High‐Resolution 2D Quasi‐Distributed Optical Sensing with On‐Chip Multiplexed FSR‐Free Nanobeam Cavity Array

Abstract

On‐chip free spectral range (FSR)‐free optical filter with a compact footprint is crucial for developing emerging sensing applications, as it allows for the optimal utilization of extensive optical bandwidth. Despite being an important area of technology, on‐chip optical sensing system has faced challenges in channel count, particularly due to the limited FSR, large footprint, and loss of the wavelength‐selective filter unit. To address these challenges, a scalable nanobeam cavity prototype with a length of ≈21 µm based on the asymmetric Bragg mirrors is presented. By engineering the band structure of the nanobeam cavity, the stopband can be flexibly tuned to achieve an FSR‐free spectral response within 350 nm. Two 25‐nanobeam‐cavity arrays are fabricated with a footprint of 215 × 120 µm2 and an average insertion loss of ≈3.2 dB over a broad wavelength range. To the best of the authors’ knowledge, this is the largest‐channel‐count multiplexed micro‐cavity array on a single waveguide, reported to date. As a proof‐of‐principle application, the 2D high‐spatial‐resolution temperature distribution sensing is experimentally demonstrated. This work provides new insight into the design of ultra‐compact FSR‐free filters and will give birth to numerous charming applications that make use of the broad bandwidth capabilities of optics while occupying minimal space.