A Coated Spherical Microresonator for Measurement of Water Vapor Concentration at PPM Levels in Very Low Humidity Environments

Abstract

This paper presents a novel approach to measurements of low relative humidity (RH) values based on the whispering gallery modes (WGMs) effect in a small silica microsphere coated with a thin layer of Agarose hydrogel. The light from a narrow-linewidth tunable laser with a few GHz tuning range is launched into an adiabatic tapered fiber, which excites WGMs in the Agarose layer via evanescent coupling. Adsorption and desorption of water vapor by the Agarose layer in response to changes in ambient humidity lead to changes in the layer's refractive index, which in turn leads to a spectral shift of the WGM resonances. We experimentally demonstrate the WGMs spectral shift over a range of RH values from 1 %RH to 25 %RH. The proposed sensor displays linear response and is capable of measurement of low concentrations of water vapor (corresponding to about 11.7±0.32 ppm water molecules in air). In addition, the proposed sensor offers the advantages of a very small form factor and good repeatability. Detailed studies of the sensor stability, cross sensitivity to temperature, response times and hysteresis are also presented.

Whispering Gallery Mode Optical Microresonators: Structures and Sensing Applications

“…Optical microspheres are used extensively for temperature, humidity, and stress sensing. The working principle of microsphere resonators sensor is that the resonant wavelength or intensity of the output signal varies with the change in external environment .…”

“…The sensor has low hysteresis, good repeatability, and low sensitivity to temperature crossover . Then, the same group updated the experimental platform with a narrow bandwidth tunable laser and got the highest water vapor detection limit reported so far, 11.7 ± 0.32 ppm . On this basis, members of the same group used a single tapered fiber coupled with two functionalized microspheres on different surfaces, and simultaneously measured ammonia (NH 3 ) concentration and RH.…”

“…Due to the ultra-high sensitivity of the microsphere resonators, the devices have been widely used in the fields of physics, biology, and chemistry. [21] Physical Sensing: Optical microspheres are used extensively for temperature, [25][26][27][28][29][30][31] humidity, [32][33][34][35][36][37] and stress [38,39] sensing. The working principle of microsphere resonators sensor is that the resonant wavelength or intensity of the output signal varies with the change in external environment.…”

“…[34] Then, the same group updated the experimental platform with a narrow bandwidth tunable laser and got the highest water vapor detection limit reported so far, 11.7 AE 0.32 ppm. [35] On this basis, members of the same group used a single tapered fiber coupled with two functionalized microspheres on different surfaces, and simultaneously measured ammonia (NH 3 ) concentration and RH. The results of the experiments showed that the sensitivity of the sensor array to ammonia was 19.07 pm ppm À1 and to RH was 1.07 pm %RH À1 .…”

High-Sensitivity Whispering Gallery Mode Humidity Sensor Based on Glycerol Microdroplet Volumetric Expansion

“…Whispering gallery mode (WGM) microresonators offer both fast measurement and high sensitivity in a larger range of humidity, as well as electromagnetic immunity [ 8 ]. The high sensitivity of WGM sensors is a result of high quality ( Q ) factors and low mode volumes [ 9 ]. Q factor largely depends on the application of different structures of resonators, such as microspheres, microbottles, microtoroids, microrings, and microdisks.…”