Page 175 - An Introduction to Microelectromechanical Systems Engineering

154 MEM Structures and Systems in Photonic Applications

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its index of refraction, dn/dT, is approximately 7 × 10 K −1 at 1,545 nm. This tem-
perature dependence results in a spectral sensitivity of 1.35 GHz/°C, thus necessitat-
ing the use of a TEC to stabilize the etalon’s temperature.
The full product is a subassembly that comprises a first glass substrate with the
diffractive optical elements, an etalon on a ceramic carrier, and two photodetectors
also mounted with the appropriate electrical contacts on a ceramic carrier. The sub-
assembly elements are epoxied together and cured. The optical path is usually void
of any epoxy to ensure long-term reliability.

Digital M × N Optical Switch
An optical switch is to fiber-optical telecommunications and light transmission what
a multithrow, multipole electrical switch is to electronic or microwave signaling. An
optical switch redirects an incoming light signal into one of many output fibers.
Arrays of optical switches allow the rapid reconfiguration of optical networks
in data communications by altering the light path in a system of intersecting
fibers—much like railroad points move train tracks to reconfigure a rail network.
They are also useful components in the addition and deletion of extra channels in
optical add/drop multiplexers (OADM) [31] for WDM—equivalent to highway
entrance and exit ramps that add or remove some of the traffic. During the boom
years of the late 1990s, venture capital funded a large number of innovative startup
companies in a race against the giants of the telecommunications industry to develop
the next generation of optical switches. But the collapse of the telecommunications
market in the following years resulted in the closure of many of these companies and
forced the remaining ones to demonstrate products that were cost effective and
highly reliable.
A key characteristic of optical switch arrays is their order (i.e., how many input
and output fibers can be independently coupled to each other). If a switch can route
the light from a single input fiber to any of N output fibers, then it is labeled 1 × N.
Generally, M × N switches are two-dimensional arrays with M input and N output
fibers. Their electronic equivalent is an analog multiplexer that selects any one of M
electrical inputs and routes its signal to any one of N output lines. The now-defunct
company Optical Micromachines, Inc., formerly of San Diego, California, was one
of the early pioneers to demonstrate arrays using surface-micromachined polysilicon
mirrors in switches reaching up to 32 × 32. Optical switches are commercially avail-
able today from such companies as JDS Uniphase of San Jose, California; DiCon
Fiberoptics, Inc., of Berkeley, California; and Sercalo Microtechnology, Ltd., of
Liechtenstein but are typically 1 × 2upto1 × 8, or they are 2 × 2 switches.
The basic architecture and technology used by Sercalo for their M × N switches
stems from prior research activities performed at the University of Neuchâtel in
Switzerland using the SOI and DRIE process introduced in Chapter 3 [32]. While
not all of the details of the Sercalo switch are publicly available, the device is nearly
identical to the work completed at the University of Neuchâtel, and we will thus
limit this discussion to the latter device.
The basic cell for a 2 × 2 switch element consists of an electrostatic comb actua-
tor controlling the position of a vertical mirror plate at the intersection of two per-
pendicular slots. Within each slot lie two optical fibers, one on each slot end (see
Figure 5.14). In the actuator’s normal unbiased position, the mirror plate sits in the