Page 177 - An Introduction to Microelectromechanical Systems Engineering

156 MEM Structures and Systems in Photonic Applications

of the fibers with respect to each other and to the mirror. Insertion loss also relies on
the mirror reflecting all of the light impinging on its surface. This essentially
requires the use of highly reflective coatings, in particular aluminum for wave-
lengths in the visible and gold in the infrared. Furthermore, the surface of the mirror
must be optically flat in order to eliminate any deleterious light-scattering effects. A
surface roughness less than 10% of a wavelength is considered to be optically flat
for most applications.
The device is fabricated on SOI wafers with a 75-µm-thick top silicon layer. The
2
overall die size is approximately 3.3 × 3.7 mm . Lithography in standard resist was
followed by DRIE down to the buried oxide. An etch step in hydrofluoric acid
removes the 2-µm buried silicon dioxide layer to release the comb actuator as well as
the mirror plate. In the research performed at the University of Neuchâtel, 50-nm-
thick aluminum is deposited on the silicon surfaces to increase reflectivity. Owing to
its small size, small mass, and stiff springs, the switch has a fast response time,
typically 500 µs.
The optical switch from the University of Neuchâtel demonstrated an overall
insertion loss of less than 1.6 dB in the bar state and less than 3.4 dB in the cross
state, losses considered excessive for optical-fiber telecommunications. The meas-
ured reflectivity of the aluminum mirror was 76%, lower than the theoretical value
of 95%, thus contributing about 1.2 dB to the loss ratio [34]. The loss was further
exacerbated by scattering from the surface roughness of the mirror plate, which was
measured at 36 nm rms.
In producing a commercial version of the switch, Sercalo had to address these
problems to offer adequate performance in the operating wavelength range from
1,300 to 1,600 nm. First, substituting gold for aluminum greatly improves reflectiv-
ity in the infrared. Second, developing a gold deposition process that yields a
smoother, fine-grain structure is necessary to reduce scattering effects. Sputtering,
evaporation, and slow (low-current) electroplating of gold are known to yield
smooth surfaces. Finally, the fibers must be aligned relative to each other with an
accuracy of less than 0.5 µm to minimize coupling losses. Because all of the slots and
mirror are precisely defined in a single lithographic and etch step, any misalignment
will typically result from variations in the fiber diameter, which can be greatly mini-
mized by using fiber from the same spool. As a result of these improvements, Sercalo
was successful in reducing the insertion loss to an average of 0.5 dB.
The measured polarization-dependent loss (PDL), which is the ratio of reflected
intensities corresponding to the two polarizations, is less than 0.1 dB. PDL is primar-
ily affected by the dependence of the reflectivity of metallic surfaces on polarization,
especially at oblique incidence. For a 45º incident angle on a gold surface, the
difference in reflectivity is about 0.5%, which corresponds to a contribution of
0.025 dB to PDL.

Beam-Steering Micromirror for Photonic Switches and Cross Connects
In M × N switch arrays, the number of individual mirrors is M·N. Arrays bigger
than 32 × 32 require a substantially large number of mirrors and become sub-
ject to sufficiently low manufacturing yields, which make them uneconomical.
Furthermore, the insertion loss increases with the array size as the optical path
length gets longer, and sensitivity to misalignment becomes more critical. A new