Time-division optical micro-area networks

Optical micro-area networks (μANs) are proposed as a way of providing flexible communications among VLSI processors and eliminate electrical I/O bottlenecks. Sharedmedium multiple access protocols in μANs can avoid the access delays associated with statistical multiple access protocols (which are unacceptable in multiprocessor applications) and increase the throughput, at the expense of wasting optical bandwidth. Time-division multiple access (TDMA) may be more practical to implement in a |iAN than other shared-medium multiple access protocols such as frequency-division or code-division. Since the total throughput of TDMA is given by the inverse of the optical pulsewidth, the throughput can be increased by making the pulse width small. Accomplishing this goal requires avoiding the use of low-bandwidth electronics in the portion of the μAN that directly processes these short pulses. Instead, optical processing can be used in those protions of the network. The architecture of a TDMA μAN which uses optical multiple access processing and is self-clocking is described in detail. Experimental demonstrations of key subsyterns for optically generating, modulating, synchronizing, delaying and correlating short optical pulses are presented. The feasibility of a variable-integer-delay line which provides rapid tuing, wide tuning range and high precision is demonstrated. A transmitter consisting of a mode-locked laser with an external modulator is used in the TDMA μAN, since arbitrarily short pulses can be controlled with a modulator that need only operate at the bit rate, which translates into extremely high total throughput. The use of an off-chip optical source and an on-chip modulator also has circuit-integration advantages over a laser diode, including lower electrical drive power, electrical drive power that is independent of output optical power, greater ease of hybrid integration on silicon substrates, and less required real-estate on the chip. Experimental measurements of the modulation depth, excess transmission loss and required electrical drive power of an MQW modulator are presented and related to system performance. An optical correlation receiver is demonstrated using 2 psec optical pulses and a two-gap photoconductive AND gate. The sensitivity, rise-time and fall-time of the device are measured. The power budget of the TDMA μAN is calculated and it is determined that 1000 nodes are feasible.