Optical Lossy-mode-resonance Relative Humidity Sensor on a Fiber Tip

TL;DR

This paper presents a novel optical fiber-tip sensor based on lossy-mode resonance for real-time relative humidity measurement in harsh environments, offering high-temperature endurance, electromagnetic interference immunity, and a compact form factor.

Contribution

The development of a D-shaped fiber-tip LMR sensor with a probe-like design for RH sensing, demonstrating high accuracy, fast response, and suitability for challenging conditions.

Findings

Linear RH response from 6.1% to 75.0%

Resolution better than 4.0% RH

Comparable response time to commercial sensors

Abstract

Real-time measurement of relative humidity (RH) is important to many physical, chemical, and biological processes. However, in processes that involve harsh conditions such as high temperatures, strong electromagnetic interferences, and complex spatial constraints, conventional electrical sensors often fall short due to their intrinsic limitations. Here, we developed an optical lossy-mode-resonance (LMR) RH sensor based on the SnO2 film coated D-shaped fiber tip. Thanks to the high-temperature endurance and electromagnetic interference immunity, the developed optical fiber-tip sensor is ideal for RH sensing in critical environments, such as in the microwave drying process. Furthermore, unlike other reported in-line LMR sensors, our sensor is located at the D-shaped fiber tip with a probe-like form factor, allowing it to be readily implemented in a spatially confined environment. We have developed a custom fabrication setup for the novel D-shaped LMR fiber-tip sensor. The LMR signal from the sensor was experimentally characterized. The lossy mode resonances are understood by the finite element analysis, the results of which agree well with the experimental measurements. The fiber-tip sensor had a linear RH response between 6.1% to 75.0% and a resolution better than 4.0% RH. The fiber-tip sensor demonstrated a response time and reversibility comparable to that of commercial electrical sensors. The novel D-shaped fiber-tip LMR RH sensor developed in this work will benefit many applications that require RH monitoring in a harsh environment. Furthermore, our innovative D-shaped fiber tip design can be readily applied to LMR-based fiber sensors in general. This will allow the design's advantages of small footprint and agile maneuverability to benefit a wide range of LMR fiber sensing applications beyond RH sensing.