Charles T. Sullivan

Optical interconnections for massively parallel architectures

This paper presents a study of board-level interconnection requirements for highly parallel and massively parallel computing. Analytical models of the I/O bandwidth of popular interconnection networks have been developed and show that current electronic technologies are poor in supporting the necessary I/O density and bandwidth. Optical interconnects appear to offer greater potential in meeting these I/O requirements. Several possible optical implementations of interconnecting a network of electronic processors are compared. The use of polymer waveguides appears to offer the best solution compatible with existing multiboard system architectures.

Planar microoptomechanical waveguide switches

Planar micromechanical waveguide switches based on lateral deflection of a cantilever beam are presented. Two material systems have been used: a GaAs-AlGaAs structure with integrated waveguides and a silicon-on-insulator (SOI), with postprocessed polymeric waveguides. The switches are characterized by low actuation voltage (3-20 V), short switching times (32-200 /spl mu/s), and low crosstalk (< -30 dB).

Optical Waveguide Circuits For Printed Wire-Board Interconnections

Integrated waveguide media based on standard polyimide process technology are described for implementation of intraboard and interboard optical interconnections. High-density router, splitter, and combiner building-block components have been developed in polyimide channel waveguides that are basically compatible with the materials and processes required for printed wire-board and multichip package fabrication. Fanout distribution circuits using 8x8 star and 1x4 tree topologies have been constructed for implementing a 1x32 global fanout control bus in a SIMD multiprocessor system. A 128 linear perfect shuffle routing circuit has been demonstrated and characterized; to our knowledge, this circuit (approximately 2300 components in 5.3x2.6 mm2) is the largest-dimension perfect shuffle and the highest-integration-level optical circuit ever reported.

Switched time-delay elements based on AlGaAs/GaAs optical waveguide technology at 1.32 μm for optically controlled phased-array antennas

Integrated optical time-shift networks consisting of cascaded pairs of 2 x 2 linear electrooptic switches and optical delay lines in GaAs waveguides at 1.32 micron are investigated for true-time optical beam forming in phased array antennas. We report new state-of-the-art results in curved waveguide and corner bend insertion loss, and preliminary results from 2-bit time delay generators (TDGs) constructed in the form of GaAs-based photonic integrated circuits utilizing these components. These results represent significant progress in our longer-term goal of demonstrating a 7-bit TDG with a loss matching monolithic microwave integrated circuit (MMIC) delay line techniques, while providing very wide bandwidth unmatched by MMIC technology.

Tapered rib adiabatic following fiber couplers in etched GaAs materials for monolithic spot-size transformation

Design details and demonstration data are presented for an (Al,Ga)As monolithic tapered rib waveguide achieving modal spot-size transformation. The tapered rib adiabatic following fiber coupler structure (TRAFFIC) achieves two-dimensional (2-D) expansion of the output optical mode of single-transverse-mode semiconductor waveguide modulators and lasers using a one-dimensional (1-D) taper between noncritical initial and final taper widths which are compatible with optical lithographic techniques. Measurements are presented of total mode expansion losses between /spl sim/1.5-2.0 dB and semiconductor to single-mode-fiber waveguide coupling losses of /spl sim/0.5-1.0 dB for doped pin optical-modulator-type waveguides using the TRAFFIC waveguide. A semiconductor laser with a TRAFFIC tapered-rib mode-expansion section and measured coupling loss between the laser output and single-mode fiber of only 0.9 dB is described. Finally, a TRAFFIC Spot-size transformer for undoped waveguide modulators with total mode expansion losses of 1.84 dB and excellent modal behavior at 1.32-/spl mu/m wavelength is presented. The TRAFFiC structure is particularly well suited for integration with both active and passive etched rib waveguide devices. Fabrication is relatively simple, requiring only patterning and etching of the tapered waveguide and uniform-width outer mesa waveguide without any epitaxial regrowth.

Polymeric optical spot-size transformer with vertical and lateral tapers

The design, fabrication, and characterization of a polymeric optical spot-size transformer with vertical and lateral tapers is reported. The vertical taper is formed by utilizing the planarization properties of a polymer, while the lateral taper is defined by photolithography. An optimization method for the taper shape is described based on fundamental results from coupled local mode theory. A total fiber to waveguide insertion loss of 0.6 dB has been measured with the integration of the transformer, an improvement of 2.1 dB. The spatial alignment tolerance for 1 dB excess loss was measured to ±2.7μm, consistent with theory.

Polymer Waveguide-Based Optical Backplane for Fine-Grained Computing

The interconnection requirements of fine-grained computing are examined and compared to the requirements of coarser grained, multiplexed systems. Specifications for the interconnection medium are developed and compared to the performance of available optical source and interconnection components. The use of polyimide waveguides for both applications is considered and the probable architecture of a multiboard fine-grained system is described.

Progress and status of guided-wave optical interconnection technology

Optical interconnects at the cabinet-to-cabinet, board-to-board, and multichip module-to- multichip module levels will enable future avionics systems requirements to be met by eliminating undesirable compromises associated with electrical interconnects. Fiber optics is the well established medium of choice for cabinet-to-cabinet applications, while planar polymeric interconnects are required at the backplane level. Significant advances have been made in demonstrating practical polymer interconnects compatible with existing board fabrication principles, however both waveguide loss and interfaces to optoelectronic components require further improvement before the technology is broadly applicable.

Guided-Wave Optical Interconnects For VLSI Systems

Electrical interconnect technology is becoming the major limitation in the realization of high-performance computing machines. The use of optical interconnections promises to alleviate key interconnect bottlenecks such as pinout, fanout, wiring density, and communication bandwidth. We present results of work on guided-wave optical interconnect circuits for eventual use within and between multichip packages. Low-loss integrated optical waveguides (<0.5 dB/cm), right-angle waveguide corner bends (<0.4 dB/cm), right-angle waveguide crossovers (<0.2 dB), and right-angle 1:2 waveguide branches (<0.5 dB) are presented.

Invited Paper Digital System Applications Of Optical Interconnections

Optical communications technology has seen highly successful implementations in telecommunications and local area networks and is now being explored for data, control, and clock applications within digital systems, such as computers and signal processors. The requirements for optical interconnections within these systems are significantly different from those applications where optical communications are used today. Digital systems require short high-speed interconnections with miniature transmit/receive modules that dissipate little power. These interconnections must also be compatible with standard digital packaging, power supply voltages, and environmental conditions. This paper explores insertion opportunities for optical interconnections within digital systems, the implementation issues, and specific examples of this technology.