  We fabricated and theoretically investigated an add-drop filter using an
on-chip whispering gallery mode (WGM) microtoroid resonator with ultra-high
quality factor (Q) side coupled to two taper fibers, forming the bus and drop
waveguides. The new device design incorporates silica side walls close to the
microresonators which not only enable placing the coupling fibers on the same
plane with respect to the microtoroid resonator but also provides mechanical
stability, leading to an add-drop filter with high drop efficiency and improved
robustness to environmental perturbations. We show that this new device can be
thermally tuned to drop desired wavelengths from the bus without significantly
affecting the drop efficiency, which is around 57%
