Abstract:
The present invention relates to the packaging of high power laser(s) in a surface mount technology (SMT) configuration at low-cost using wafer-scale processing. A reflective sidewall is used to redirect the output emission from edge-emitting lasers through an optical element (e.g., a diffuser, lens, etc.). A common electrical pad centered inside the package provides p-side connection to multiple laser diodes (i.e. for power scalability). Thick plating (e.g. 75 um to 125 um) with a heat and electrically conductive material, e.g. copper, on a raised bonding area of a substrate provides good heat dissipation and spreading to the substrate layer during operation. The composite CTE of the substrate layer, e.g. AlN, and the heat/electrical conductive plating, e.g. Cu, substantially matches well with the laser substrates, e.g. GaAs-based, without the requirement for an additional submount.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention claims priority from U.S. Patent Application No. 61/509,771 filed Jul. 20, 2011, which is incorporated herein by reference for all purposes. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to laser packaging, and in particular to a the packaging of high power lasers in a surface mount technology (SMT) configuration at low-cost. 
       BACKGROUND OF THE INVENTION 
       [0003]      FIG. 1  is a perspective view of a conventional transistor outline (TO) can optical module package  100 . The conventional TO-can package  100  comprises a base  101 , with a heat sink (e.g. copper) block  111  and/or ceramic submount extending upwardly therefrom, and a plurality of electrical leads  102  extending therethrough. Typically, the leads  102  comprise two leads for driving a laser diode (LD)  103  and two leads for biasing a monitor photodiode (MPD)  104 . The LD  103  and the MPD  104  are arranged on the surface the base  101  so that the mounted LD  103  points upwardly so that light emission is along the optical axis of the TO-can  100 , and so that the MPD  104  receives a portion of the light emitted from the rear of the LD  103 . In particular, LD  103  is mounted on the submount  111 , e.g. with a hard solder (AuSn), with the MPD  104  disposed on the base  101  directly below the LD  103 . The LD  103  and the MPD  104  are connected to the leads  102  by, for example, wire bonding. 
         [0004]    The leads  102  are coaxially aligned via through-holes  113 , which extend through the lower and upper surfaces of the base  101 . The through-holes  113  are filled with a glass sealant  105 , which is in a heated, fluid state during assembly, cools to a solid state, thereby fixing the leads  102  to the base  101  and hermetically sealing the through holes  113  at the same time. A cap  115 , typically constructed from a material, e.g. Kovar, with a coefficient of thermal expansion matched to that of silica or glass, is mounted on the base  101  over top of the aforementioned electro-optical elements with some form of hermetical seal. Lensing  116  is typically provided along the optical axis of the TO-can  100  to control, e.g. focus, collimate, the light exiting the LD  103 . 
         [0005]    Unfortunately, TO-can packages do not scale well for high average power, and do not enable laser drivers to be positioned in close proximity to the laser. Accordingly, recent demand for LDs and multi-LD packages operating at high bit rates (&gt;10 Gb/s) have necessitated modifications to the conventional TO can arrangement. For example, the number of leads must be increased to at least six, and the lengths of the leads extending from the TO can must be minimized. The amount of heat dissipated from the TO can must be increased. Moreover, it is highly beneficial for some of the electrical components to be disposed adjacent the laser, which is impossible with the current TO can structure. 
         [0006]    An object of the present invention is to overcome the shortcomings of the prior art by providing a compact laser package with up to a plurality of laser diodes with minimal lead lengths utilizing a reflective ring to redirect light perpendicular to the substrate and out of the package. 
       SUMMARY OF THE INVENTION 
       [0007]    Accordingly, the present invention relates to a laser emitter package comprising: 
         [0008]    a substrate including a thermally and electrically conductive plating on an upper surface thereof; 
         [0009]    a plurality of laser emitters disposed on the substrate for emitting light parallel to the upper surface of the substrate; 
         [0010]    a reflector ring for reflecting the light from laser emitters; and 
         [0011]    a laser driver disposed on the substrate between the plurality of laser drivers for driving the plurality of laser emitters. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein: 
           [0013]      FIG. 1  is a isometric view of a conventional laser drive package; 
           [0014]      FIG. 2  is a cross-sectional view of a laser emitter package in accordance with the present invention; 
           [0015]      FIG. 3   a  is a top view of a first embodiment of the laser emitter package of  FIG. 2 ; 
           [0016]      FIG. 3   b  is a top view of a second embodiment of the laser emitter package of  FIG. 2 ; 
           [0017]      FIGS. 4   a  and  4   b  illustrate alternate embodiment of the reflecting ring; 
           [0018]      FIG. 5  is a top view of the second embodiment of  FIG. 4  including electrical circuitry; 
           [0019]      FIG. 6  is a bottom view of the laser emitter package of  FIG. 1 ; 
           [0020]      FIG. 7  is a schematic diagram of the electrical circuitry for controlling laser emitters in accordance with the present invention; and 
           [0021]      FIGS. 8   a  to  8   i  illustrate the manufacturing process of the laser emitter package of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    The laser package  1 , in accordance with the present invention, with reference to  FIG. 2 , includes a plurality of, e.g. four or more, laser diodes (LD)  2  directly attached to a thermally and electrically conductive substrate  3  with a solder, e.g. AuSn. Each edge or side emitting LD  2  is mounted parallel to the substrate  3 , so that light is emitted perpendicular to the longitudinal optical axis of emission OA. 
