Three-Dimensional MEMS Device Switches 100 Channels
A compact, 100-channel, microelectromechanical systems (MEMS) optical cross connect has been designed and built by a team of NTT Corp. engineers in Kanagawa, Japan. The device utilizes a pair of 10 X 10 MEMS micromirror arrays and can switch the signal on any of 100 input optical fibers to any of 100 output fibers.
High-capacity switches route telephone calls and data passing across any telecommunications system. Today, even though much long-haul transmission takes place over optical fibers, nearly all switching is done with conventional electromechanical devices. The optical pulses must be co>nverted to electrical signals, switched to the appropriate routes and converted back to optical pulses for the next transmission stages. In recent years, much experimental effort has been devoted to designing high-capacity, purely optical switches for commercial networks.
In the NTT switch, the two 10 X 10 micromirror arrays face each other so that each of the 100 input fibers and each of the 100 output fibers has its own dedicated mirror (see figure).
Each MEMS micromirror has two independent tilt axes.
Each micromirror comprises a reflective surface and an electrode substrate, fabricated from single-crystal silicon. The mirror assembly has two tilt axes, each defined by a pair of torsion springs. The 6:1 aspect ratio of these springs makes them very flexible, while still providing the strong physical mirror support required for reliable operation.
Tilting is accomplished by electrostatic force between the mirror and the electrode substrate. Each mirror is 600 µm in diameter and 10 µm thick and is spaced by 1.3 mm in the 10 X 10 array.
Precise positioning of the fibers with respect to the mirrors is crucial in three-dimensional MEMS switches and is an expensive step in the production of such devices. To hold down the cost of their switch, the researchers designed a two-dimensional array of single-mode fibers that can be passively assembled. They placed a metal microferrule on the tip of each fiber and inserted each fiber into a polymer substrate having an array of holes precisely aligned on 1.3-mm centers. They positioned a microlens array atop the polymer substrate, using a pair of dowel pins for alignment. They fabricated the microlens array using a precise injection-molding method.
Fiber alignment was accurate to within ±2 µm, ensuring low-loss transport across the switch. To reduce the loss further, the microferrules and both sides of the microlens arrays were antireflection-coated.
The entire MEMS switch module fits into a package that measures 8 X 6 X 3.5 cm. The researchers measured an insertion loss of only 4 dB, and the crosstalk between channels was less than –60 dB. The measured switching speed was 3 ms, with only slight ringing in some cases.
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