VCSEL-based Free-Space Optical Interconnect with Integrated Microlenses

Abstract

Increasing computer clock speed is the motivation for investigating the use of optics in comupter interconnects. The technology that will eventually replace electrical interconnects must feature high speed and low power consumption at a low cost to the end user. Vertical cavity surface emitting laser (VCSELs) arrays have become viable candidates for transmiters in optical links [1-2]. Their low threshold currents (< 300 µA for ~ 4 µm diameter devices), relatively low power consumption, and low operating voltage (< 2 V) [3-4], make them compatible with simple drive circuits. High modulation bandwidth can be achieved without additional pre-bias circuitry. VCSELs with a record 3 dB bandwidth of 15.3 GHz at only 2.1 mA of bias current have been reported [5]. Furthermore, highly divergent beams from small diameter devices are compatible with microoptics integrated on the same substrate. The beams reaching the back side of the substrate fill integrated microlenses enabling high numerical aperture (NA) focusing or collimation into low diffraction beams. With these integrated components, coupling into fibers and collimation for free-space systems can be realized with no external optics. We demonstrate a free-space optical link consisting of arrays of substrate emitting VCSELs as transmitters and back-side illuminated double-pass Schottky photodiodes as receivers. These devices are integrated on-chip with microlenses etched on the back side of the substrate [6]. Analysis and test results of the system’s tolerance to mechanical misalignments are presented.