The present invention relates generally to semiconductor lasers and, more particularly, to an end-pumped solid state laser utilizing a laser diode stack as the pump source.
High power laser diodes, due to their size, efficiency and wavelength range, are well suited for pumping high power solid state lasers. In such laser systems the output from one or more laser diodes is coupled into a laser gain medium, the gain medium contained within a laser cavity defined by a pair of mirrors or reflective coatings disposed at either end of the medium. The laser diode output may be coupled into either an end surface of the gain medium, creating an end-pumped laser, or into one or more side surfaces of the gain medium, creating a side-pumped laser. End-pumped lasers are typically of lower power than side-pumped lasers due to the difficulty in coupling the output from multiple laser diodes into the relatively small end surface of the gain medium.
U.S. Pat. No. 4,653,056 discloses a neodymium YAG (Nd:YAG) laser that is end-pumped by a gallium aluminum arsenide (GaAlAs) diode array. A first lens collimates the diverging beam emitted by the diode array while a second lens focuses the beam into the back end of the Nd:YAG crystal. The pumping volume was matched to that of the lasing volume in order to optimize pumping efficiency.
An alternate pumping configuration is disclosed in U.S. Pat. No. 4,665,529. In the disclosed system, the output of the pump laser diode is coupled to the laser head using a removable optical fiber with a focusing sphere imaging the pump radiation into the rod-shaped laser gain medium. The pumping volume of the laser diode is matched to the lasing volume of the gain medium. A goal of the disclosed system is to provide a versatile system in which multiple laser heads can be interchanged with a single pump source. Additionally by separating the pump source from the laser head via an optical fiber, the size of the laser head could be optimized for a variety of applications.
In order to overcome the limitations imposed by the relatively small size of the end surface of a laser gain medium and yet still end-pump the medium, U.S. Pat. No. 4,837,771 discloses using a laser cavity with a tightly folded zig-zag configuration within a block of the gain medium. By folding the cavity, the longitudinal axis of the resonator is substantially normal to the side surface of the gain medium. As a result, a laser bar in proximity to the side of the gain medium can be used to pump the cavity at a number of spaced intervals.
U.S. Pat. No. 5,170,406 discloses another configuration to efficiently couple pump energy into a laser gain medium. As disclosed, pump energy from two groups of laser diode bars is directed onto opposite end surfaces of the gain medium using an off-axis, geometric multiplexing configuration. The laser diode bars are circumferentially distributed about the optical axis in a uniform pattern and at the same distance along the optical axis from the gain medium.
Although there are a variety of end-pumped, solid state laser configurations, typically they suffer from low power, excessive complexity and excessive heat build-up. Accordingly, what is needed in the art is an end-pumped, solid state laser that overcomes these issues. The present invention provides such a system.
The present invention provides an end-pumped solid state laser utilizing a laser diode stack of laser diode subassemblies as the pump source. The laser gain medium of the solid state laser is contained within a laser cavity defined by a pair of reflective elements. Each laser diode subassembly includes a submount to which one or more laser diodes are attached. The fast axis of each laser diode's output beam is substantially perpendicular to the submount mounting surfaces. 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. The laser diodes can be of one wavelength or multiple wavelengths. 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. An electrically isolating pad is attached to the same surface of the submount as the laser diode. A metallization layer is deposited onto the outermost surface of the electrically isolating pad, to which an electrical contact pad is bonded. Electrical interconnects, such as wire or ribbon interconnects, connect the single emitter laser diode 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 subassembly, 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.
In at least one embodiment of the invention, coupling optics are interposed between the end surface of the laser gain medium and the laser diode stack.
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.
As the mode volume of a gain medium, for example a cylindrical gain medium, is quite limited, the power of a laser diode or a laser diode array that can be coupled into the end surface of the gain medium is determined not by the output power of the laser diode/array, but rather by the overlap of the mode of the laser diode/array and the mode volume of the gain medium. Accordingly, since the critical parameter is the gain/mode overlap efficiency, the important characteristics of a pump source are not only its power, but also the brightness and the symmetry of the output beam.
A laser bar, which is approximately 1 centimeter in width, typically includes between 10 and 80 emitters, the emitters laterally spread across the width of the bar. Although the lateral aperture of the individual emitters is typically on the order of 50 to 300 microns, the lateral aperture of the bar is on the order of 0.8 to 0.9 centimeters. The vertical aperture of a laser bar, i.e., measured in the fast axis, is on the order of 1 to 2 microns.
