Tunable narrow linewidth all-buried heterostructure ring resonator filters using Vernier effects

Channel configurable optical filters are realized by using buried heterostructure semiconductor ring resonators. Two rings having slightly different radii are laterally coupled to bus waveguides in a cascaded manner, which affords free spectral range (FSR) expansion and channel configuration by Vernier effects. The effective FSR and spectral linewidth at resonance measured from a drop port are 10.2 and 0.017 nm, respectively, that corresponds to a finesse (F) of 600. By shifting the resonant wavelength of one of the resonators with free carrier injection, we demonstrate digital tuning filters where a distinct channel isolation of 15-20 dB is achieved with 0.68-nm spectral spacing.