         [0023]    Ideally, the substrate  3  includes a base  4  with a thin layer of plating  6 , e.g. from 75 um to 150 um thick of a metallic material, e.g. copper, silver or gold. The material making up the base  4  of the substrate  3  can be any suitable material, but ideally the composite material, i.e. plating  6  and base  4 , is nominally CTE-matched to the material forming the LD  1 , e.g. GaAs, which enables improved heat sinking, as well as eliminates the need for a ceramic submount between the LD  2  and the substrate  3 , thereby reducing cost. The substrate  3  includes a raised or stepped bonding pad section  5  extending upwardly into the body of the laser package  1  for supporting the LDs  2 . The raised section  5  is ideally cylindrical with a circular upper surface for supporting the LDs  2 , which are equally spaced apart and radially extending around the circumference of the upper surface, but could also be comprised of a multi-facet structure. 
         [0024]    A highly reflective slanted ring  7 , e.g. Ag- or Au-plated metal or plastic ring, surrounding the LD′s  1  and the raised section of the substrate  3 , is used to redirect the laser output beams substantially perpendicularly to the LD′s  1  and vertically out of the laser package  1 . The ring  7  can be circular or annular, i.e. cylindrical with a substantially triangular cross section, as shown in  FIG. 3   a . An alternate embodiment of the laser package, generally indicated by  1 ′, is illustrated in  FIG. 3   b , in which a ring  7 ′ is comprised of multiple sidewalls, such that each LD  2  has its own unique reflector sidewall. In this embodiment, the substrate  3  includes an octagonal raised section  5 ′, or some other multi-sided shape, depending on the number of LD″s  2  in the laser package  1 ′. The sidewall angle and profile of the ring  7  and  7 ′ can be planar, e.g. at an angle between 30° and 60° (preferably 45°) to the horizontal, or tailored to provide optical power for the specific optical configuration depending on the optics design requirements. For example, with reference to  FIGS. 4   a  and  4   b , a concave ring  7 ″ or a convex ring  7 ′″ having optical power can be provided for collimation, coupling, imaging, etc. Alternatively, the ring  7  can actually be modified into a linear array, where the laser diodes  2 , drivers and single mirror are all of nearly the same length. 
         [0025]    The height of the raised section  5  of the substrate  3  is raised (i) to allow for the LD′s  2  to overhang the edge thereof to prevent solder bridging; and (ii) to prevent obstruction of the laser output beams when they diverge from the front facet of the LD′s  2 . The reflective ring  7  is mounted on a lower section of the base  4  of substrate  3  surrounding the raised section  5 , whereby the light is incident on the reflective ring  7  proximate the middle thereof. Since LD′s  2  along a fast and a slow axis with high divergence, the reflective surface(s)  7  will reflect that portion of the light within a desired or predetermined numerical aperture defined by the mirrored surface of the ring  7 . 
         [0026]    In a preferred embodiment an optical element  8 , e.g. lens, lens array for multiple emitters, diffuser, waveplate, etc, is mounted in the opening of the package  1 , e.g. on top of the reflective ring  7  in the path of the output light. The optical element  8  can be used to shape the far-field of the output beam. 
         [0027]    The package  1  is processed with wafer-scale technology, thereby facilitating high-volume, low-cost packaging. The LD′s  2  use similar packaging technology; however, LDs can utilize epoxy for die attach or can be AuSn bonded directly onto the composite substrate  3 , since they are CTE matched, which provides ideal thermal conductivity and heat spreading. 