Due to the large lateral aperture and the slow axis divergence of a laser bar, as noted above, the brightness of a high power laser bar is relatively low. For example, assuming an output power of 100 watts, a lateral aperture of 0.8 centimeters, a vertical aperture of 1 micron, a slow axis beam divergence of 10° (174.53 milliradians) and a fast axis divergence of 50° (872.66 milliradians), the brightness of such a laser bar is only 0.08 watts/(mm-mrad)2. In comparison, approximately the same brightness can be achieved with a much smaller, lower power array. For example, assuming an array of 2 emitters with a total output power of 12 watts, a lateral aperture of 0.1 centimeters (100 micron emitters on 1 millimeter centers), a vertical aperture of 1 micron, a slow axis beam divergence of 10° (174.53 milliradians) and a fast axis divergence of 50° (872.66 milliradians), the brightness is 0.08 watts/(mm-mrad)2.
Although in the above example the laser bar and the 2 emitter array exhibit approximately equivalent brightness, the laser bar suffers from a variety of drawbacks that make it less desirable for end-pumping a gain medium. One issue with the laser bar is its mode volume in comparison to that of a gain medium. While the mode volume of the laser bar in the above example is 1218 (millimeters-milliradian)2, the mode volume of the above-described 2 emitter array, which exhibits comparable brightness to the laser bar, was only 152 (millimeters-milliradian)2. Therefore eight of the 2 emitter arrays have approximately the same mode volume as the laser bar. As such, it is possible to couple much more pump power into the laser rod with smaller arrays than with a laser rod.
In addition to increasing the amount of power that can be pumped into the laser rod, a smaller array such as the one noted above also has significant heat dissipation advantages over a laser bar. In a laser bar the center-to-center spacing of the emitters is relatively small, thus providing the desired packing density of emitters within the bar. As a result, heat generated by the individual emitters does not have any space available for lateral heat spreading, thereby requiring all of the generated heat to be vertically dissipated through the bottom surface of the device. Therefore a laser bar with an 80 percent fill factor, 100 micron wide emitters, center-to-center spacing of 125 microns, and a 6 watt per emitter heat load has a calculated maximum emitter temperature of 105° C. Accordingly the laser bar must be operated at a lower power per emitter in order to achieve an acceptable temperature level. In contrast, the individual emitters in the above exemplary array with only two 6 watt emitters separated by a millimeter can dissipate the generated heat both vertically and laterally, with the two emitters exhibiting minimal thermal cross talk and operating at a maximum temperature of approximately half that of the emitters in the laser bar. Due to the lower operating temperature, not only can the emitters in the array operate at full power, they are also less likely to suffer from heat induced damage. Additionally the heat dissipation requirements placed on the laser diode system are less demanding, thus allowing a smaller, more robust system to be utilized.
Another issue that affects the performance of laser bars more than small arrays is smile, a term that refers to the warping of a laser diode during processing. In general, the larger the device, the greater the degree of smile experienced during processing. Accordingly, a laser bar will typically experience a greater degree of smile than a small multi-emitter array such as the exemplary array discussed above. Since smile causes the individual emitters to be at different heights, the primary effect is to increase the effective vertical aperture of the laser bar, thereby decreasing the brightness of the bar. For example, assuming a modest smile in the exemplary laser bar described above, the effective vertical aperture changes from 1 micron to 2 microns. This, in turn, causes a decrease in the brightness by 50 percent (i.e., to 0.04 watts/(mm-mrad)2) and a doubling of the mode volume (i.e., to 2,436 (millimeters-milliradian)2). Thus in this instance not only has the smile decreased the brightness of the laser bar to half that of the 2 emitter array, it has also doubled the bar's mode volume, giving the exemplary 2 emitter array a factor of 16 better performance in terms of mode volume. Note that small arrays such as the previously described 2 emitter array, due to its very small size, will typically experience an inconsequential degree of smile.
In light of the noted deficiencies of laser bars, the present invention utilizes a laser diode pump assembly consisting of at least two, and preferably more than two, array subassemblies where each array subassembly includes a submount and at least one single emitter, and preferably at least two single emitters. The array subassemblies cannot use diode laser bars. The single emitters of each array can be either single mode single emitter laser diodes or broad area multi-mode single emitter laser diodes. Furthermore the multiple emitters of each individual array can either be fabricated on a single substrate or on individual substrates.
