This invention relates, in general, to transmitter optoelectronic integrated circuits, and more particularly to a distributed drive, vertically integrated, transmitter optoelectronic integrated circuit for mating with a plastic optical fiber.
Optoelectronic transmission systems have emerged as a prominent technology in a variety of disciplines including: automotive, computer, medical, and communications. Typically, an optoelectronic system comprises a transmitting optoelectronic integrated circuit, or transmission source, coupled to a receiving integrated circuit via an optical fiber. Two key indices of an optoelectronic signal transmission system are the coupling efficiency between the transmission source and the fiber optic cable, and the power level of the signal generated by the transmission source.
Coupling efficiency is a figure of merit that indicates how much of the optical signal generated by the transmission source is conducted by the optical fiber. The theoretical maximum coupling efficiency is a function of the dimensions of both the optical fiber and the transmission source as shown by Hudson in his paper "Calculation of the Maximum Optical Coupling Efficiency into Multimode Optical Waveguides" (Applied Optics Vol. 13, No. 5, May 1974). In particular, the maximum coupling efficiency is determined by the diameter and numerical aperture of the optical fiber as well as the diameter of the emission source. Hence, the optical fiber is an important component in the optoelectronic transmission system.
Generally, optical fibers are cylindrically shaped with an inner core surrounded by an outer core, commonly referred to as a cladding layer. Two optical fiber parameters which strongly influence maximum coupling efficiency are the inner core diameter and the numerical aperture. In the past, the preferred material for the optical fiber has been glass; a material in which both the inner core diameter and numerical aperture are relatively small. Further, the use of glass optical fibers requires that the transmitting and receiving portions of these systems employ expensive packaging materials to ensure adequate coupling between these two components.
According to the mathematical relationship derived by Hudson, selecting an inner core diameter and a numerical aperture of the optical fiber constrains the diameter of the transmission source for a selected maximum coupling efficiency. Further, to obtain an acceptable maximum coupling efficiency, the relationship between the inner core diameter and the numerical aperture of the optical fiber limits the transmission source diameter to be a small percentage of the inner core diameter. As an example, for a typical glass fiber with an inner core diameter of 50 micrometers and an numerical aperture of 0.21, the transmission source diameter is limited to 20 percent of the optical fiber inner core diameter for the theoretical maximum coupling efficiency.
The primary disadvantage of a small transmission source diameter is that the injection current density in the transmitting device must be relatively high to achieve an acceptable minimum coupled power from the transmitting optoelectronic integrated circuit to the optical fiber. Moreover, thermal properties of both transmitter optoelectronic semiconductor devices, and the lower cost plastic packages used for encapsulating the devices, limit the maximum injection current. Hence, theoretical maximum coupled power has been constrained by physical limitations of the transmitting semiconductor device in addition to those posed by the optical fiber cable.
Further, the use of glass optical fibers has limited the topography of the circuitry associated with transmitter optoelectronic integrated circuits such that current must be collected at contact regions of a transmission source transistor, and transported through metal interconnects. Ultimately, the current must be redistributed by an optimized ohmic contact pattern on an optical emission device. Associated with this current distribution scheme are parasitic resistances and capacitances that degrade the performance of the transmitter optoelectronic integrated circuit. And as discussed previously, the use of glass optical fibers requires very sophisticated and expensive optoelectronic circuit packaging material to achieve the theoretical maximum coupling efficiency.
More recently plastic optical fibers have gained widespread acceptance as an alternative to glass optical fibers. Plastic optical fibers have both a larger inner core diameter and numerical aperture than do their glass counterparts. Hence, for a similar maximum coupling efficiency, the transmission source diameter for a plastic optical fiber may be greater than that of a glass optical fiber. As an example, to achieve the theoretical maximum coupling efficiency, a typical plastic optical fiber with an inner core diameter of 1000 micrometers and a numerical aperture of 0.47 the transmission source diameter can be up to 50 percent of the diameter of the plastic optical fiber. Hence, one advantage derived by using plastic optical fibers is that the larger allowable transmission source diameter provides the option for a lower injection current density. Further, plastic optical fibers make the use of lower cost plastic packaging for the transmission optoelectronic devices feasible. Unfortunately, the present methods for fabricating the transmission optoelectronic devices still renders the use of high current densities in localized portions of the optoelectronic devices. Further, the parasitic resistances and capacitances associated with current collection, transport, and redistribution in the optoelectronic devices still exist.
Accordingly, it would be beneficial to have a transmitter optoelectronic integrated circuit capable of achieving high output current levels with relatively low current densities; while simultaneously taking advantage of the increased transmission source area afforded by using plastic optical fibers to optimize the optoelectronic integrated circuit layout wherein the circuit does not suffer from the performance degradations caused by parasitic resistances and capacitances associated with current collection, transport, and redistribution.