In this paper, we propose a novel approach to measurements of low relative humidity (RH) levels based on the whispering gallery modes (WGMs) phenomenon in a silica microsphere resonator coated with Agarose. The spectral dips of the WGM resonances excited in the proposed sensor depend strongly on the changes in the refractive index (RI) of the coating material as well as the surrounding RI. A study of the humidity-induced RI changes in a thin Agarose layer, applied to the surface of a 162 μm-diameter silica microsphere was carried out by correlating the experimental results and numerical simulations performed using the perturbation theory. We experimentally demonstrate a linear sensing characteristic in a low-humidity range from 10% to 45% RH. The estimated quality factor of the micro-resonator is 2.82×10<sup>6</sup> and detection limit for the sensor is 0.057 %RH, corresponding to the RI resolution of 8.4×10<sup>-7</sup> RIU.
A new type of fiber optic sensor for the detection and quantification of ammonia (NH3) vapor levels is proposed and experimentally demonstrated. This sensor is based on a spherical silica micro resonator coated with porous silica gel. Whispering gallery modes (WGMs) in the micro resonator are excited by evanescent coupling to a tapered fiber with a 3.3 μm waist diameter. The optical properties of the porous silica layer change when it is exposed to ammonia vapor, leading to a spectral shift of the WGM resonant wavelengths. The sensitivity of the proposed sensor has been tested by exposing it to different low level concentrations of ammonia: 4 ppm, 8 ppm, 12 ppm and 30 ppm at a constant relative humidity (50% RH) and constant temperature (23°C). The detection limit is calculated from experimental results as 57 ppb of ammonia for a 282 μm diameter porous silica coated microsphere.
This paper experimentally demonstrates a method for geometrical profiling of thin microfiber tapers with small waist diameters ranging from 10 to 40 μm with submicron accuracy. The method is based on analysis of whispering gallery mode resonances excited in cylindrical fiber resonators as a result of evanescent coupling of light propagating through the fiber taper under test. Measurement results have been verified by optical and SEM microscopic studies. The proposed method can be applied in the fabrication of fiber micro tapers for accurate control of the taper profile and in sensing of biochemical species on the surface of the microfiber.
We studied the excitation of whispering gallery modes (WGMs) in a cylindrical microresonator formed by a short length of a multimode fiber. The light was evanescently coupled into the fiber from a micro-cylinder whose tilt angle was varied from perpendicular to parallel with respect to the fiber taper. Narrow WGM resonances are observed in the transmission spectrum of the fiber taper with a maximum Q factor of ~10<sup>5</sup>. Evolution of the transmission spectrum with the change of tilt angle involved changes in the number of WGMs in the spectrum for smaller tilts and the disappearance of WGM resonances at large tilt angles.