors. 
         FIG. 10  illustrates a further embodiment in accordance with aspects of the invention. 
         FIG. 11  illustrates multiple MEMs alignment chips in accordance with aspects of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1   a  shows a simple way that currently broad area lasers are coupled to a multimode fiber. In the drawing, a 250 micron diameter fiber  10  has a hundred micron diameter core  20 . A broad area laser  30  has an active stripe  40  with a width of one hundred microns that emits light from a front facet  50 . In the configuration shown in  FIG. 1   a , a cylindrical lens  60  reduces the divergence in the vertical direction to match the NA of the fiber, while the light is not collimated in the lateral direction. The lens  60  is optional, as the laser can be positioned directly against the fiber, in a manner known as butt coupling, such that the emitting aperture is aligned to the center of the fiber core. However, the lens  60  does improve the coupling efficiency. In both cases, the laser illuminates the central part of the core, as shown in  FIG. 1   b . The rectangle  70  is the area illuminated by the broad area laser. This area quickly spreads out in the fiber and the modes move and mix so after a given length of fiber the entire core is illuminated, but at a reduced brightness, since the concentrated initial area  70  is dispersed throughout the core. As explained previously, this reduces the utility of the light. 
     Though not commercially available, there are ways to couple multiple broad area lasers to the same fiber core simultaneously.  FIG. 2  shows a possible configuration where three lasers  30 ,  210 ,  220  are all coupled to a multimode fiber simultaneously. In this case a small cylindrical lens  60  collimates the fast axis of the laser, then a second lens  210  forms an image on the fiber face after reflecting from a mirror  225 . The optics of the three lasers are identical, except that the images are offset in the vertical direction by a few microns, such that all three images fit on the fiber core, as shown in  FIG. 2   b . In this case the image of the light from laser  30  forms the rectangle  270 , that of laser  210  forms the rectangle  280 , and that of  220  forms the rectangle  290 . To prevent blocking of the images, ideally the lasers themselves are vertically offset and the mirrors are made narrow in the vertical direction, such that the light from the back lasers can pass above or below the mirror. Alternatively, the mirrors can be offset linearly such that the light from the rear lasers passes to the right or left of the mirror. That causes a change in the incidence angle onto the fiber, but the effect on coupling efficiency is minimal. The advantage of the structure shown in  FIG. 2  is that the light exiting the fiber is considerably brighter, with three times as much power coupled into the same 100 micron diameter core. With proper optics and mode matching it should be possible to couple, for example, up to 10 such broad area lasers efficiently into the fiber. 
       FIG. 3   a  shows a simpler way to couple multiple broad area emitters into a multimode fiber. The same lasers,  30 ,  210 , and  220  are used with a cylindrical lens, the first cylindrical lens marked  60  in the figure, to couple light into a planar lightwave circuit (PLC)  300 , which are lithographically defined planar waveguides typically fabricated on silicon wafers using germanium doped silicon dioxide. The wafer contains multiple broad area waveguides  310 , each single mode in the vertical direction and multimode in the horizontal or planar direction. Three are shown in  FIG. 3   a . Typical dimensions of the cores of each waveguide would be 100 um wide and 10 microns thick. On the PLC the three waveguides come together to form a very wide single waveguide  320  at the exit of the PLC. Since there is no change in the emitting area, brightness is conserved on the PLC. Then to couple out of the PLC, a flattened fiber  330  is used. If the index difference between the core and the cladding stays the same in the PLC as in the tapered/flattened fiber, then the two can be butt-coupled, or simply placed or epoxied together with no intermediate optics.  FIG. 3   b  shows an exaggerated view of the PLC  300 , showing that the output waveguide width  320  is simply n times the width of the input waveguide, where n is the number of input waveguides. The purpose of the PLC is simply to eliminate or reduce the dead space between the modes. 
     The flattened tapered fiber is better shown in  FIG. 4   a - f . Such tapers are relatively easy to make in optical fibers and may be realized by stretching or squeezing the fiber while the temperature is elevated to soften the glass. In this particular case pressure is applied to flatten the fiber, thus producing a structure where, for an end  411  of the fiber, the width has increased and the thickness has decreased, as compared to an extending cylindrical portion  413  of the fiber. In the embodiment show in  FIGS. 4   a - f , relative expansion in the lateral direction and the contraction in the vertical direction for the end of the fiber, compared to the extending cylindrical portion, is a factor of 5. In one example the diameter of cylindrical fiber is 250 μm core, with a 100 μm core, with the core at the end of the fiber being 500 μm in the lateral direction and 20 μm in the vertical direction. 
