Page 204 - Optofluidics Fundamentals, Devices, and Applications

Adaptive Optofluidic Devices 179

promise to benefit optical protection switches. Such switches recon-
figure the interconnection of N × N input/output ports in case of an
interruption of service, while one or more faulty optical transmis-
sion lines are repaired or replaced. The major advantage of optoflu-
idic switches in such applications is a transparency over a wide
bandwidth and insensitivity to polarization.

8-1-1 Switches Based on Total Internal Reflection
Total internal reflection (TIR) off an otherwise transparent surface
was achieved by replacement of one fluid by another [22], bubble
generation [23] and manipulation [24–27], fluid-fluid interface con-
trol using electrowetting [28–31], thermocapillary effect [32], and
hydrodynamic spreading [33].
A concept of TIR by bubbles was commercially developed by
Agilent Technologies (Palo Alto, CA) [34,35]. Multiple waveguides
are created in the planar-lightwave circuits, intersecting at several
cross points (see Fig. 8-1a). At those cross points, the light travels
through a fluid whose refractive index is matched to the waveguide.
As a result, the optical mode travels unimpeded through the cross
point. When a bubble is inserted into the cross point, the light is
reflected into another waveguide. These bubbles can be formed and
removed hundreds of times per second, providing a fast and reli-
able switching function. The technology is similar to that used in
ink-jet printers, indicating that such bubble switches should be
mass-producible. Traditional bubble generation by resistive heaters
was recently replaced by more efficient laser-activated heaters [36].
Heat and fluid flow models provided insights into the behavior of
the bubble [27].
Another approach was based on oil latching interfacial tension
variation effect (OLIVE) [32]. The switch is based on thermal-capil-
lary effect to move trapped bubbles. The light path is switched when
the refractive-index-matching oil moves in the slit due to surface-tension
variation caused by heating (thermocapillarity). High extinction ratio
(>50 dB), low crosstalk (<−50 dB), and a response time below 10 ms were
achieved in 16 × 16 switch [37,38]. Surface wettability was shown to be
crucial for fast bubble manipulation [26].
A 2 × 2 TIR optical switch was demonstrated by Campbell et al.
and operated in free-space configuration [22]. The switch had an
insertion loss smaller than 1 dB and extinction ratio on the order of
20 dB. The device could switch between transmission (bypass) and
reflection (exchange) modes within less than 20 ms. The device,
shown schematically in Fig. 8-1c and 8-1d, has two distinct layers of
microchannels made in polydimethylsiloxane (PDMS). Channels of
one layer (the flow layer) are used to deliver the liquids into the
mirror channel. Channels of the second layer (the control layer) are
used to actuate the microvalves, enabling fully controlled manipu-
lation of the liquid in the mirror channel [39].