Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-04-27
2002-01-22
Arroyo, Teresa A. (Department: 2887)
Coherent light generators
Particular active media
Semiconductor
C372S023000
Reexamination Certificate
active
06341137
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a semiconductor device that includes an array of long-wavelength vertical cavity lasers.
2. Description of the Related Art
A vertical cavity surface emitting laser (VCSEL) is a semiconductor laser including a semiconductor layer of optically active material, such as gallium arsenide or indium phosphide. The optically active material is sandwiched between mirrors formed of highly reflective layers of metallic material, dielectric material, or epitaxially-grown semiconductor material. Conventionally, one of the mirrors is partially reflective so as to pass a portion of the coherent light which builds up in a resonating cavity formed by the mirrors sandwiching the active layer.
Lasing structures require optical confinement in the resonating cavity and carrier confinement in the active region to achieve efficient conversion of pumping electrons into stimulated photons through population inversion. The standing wave of reflected optical energy in the resonating cavity has a characteristic cross-section giving rise to an optical mode. A desirable optical mode is the single fundamental transverse mode, for example, the HE
11
mode of a cylindrical waveguide. A single mode signal from a VCSEL is easily coupled into an optical fiber, has low divergence, and is inherently single frequency in operation.
In order to reach the threshold for lasing, the total gain of a VCSEL must equal the total loss of the VCSEL. Unfortunately, due to the compact nature of VCSELs, the amount of gain media is limited. For efficient VCSELs, at least one of the two required mirrors must have a reflectivity greater than approximately 99.5%. It is more difficult to meet this requirement in long-wavelength VCSELs than in short-wavelength VCSELs because such high reflectivity mirrors are difficult to grow in the same epitaxial step as the long-wavelength active region. Because epitaxially-grown mirrors often do not enable sufficiently high reflectivity, some VCSELs are formed by wafer fusing the top and bottom mirrors to the active region.
Wafer fusion is a process by which materials of different lattice constant are atomically joined by applying pressure and heat to create a real physical bond. Thus, wafer fusion of one or both of the mirrors to the active region is used to increase the reflectivity provided by either or both of the mirrors to compensate for the small amount of gain media so that the lasing threshold can be reached and maintained.
A long-wavelength VCSEL can be optically coupled to and optically pumped by a shorter wavelength, electrically pumped VCSEL. U.S. Pat. No. 5,513,204 to Jayaraman entitled “LONG WAVELENGTH, VERTICAL CAVITY SURFACE EMITTING LASER WITH VERTICALLY INTEGRATED OPTICAL PUMP” describes an example of a short-wavelength VCSEL optically pumping a long-wavelength VCSEL.
Various schemes have been proposed to make wavelength-division-multiplexed (WDM) arrays of vertical cavity surface emitting lasers (VCSELs) monolithically on a single substrate. One technique introduces temperature gradients across the wafer during epitaxial growth in order to change growth rate and lasing wavelength across the wafer. In another approach, the bottom mirror and active region of the VCSEL are grown first. The active region is then etched different amounts in different areas, after which the top mirror is re-grown to complete the wavelength-stepped VCSEL array. Another technique takes advantage of the wavelength dependence on size, to create an array of different sized devices which have different wavelengths. Still another approach uses a tapered thickness oxidation layer to shift wavelength different amounts at different parts of the taper.
All of these schemes have been applied to short-wavelength (e.g., 850 nm or 980 nm) VCSELs. Work in WDM arrays of long-wavelength (e.g., 1300 nm or 1550 nm) VCSELs has progressed more slowly, however.
FIG. 1
illustrates the making of a double-fused WDM array. A 1550 nm active region
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has been wafer-fused to a first GaAs/AlGaAs mirror
12
. A stepped surface
14
is formed (e.g., by etching) in the top surface of the active region
10
. A second GaAs/AlGaAs mirror
16
is subsequently fused to the active region
10
. The presence of the stepped surface
14
complicates the wafer fusion, which requires nominally planar surfaces to work with high yield.
The physics of wafer fusion to a stepped surface are not well-understood, but probably involve mass transport which planarizes the surface being fused during fusion. This leads to uncertainty in the ultimate wavelength of the fused device. It also makes it difficult to create large wavelength steps for wide channel spacing systems, since this requires deep etches which may not fully planarize, leading to a poor quality bond.
SUMMARY OF THE INVENTION
A semiconductor device includes a wavelength-division-multiplexed (WDM) array of long-wavelength vertical cavity lasers. Each vertical cavity laser includes at least one wafer-fused interface and a buried semiconductor recess adjacent the wafer-fused interface. In an exemplary embodiment, the long-wavelength vertical cavity laser is a vertical cavity surface emitting laser (VCSEL) that includes a wafer-fused interface and a buried semiconductor recess disposed at the wafer-fused interface. The buried semiconductor recess substantially overlaps a transverse modal profile of the VCSEL.
A first portion of the VCSEL is provided on a first wafer. A second portion of the VCSEL is provided on a second wafer. At least one recess is etched in one of the first or second portions. The first and second wafers are bonded such that the buried semiconductor recess is formed in a location that will substantially overlap a transverse modal profile of the VCSEL in operation.
Other features and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawing, which illustrate, by way of example, the features of the invention.
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Fiore A et al. Multiple-wavelength vertical-cavity laser arrays based on postgrowth lateral-vertical oxidation of AlGaAs. Applied Physics Letters 1998; 73(3) 282-284.
Imada M et al. Distributed feedback surface-emitting laser with air/semiconductor gratings embedded by mass-transport assisted wafer fusion technique, IEEE Photonics Technology Letters 1997; 9(4) 419-421.
Geske Jonathan
Jayaraman Vijaysekhar
Peters Frank
Arroyo Teresa A.
Genco Vic
Gore Enterprise Holdings Inc.
Monbleau Davienne
Sheets Eric
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