Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2001-10-18
2003-09-23
Wille, Douglas A. (Department: 2814)
Optical waveguides
With optical coupler
Particular coupling structure
C385S088000, C438S039000
Reexamination Certificate
active
06625357
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a technique for fabricating fiducials in a buried heterostructure edge emitting laser for alignment of the device in a passive manner.
BACKGROUND OF THE INVENTION
The present invention is related to U.S. Pat. No. 5,981,975 to Imhoff, filed Feb. 27, 1998 as well as to U.S. patent application Ser. No. 60/079,910 filed on Mar. 30, 1998, the disclosures of which are specifically incorporated herein by reference. Light emitting devices often utilize double heterostructures or multi-quantum well structures in which an active region of a III-V semiconductor is sandwiched between two oppositely doped III-IV compounds. By choosing appropriate materials for the outer layers, the band gaps are made to be larger than that of the active layer. This procedure, well known to one of ordinary skill in the art, produces a device that permits light emission due to recombination in the active region, but prevents the flow of electrons or holes between the active layer and the higher band gap sandwiching layers due to the differences between the conduction band energies and the valence band energies, respectively. Light emitting devices can be fabricated to emit from the edge of the active layer, or from the surface. Typically, a first layer of material, the substrate, is n-type indium phosphide (InP) with an n-type buffer layer disposed thereon. This buffer layer again is preferably InP. The active layer is typically indium gallium arsenide phosphide (InGaAsP) with a p-type cladding layer of InP disposed thereon. One potential pitfall of double heterostructure lasers is often a lack of means for confining the current and the radiation emission in the lateral direction. The result is that a typical broad area laser can support more than one transverse mode, resulting in unacceptable mode hopping as well as spatial and temporal instabilities. To overcome these problems, modern semiconductor lasers employ some form of transverse optical and carrier confinement. A typical structure to effect lateral confinement is the buried heterostructure laser. The buffer, active and cladding layers are disposed on the substrate by epitaxial techniques. The structure is then etched through a mask down to the substrate level leaving a relatively narrow (roughly on the order of several microns) rectangular mesa composed of the original layers. A burying layer is then regrown on either side of the mesa resulting in the buried heterostructure device. The important feature of a buried heterostructure laser is that the active layer is surrounded on all sides by a lower index material so that from an electromagnetic perspective the structure is that of a rectangular dielectric waveguide. The lateral and transverse dimensions of the active region and the index discontinuities are chosen so that only the lowest order transverse mode can propagate in the waveguide. Another very important feature of the structure and that which is required to effect lasing is the confinement of injected carriers at the boundaries of the active region due to the energy band discontinuities at the interface of the active region and the InP layers. These act as potential barriers inhibiting carrier escape out of the active region.
One area of optoelectronics which has seen a great deal of activity in the recent past is in the area of passive alignment. Silicon waferboard, which utilizes the crystalline properties of silicon for aligning optical fibers, as well as passive and active optical devices, has gained a great deal of acceptance. One technique for aligning an optoelectronic device to an optical fiber and other passive and/or active elements is the use of an alignment pedestal for lateral planar registration and standoffs for height registration. By virtue of the sub-micron accuracy of photolithography used to define and align these pedestals and standoff features, the application of this approach has proven to be a viable alignment alternative. By effecting alignment in a passive manner, the labor input into the finished product can be reduced, resulting in lower cost of the final product.
One example of such an alignment scheme can be found in U.S. Pat. No. 5,163,108 to Armiento, et al., the disclosure of which is specifically incorporated herein by reference. The reference to Armiento, et al. makes use of an alignment notch on the active device which is designed to mate with alignment pedestals and standoffs on the silicon waferboard. This particular structure is used for aligning an optical fiber array to an array of light emitting devices.
FIG. 1
is a perspective view of a laser array die
102
which is to be mounted on a silicon substrate
100
such that the active region
106
of the laser die
102
accurately aligns with a fiber to be placed in a v-groove
105
on the silicon substrate
100
. As shown, the die
102
has a notch
101
that has been etched therein to be an accurately controlled distance from the laser active region
106
. Further, pedestals
103
,
104
,
108
, and
109
have been fabricated on the substrate at predetermined locations to serve as mechanical fiducials for the laser die
102
, i.e., the laser will be aligned by virtue of contact with the fiducial. In particular, the laser die is placed on the silicon substrate
100
generally in the vicinity of fiducials
103
,
104
,
108
, and
109
so that the active region
106
roughly aligns with the v-groove
105
. The laser die
102
is then pushed in the z direction so that the front surface
107
of the die
102
abuts mechanical fiducials
108
and
109
and in the x direction so that the surface
112
of notch
101
abuts the surface
113
of mechanical fiducial
103
, thereby precisely aligning the laser die
102
on the silicon in the x and z directions in a position dictated by the placement of the mechanical fiducials
103
,
104
,
109
, and
110
, (and notch
101
).
Unfortunately, one problem with structures like the one shown in the reference to Armiento, et al., is that it pertains only to ridge laser structures. This is because, in a ridge waveguide laser structure, the patterning photolithography step that defines the active waveguide is simultaneously used to define the alignment notch in the same mask level, resulting in an alignment of the notch and active waveguide that is limited only by the variations in the photolithography mask. However, it is advantageous from a performance standpoint to be able to utilize lasers and other active devices that incorporate a regrowth step, such as the buried heterostructure laser described above. For this class of devices, the subsequent regrowth step(s), bury the active waveguide mesa and, hence, also the notch. Accordingly, fabrication is complicated because the alignment notch must be made after the regrowth since the notch patterning step must occur in a photolithography step subsequent to the one in which the active waveguide is defined. Moreover, a notch patterning step on the regrown surface of the wafer is difficult because the mesa is not a visible re-alignment feature using the conventional technique of optical alignment methods. Even further, creating the notch using a different photolithography step and mask than was used to create the mesa increases the potential misalignment between the mesa and the notch. Particularly, in such situations, the tolerances of the masks are essentially cumulative. Further, additional error is introduced by misalignment of the masks to one another.
Another known scheme for passively aligning an optoelectronic device to an optical fiber on a silicon waferboard is the use of visual fiducials and an optical detection system. In this technique, visible markings (the fiducials) are made on the surfaces of the optoelectronic device and mating markings are made on the silicon waferboard. The visual fiducials usually are made by etching through at least the outermost layer of the optoelectronic device and the silicon waferboard to leave an aperture that can be detected by an optical detection system. The fiducials
Anderson Richard
Bowen Terry Patrick
Breedis John Baker
Jiang Ching-Long
Ring William Sean
Tyco Electronics Corporation
Wille Douglas A.
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