RUGGED BOARD-TO-BOARD OPTICAL INTERCONNECT WITH CLOSED-LOOP MICROLENS SCANNER

This paper discusses a free-space optical interconnect system capable of dynamic closed-loop optical alignment using a microlens scanner and a proportional integral and derivative (PID) controller. Electrostatic microlens scanners based on combdrive actuators are designed, fabricated, and characterized with vertical cavity surface emitting lasers (VCSELs) for adaptive optical beam tracking in the midst of mechanical vibration noises. We demonstrate optical beam positioning noise reduction of approximately 20 dB with a 400 Hz bandwidth in the presence of up to a 3g (g = 9.8m/s) vibration. INTRODUCTION Optical interconnect technologies can significantly increase the chip-to-chip and board-to-board communication bandwidth, relieving the bottleneck of traditional backplane-based computer systems [1]. Especially, free-space optical interconnects using arrays of VCSELs and photo-receivers allow for cheaper, lower power, higher bandwidth, and more compact alternatives to traditional copper-based interconnects [1-4]. However, alignment between the optical source and detector is critical for optical interconnect applications, and mechanical noises due to vibration and temperature variation inside the computer systems have prevented the wide deployment of such technology. We present an adaptive free-space optical interconnect using electrostatic microelectromechanical systems (MEMS) lens scanners with closed-loop control to circumvent such difficulties. Although various strategies to adaptively compensate for the misalignment using MEMS devices [3, 4] with feed-forward [5] and feedback control [6] have been attempted, a vibration-resistant free-space optical interconnect system with an intensity-modulated optical beam has never been fully demonstrated. Figure 1 shows the schematic view of our device correcting a misalignment Δx by steering the optical beam across the board-to-board gap. Figure 1: Schematic diagram of MEMS based free-space board-to-board optical interconnect. Although the optical transmitter and receiver are laterally misaligned by Δx, the MEMS microlens scanner steers the optical beam to the correct position. DEVICE DESIGN AND FABRICATION The light source (VCSEL) is located near the back focal plane of the polymer microlens (focal length: f), and the beam deflection angle due to the MEMS lens scanner is approximately given by (θX, θY)=(dX/f, dY/f), when the microlens lateral displacement on the X and Y direction are dX and dY, respectively (f ≫ dX ,dY) [3, 7]. For example, to compensate for the misalignment of Δx at the board-to-board spacing of d, as schematically depicted in Fig. 1, the microlens should be laterally translated by Δxf/d toward the photodetector (PD). Design parameters for the microlens size, displacement requirement, and footprint are summarized in Table 1. Table 1: Design parameters for MEMS lens scanner devices.