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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.

Optical Cross Connect

Optical Switch

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 cost.

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.

Switch Insertion Loss

MEMS Optical Swicth

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 reliability.
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.
The ultimate 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

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paul.mcwhorter@memx.com

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