Many properties of an optically interconnected system can be improved through the use of a modelocked laser. The short pulse duration, high peak power, wide spectral bandwidth, and low timing jitter of such a laser lead to these benefits. Timing advantages include simplified synchronization across large chip areas, receiver latency reduction, and data resynchronization. Lower power dissipation may be achieved through improved receiver sensitivity. Additional applications of short optical pulses include time-division multiplexing, single-source wavelength-division multiplexing, and precise time-domain testing of circuits. Several of these concepts were investigated using a high-speed chip-to-chip optical interconnect demonstration link. The link employs a modelocked laser and surface-normal optoelectronic modulators that were flip-chip bonded to silicon CMOS circuits. This paper outlines experiments that were performed on or simulated for the link, and discusses the important benefits of ultrashort optical pulses for optical interconnection.

Abstract-We present a new technique of injecting clocks optically onto CMOS chips without the use of a receiver amplifier. We discuss the benefits of such a direct approach and present proof-ofprinciple experiments of the technique. We analytically compare a receiver-less optical clock distribution and an electrical clock distribution in a fan-out-of-four clock tree to evaluate the timing and power benefits of the optical approach for present microprocessors. We also compare receiver-less direct injection of optical clocks to trans-impedance receiver based injection within the same distribution framework.

A multi-channel free-space micro-optical module for dense MCM-level optical interconnections has been designed and fabricated. Extensive modeling proves that the module is scalable with a potential for multi-Tb/s.cm 2 aggregate bit rate capacity while alignment and fabrication tolerances are compatible with present-day mass replication techniques. The micro-optical module is an assembly of refractive lensletarrays and a high-quality micro-prism. Both components are prototyped using deep lithography with protons and are monolithically integrated using vacuum casting replication technique. The resulting 16-channel high optical-grade plastic module shows optical transfer efficiencies of 46% and interchannel cross talks as low as-22 dB, sufficient to establish workable multi-channel MCM-level interconnections. This micro-optical module was used in a feasibility demonstrator to establish intra-chip optical interconnections on a 0.6µm CMOS opto-electronic field programmable gate array. This opto-electronic chip combines fully functional digital logic, driver and receiver circuitry and flip-chipped VCSEL and detector arrays. With this test-vehicle multi-channel on-chip data-communication has been achieved for the first time to our knowledge. The bit rate per channel was limited to 10Mb/s because of the limited speed of the chip tester.

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