An Ultra-Compact Rectangular Filter Featuring Optimized Local Edge Steepness

Abstract

Rectangular filters are widely employed in diverse fields, including communications, sensing, and healthcare. We propose and demonstrate an ultra-compact rectangular filter with high shape factor tunable local edge steepness. By utilizing a single microring resonator embedded with a slot, the slot-induced reflection of optical signals enables the rectangular filtering response. The filter occupies a core area of less than $900~\mu $ m2 and achieves a shape factor (SF, 10-dB bandwidth/3-dB bandwidth) of 1.59. By thermal tuning the heaters in the filter, the local edge steepness near the 10-dB, 15-dB, and 20-dB bandwidths becomes adjustable. Our filter combines advantages such as compact footprint, remarkable SF, and fabrication simplicity. Moreover, its controllable edge steepness offers unique potential for applications in mid-infrared gas absorption spectroscopy, medical imaging, and laser ranging.