1. Field of the Invention
This invention relates generally to the field of optoelectronics and more specifically is directed to an integrated assembly for coupling laser diodes or photodetectors to optical fibers.
2. State of the Prior Art
Optical interconnection of electronic data and communication systems is of great interest as it offers use of small, relatively lightweight cable, large transmission capacity, long transmission distance, and immunity to electromagnetic noise, compared with electrical cables using metallic conductors. Potential applications include, but are by no means limited to, telephone transmission lines, subscriber television cable service, and interconnection of subsystems in large computer architectures at different packaging hierarchies, such as chip-to-chip, board-to-board, card-cage to card-cage, and inter-cabinet connections.
Optical fibers are continuous lengths of finely drawn, highly transparent glass material which can transmit light over long distances. Optical fibers must be interfaced to the electronic circuits which generate and process the electrical signals carried by the fibers. At the transmitter end, the electronic circuits drive light emitting semiconductors, such as laser diodes, which produce light pulses fed into the optical fibers. At the receiving end, the light signals carried by the optical fibers are directed onto photodetectors which convert the optical, i.e. light signals to an electrical signal for further processing by electronic receiver circuitry.
Typically, optical fiber transmission lines are made up of bundles of such fibers for carrying parallel data. Data is fed to each optical fiber by a corresponding light emitting element which is electrically modulated such that the light output corresponds to an electrical signal input. The light emitting elements are typically formed on a single semiconductor chip, such as a laser diode array, which may for example have twelve mutually independent laser diode emitters spaced along a line on a common face of the chip. The individual laser diodes of the array are closely spaced to each other and each emits a cone of light. The cones spread out and begin to overlap at a short distance from the laser chip. In order to avoid cross-over and interference between the optical signals, the laser diode emitters must be positioned closely to the end faces of the optical fibers comprising the optical transmission link. The close spacing between the laser diode emitters, coupled with the small diameter of the fiber end faces and even smaller core diameters, requires precise alignment of the diode array in relation to the optical fiber holder, so that a maximum amount of laser light illuminates the corresponding optical fiber core. Misalignment between the laser diode and the fiber core results in wasted laser output and a consequent weakening of the transmitted light signal. Proper alignment of the laser diode array is specially important when single mode optical fibers are employed, due to the very small diameter of the core of such fibers. Multi-mode optical fiber cores are substantially larger in diameter, and alignment of the laser diode array is easier in such case.
For the potential of optoelectronic communication to be fully realized improvement is needed in the packaging of the optoelectronic components, particularly the assemblies used for creating the interface between the electrical and optical portions of the system. In particular, improvement is needed in regard to the mounting of the light emitting and photo detector devices and their coupling to the optical fibers in order to improve the efficiency and reliability of the optoelectronic interconnects, and to reduce the cost of current assemblies which rely on discrete, precision machined components, require costly, labor intensive active alignment, and offer inadequate thermal and mechanical stability.
One type of widely used optical fiber connector, the MT (Mechanically Transferable) type connectors, has an optical fiber holder where the fibers are captive in channels defined between an upper substrate and a lower substrate. Typically, one substrate has a surface traversed by parallel grooves, each groove having a V-shaped cross-section. A flat surface of the other substrate is joined against the grooved surface to define between the two substrates parallel channels of triangular cross-section. A fiber ribbon or cable containing one or more optical fibers is clamped between the two substrates and individual fibers extend from the cable or ribbon within corresponding channels and terminate at a common plane surface defined by the two substrates, the fiber ends being arranged along a straight line formed by the junction of the two substrates. Each optical fiber has a light transmitting core surrounded by a cladding. The diameter of the fiber is normally 125 microns including the cladding. The core diameter is 62.5 microns for multi-mode fibers, and only 10 microns for single mode fibers. The interior dimensions of the triangular channels are held to very close tolerances so that in cross section the cylindrical fibers make tangential contact with the center of each side surface of the channel. The fiber ends as well as the common plane surface of the substrates are highly polished and flat to facilitate close physical contact with a second similar holder for making an optical connection between two lengths of fiberoptic cable. Alignment of the fiber ends between the two connectors is ensured by precisely machined guide pins on one connector mated to equally precise guide holes in the other connector. This type of optical fiber connector is available from NGK of Japan with the two substrates made of ceramic material, and in a precision plastic version from US Conec Ltd., of Hickory, N.C.
Current engineering practice is to mount the light emitting and photo detector elements separately from the optical fiber holder or connector, on a submount which is part of the optoelectronic package containing the transmitter/receiver electronics. The fiber optic connector is mechanically engaged to the housing of the optoelectronic package and is held in alignment with the photo emitter or detector by alignment pins or other mechanical expedient. Such assemblies typically require active alignment, i.e., the laser diode is powered up and adjusted until the light output of the optical fibers is maximized. Prior art optoelectronic couplings and connectors which rely on a combination of structural materials having different coefficients of thermal expansion tend to suffer from thermal instabilities. As different parts of a coupling or connector expand at different rates with temperature changes, the optical alignment between the light emitting device and the optical fibers can be affected, diminishing the power delivered by the light emitting device to the optical fibers and in extreme cases disrupting the optoelectronic link.
What is needed is an optoelectronic coupling with thermally stable and mechanically dependable integrated mounting of the laser diode/photo detector and optical fiber holder on a common base. Past efforts in that direction have produced laboratory prototypes with technologies which have proven prohibitive and impractical for manufacture in commercial quantities. In one such prior effort, a number of parallel grooves were chemically etched in the surface of a silicon base, and optical fibers were laid in the grooves and secured in place with adhesive. The groove depth was such that only the fiber cladding was recessed below the silicon surface and the fiber cores remained above surface. A laser diode array was mounted on the silicon base in alignment for illuminating the end surfaces of the optical fiber cores. Power was supplied to the laser diode through metal film electrodes deposited on the silicone base and connected by wire bonding to corresponding electrodes of the laser diode. This assembly, which performs reasonably well under laboratory conditions, is impractical for commercial applications. Firstly, the chemical etching technology needed to make the precisely dimensioned grooves is very difficult to control, and is an expensive process not suited to commercial production. Further, silicon has a rather large coefficient of thermal expansion, which tends to make the assembly thermally unstable, and lacks adequate mechanical strength for field use in connector applications.
A continuing need exists for a commercially practical and reasonably priced integrated optoelectronic coupler and connector offering good thermal stability, satisfactory mechanical strength and advantageous electrical properties.