Depending upon the mode volume of the laser gain medium as well as the power requirements of the system, a variety of laser diode pump configurations can be used. For example, and as illustrated in
In addition to providing a pump laser that can be sized to efficiently couple into the gain medium, the present invention also provides a means of compensating for temperature induced variations in the pump wavelength. As is well known by those of skill in the art, since the output wavelength of a laser diode varies with temperature, the pumping efficiency may vary as the system changes temperature and the pump wavelength varies from the optimal wavelength. As a result of this variation, the output of a conventional solid state laser may also vary with temperature. The laser diode stack of the present invention, however, can be designed to operate at multiple wavelengths simply by including emitters of different wavelengths. Thus, for example, one group of emitters can be the primary pump source at the initial temperature, then a second group of emitters can become the primary pump source as the system temperature increases with time, then a third group of emitters can become the primary pump source as the temperature increases further, etc. These wavelength-grouped emitters are preferably spread throughout the entire laser diode stack, thus insuring that the entire volume of the gain medium is efficiently pumped. In a preferred configuration, each subassembly includes multiple laser diode emitters, preferably on individual substrates, each operating at a different wavelength. It will be appreciated that there are a variety of possible configurations depending upon the number of desired wavelengths, the number of subassemblies, and the number of emitters per subassembly.
Regardless of the laser diode pump configuration, and as previously noted, in a typical configuration there is at least one coupling optic interposed between the output of each laser emitter and the laser resonator cavity/gain medium. For example, assuming an array such as the one shown in
On top of submount 1101 is a spacer that is preferably comprised of a first contact pad 1105, preferably used as the N contact for the laser diode, and an electrically insulating isolator 1107 interposed between contact pad 1105 and submount 1101. Preferably insulating isolator 1107 is attached to submount 1101 via solder layer 1103. Preferably contact pad 1105 is attached to isolator 1107 using the same solder material as that of layer 1103 (e.g., Au—Sn solder). Also mounted to submount 1101 via solder layer 1103 is a laser diode 1109 positioned such that the radiation-emitting active layer of the laser is substantially parallel to the mounting surfaces of submount 1101 and the fast axis corresponding to the output beam of the radiation-emitting active layer is substantially perpendicular to the mounting surfaces of submount 1101 (e.g., surfaces 808 and 809 of
After completion of subassembly 1100, preferably the laser diode or diodes 1109 attached to the submount are tested. Early testing, i.e., prior to assembly of the entire laser diode pump assembly, offers several advantages over testing after assembly completion. First, it allows defective laser diodes to be identified prior to stack assembly, thus minimizing the risk of completing an assembly only to find that it does not meet specifications due to one or more defective laser diodes. Thus the present stack 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 grouped together in the desired order.
During the next series of steps the laser diode stack, which is comprised of a stack of laser diode subassemblies 1100, is fabricated. The perspective view of
In a preferred embodiment of the invention, laser diodes 1109 are serially coupled together. In this embodiment the individual submount assemblies 1100 are combined into a single assembly by bonding the upper surface of each contact pad 1105 to a portion of the lower surface of the adjacent submount 1101, submounts 1101 being comprised of an electrically conductive material. Preferably solder 1203 coupling contact pads 1105 to submounts 1101 has a lower melting temperature than the solder used to fabricate subassembly 1101, thus insuring that during this stage of assembly the reflow process used to combine the subassemblies 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 1103.
In the next series of processing steps, illustrated in
In an alternate embodiment of the invention laser diodes 1109 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 subassemblies must be severed, for example using a pad 1105 that is not electrically conductive, and/or using a submount 1101 that is not electrically conductive, and/or placing an electrically isolating layer between submounts 1101 and pads 1105 within assembly 1200. Parallel connections as well as individual laser diode connections can be made, for example, by coupling interconnect cables to metallization layers 1103 and 1111. Additionally one or more of members 1301, 1401 and 1403 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 1401 and 1403 to submounts 1101 is also used to attach contact assemblies 1405 and 1407 to the laser diode package. Preferably contact assemblies 1405 and 1407 are assembled in advance using a higher melting temperature solder such as a gold-tin solder. Each contact assembly 1405/1407 includes a wire 1409, covered with an insulator 1411 (e.g., Kapton), and a contact (or contact assembly) 1413.
In the preferred embodiment, the laser diode assembly, shown 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 | |
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Parent | 11384940 | Mar 2006 | US |
Child | 11436232 | May 2006 | US |