         [0028]      FIGS. 5 and 6  shows the top and bottom view of the proposed package with four LD′s  2  electrically-connected in parallel. If metal breaks are fabricated inside the package base and wirebonds are added between the cathode of one LD  2  to the anode diode pad of the adjacent LD  2 , then series connection is possible, which may be advantageous for power dissipation/speed. 
         [0029]    In general, any number of LD′s  2  can be packaged, limited by space considerations.  FIG. 5  shows an example of four LD′s  2  each with its own reflective sidewall  7 . Also shown in  FIG. 5  is a metal-oxide-semiconductor field-effect transistor  16  (MOSFET) and a MOSFET driver  17  mounted on a ground pad  18 . For high-speed (rise time and fall time shorter than 10 ns) performance, it is advantageous to place the driving electronics  16  and  17  near the LD′s  2 , e.g. preferably less than 0.5 mm for each current conducting path  19 , e.g. wirebond, to minimize inductance from the electrical current path. Low inductance, e.g. less than 5 nH, is required for high speed performance, and typically 1 mm of current conducting path provides 1 nH of inductance desirable for higher frequency operation. The driving electronics  16  and  17  for the LD′s  2  can be constructed, but not limited, by MOSFETs and bipolar transistors. 
         [0030]    With reference to  FIG. 7 , the encircled components represent illustrative components included in this embodiment, while other components, e.g., resistor and capacitor, will be needed outside of the laser-emitter package  1 . In this embodiment, the following components are illustrated: a MOSFET and driver  16 , a laser driver  17  and a capacitor  20 . The laser driver  17  includes a voltage source connection V CC , and trigger, gate and output connections, as is well known. 
         [0031]    In the example shown in  FIG. 5 , one set of laser drivers  17  is shared by four LD′s  2 . The laser driver  17  is disposed proximate, or in, the center of the raised section  5  of the substrate  3  with the plurality of LD′s  2  extending radially outwardly. Preferably, the LD′s  2  are equally spaced, i.e. separated by an equal angle, e.g. 90° for four LD′s, 120° for three LD′s. However, each laser driver design may have different driving capacity, which also depends on the electrical and optical properties of the particular laser diode used in the circuit. One driver  17  may be shared for all laser diodes  2 ; alternatively, each laser diode  2  may have its own drivers  17 . In the latter case, it may be more desirable to modify the design into a linear array, whereby the lasers  2 , mirrors  7 , drivers  17  all are aligned along a single axis over a similar length. 
         [0032]    A highly thermally-conductive material, i.e. &gt;100 W/mK and preferably &gt;=200 W/mK, e.g. copper (400 W/(m.K) @25° C.), SiC (120-200 W/mK), AlN (160 @/mK), CuW (200 W/mK), BeO (250 W/mK), diamond (2000 W/mK), fills the multiple vias  25  that provide interconnect between the electrical and electro-optic elements on the upper surface of the raised section  5  to electrical connection pads  26  on the bottom of the substrate  3 . The electrical connection pads  26  are then connected to external power and control sources (not shown). 
         [0033]    Packaging and process steps are illustrated in  FIGS. 8   a  to  8   i.    
         [0034]    Step a): Substrate lamination, the raised sections  5  of the substrates  3  are formed by, e.g. laminating a smaller top layer at a green stage, e.g. before a ceramic co-firing; 
         [0035]    Step b): Holes are formed, e.g. punched, through the raised sections  5  of the substrate  3  and filled with the thermally and electrically conductive material to form vias  25 , e.g. Copper; 
         [0036]    Step c): The upper surface of the raised sections  5  of the substrate  3  are plated with an electrically and thermally conductive material, e.g. copper, forming the plating  6 ; 
         [0037]    Step d): The LD′s  2 , MOSFET transistors  16  and driver  17  are fixed to the copper plating  6  using a solder, e.g. AuSn, preferably with the emitting facet of the LD′s  2  overhanging the edge of the raised section  5 ; 
         [0038]    Step e): The reflector rings  7  are fixed to the lower sections of the substrate  3  around the raised sections  5 ; 
         [0039]    Step f): Wire bonds  19  are used to electrically connect LD′s  2 , MOSFET transistors  16  and driver  17  according to bond diagram  FIG. 7 ; 
         [0040]    Step g): The optical element  8 , e.g. lens or diffuser, is mounted in the opening of the package  1  on the upper edge of the reflective ring  7 ; 
         [0041]    Step h): The LD′s  2  are tested and burned in. 
         [0042]    Step i): The laser packages  1  are separated from each other in a singulation step, e.g. mechanical breaking or a sawing/dicing process.