Robust free space board-to-board optical interconnect with closed loop MEMS tracking

Applied Physics A, Jun 2009

We present a free-space optical interconnect system capable of dynamic closed-loop optical alignment using a microlens scanner with a proportional integral and derivative controller. Electrostatic microlens scanners based on combdrive actuators are designed and characterized with vertical cavity surface emitting lasers (VCSELs) for adaptive optical beam tracking in the midst of mechanical vibration noise. The microlens scanners are fabricated on silicon-on-insulator wafers with a bulk micromachining process using deep reactive ion etching. We demonstrate dynamic optical beam positioning with a 700 Hz bandwidth and a maximum noise reduction of approximately 40 dB. Eye diagrams with a 1 Gb/s modulation rate are presented to demonstrate the improved optical link in the presence of mechanical noise.

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Robust free space board-to-board optical interconnect with closed loop MEMS tracking

Jeffrey Chou Kyoungsik Yu David Horsley Brian Yoxall Sagi Mathai Michael R.T. Tan Shih-Yuan Wang Ming C. Wu 0 1770, Berkeley, CA 94720-1770, USA We present a free-space optical interconnect system capable of dynamic closed-loop optical alignment using a microlens scanner with a proportional integral and derivative controller. Electrostatic microlens scanners based on combdrive actuators are designed and characterized with vertical cavity surface emitting lasers (VCSELs) for adaptive optical beam tracking in the midst of mechanical vibration noise. The microlens scanners are fabricated on siliconon-insulator wafers with a bulk micromachining process using deep reactive ion etching. We demonstrate dynamic optical beam positioning with a 700 Hz bandwidth and a maximum noise reduction of approximately 40 dB. Eye diagrams with a 1 Gb/s modulation rate are presented to demonstrate the improved optical link in the presence of mechanical noise. - PACS 42.15.-i 42.55.Px 1 Introduction Optical interconnect technologies can significantly increase the chip-to-chip and board-to-board communication band width, relieving the bottleneck of traditional electrical backplane-based computer systems [1]. Specifically, freespace optical interconnects using arrays of vertical cavity surface-emitting lasers (VCSELs) and photo-receivers allow for cheaper, lower power, and higher bandwidth alternatives to traditional copper-based electrical interconnects [13]. When compared to waveguide-based optical interconnect technologies, free-space optical interconnects provide a number of advantages in communication capacity, density, and scalability due to their parallelism [4]. However, alignment between the optical source and detector is critical for high-performance, reliable optical interconnect applications, and mechanical noises due to vibration and temperature variation inside the computer systems have prevented the wide deployment of such technology. Optical misalignment introduces higher insertion loss and crosstalk between optical links, which can severely impact the system performance and reliability [5, 6]. Various strategies to adaptively compensate for the misalignment in free-space board-to-board optical interconnects have been demonstrated, including bulk optic Risley prisms [7, 8], mechanical translational stages [9], liquid crystal spatial light modulators [10, 11], and microelectromechanical systems (MEMS) devices [12, 13]. Among these approaches, MEMS technology offers faster speed, low optical loss, and small form factor that can be directly integrated on top of VCSEL arrays [13]. However, a vibration-resistant free-space optical interconnect system with an intensitymodulated optical beam using real-time feedback control has never been demonstrated with dynamic MEMS devices. In this paper, we present an adaptive free-space optical interconnect using electrostatic MEMS lens scanners with closed-loop control to circumvent misalignment difficulties in free-space optical interconnect systems. Fig. 1 Schematic diagram of MEMS-based free-space board-to-board optical interconnect. Although the optical transmitter and receiver are laterally misaligned by x and , the MEMS microlens scanner steers the optical beam to the correct position Figure 1 shows the schematic view of our proposed freespace optical interconnect system correcting a lateral and tilt board misalignment ( x and ) by steering the optical beam path across the board-to-board gap with an MEMS microlens scanner. The beam scanning range on the receiving board is amplified by the board-to-board distance, allowing for small microscale lens scanning to compensate for larger lateral misalignments. This paper assumes an optical interconnect setup with one microlens scanner per VCSEL to avoid the use of large optics on the MEMS translational stages and thus allow for higher operating speeds. We also assume that the misalignments are constrained in only one dimension along the X-axis as shown in Fig. 1. However, it is possible to extend our design for other optical configurations where multiple VCSELs are relayed by a bigger lens or multiple intermediate lenses [6]. It is also straightforward to improve our devices to scan two orthogonal axes as discussed in Sect. 3. 2 Device design and fabrication 2.1 Optical design The microlens scanner design is based on the chosen parameters for board-to-board interconnects summarized in Table 1. In our optical design, the light source (VCSEL) is located near the back focal plane of the polymer microlens with a focal length of f . Assuming Gaussian beam propagation, we calculate the minimum lens diameter given the VCSEL wavelength and board-to-board spacing listed in Table 1. To collimate the beam between the two lenses, we set the confocal length equal to half the board-to-board spacing Table 1 Design parameters beam diameter at the microlens must be 220 = 2 d , or approximately 165 m when the VCSEL wavelength, , and the (...truncated)


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Jeffrey Chou, Kyoungsik Yu, David Horsley, Brian Yoxall, Sagi Mathai, Michael R. T. Tan, Shih-Yuan Wang, Ming C. Wu. Robust free space board-to-board optical interconnect with closed loop MEMS tracking, Applied Physics A, 2009, pp. 973, Volume 95, Issue 4, DOI: 10.1007/s00339-009-5126-1