Page 87 - Optofluidics Fundamentals, Devices, and Applications

68 Cha pte r F o u r

In parallel to the absorption filters, various optofluidic interfer-
ence filters were recently demonstrated; some of them are also tunable.
For example, Mach et al. [30] demonstrated a tunable optofluidic
microstructured fiber. This device combined long-period Bragg grat-
ings and inner microchannels in the fiber. The tuning liquids con-
sisted of adjacent segments of low index (n = 1.28) and high index
(n = 1.73) immiscible microfluidic plugs. The liquids are pulled into
the fiber one after another and positioned such that the interface
between the liquids lies at the edge of the long-period Bragg grating.
By using independent control mechanism based on microheaters it is
possible to tune the transmission and the resonant wavelength inde-
pendently. With this approach a tuning range of about 12 nm and
attenuation of about 12 to 15 dB was demonstrated.
Another interference filtering scheme is based on the use of a dif-
fraction grating [31]. With such an approach, Domachuk et al. [31]
demonstrated an optofluidic on-chip spectrometer made by the inte-
gration of a diffraction grating with a microfluidic channel using soft
lithography in PDMS. The device was calibrated by couple of spectral
filters in different spectral regimes. Resolving power was estimated
to be ~330. The functionality of the integrated device was demon-
strated by performing a spectral analysis of chlorophyll probed using
supercontinuum light source. The measured absorption data show
reasonable agreement with previously reported absorption data.
Narrow-linewidth optical interference filters can be realized on a chip
by the use of integrated resonators. Specifically, the microring resonator is
of major importance for on-chip filtering applications. The MRR is very
popular for on-chip realization of optical filters because of its robustness,
flexibility, and the potential for dense integration of arrays of MRRs on
chip. A modified version of the MRR is the microtoroid resonator, demon-
strated by Armani et al. [32], with the advantage of ultrahigh Q factors. An
MRR can operate in notch filtering mode or in add/drop filtering mode,
depending on the number of bus waveguides coupled to the MRR.
Recently, Levy et al. [33] demonstrated an on-chip tunable optofluidic
notch filter by integrating a polymer MRR with a microfluidic channel
network. The work was motivated by the need to achieve fine-tuning of
an optical MRR. Tuning was obtained by dynamic variation of refractive
index of the medium surrounding its waveguides. A magnified image
showing a section of the fabricated device is shown in Fig. 4-4 (left).
The MRR was positioned at the bottom of a flow-through micro-
channel which is a part of a microfluidic chip. The liquid injected into the
microchannel constitutes the upper cladding of the MRR waveguides.
Variation of the refractive index of the liquid was achieved by on-chip
mixing of two source liquids with different indices of refraction. The liq-
uids injected into the inlets flow through a microchannel network of the
type introduced by Whitesides [34]. The network generates repeated
splitting and mixing, such that the concentration of the solute linearly
varies across the stream emerging from the network (along the dashed
line 1 in Fig 4-4 left). The stream further follows to a crossroad, where