Micro-racetrack notch filters based on InGaAsP/InP high mesa optical waveguides

Experimental and theoretical investigations were carried out on the spectral responses of micro-racetrack notch filters based on InP/InGaAsP high mesa optical waveguides that were fabricated by utilizing electron beam lithography and inductively-coupled-plasma reactive ion etching (ICP-RIE) technique. The critical factors determining the performance of micro-racetrack resonator-coupled devices were identified to be the optical power coupling efficiency between the bus waveguide and the racetrack resonator and the round-trip loss of the racetrack resonator. These parameters were extracted from the measured spectral responses of three single resonator-coupled waveguide devices with different gap spacings between the bus waveguides and the racetrack resonators, which were 0.2, 0.35, and 0.5 mu m. These extracted parameters can be used to uniquely determine the pole and zero locations of the unit racetrack-coupled waveguide filter. The phase responses as well as the magnitude responses of optical filters can be calculated using the uniquely determined pole and zeros. The extracted parameters were used to calculate the spectral responses of a high-order racetrack-coupled device that was designed by cascading the three single racetrack-coupled devices having gap spacings of 0.2, 0.35, and 0.5 mu m. The calculated and measured spectral responses of the high-order filters were compared to verify the parameter extraction process. The measured spectral response of the filters matched very well with the theoretically calculated response using the extracted parameters from the first-order racetrack resonator-coupled devices.