Microfiber evanescent-field photothermal gas detection using acoustic-induced mode-dependent frequency shift

Abstract In this study, we experimentally showcase the microfiber evanescent-field photothermal gas detection by exploiting all-fiber MHz-level frequency shift scheme. Based on acousto-optic interaction effect, low-frequency shifts of 0.9 MHz and 1.83 can be obtained through cyclic conversion between transverse core modes LP 01 11 in few-mode fiber. Our proposed shifters show flexible up(down) with superior sideband rejection ratio (over 40 dB) low insertion loss (less than 1 dB). Furthermore, an heterodyne interferometric system is implemented leveraging above shifters, which around 1-μm-diameter investigated for detection. A pump-probe configuration employed to obtain effect induced absorption modulated evanescent field. By demodulating phase beat signal output interferometer, equivalent limit (1 σ ) 32 ppm a response time 22 s are achieved ammonia, as well 0.24 % instability within 48 pump cycles. Given its compact high sensitivity fast response, experimental results pave way widespread applications like detection, fiber optical sensors, interplanetary coherent communications.