Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-12-29
2004-08-24
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S096000
Reexamination Certificate
active
06782027
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the field of optoelectronic devices, and more particularly to resonant reflectors for use with optoelectronic devices.
Various forms of optoelectronic devices have been developed and have found widespread use including, for example, semiconductor photodiodes, semiconductor photo detectors, etc. Semiconductor lasers have found widespread use in modem technology as the light source of choice for various devices, e.g., communication systems, compact disc players, and so on. For many of these applications, a semiconductor laser is coupled to a semiconductor detector (e.g., photodiode) through a fiber optic link or even free space. This configuration provides a high-speed communication path, which, for many applications, can be extremely beneficial.
A typical edge-emitting semiconductor laser is a double heterostructure with a narrow bandgap, high refractive index layer surrounded on opposed major surfaces by wide bandgap, low refractive index layers often called cladding layers. The low bandgap layer is termed the “active layer”, and the cladding layers serve to confine both charge carriers and optical energy in the active layer or region. Opposite ends of the active layer have mirror facets which form the laser cavity. When current is passed through the structure, electrons and holes combine in the active layer to generate light.
Another type of semiconductor laser is a surface emitting laser. Several types of surface emitting lasers have been developed including Vertical Cavity Surface Emitting Lasers (VCSEL). (See, for example, “Surface-emitting microlasers for photonic switching and interchip connections”,
Optical Engineering,
29, pp.210-214, March 1990, for a description of this laser). For other examples, note U.S. Pat. No. 5,115,442, by Yong H. Lee et al., issued May 19, 1992, and entitled “Top-emitting Surface Emitting Laser Structures”, which is hereby incorporated by reference, and U.S. Pat. No. 5,475,701, issued on Dec. 12, 1995 to Mary K. Hibbs-Brenner, and entitled “Integrated Laser Power Monitor”, which is hereby incorporated by reference. Also, see “Top-surface-emitting GaAs four-quantum-well lasers emitting at 0.85 &mgr;m”,
Electronics Letters,
26, pp. 710-711, May 24, 1990.)
Vertical Cavity Surface Emitting Lasers offer numerous performance and potential producibility advantages over conventional edge emitting lasers. These include many benefits associated with their geometry, including their amenability to one- and two-dimensional arrays, wafer-level qualification, and desirable beam characteristics, typically circularly symmetric low-divergence beams.
VCSELs typically have an active region having bulk or one or more quantum well layers. On opposite sides of the active region are mirror stacks, often formed by interleaved semiconductor layers each a quarter wavelength thick at the desired operating wavelength (in the medium). The mirror stacks are typically of opposite conductivity type on either side of the active region, and the laser is typically turned on and off by varying the current through the mirror stacks and the active region.
High-yield, high performance VCSELs have been demonstrated and exploited in commercialization. Top-surface-emitting AlGaAs-based VCSELs are producible in a manner analogous to semiconductor integrated circuits, and are amenable to low-cost high-volume manufacture and integration with existing electronics technology platforms. Moreover, VCSEL uniformity and reproducibility have been demonstrated using a standard, unmodified commercially available metal organic vapor phase epitaxy (MOVPE) chamber and molecular beam epitaxy (MBE) giving very high device yields. VCSELs are expected to provide a performance and cost advantage in fast (e.g., Gbits/s) medium distance (e.g., up to approximately 1000 meters) single or multi-channel data link applications, and numerous optical and/or imaging applications. This results from their inherent geometry, which provides potential low-cost high performance transmitters with flexible and desirable characteristics.
A related photodetector is known as a resonant cavity photo detector (RCPD). Resonant cavity photodetectors are typically constructed similar to VCSELs, but operate in a reverse bias mode. A resonant cavity photodetector may be more efficient than a standard photodiode because the light that enters the optical cavity, through one of the mirrors, may be effectively reflected through the active region many times. The light may thus be reflected between the mirror stacks until the light is either absorbed by the active region or until it escapes through one of the mirror stacks. Because the mirror stacks are typically highly reflective near resonance, most of the light that enters the cavity is absorbed by the active region.
For many optoelectronic devices that have a resonant cavity, the top and/or bottom mirror stacks are Distributed Bragg Reflector (DBR) mirrors. DBR mirrors AlGaAs and AlAs. Often, both the top and bottom mirror stacks include a significant number of DBR mirror periods to achieve the desired reflectance. One way to reduce the number of DBR mirror periods that are required is to replace some of the DBR mirror periods with a resonant reflector. Such a configuration is disclosed in U.S. Pat. No. 6,055,262, entitled “Resonant Reflector For Improved Optoelectronic Device Performance And Enhanced Applicability”, which is incorporated herein by reference. A typical resonant reflector may include, among other things, a waveguide and a grating.
Despite the advantages of using a resonant reflector in conjunction with a DBR mirror stack, it has been found that the reflectivity of the resonant reflector can be limited if it is not properly isolated from adjacent conductive layers. Too much energy in the guided-mode in the waveguide overlaps into the lossy, conductive DBR films of the optoelectronic device. What would be desirable, therefore, is an optoelectronic device that provides isolation between the resonant reflector and adjacent conducting layers of the optoelectronic device.
SUMMARY OF THE INVENTION
The present invention overcomes many of the disadvantages of the prior art by providing an optoelectronic device that provides isolation between a resonant reflector and an adjacent conducting layer of the optoelectronic device. Isolation is preferably accomplished by providing a dielectric buffer or cladding layer between the resonant reflector and the adjacent conducting layer of the optoelectronic device. The cladding or buffer layer is preferably sufficiently thick, and/or has a sufficiently low refractive index relative to the refractive index of the waveguide of the resonant reflector, to substantially prevent energy in the evanescent tail of the guided mode in the waveguide from entering the adjacent conductive layer of the optoelectronic device.
In one illustrative embodiment of the present invention, an optoelectronic device includes a top mirror and a bottom mirror, with an active region situated therebetween. The top mirror and bottom mirror are Distributed Bragg Reflector (DBR) mirrors made from alternating layers of semiconductor materials that are doped to be at least partially conductive. Current can be provided through the active region and DBR mirrors to activate the device.
A resonant reflector is positioned adjacent a selected one of the top or bottom mirrors of the optoelectronic device. The resonant reflector preferably has a waveguide and a grating. The waveguide and grating are preferably configured such that a first-diffraction order wave vector of the grating substantially matches the propagating mode of the waveguide. A cladding or buffer layer is positioned between the resonant reflector and the selected top or bottom mirror. The cladding or buffer layer is preferably sufficiently thick, and/or has a sufficiently low refractive index relative to the refractive index of the waveguide, to substantially prevent energy in the evanescent tail of the guided mode in the waveguide from entering the select
Cox James Allen
Morgan Robert A.
Finisar Corporation
Leung Quyen
Workman Nydegger
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