Page 277 - Optofluidics Fundamentals, Devices, and Applications

Optofluidic Dye Lasers 251

dye lasers are tuned by mechanically altering the feedback conditions,
for example, by adjusting the angle of incidence on a grating, which
acts as wavelength selective element in the laser resonator.
The free spectral range of the optical resonator determines the size
of the tuning range. If the FSR, or mode-spacing, is larger than the gain-
width of the dye, the same mode of the optical resonator can ideally be
frequency tuned across the full gain region of the laser dye without
other modes entering the gain region. If the FSR is smaller than the
width of the gain region, two or more optical resonator modes overlap
with the gain region, and the laser oscillations will be able to jump
from one mode to another. This is called mode hopping, and will result
in abrupt jumps in lasing frequency when the laser is frequency tuned.
Considering the optical feedback schemes and resonance conditions,
[Eqs. (10-3), (10-4), and (10-6)] the miniaturization of optofluidic dye
lasers becomes an advantage. For the optofluidic lasers discussed in
this chapter, the resonance frequency is controlled by the optical path-
length. The output frequency of the optofluidic dye lasers can therefore
be tuned in two ways: mechanically to alter geometric pathlength(s) or
by fluidics to modify refractive indices of fluids.
The microfluidic platform offers the ability to precisely control a
liquid medium and thereby control optical properties, such as refrac-
tive index and optical gain media concentration. These parameters
can be altered in real time through, for example, microfluidic mixing
of liquids in a precise and predictable fashion [13].
The refractive index of the liquid laser dye can be tuned by apply-
ing different solvents and mixtures thereof. Optofluidic dye lasers
have been operated with the laser dye rhodamine 6G dissolved in
different solvents, listed in Table 10-1. Continuous tuning of the
refractive index is obtained by applying mixtures of solvents. As an
example, mixtures of refractive index between 1.43 and 1.538 can be
obtained by mixing ethylene glycol and benzyl alcohol, where the

Solvent Refractive Index
Methanol 1.33
Ethanol 1.33
Ethylene glycol 1.43
DMSO 1.478
Benzyl alcohol 1.538

TABLE 10-1 Solvents Applied with Rhodamine 6G Laser
Dye for Microfluidic Refractive Index Tuning of Optofluidic
Dye Lasers