Low-impedance CMOS optical receivers and substrate-mode holograms for optical interconnects.

Abstract

The performance of modern VLSI systems is limited by signal transmission characteristics of electrical interconnections. Free-space optical interconnects have been suggested as a method to solve these problems. In this work, holographic optical elements (HOEs) for use as free-space interconnects and optical receivers compatible with CMOS systems are investigated. First, the switching characteristics of two basic CMOS receivers without a gain stage are investigated. A simple low-impedance load configuration for high-speed operation is introduced which requires only one photodetector to receive optical signals, and one transistor to provide a desired bias. The optimization and various operating characteristics of this receiver are discussed in detail. SPICE simulations and experimental results using discrete components demonstrate that this receiver can operate at high-frequencies with reasonable optical input power. However, the system fan-out is limited by the available optical power. In order to improve system fan-out, the basic low-impedance load CMOS receiver is extended with a simple gain stage without significantly increasing the complexity of the receivers. Addition of the gain stage is shown to improve fan-out by one order of magnitude at a fixed operating frequency. Free-space interconnects using HOEs have many advantages such as combining several optical functions into a single thin film element. However, the realization of these unique features is limited by image degradation effects due to misalignment and wavelength variations of the light source. Substrate-mode holograms (SMHs) are used to minimize these image degradation effects. Methods for recording SMHs in dichromated gelatin (DCG) are described. Techniques for fabricating SMHs and controlling the angular bandwidth are presented. Experimental results for different combinations of DCG SMHs show that these high-performance elements can function as free-space optical interconnects. In addition, highly polarization-selective substrate-mode holograms were investigated and fabricated. A demonstration of beam switching using polarization selective and non-selective elements with an electro-optic halfwave plate is also given.