Chemical modified fiber Fabry–Pérot interferometer by silver mirror reaction for hot-wire anemometry

Abstract

This study reports the first chemically modified polymer air-gap fiber Fabry–Pérot interferometer (PAGFFPI) with a silver-coated film by silver mirror reaction (SMR) for achieving the hot-wire anemometry (HWA). The main sensing element, PAGFFPI based on a high thermal expansion polymer has a high temperature (T)-sensitivity; however, if it is simply fabricated with NOA65 polymer and fused silica the heating process with a commercial pumping laser diode (LD) can not be applied. Thus, a chemically modified PAGFFPI with Ag thin film based on SMR is proposed to significantly improve the absorption efficiency of laser heating energy and to obtain the steady-state high T which exceeds that of the surrounding. The PAGFFPI's interference wavelength fringes shift due to its T decrease when being cooled by the airflow. The cooling effect on heated PAGFFPI is largely determined by the airflow speed and it is converted into a wavelength shift in the interference fringes. According to experimental results, the chemically modified PAGFFPI by SMR can be successfully heated by LD with rise and fall times of 0.14 s and 0.98 s, respectively. The SMR with an Ag thickness of 270 nm can achieve a better airflow measurement than that with the 150 nm. Over the airflow range of 0–20 m/s, SMR-thickness of 150 and 270 nm achieve average sensitivities of 0.702 nm/(m/s) and 3.302 nm/(m/s), respectively.