Optical All-Pass Filter in Silicon-on-Insulator

Optical all-pass filters, which exhibit constant amplitude response and pure phase variation, are extremely desired when only optical phase manipulation is required. However, many reported all-pass filters are accompanied by amplitude variations, which could degrade their performance. In this study, an all-pass filter is designed with constant amplitude and pure phase variation based on the analysis of the microring resonator response in the complex plane. The all-pass filter makes use of optical interference between the outputs of an all-pass microring resonator and a straight waveguide. Based on a silicon-on-insulator wafer, the all-pass filter is fabricated and its application to a microwave photonic phase shifter is also investigated. The all-pass filter exhibits a maximal magnitude variation of approximately 1.2 dB and a phase shift of 1.97π and can be continuously and widely tuned. This work offers a new approach to manipulate pure optical phases to improve the performance of optical devices and systems.