Rohit Nair

Multiscale free-space optical interconnects for intrachip global communication: motivation, analysis, and experimental validation.

The use of optical interconnects for communication between points on a microchip is motivated by system-level interconnect modeling showing the saturation of metal wire capacity at the global layer. Free-space optical solutions are analyzed for intrachip communication at the global layer. A multiscale solution comprising microlenses, etched compound slope microprisms, and a curved mirror is shown to outperform a single-scale alternative. Microprisms are designed and fabricated and inserted into an optical setup apparatus to experimentally validate the concept. The multiscale free-space system is shown to have the potential to provide the bandwidth density and configuration flexibility required for global communication in future generations of microchips.

Hybrid chip-scale optical interconnects using multiple quantum well devices bonded to silicon

A hybrid MQW-device-based chip-scale optical interconnect is demonstrated. Novel small-footprint couplers are fabricated with gray-scale lithography to enable high-density fabrics and low-capacitance devices, with sub-pJ/b link performance. Contrast ratio and refined coupler results are presented.

Cost-effective integration of plastic optical fiber and total internal reflection mirrors in printed circuit boards for parallel optical interconnects

A novel method of integrating a total internal reflection (TIR) mirror into an optical waveguide embedded in a printed circuit board (PCB) was demonstrated for application in chip-to-chip optical interconnects. Plastic optical fiber (POF) was placed into channels that were mechanically machined into FR-4 composite plates. The TIR mirror was created by a second mechanical machining. The mirror loss was measured to be approximately −1.6 dB per reflection. The use of POF resulted in attenuation losses in the waveguide over an order of magnitude lower than what is obtainable with typical planar waveguides that are integrated into PCBs. Monte Carlo ray tracing was used to determine the theoretical efficiency of the system as well as cross talk between channels due to optical device alignment tolerances.

Prismatic Coupling Structure for Intrachip Global Communication

Metal wires for global communication on integrated circuits have become problematic as device integration densities scale with rapid advancements in CMOS technology. They may not be able to deliver the growing bandwidth requirements of future microprocessors. Optical interconnect technologies may provide a solution to meet this challenge and extend Moores law. In this paper, a novel guided-wave optical interconnect fabric aiming to replace the slow global metal interconnections is proposed and analyzed. The reflection-mode multiple-quantum-well-modulator-based optical interconnection approach is projected to achieve high coupling efficiency and be compatible with standard CMOS processes. The key notion is a prismatic coupling structure that is embedded in the optical waveguide and therefore has a very small footprint in the circuit. Ray-trace and finite-difference time-domain simulation results predict high coupling efficiency of this structure.

Chip-Level Multiple Quantum Well Modulator-Based Optical Interconnects

High performance computing systems are becoming increasingly limited by the capacity of interconnects due to the continued scaling down of CMOS critical dimensions, resulting in the implementation of optical interconnects at ever decreasing distances. At the chip level, the communication bottleneck and energy consumption per bit are now major limitations to the continued performance scaling of microprocessors. In this paper, a novel integrated photonic approach is presented that uses polymer waveguides and surface-normal GaAs/AlAs multiple quantum well devices integrated directly onto a silicon chip. The concept provides sub-pJ/b performance and seamless interfacing between the on- and off-chip domains. This is the first demonstrated waveguide-coupled surface-normal MQW-based approach to be fully integrated within a photonic layer and to a large extent mitigates packaging issues for future photonics systems integrated with Si chips. Key aspects of the architecture are efficient and minimum-footprint optical fabrics and low-power-consuming optical transceivers. Gray-scale lithography is used to fabricate the 3-D coupling structures directly in the waveguide polymer layer. Analyses and experimental results show that the optical fabric concept provides the necessary bandwidth density and low power consumption for future chip-scale interconnections.

Integrated free-space optical interconnects: All optical communications on- and off-chip

Compact free-space optical interconnects using MQW modulators heterogeneously integrated on silicon with novel waveguide-coupled free-space optical fabrics are presented. The goal is to provide seamless ultra-high density links spanning the intra- and inter-chip levels.

Demonstration of chip-scale optical interconnects based on the integration of polymer waveguides and multiple quantum well modulators on silicon

Key elements of a multi-material hybrid integration concept for implementing high-density chip-scale optical interconnects are demonstrated. Polymer waveguides with curved facets are used to couple light in and out of the MQW devices flip-chip bonded to silicon.

Coupling Structure for Intrachip Optical Global Communication: Design and Simulation

Hybrid optical interconnects are a potential solution to the density, latency and power consumption issues of on-chip global wires. Simulations and analysis of an efficient coupling scheme for a MQW modulator-based waveguide fabric are presented

On-chip guided-wave optical interconnects using multiple quantum well modulators

Novel guided-wave optical interconnect fabrics providing minimum footprints for MQW modulators-based systems are introduced, enabling high-density and low-power photonic links, to prevent performance saturation in microprocessors at the chip-level.

Printed circuit board based modulated retroreflector using three large-area quantum well modulators

We demonstrate a modulated retroreflector that utilizes large- area multiple quantum well modulators on all three faces of a retroreflec- tor. The large-area devices, fabricated by metalorganic chemical vapor deposition, are characterized in terms of the yield and leakage currents. A yield higher than that achieved previously using devices fabricated by molecular beam epitaxy is observed. The retroreflector module is con- structed using a standard FR4 printed circuit board PCB technology, thereby simplifying the wiring issue. A high optical contrast ratio of 8.23 dB is observed for a drive of 20 V. A free-standing PCB retroreflec- tor is explored and found to have insufficient angular tolerances ±0.5 deg. We show that the angular errors in the corner-cube con- struction can be corrected for using off-the-shelf optical components as opposed to mounting the PCBs on a precision corner cube, as has been done previously.