Coherent light generators – Particular temperature control – Heat sink
Reexamination Certificate
1999-04-05
2001-10-30
Font, Frank G. (Department: 2877)
Coherent light generators
Particular temperature control
Heat sink
C372S045013, C372S050121, C372S075000
Reexamination Certificate
active
06310900
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to lasers diodes and, in particular, to a package for a laser diode that is easy to manufacture and has a low thermal resistance.
BACKGROUND OF THE INVENTION
Semiconductor laser diodes have numerous advantages. They are small in that the widths of their active regions are typically submicron to a few microns and their heights are usually no more than a fraction of a millimeter. The length of their active regions is typically less than about a millimeter. The internal reflective surfaces, which produce emission in one direction, are formed by cleaving the substrate from which the laser diodes are produced and, thus, have high mechanical stability.
High efficiencies are possible with semiconductor laser diodes with some pulsed junction laser diodes having external quantum efficiencies near 50%. Semiconductor lasers produce radiation at wavelengths from about 20 to about 0.7 microns depending on the semiconductor alloy that is used. For example, laser diodes made of gallium arsenide with aluminum doping (AlGaAs) emit radiation at approximately 0.8 microns (~800 nm) which is near the absorption spectrum of common solid state laser rods and slabs made from Neodymium doped, Yttrum-Aluminum Garnet (Nd:YAG), and other crystals and glasses. Thus, semiconductor laser diodes can be used as the optical pumping source for larger, solid state laser systems.
Universal utilization of semiconductor laser diodes has been restricted by thermally related problems. These problems are associated with the large heat dissipation per unit area of the laser diodes which results in elevated junction temperatures and stresses induced by thermal cycling. Laser diode efficiency and the service life of the laser diode is decreased as the operating temperature in the junction increases.
Furthermore, the emitted wavelength of a laser diode is a function of its junction temperature. Thus, when a specific output wavelength is desired, maintaining a constant junction temperature is essential. For example, AlGaAs laser diodes that pare used to pump a Nd:YAG rod or slab should emit radiation at about 808 nm since this is the wavelength at which optimum energy absorption exists in the Nd:YAG. But, for every 3.5° C. to 4.0° C. deviation in the junction temperature of the AlGaAs laser diode, the wavelength shifts 1 nm. Accordingly, controlling the junction temperature and, thus, properly dissipating the heat is critical.
When solid state laser rods or slabs are pumped by laser diodes, dissipation of the heat becomes more problematic since it becomes necessary to densely pack a plurality of individual diodes into arrays which generate the required amounts of input power for the larger, solid state laser rod or slab. However, when the packing density of the individual laser diodes is increased, the space available for extraction of heat from the individual laser diodes decreases. This aggravates the problem of heat extraction from the arrays of individual diodes.
One known package which attempts to resolve these thermally-related problems includes the use of a thin, thermally conductive ceramic structure, like beryllium oxide. The ceramic structure includes a plurality of grooves which are cut, etched or sawed therein. A metallized layer extends from groove to groove to conduct electricity therethrough for supplying electrical power to the plurality of laser diodes which are soldered to the metallized layers in the grooves. This type of package is generally disclosed in several U.S. patents to Karpinski including, for example, U.S. Pat. Nos. 5,128,951 and 5,040,187.
However, this known package has several problems. For example, laser diodes typically have an inherent curvature due to the process by which they are made. Placing a curved laser diode in the straight groove of this known package results in additional stress on the laser diode and often an uneven solder bond along the length of the laser diode which can lead to failure. Because the grooves are typically deeper than the laser diodes, it can be difficult to control the location of the emitting surface of the laser diode in this known package. If beryllium oxide is the material used in this package, further problems arise since it is a toxic material and cutting grooves produces airborne dust particles.
SUMMARY OF THE INVENTION
A laser diode assembly includes a laser diode having an emitting surface and a reflective surface opposing the emitting surface. Between the emitting and reflective surfaces, the laser diode has first and second surfaces to which a first heat sink and second heat sink are attached, respectively, via a solder bond.
A spacer element is disposed between the first and second heat sinks. The spacer element is positioned below the laser diode and contacts the reflective surface of the laser diode. The spacer element has a width that is chosen to provide optimum spacing between the first and second heat sinks. Furthermore, the spacer element has a height that is chosen to place the emitting surface of the laser diodes at a position that is substantially flush with the upper surfaces of the heat sinks. Preferably, the spacer element is made of a material that is rigid or at least semi-rigid so that its function in establishing the optimum spacing between the components is not compromised when the heat sinks sandwich the spacer element. The material of the spacer element should also be compatible with the material of the laser diode against which the spacer element is positioned. In one embodiment, the spacer element is made of gallium arsenide, the same fundamental material within the laser diode.
A substrate is positioned below the first and second heat sinks and is attached to these two components usually via a solder bond. The substrate is preferably made of a nonconductive material so that electrical current flows from the first heat sink, into the laser diode and finally into the second heat sink. To properly locate the spacer element, the substrate may include a locating channel into which the spacer element fits.
The components of the laser diode assembly are made integral with each other by one heating step. Each of the heat sinks is coated with a solder layer prior to assembly. Once the components are placed in their basic assembly position, the heating step causes the solder layer on the heat sinks to reflow so that each heat sink attaches to the adjacent laser diodes and to the substrate.
The laser diode assembly can also be produced with each assembly having multiple laser diodes. Again, one heating step is used to make all components integral with each other.
The resulting laser diode assembly can be used for continuous wave (CW) modes of operation or for pulsed modes of operation. In either operational mode, the lower surface of the substrate is attached, usually by soldering, to a thermal reservoir such as a heat exchanger.
The above summary of the present invention is not intended to represent each embodiment, or every aspect, of the present invention. This is the purpose of the figures and the detailed description which follow.
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Boxell Alan D.
Micke Dean W.
Stephens Edward F.
Cutting Edge Optronics, Inc.
Font Frank G.
Jenkens & Gilchrist
Rodriguez Armando
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