Highly sensitive multi-frequency acoustic sensor based on a high-Q graded hollow-core optical microcavity

A highly sensitive multi-frequency acoustic sensor based on a high-quality-factor (Q-factor) graded hollow-core optical microcavity (GHCOM) is proposed and experimentally demonstrated. The GHCOM, fabricated by a pressure-controlled fusion-tapering method, exhibits an ultrahigh Q-factor exceeding 107. The graded geometry, with a high aspect ratio of >21, significantly enhances pressure sensitivity. Theoretical analysis reveals that reduced wall thickness of the GHCOM results in increased vibration amplitude and the eigenfrequency under identical acoustic excitation. Through optimized design, a GHCOM with an outer diameter of 138 μm and a wall thickness of 2 μm is fabricated. The measured frequency response spans from 0.2 to 19 kHz, covering nearly the entire audible frequency range. A signal-to-noise ratio (SNR) of 50 dB is achieved at 1 kHz, corresponding to a minimum detectable pressure of approximately 14.7 μPa/Hz1/2. Moreover, the sensor exhibits excellent multi-frequency detection capability, with SNRs of 51 and 55 dB for dual-frequency signals at 6 and 7 kHz, respectively, and 49, 50, and 46 dB for triple-frequency signals at 7, 8, and 9 kHz, respectively. Directional response measurements indicate SNRs exceeding 24 dB for acoustic signals incident from all angles, demonstrating the sensor's robust and comprehensive acoustic sensing performance.