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)