Agarose coated spherical micro resonator for humidity measurements

Abstract

Abstract:A new type of fiber optic relative humidity (RH) sensor based on an agarose coated silica microsphere resonator is proposed and experimentally demonstrated. Whispering gallery modes (WGMs) in the micro resonator are excited by evanescent coupling using a tapered fiber with ~3.3 µm waist diameter. A change in the relative humidity of the surrounding the resonator air induces changes in the refractive index (RI) and thickness of the Agarose coating layer. These changes in turn lead to a spectral shift of the WGM resonances, which can be related to the RH value after a suitable calibration. Studies of the repeatability, long-term stability, measurement accuracy and temperature dependence of the proposed sensor are carried out. The RH sensitivity of the proposed sensor depends on the concentration of the agarose gel which determines the initial thickness of the deposited coating layer. Studies of the micro-resonators with coating layers fabricated from gels with three different Agarose concentrations of 0.5%, 1.125% and 2.25 wt./vol.% showed that an increase in the initial thickness of the coating material results in an increase in sensitivity but also leads to a decrease of quality factor (Q) of the micro resonator. The highest sensitivity achieved in our experiments was 518 pm/%RH in the RH range from 30% to 70%. The proposed sensor offers the advantages of a very compact form factor, low hysteresis, good repeatability, and low cross sensitivity to temperature.

Recent Developments in Fiber Optics Humidity Sensors

“…A tapered optical fiber was used as the waveguide and a silica microsphere was used as the resonator structure [91,92]. The resonator was coated with SiO 2 nanoparticles by layer-by-layer nano-assembly.…”

“…This OFHS has a resolution of 0.003%RH [91] using the coated WGM microspheres at low RH (0%–12%RH). The other material tested using a similar structure was agarose, which provides a sensitivity of 518 pm/%RH [92]; the obtained results are shown in Figure 15.…”

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 .…”

“…In the range of 30–70%, the highest sensitivity was 518 pm %RH −1 . 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 .…”

“…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.…”