Effect of hygroscopic polymer-coatings on the performance of relative humidity sensor based on macro-bend single-mode fiber

Abstract

In this paper, we present an investigation on the sensing performance of a relative humidity (RH) sensor based on macro-bending structure-induced fiber Mach-Zehnder interferometer (MZI). The macro-bending structure was formed by bending a section of single-mode fiber (SMF) into a droplet-shape without any splicing. By properly adjusting the bending diameter of the droplet-shaped fiber section, interference has been achieved between the whispering gallery mode (WGM) and the core mode. Four sensing probes based ondifferent SMFs were fabricated and compared to investigate the effect of polymer coatings on the sensing properties towards RH variation ranges from 35% to 95%; namely, probe A composed of a bare SMF with its original protective polymer coating, probe B based on polymer jacket stripped SMF, probe C based on polyethylene oxide (PEO)-coated SMF, and probe D based on polyvinyl‐alcohol (PVA)-coatedSMF. The sensitivity increased after the deposition of PEO and PVA coating on the macro-bending structures. Through a comparative investigation, experimental results revealed that sensor D performed better than other probes and possessed an improved sensitivity of −0.6 nm/RH% and an ultrafast response time of 1.33 ms. To the best of our knowledge, this is the first time that two main roles of the original polymer jacket of the SMF and the deposited polymer coatings on the SMF have been investigated in fabricating and optimizing the RH sensing performance of a droplet-like interferometer.