Patent Application
20070217468
The present invention relates generally to semiconductor lasers and, more particularly, to a laser diode package that provides improved performance and reliability.
High power laser diodes have been used individually and in arrays in a wide range of applications including materials processing, medical devices, printing/imaging systems and the defense industry. Furthermore due to their size, efficiency and wavelength range, they are ideally suited as a pump source for high power solid state lasers. Unfortunately reliability issues have prevented their use in a number of critical applications such as space-based systems in which launch costs coupled with the inaccessibility of the systems once deployed requires the use of high reliability components.
During operation, a laser diode produces excessive heat which can lead to significant wavelength shifts, premature degradation and sudden failure if not quickly and efficiently dissipated. These problems are exacerbated in a typical laser diode pump array in which the laser diode packing density reduces the area available for heat extraction. Additionally as most high energy pulse lasers require a quasi-CW (QCW) laser diode pump, the extreme thermal cycling of the laser diode active regions typically leads to an even greater level of thermal-mechanical stress induced damage.
One approach to overcoming some of the afore-mentioned problems is a laser diode package (e.g., a G package) in which an efficient heat extracting substrate (e.g., beryllium oxide, copper, copper tungsten, etc.) includes multiple grooves into which individual laser diode bars are soldered using an indium solder. Although this package has improved heat dissipation capabilities, it still suffers from numerous problems. First, the coefficient of thermal expansion (CTE) of the solder does not provide a good match with that of the substrate, leading to solder delamination during thermal cycling. Solder delamination is problematic due to the high drive currents that the solder must conduct into the laser diode as well as the heat which the solder must efficiently transfer from the laser diode to the heat extracting substrate. Second, it is difficult to test the individual laser diode bars before installing them into the grooved substrate, potentially leading to arrays in which one or more of the laser diode bars is defective (i.e., non-operational or out of spec.). Third, mounting the laser diode bars into the individual grooves of the substrate may lead to further stresses if the laser diode bars exhibit any curvature.
Accordingly what is needed in the art is an alternate laser diode package that overcomes the problems inherent in the laser diode packages of the prior art, thereby providing improved reliability and performance. The present invention provides such a laser diode package.
The present invention provides a laser diode package which includes a stack, either a horizontal stack or a vertical stack, of laser diode submount assemblies. Each laser diode submount assembly is comprised of a submount. At least one laser diode is attached to a front portion of each submount. Exemplary laser diodes include single mode single emitter laser diodes, broad area multi-mode single emitter laser diodes, and multiple single emitters fabricated on either a single substrate or on multiple substrates. Preferably the submount has a high thermal conductivity and a CTE that is matched to that of the laser diode. In an exemplary embodiment the submount is fabricated from 90/10 tungsten copper and the laser diode is attached to the submount with a gold-tin solder. A spacer, preferably comprised of an electrically isolating pad, a metallization layer and an electrical contact pad, is attached to the rear portion of the same surface of the submount as the laser diode. Electrical interconnects, such as wire or ribbon interconnects, connect the laser diode (or diodes) to the metallization layer. Preferably the laser diode stack is formed by electrically and mechanically bonding together the bottom surface of each submount to the electrical contact pad of an adjacent submount assembly, for example using a silver-tin solder.
To provide package cooling, the laser diode stack is thermally coupled to a cooling block, the cooling block preferably including a slotted region into which the laser diode stack fits. In at least one preferred embodiment of the invention, thermally conductive and electrically isolating members are first bonded to the bottom and side surfaces of each submount and then bonded to the cooling block, the members being interposed between the laser diode stack and the cooling block. Preferably the cooling block is comprised of a pair of members, thus insuring good thermal coupling between the laser diode stack and the cooling block.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.
The present invention provides a vertical or horizontal stack of laser diode submount assemblies, each submount assembly including at least one laser diode. In a preferred embodiment, each laser diode of each submount assembly operates at the same wavelength. In an alternate embodiment, the laser diode or diodes of each submount assembly operate at a different wavelength. In yet another alternate embodiment, the stack includes groups of laser diodes where each group operates at a preset wavelength (e.g., 635 nm, 808 nm, 975 nm, 1470 nm, 1900 nm, etc.). It will be appreciated that there are a variety of possible configurations depending upon the number of desired wavelengths and the number of submount assemblies within the laser diode package.
