Page 65 - Optofluidics Fundamentals, Devices, and Applications

46 Cha pte r T h ree

Diffusion-Controlled Splitter
2
Diffusion-controlled splitter consists of two parallel L waveguides [11].
The rate of flow is sufficiently low to allow complete diffusive mixing of
the liquids as they reach the end of the channel. As a result, the two core
2
streams merge smoothly into a single L waveguide. Light propagates in a
direction opposite to that of the flow of liquids, that is, in the direction of
decreasing extent of diffusive mixing. This system has been demonstrated
to split a single input beam into two output beams with equal intensities.
Advantages and Disadvantages of L Waveguides
2
2
To conclude our discussion of these systems, L waveguides have two
main advantages:
1. They are dynamically reconfigurable. Their structure and
function depend on a continuous, laminar flow of the core
and cladding liquids, and can therefore be reconfigured and
adapted continuously in ways that are not possible with
solid-state waveguides.
2. They are simple to fabricate. The roughness of the wall of the
channel does not affect the smoothness of the laminar inter-
face between the core and the cladding streams, and does not
lead to the scattering of light or degradation in the perfor-
2
mance of waveguides. L waveguides can therefore be fabri-
cated easily and rapidly in organic polymers by using the
convenient techniques of rapid prototyping [12].
2
The L waveguides also have prominent disadvantages:
1. A constant supply of fluids is necessary to maintain the
waveguiding streams (a supply of 144 mL is necessary to run
one stream at 100 μL/min for 24 h).
2
2. L systems using water and PDMS are unable to guide light
in the infrared (λ = 1300–1600 nm) used in telecommunica-
tions applications because of large absorptive losses in both
the fluids and in the PDMS.
3. The speed of optical switching is ~ 0.1 Hz. This value is much
slower than switching in conventional planar waveguides
(~ 1–100 GHz). Nevertheless, the system should meet the
demands of applications that do not require fast switching,
such as optical sensing and bioassays.

2
3-5-2 L Lenses
Design
2
2
The design of the L lens is similar to that of the L waveguide: it is formed
by laminar flow of three streams of fluids; the index of refraction of the