High-sensitivity thermally stable acoustic fiber sensor

A new design of miniature fiber acoustic sensors with high sensitivity and greatly improved thermal stability and reproducibility of the operating wavelength is presented. It consists of a high-finesse Fabry-Perot (FP) made of a photonic crystal (PC) reflector fabricated on a compliant Si membrane and placed in close proximity to the reflective end of a fiber. An incident acoustic wave deflects the membrane, which shifts the FP reflection spectrum. This shift is detected as a change in the power reflected by the FP at a fixed wavelength. The reported improvements over an earlier design include (1) the use of photolithography to fabricate the PC diaphragm (higher accuracy and yield), (2) making the sensor chip out of silica instead of Si (higher thermal stability and reproducibility of the FP spectrum), and (3) using silicate bonding to assemble the sensor (higher thermal stability). An experimental fiber acoustic sensor with a finesse of ~6 is shown to measure pressures in air with a resolution from 180 to 27 μPa/VHz from ~700 Hz to ~8.6 kHz, and as low as 5.6 μPa/VHz at 12.5 kHz. This last value is ~4 times better than previously reported. The measured thermal stability is ~70 times better than that of the earlier Si-based design.