     The structure of  FIG. 3  is particularly amenable to MEMS based coupling techniques where each cylindrical lens can be aligned in x y and z using micromechanical levers. Such a MEMS alignment unit is shown in  FIG. 5   a  (top side) and  5   b  (bottom side). It is fabricated on SOI with patterning on both front and backside. The front side includes a platform  550 , on the backside of which is mounted a cylindrical lens  580  (shown in  FIG. 5   b ). The platform is attached to two silicon levers with handles on the ends  510 ,  520 . These handles are manipulated simultaneously that in turn move the platform. The platform is connected to the body through two springs  560  and  570 , with the springs being on an opposite side of the platform than the levers in the embodiment illustrated in  FIGS. 5   a  and  b . The silicon springs can stretch and bend to allow the platform  550  and thus the lens  580  under it to move. If the two handles are pulled or pushed as to stretch the two springs uniformly, the lens moves generally in a corresponding direction, which may be considered in and out. If the two handles are moved to one side or vertically, the lens moves correspondingly. Finally, if one handle is moved down, while the other is moved up, the lens tilts. Once optimal alignment has been achieved, the handles are locked down using solder balls such as  530 . Electric current may be passed through heaters  540  such that the heat melts the solder ball  530 . In this particular design, the parts are initially placed such that proper alignment will include moving the extension towards the solder. Thus, the solder ball may be melted, and moving the handle to the right (as seen in  FIG. 5   a ), moves an extension  590  on the handle into the molten solder ball  530 . At this point, the current is removed, the solder freezes, and the handle is locked into place. The advantage of this technique is that submicron tolerances for moving and locking down the optical part are eased to many microns as motions are amplified by the arm and handle. 
     The final assembly looks similar to  FIG. 6 , where the PLC  300  and the broad area laser  30  are shown with respect to the MEMs alignment unit including lens  880 . The submount for the laser and the base of the assembly are not shown. 
     The MEMS alignment assembly can also be used with the mirror configuration shown in  FIG. 2 . In this case, the lenses are positioned using the MEMs manipulators so that the beams are aligned with mirrors shown or for example fold mirrors or stacked mirrors in  FIG. 2 . In this embodiment the lens and MEMS chip would not be mounted on or with respect to a PLC, but on a base, and lenses would collimate the beam from the laser, rather than focus the beam onto a PLC. The collimated beam is reflected toward an output fiber. A coupling lens is placed in front of the output fiber to focus the beams into the fiber. 
     In one embodiment, as shown in  FIG. 10 , light from a laser  1001  is collimated by a fast axis collimating lens  1002  and a slow axis collimating lens  1003 , with the light shown as optical rays  1008 . The collimated light is directed by a mirror  1004  to a coupling lens  1005 . The coupling lens focuses the light into an output fiber  1006 . 
     The collimating lenses are on a MEMs alignment chip  1007 , as illustrated in  FIG. 10 . The MEMs alignment chip may be as previously discussed, for example with both collimating lenses on a single platform. Alternatively, each lens may be on a separate independent platform. 
       FIG. 11  illustrates an embodiment in which each lens is provided its own MEMs alignment chip, although in some embodiments the silicon structures may be included on a single chip. In the embodiment of  FIG. 11 , each MEMs alignment chip includes a silicon portion, with a platform in what may be considered a cutout of the silicon. The cutout may be formed by etching for example. The platform is coupled to an edge of the cutout by springs in the silicon. Dual extensions extend from the platform, with each extension having an arm near a handle terminating the extension. Movement of the extensions, for example using the handles, allows for positioning of a lens on the platform, with one of the platforms carrying the fast axis collimating lens and the other of the platforms carrying the slow axis collimating lens. Each arm of the extension extends towards a corresponding solder ball, which can be melted to capture at least a portion of the arm, and to fix the extension, and hence the platform, in position when the solder thereafter solidifies. 
     One problem with the approach of  FIG. 10  is that the tolerances on the alignment of fold mirrors used, for example as the mirror  1004  in  FIG. 10 , are tight. In practice, the fold mirrors would typically require precise angular adjustment. The problem is especially pronounced for mirrors spaced at a large distance from the final coupling lens and output fiber. 
     One way to reduce the tolerances is to position the fold mirrors closer to the output fiber. This case is shown in  FIG. 7 . Ideally, each submount, shown representatively by lenses  705 , and collimated beam would be slightly angled, to impinge on a spiral stack of fixed mirrors  710 . A focusing lens  720  would focus the beams on the fiber, as shown in  FIG. 2   b . One possible configuration of a stack of four mirrors  810  is shown in  FIG. 8 . In an alternate embodiment shown in  FIG. 9 , adjustability of the mirror angles can be retained by using a 1-D array of MEMs mirrors  910 . This case is shown in  FIG. 9 . The beams are directed to the MEMs mirrors  920 , mounted in a MEMs structure  930  allowing for individual positioning of the mirrors, by way of electrostatic force for example. Each mirror can be adjusted independently to direct the light to the focusing lens. 
     Although aspects of the invention have been described with respect to varying embodiments, it should be recognized that the invention comprises the novel and non-obvious claims and their insubstantial variations supported by this disclosure.