MEMS Adaptive Optics
MEMS Optical Cross Connect:
MEMS mirrors can be used to create high performance optical cross
connects for telecommunications applications and
communications equipment. The product, pictured at the right,
is capable of redirecting the data streams from any one of 32 input fibers
to any one of 32 output fibers. The unit receives input from an
Ethernet port in the rear of the box. The device has under 2 dB of
loss through the system, including connectors. The system can be
completely reconfigured between any two states in under ten milliseconds,
without any "ringing" in the signal.
|The image at the left shows the
optics module which serves as the heart of the system. The optics
module is hermetically sealed, and has two MEMS chips. Each chip
contains 36 high performance beam steering mirrors. The optics module
has 32 input fibers and 32 output fibers. Electrical connections are
made to the MEMS chips via flex cables. By correctly pointing the
mirrors, optical signals are redirected from any input fiber to any output
fiber. The mirrors are controlled by a DSP. Because of the intrinsic
stability of our MEMS chips, the system demonstrates extremely stable open
loop performance. While the system has closed loop control of mirror
position, we have demonstrated less than .1 dB of drift operated for 16
hours open loop. This high degree of mirror stability allows us to "share"
the DSP among 64 mirrors, leading to a small form factor system, and low
A unique aspect of the this optical cross connect system is
its low insertion loss. This comes from the sophisticated performance
of the core MEMS mirror design. Our mirrors can steer an optical beam
through a 40 degree light cone, and require only a 30 volt control signal.
This large tilt range enables us to design the optical module with a very
short optical path leading to very low insertion loss. The figure at
the right shows the insertion loss through the system for all 1024 possible
optical paths. The data illustrates an average insertion loss of 1 dB and
worst case insertion loss of 1.5 dB. The variation in insertion loss
is a result of variation in the optical path length for different optical
paths through the module.
Click on Image to View Video of Chip Operating
|The photograph to the left
shows the MEMS optical cross connect chip. Clicking on the image will
show a video of the mirrors in operation, with 36 out of 36 mirrors
operating. The large tilt angles achieved by the individual mirrors
can be seen in the video. These mirror designs are unique, and have a
linear transfer function. They do not suffer from the "snap down"
problem found in competing gimbaled mirror approaches. Traditional
MEMS mirrors operate by applying an electrostatic force between the mirror
and the substrate. As the mirror begins to tilt, the gap gets smaller,
the field goes up, and the mirror crashes into the substrate. The
mirror has a much more sophisticated actuation scheme, resulting in mirrors
which are easy to control to very precise positions. For the case of
the optical cross connect, mirror position can be controlled to within a few
angstroms. While these mirror designs are very complex, they do not
have any rubbing or touching mechanical elements, enabling high levels of
|The data to the right presents
switching times for the cross connect mirrors. The purple signal
shows X position on a PSD, while the Green curve shows Y position on the PSD.
The blue curve shows the X drive signals applied to the mirrors and the
yellow curve shows the Y drive signal applied to the mirrors. This
chart illustrates that the mirrors can switch between the two extreme
positions (a corner-to-corner transition) in 5 milliseconds with no ringing.
The mirrors are fabricated using the SUMMiT V technology, which enables
the creation of stiff, lightweight mirrors. Fast switching is a key
requirement for the optical cross connect product.
performance of a MEMS-based product is determined by the performance of the
core MEMS chip. The optical cross connect product is an example of how