  Fully reconfigurable add-drop filters (ADFs) have important applications in
optical communication and information processing. Here we demonstrate a broadly
tunable add-drop filter based on a double-disk cavity optomechanical system,
side-coupled with a pair of tapered fiber waveguides. By varying the coupling
rates between the cavity and the two waveguides, we investigate the dependence
of the through (drop) efficiency on the coupling rates, which agrees well with
the theoretical results. By optimizing the cavity-waveguide coupling rates, a
drop efficiency of 89% and a transmission of 1.9% have been achieved.
Benefiting from the large optomechanical coupling coefficient of the
double-disk microcavity, a tuning range of 8 nm has been realized, which is
more than one free spectral range (FSR) of the cavity. This is realized by
changing the air gap of the double disk using a fiber tip, which is controlled
by a piezoelectrical nanostage, with a required voltage of 7 V. As a result,
both the through and drop signals can be resonant with any wavelength within
the transparent window of the cavity material, which indicates that the ADF is
fully reconfigurable.
