Optical All-Pass Filter Realized by Self-Compensation of Loss

Abstract

Filters are key devices in optical systems and are usually applied to signal suppression or selection based on their amplitude frequency responses. Different from amplitude filtering, an all-pass filter (APF) is a unique type of filter that exhibits a uniform amplitude and variable phase in the frequency response. Traditionally, an APF is designed to be ideal and waveguide loss as well as coupling loss are omitted, which is impossible in practice. Here we demonstrate a method of realizing an APF even when waveguide loss and coupling loss exist, and a feasible structure for realizing the APF with waveguide loss and coupling loss is proposed. The method indicates that an APF can be realized by introducing self-compensation of loss to the structure of an ideal single-stage APF, and further, an APF based on silicon-on-insulator foundry is fabricated and demonstrated. The method of self-compensation of loss can also be used to realize APFs in the electrical domain. We anticipate that our work will inspire realizations of more APF-related devices that once seemed impossible in practice, as well as loss related devices or structures.