On top of submount 101 is a spacer 105. In the preferred embodiment, the spacer is comprised of a first contact pad 107, preferably used as the N contact for the laser diode, and an electrically insulating isolator 109 interposed between contact pad 107 and submount 101. Preferably insulating isolator 109 is attached to submount 101 via solder layer 103. Preferably contact pad 107 is attached to isolator 109 using the same solder material as that of layer 103 (e.g., Au-Sn solder). Also mounted to submount 101 via solder layer 103 is a laser diode 111, positioned such that the emitting facet 113 is substantially parallel with end face 115 of submount 101. Exemplary laser diodes include both single mode single emitter laser diodes and broad area multi-mode single emitter laser diodes. Additionally, multiple single emitters, either fabricated on individual substrates or on a single substrate, can be mounted to submount 101, thereby forming an array of single emitters on a single submount assembly. Laser bars, due both to their size (i.e., 1 centimeter) and their poor heat dissipation characteristics resulting from close emitter spacing, are not used with the submount assemblies of the invention. In this embodiment of the invention one contact of laser diode 111, preferably the P contact, is made via submount 101, while the second contact, preferably the N contact, is made using wire bonds, ribbon bonds, or other electrical connector which couple the laser diode to metallization layer 117. For illustration purposes, both representative wire bonds 119 and a representative contacting member 121 are shown in
In the preferred embodiment of the invention, as illustrated in
After completion of submount assembly 100, preferably the laser diode or diodes 111 attached to the submount are tested. Early testing, i.e., prior to assembly of the entire laser diode package, offers several advantages over testing after package completion. First, it allows defective laser diodes to be identified prior to package assembly, thus minimizing the risk of completing a package assembly only to find that it does not meet specifications due to one or more defective laser diodes. Thus the present package assembly improves on assembly fabrication efficiency, both in terms of time and materials. Second, early testing allows improved matching of the performance of the individual laser diodes within an assembly, for example providing a means of achieving improved wavelength matching between laser diodes or allowing laser diodes operating at different wavelengths to be coupled together in the desired order.
During the next series of steps the laser diode package, which is comprised of a stack of laser diode submount assemblies 100, is fabricated. The perspective view of
One advantage of the laser diode package of the present invention is illustrated in
In a preferred embodiment of the invention, laser diodes 111 are serially coupled together. In this embodiment the individual submount assemblies 100 are combined into a single assembly by bonding the upper surface of each contact pad 107 to a portion of the lower surface of the adjacent submount 101, submounts 101 being comprised of an electrically conductive material. Preferably solder 203 coupling contact pads 107 to submounts 101 has a lower melting temperature than the solder used to fabricate submount assembly 101, thus insuring that during this stage of assembly the reflow process used to combine the submount assemblies will not damage the individual assemblies. In a preferred embodiment of the invention, a silver-tin solder is used with a melting temperature lower than that of the Au-Sn solder preferably used for solder joint 203.
In the next series of processing steps, illustrated in
In an alternate embodiment of the invention laser diodes 111 are not serially coupled together, rather they are coupled together in parallel, or they are individually addressable. Individual addressability allows a subset of the total number of laser diodes within the stack to be activated at any given time. In order to achieve individual addressability, or to couple the laser diodes together in a parallel fashion, the electrically conductive path between individual submount assemblies must be severed, for example using a pad 107 that is not electrically conductive, and/or using a submount 101 that is not electrically conductive, and/or placing an electrically isolating layer between submounts 101 and pads 107 within assembly 200. Parallel connections as well as individual laser diode connections can be made, for example, by coupling interconnect cables to metallization layers 103 and 117. Additionally one or more of members 601, 701 and 703 can be patterned with electrical conductors, thus providing convenient surfaces for the inclusion of circuit boards that can simplify the relatively complex wiring needed to provide individual laser diode addressability.
In the preferred package assembly process and assuming that the laser diode subassemblies are serially coupled together, the same mounting fixture that is used to attach side members 701 and 703 to submounts 101 is also used to attach contact assemblies 705 and 707 to the laser diode package. Preferably contact assemblies 705 and 707 are assembled in advance using a higher melting temperature solder such as a gold-tin solder. Each contact assembly 705/707 includes a wire 709, covered with an insulator 711 (e.g., Kapton), and a contact (or contact assembly) 713.
In the preferred embodiment, the laser diode submount stack assembly, shown in
In an alternate assembly in which submounts 101 are comprised of a non-electrically conductive material, for example to fabricate an assembly in which the laser diodes of the subassemblies are not serially coupled together, the laser diode submount stack assembly is preferably directly attached to the cooler body as illustrated in
As will be understood by those familiar with the art, the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosures and descriptions herein are intended to be illustrative, but not limiting, of the scope of the invention which is set forth in the following claims.
This application is a continuation-in-part of U.S. patent application Ser. No. 11/384,940, filed Mar. 20, 2006, the disclosure of which is incorporated herein by reference for any and all purposes.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11384940 | Mar 2006 | US |
| Child | 11417581 | May 2006 | US |
