Whispering gallery mode micro resonators for multi-parameter sensing applications

Abstract

A novel fiber optic sensing configuration for simultaneously measuring ammonia vapor (NH 3) concentration and relative humidity (RH) in air is proposed and experimentally demonstrated. The system comprised two silica whispering gallery mode (WGM) microsphere resonators coated with different polymer layers. One of the microspheres was dip-coated with sol gel silica polymer and another with a 0.5 % wt./vol. agarose hydrogel. WGMs in both microspheres were excited simultaneously by evanescent coupling using a single adiabatic fiber taper. The optical properties of both coating layers change due to their exposure to ammonia and water molecules in the surrounding atmosphere, resulting in the spectral shifts of the WGM resonances relevant to each of the microspheres. By measuring the relevant WGMs' spectral shifts, the NH 3 concentration in air and the RH can be determined simultaneously. The experimentally demonstrated sensitivity of the proposed sensor array to ammonia was estimated as 19.07 pm/ppm (NH 3 molecules in air) and its sensitivity to relative humidity as 1.07 pm/% RH. Detailed studies of the coatings' cross-sensitivity and temperature dependence are also presented. The proposed sensor array is compact, highly sensitive and potentially low cost.

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 .…”

“…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 −1 and to RH was 1.07 pm %RH −1 . Most experiments are based on polymer coating.…”

“…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.…”

“…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 . [36] Most experiments are based on polymer coating. Although it has good sensitivity and selectivity, the repeatability and long-term stability of the sensor are not ideal.…”

Multi-Parameter Sensing in a Multimode Self-Interference Micro-Ring Resonator by Machine Learning

“…Therefore, the WGM sensor must address an open challenge to achieve the multi-parameter detection ability for practical applications [3,[34][35][36]. Although a micro-ring resonator biosensing array is exploited to realize multi-parameter biosensing using wavelength division multiplexing technology [37][38][39][40][41][42], its complexarray structure and detecting cost hinder its further development. Hence, it is critical for WGM sensors to exploit a universal multi-parameter detection mechanism with low detection complexity.…”