Coherent light generators – Particular temperature control – Heat sink
Reexamination Certificate
1998-06-16
2001-01-09
Davie, James W. (Department: 2881)
Coherent light generators
Particular temperature control
Heat sink
C372S075000, C372S109000
Reexamination Certificate
active
06172997
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to solid state lasers and, more particularly, to a method and apparatus for fabricating integrated semiconductor diode laser pumped solid state lasers.
Semiconductor lasers of GaAlAs, GaAs, InGaAs and other materials have been used to excite solid state media to achieve laser oscillation. Typically solid state laser media are comprised of a doped crystalline, polymer or glass host material, for example, Nd:YAG, Nd:YLF, Nd:YVO
4
, or Yb:YAG. In order to obtain laser oscillation, the laser media is placed within an optical resonator cavity that includes appropriate cavity optics.
In designing a diode pumped solid state laser, the various optical components must be aligned to each other with positional tolerances on the order of 10's of micrometers. Conventional solid state lasers use mechanical mounting components and structures, many with adjustment capabilities, to accurately position the optical components. These mechanical mounts are typically manufactured using conventional metal fabrication techniques (e.g., grinding, lapping, milling, turning, etc.), thereby resulting in dimensional imperfections. As a result of these imperfections, the optical components of the solid state laser must be manually aligned.
A variety of systems have been designed to precision align various optical components, thus requiring little if any manual alignment. In U.S. Pat. No. 4,731,795 a monolithic support structure fabricated of metal or plastic is disclosed that automatically places the individual components into the proper relationship to one another. In at least one embodiment, the support structure is comprised of a substantially tubular housing of metal, ceramic, glass, thermoplastic material or thermosetting material and is formed using conventional fabrication techniques such as machining, injection molding, or die casting. Within the housing is an optical pump comprised of a laser diode, a heat sink, a lens, a laser medium, and an output coupler. The system may also include a nonlinear optical material.
A variety of precision optical assemblies have also been fabricated that take advantage of miniature optical components. For example, in U.S. Pat. No. 4,079,404 an optical assembly is disclosed in which the optical components are aligned and supported by one or more wafers. In order to align and orient the various optical components, a series of V-shaped grooves are etched into the wafer surface using conventional photolithographic techniques. Into one series of parallel grooves are placed fiber optic waveguides, the waveguides abutting a cylindrical lens laid into a perpendicular groove. A laser package such as a GaAlAs double heterostructure laser is attached to the wafer. In order to attach the laser, complementary grooves are etched into the wafer and the laser. Within the complementary grooves are placed locating fibers.
U.S. Pat. No. 4,904,036 discloses optoelectronic chips in which lasers and photodiodes are mounted onto single crystal silicon bases. The lasers and photodiodes are interconnected using silica waveguides and couplers integrally formed onto the base. In one embodiment a pair of tandem V-shaped grooves are etched into the base and aligned with the optical axis of the waveguide. The size of one of the grooves is adapted to receive the bare portion of an optical fiber while the size of the second groove is adapted to receive the coated portion of the optical fiber. Thus the grooves allow a fiber to be easily aligned with the waveguide.
U.S. Pat. No. 5,268,066 discloses an optical assembly system that utilizes mechanical registration features to facilitate the passive alignment of lasers integrated on a chip to optical fibers in integral contact with the substrate. The system includes pedestal structures formed on the substrate that define various mounting regions. The pedestal structures are formed through a combination of etching, masking, and polyimide deposition steps. The assembly and registration system also includes a series of V-shaped grooves etched into the substrate, the grooves being used to register the optical fibers.
U.S. Pat. No. 5,548,605 discloses a monolithic microchannel heat sink for use with diode lasers. A series of slots are sawn into a silicon wafer, thus allowing the diode laser to be mounted in contact with the silicon. In order to provide cooling, a series of microchannels are etched into the back of the wafer. The channels are rotated by an angle perpendicular to the diode bars, thereby providing increased penetration between the mounted diode bars. The microchannel heat sinks have low thermal resistance due to the close proximity of the microchannels to the laser diode being cooled.
Although a variety of optical assemblies have been fabricated using miniature optical components and monolithic support structures, a method of economically packaging semiconductor pump lasers with solid state gain media is desired.
SUMMARY OF THE INVENTION
The present invention provides a method and apparatus for fabricating extremely robust opto-electronic devices contained on a monolithic support structure. Incorporated into the support structure are registration structures that are used to quickly and accurately align the various components associated with the device, typically eliminating the need for manual component alignment. The registration structures are fabricated using conventional lithographic techniques, offering alignment accuracy of a micrometer or less. A variety of registration structures may be used, including V-grooves, wells, tapered wells, and stops.
In one aspect of the invention, a gain module is fabricated. The components of the gain module are bonded to a monolithic substrate, preferably of silicon although other materials may be used. Prior to bonding, registration structures are formed on the surface of the substrate. The gain module components are comprised of a pump laser, an optical element (e.g., a lens), a waveplate, and a solid state gain medium. The pump laser is preferably a semiconductor diode laser that pumps the edge of the gain medium, thus eliminating many of the difficulties that arise from end pumping the medium. The optical element, for example a collimating lens, is interposed between the pump laser and the gain medium and collimates and/or focuses the emissions from the pump laser onto the laser medium in such a manner as to optimize the efficiency of the module. The gain module may also include other components, for example a waveplate to provide the gain medium with pump light of the desired polarization. A single gain module may also include several pump lasers and/or gain media, thus increasing the achieved output power.
In another aspect of the invention, a gain module is mounted to a heat spreader, thus inhibiting the development of hot spots in the substrate due to localized heating within either the gain media or the pump laser. To further inhibit the development of hot spots, the substrate is preferably thinned to the extent possible without sacrificing module rigidity and robustness. In addition, the surface of the substrate to be bonded to the heat spreader may first be coated with a material offering improved thermal conductivity such as diamond or copper. If the gain module is to be operated at sufficiently high powers and sufficiently extended times as to affect module life, the heat spreader is preferably attached to a heat sink. Depending upon the desired application, the heat sink may be either passive or active.
In another aspect of the invention, a gain module attached to a heat spreader is projected through a cutout in a miniature optical bench. Although it is desirable to rigidly attach the gain module to the optical bench, preferably they are thermally isolated from one another. The optical bench is comprised of a material with a very low coefficient of thermal expansion, thus providing thermal stability. The surface of the optical bench is metallized, preferably in a pattern of gold pads. Optical components to be attache
Miyake Charles I.
Pearson Leonard P.
Pierce Jeffrey
Aculight Corporation
Beck David G.
Davie James W.
McCutchen, Dole, Brown & Enerson, L.L.P.
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