Page 89 - Optofluidics Fundamentals, Devices, and Applications

70 Cha pte r F o u r

the use of electrowetting for the realization of on-chip tunable opto-
fluidic filter. Electrowetting is described in details in Chap. 19. Here,
we focus on a recent demonstration of an on-chip tunable MRR that
is actuated by electrowetting [38]. Tunability is achieved by control-
ling the wetting angle of a droplet that is partially covering an MRR
made of polymer waveguide. By applying a voltage to the droplet, its
wetting angle is modified, and the droplet covers larger area of the
MRR. This results in an increase in the effective refractive index of the
MRR waveguide, thus the resonant wavelength and the transmission
through the device can be modified. In addition to the tuning of the
resonant wavelength, the authors also demonstrated a significant
tuning of the extinction ratio by positioning the droplet on top of the
coupling region between the MRR and the bus waveguide, thus allow-
ing controlling the coupling coefficient of the device. Figure 4-5 shows

–35

Transmission (dB) –45

–55
Off
On – V = 285(V) RMS
1545 1546 1547 1548
Wavelength (nm)
(a)

Off On

(b) (c)
FIGURE 4-5 (a) Transmission spectrum of the device in the off (dashed curve) and the
on (solid curve) states. (b) and (c) Microscope images show the MRR and the droplet
in the off and the on states, respectively. (R. Shamai and U. Levy, “On chip tunable
micro ring resonator actuated by electrowetting,” Opt. Exp. 17, (2009), 1116–1125.)