Proc. SPIE 0466, Will Presently Available System Design Approaches And Electrical Interconnect Rules Fulfill The Performance Requirements Of High Clock Rate Digital Signal Processors Based On Gaas?, 0000 (10 May 1984); doi: 10.1117/12.941555
The emerging application of Gallium Arsenide digital integrated circuits to signal processing problems will require the development of architectures quite different from those used for highly parallel silicon VLSI and VHSIC components. Much higher system clock rates will be employed in GaAs based systems than will be common in silicon VLSI, placing much more stress on interchip signal protocols, and on the design of first and second level packaging, i.e., at the chip carrier and logic board levels.
Conventional interconnect and switching technology is rapidly becoming a critical issue in the realization of systems using silicon-based VHSIC/VLSI chips or GaAs integrated circuits. Optical interconnect technology promises to enhance performance significantly, provide relief from the pinout problem, decrease implementation complexity, and provide improvements to the flexibility and real-time reconfigurability of these systems. Furthermore, by releasing the bandwidth constraints on interconnects, the full processing speed capabilities of silicon logic could be exploited to improve system throughput dramatically. This paper outlines the key issues involved in implementing optical interconnects and the impact of exploiting this concept on the architecture of digital systems.
GaAs is an ideal material for use in Very-High-Speed Integrated Circuits for two fundamental reasons: (1) such circuits have great potential for high-speed performance because of the high intrinsic mobility and saturation velocity of GaAs, and (2) efficient optical devices can be fabricated in this material, providing the potential for optical interconnects. In this paper, the integration of optoelectronic devices utilizing geometries that lend themselves to inter- and intra-chip interconnects will be discussed. Monolithic integration of active devices parallel to the plane of the epitaxial layers will be described, along with recent schemes developed for output couplers transverse to the layers.
For the case of monolithic integration within the epitaxial layers, the development of Distributed Feedback and other devices utilizing periodic corrugations will be reviewed briefly. More attention will be devoted to the formation of Fabry-Perot resonators using both wet and dry etching techniques, and the monolithic integration of these devices into optical circuits. Results in both GaAs and InP-based materials will be reported. Attempts to produce ring lasers or other structures utilizing curved waveguides will also be described, along with investigations of techniques to produce optical circuits directly during the crystal-growth procedure by utilizing various types of masks.
Output couplers will also be considered in this paper. The use of gratings can result in coherent, parallel output beams that are easily coupled into optical fibers. Recent attempts to fabricate Fabry-Perot resonators in the direction transverse to the epilayers will also be reviewed.
One of the main advantages which integrated optoelectronic circuits (IOEC's) have over discrete circuits is the ability to achieve high-frequency modulation due to a significant reduction in parasitic reactances. This is due to the elimination of bond wire connections which have high inductances and to the use of semi-insulating substrates which reduces the parasitic capacitances of bonding pads. While the active electronic devices incorporated in such circuits, e.g., GaAs metal-semiconductor field-effect transistors (MESFET) can be modulated at frequencies exceeding 30 GHz, the overall modulation bandwidth of the integrated optoelectronic circuit is limited, in general,by the frequency response of the light source (i.e., laser) and the photodetector.
This paper describes the electro-optic and modulation characteristics of GaInAsP/InP buried heterostructure injection lasers. A network model for the laser is derived from analytic solutions to the rate equations that govern its dynamic response. Parasitic elements introduced during device packaging are identified and incorporated into a circuit model for the laser package. The circuit model is used in conjunction with SPICE to simulate the effect of transmitter circuitry on their modulation characteristics at high frequencies.
Q-switched diode lasers are of interest as a source in optical communication systems and in very high speed optical interconnects between digital circuits and systems. Buried-heterostructure Q-switched diode lasers have been made with thresholds as low as 15 mA. The lasers operate continuously at room temperature. Modulation has been observed at rates up to 7 GHz. Evidence of several modes of Q-switching has been obtained including a new mode of operation which should permit modulation at rates of several tens of gigahertz.
The interconnection of electronic system modules, boards, and chips will become increasingly difficult using conventional electrical approaches as the data rates and the number of input/output ports increase. Optical interconnection implementing high-speed (multi-Gb/s) optical links will be an attractive and viable alternative technique. High-performance hardware for such optical interface applications are anticipated in the form of single-chip GaAs/GaAlAs integrated optoelectronic transmitters, receivers, and transceivers. GaAs optical and electronic device technology has advanced to the stage where such integrated optoelectronic devices are now possible. Key integration techniques developed and verified with simple operational transmitters are the basis for realizing more complex integrated optoelectronic circuits with sophisticated optical and electronic functions.
We have demonstrated the first operational integrated optoelectronic circuit which combines a semiconductor laser diode with small-scale (36 gates) GaAs circuitry. The successful development of this technology has demonstrated one of the key elements for high speed optical interconnects using fabrication techniques which are compatible with medium to large scale integration. Due to the different and often conflicting requirements for high performance lasers and electronics, only very elementary integrated laser/driver structures (less than six transistors) have been reported until now. We have designed and fabricated an integrated optoelectronic transmitter containing a tranverse junction stripe (TJS) laser, a field effect transistor (FET) driver, and a 4:1 multiplexer (Mux). The Mux and driver active regions are formed by selective ion implantation while the TJS laser is fabricated in epitaxial layers grown in a well that is etched into the substrate. The Mux consists of 36 SDFL (Schottky diode FET logic) NOR gates. The MESFETs in the GaAs circuit have 1 micron gate lengths and a 1.2V threshold voltage. The transmitter chip has been tested at 160 MHz clock rates. Higher speeds are possible.
The fabrication and performance of a monolihtic optical receiver chip consisting of a pin photodiode and a transimpedance preamplifier on a GaAs semi-insulating substrate is reported. The epitaxial layers for the photodiode are grown by hydride vapor phase epitaxy and the circuit elements are fabricated by selective ion implantation in the semi-insulating substrate. The integration scheme results in a planar surface which simplifies the processing of optoelectronic integrated chips.
A low threshold buried heterostructure laser, a metal-semiconductor field effect transistor (MESFET), and a photodiode, have for the first time, been monolithically integrated on a semi-insulating GaAs substrate. This integrated optoelectronic circuit (IOEC) was operated as a rudimentary optical repeater. The incident optial signal is detected by the photodiode, amplified by the MESFET, and converted back to light by the laser. The gain bandwidth product of the repeater was measured to be 178 MHz.
Integrated optoelectronic logic circuits which convert the electrical signal inputs into the logic gated optical signal output are discussed. These circuits include OR, AND, NOR, NAND, Exclusive OR and D/A Converter. The technology required to develop these circuits is not ready at present. However, initial results on the optoelectronic integration effort are encouraging. If the development is successful, there will be a wide range of applications for these devices.
A high speed fiber optic receiver is described which uses a single chip to perform all of the electronic functions. These functions include low noise preamplification, amplification to a logic compatible signal level, and automatic level restoration which maintains the correct receiver decision point over a large range of optical input power. The receiver has operated at bit rates from 50 to 400 Mbit/sec with sensitivities as high as -33 dbm.
A dynamically programmable IOC vector matrix multiplier for high data rate transmission and high throughput is proposed. The multiplier is based on a channel configuration and is compared to previous systems utilizing planar geometries. The extension of this device to digital operation is considered and a discussion of the necessary requirements for optical or electrooptical algebraic processors to supercede electronic digital computers is undertaken.