1. Technical Field
The present invention relates to a semiconductor optical device mounting structure and, more particularly, to a silicon-based stripline structure for providing an interconnection between the semiconductor optical device and a high frequency modulation current source.
2. Description of the Prior Art
Semiconductor optical devices, such as lasers and light emitting diodes (LEDs), are used in a wide variety of applications, due to their compactness, relatively high efficiency, and well-controlled output. However, a number of requirements are imposed upon these devices. For durability, cooling of the optical device is necessary, since prolonged high temperature operation can seriously damage and even destroy the device. Further, since the output light intensity from the device is a function of its junction temperature, the supporting structure must be able to dissipate the tremendous amount of heat generated by the high current density in the device in its operating state.
While it is relatively simple to solve these temperature related problems (by use of a thermo-electric cooler, for example), other problems develop when a semiconductor optical device is operated at extremely high bit rates, for example, above 1 Gb/s. At these speeds, the low impedance of the laser relative to that of the modulation current source, as well as the parasitics associated with the interconnecting network, become critical factors. Minimizing these parasitics by matching the network impedance to the optical device impedance over a broad bandwidth must be performed in order to achieve acceptable performance. It is well-known that semiconductor devices such as lasers exhibit an impedance in the range 2-8.OMEGA., while most high frequency modulation current sources have a relatively high output impedance (&gt;&gt;50 .OMEGA., typically). The impedance mismatch between the laser and the modulation current source would cause multiple reflections of the signal from the laser and consequently severe distortion in the signal waveform applied to the laser. This distortion, in turn, degrades the bit error rate (BER) of the system incorporating the laser. One approach to this problem is disclosed in U.S. Pat. No. 4,097,891 issued to P. R. Selway et al. on Jun. 27, 1978. Selway et al. disclose a particular laser stud mount design which utilizes a stripline (a conductive strip held between a pair of ceramic annuli) as one terminal connection for the laser. The stripline may then be correctly sized (in terms of thickness and width) to provide impedance matching between the signal source and the laser.
A problem with this approach is that since every laser exhibits a slightly different impedance (related to processing variations), each stripline of the Selway et al. design would have to be individually designed to match the particular laser to which it is being coupled. Further, in trying to use the stripline to directly match the impedance of the laser to the modulation current source, other problems develop. In general, to provide impedance matching, the ideal solution is either to decrease the impedance looking back from the laser to the source (so that the source appears to the laser as an equivalent load) or, alternatively, to increase the resistance of the laser as seen by the supply. For many reasons, however, these alternatives are impractical, if not impossible. In particular, to decrease the source impedance as seen by the laser would require the use of an extremely thin stripline, making the device too fragile for practical applications. In contrast, providing a stripline which increases the laser's impedance would greatly increase the necessary voltage swing of the modulation current source required to deliver the same current to the laser. Additionally, these arrangements, by nature of their design, introduce parasitic inductive elements into the interconnection network. At high bit rates, greater than 1 Gb/s, for example, these parasitics seriously load the signal being applied to the laser and severely degrade the performance of the system.
Thus, a need remains in the prior art for a means of interconnecting a semiconductor optical device to a high frequency modulation current source which allows the laser to operate at high bit rates, e.g., speeds exceeding 1 Gb/s without introducing excessive parasitic inductive elements into the interconnecting network.