This invention relates generally to optical communications, and more particularly to the alignment of optical fibers to photodetectors.
As compared to traditional communication mediums such as twisted pair or coaxial cable, optical fibers provide much greater data-carrying capacity. Many data-carrying channels, each centered on its own wavelength may be multiplexed onto a single optical fiber using, for example, dense wavelength division multiplexing. Data represented by optical signals on the fiber must be converted into electrical form by a fiber optic detector before it may be received by a user.
Fiber optic detectors include a photodetector such as a PIN phototodiode or an avalanche photodiode to convert the received optic signal into an electrical signal. PIN photodiodes are favored for low-speed data traffic whereas avalanche photodiodes are favored for high-speed data traffic. Regardless of the type of photodiode incorporated into a fiber optic detector, its performance depends upon a precise alignment of the optical fiber to the photodiode. A photodiode has an active area that reacts to light to produce electrical carriers. Because of edge effects, the edge of the active area may have a greater responsivity to light than the active area's center. Alternatively, depending upon the photodiode's construction, the responsivity may be approximately constant across the active region. During the alignment of an optical fiber to a photodiode, the increased responsivity caused by an optical fiber being aligned with the edge of the active area may fool a manufacturer into believing that the alignment is optimal. However, the edge of the active area responds much more slowly than the center so that an edge-aligned photodetector will “smear” the bit transitions in the received signal. Thus, an optical fiber must be carefully aligned with the center of a photodiode's active area for proper operation.
This alignment is hampered by the components' miniature dimensions. The core of a single-mode optical fiber typically has a diameter of between 8 and 9 microns. The center region of a photodiode's active area is only slightly larger, typically being about 25 microns in diameter. Performing the alignment manually is quite slow, labor intensive, and error prone. Because of the close tolerances, automated assembly equipment that have been developed to perform this alignment are quite expensive. Regardless of whether an automated or manual process is used, a proper alignment is an active process in that the photodiode must be powered and responding to a light signal from the optical fiber's core during assembly. For example, in an automated process, the alignment apparatus moves the optic fiber in a preset pattern with respect to the photodiode until the detected signal strength and response speed are maximized. The fiber and photodiode are then fixed into place.
Accordingly, there is a need in the art for improved fiber alignment techniques for photodetectors.
In accordance with one aspect of the invention, an integrated fiber alignment photodiode is provided including: a first substrate including a photodiode, the photodiode having an optically-active area; and a second substrate having a through hole defined through the substrate, the second substrate being bonded to a surface of the first substrate such that the through hole is aligned with the optically-active area, the through hole having a cross section sized to accept an optical fiber.
In accordance with another aspect of the invention, a wafer-scale fiber alignment photodiode assembly is provided that includes: a first wafer including a plurality of photodiodes, each photodiode having an optically-active area, the optically-active areas being arranged according to a predetermined pattern; a second wafer including a plurality of through holes defined through the second wafer, the through holes being arranged according to the arrangement of the optically-active areas such that each through hole corresponds on a one-to-one basis with an optically-active area, the second wafer being bonded to a surface of the first wafer such that each through hole is aligned with the corresponding optically-active area, each through hole having a cross section sized to accept an optical fiber.
a is a cross-sectional view of a fiber alignment photodiode coupled to an optical fiber using a through hole with a trapezoidal cross section in accordance with an embodiment of the invention.
b is a cross-sectional view of a fiber alignment photodiode coupled to an optical fiber using a through hole with a uniform cross section in accordance with an embodiment of the invention
Embodiments of the present invention and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures.
Referring now to the drawings, the active side of an InP wafer 100 is shown in
Once through holes 205 have been etched into wafer 200, it may be bonded to a surface of wafer 100 so that optical fibers may be fixed within through holes 205. A number of bonding techniques may be used to bond wafers 100 and 200. For example, as known in the art, flip-chip bonding tools may be used to provide alignment tolerances of approximately 1 micron or less. Using either infra-red or mechanical alignment techniques, a flip-chip assembly tool would align wafer 200 so that through holes 205 are substantially centered with respect to active areas 205. A suitable adhesive such as an ultraviolet-light-curable optical epoxy bonds wafers 100 and 200 together.
Once wafers 100 and 200 have been bonded together, individual die may be diced from the completed wafer. For example, an expanded view of the silicon side of a completed wafer 300 is shown in
The geometry of each through hole depends upon the etching process used. Should the silicon wafer have a (100) lattice orientation, a wet etch produces a through hole 315 having a trapezoidal cross section. Alternatively, a dry etch on silicon wafer 200 produces a through hole 325 having a constant diameter. A cross-sectional view of the resulting through holes is shown in
The diameter of dry-etched through hole 325 should equal that of optical fiber 427 plus an acceptable tolerance. Wet-etched through hole 315 has a beginning diameter that is larger than its ending diameter. To receive optical fiber 427, the dimensions for the inner and outer diameters should be such that an intermediate diameter falling approximately half way between these inner and outer diameters also equals the diameter of optical fiber 427 plus an acceptable tolerance. As shown in
Those of ordinary skill in the art will appreciate that many modifications may be made to the embodiments described herein. For example, as seen in