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Investigation of a Micro-Optical Species Concentration Sensor Based on Whispering Galley Mode Resonators

2007; Linguagem: Inglês

10.2514/6.2007-1199

N.K. Das, Tindaro Ioppolo, M. V. Ötügen,

Mechanical and Optical Resonators

This paper investigates the feasibility of a micro -optical sensor for species concentration. A theoretical analysis was carried out to model the sensor performance followed by a series of experiments. The measurement concept is based on optical mode shifts of dielectric spheres with diameters of t he order of 100 ∝m. The optical modes, which are also called the “whispering gallery modes” (WGM), are excited by coupling light into the sphere using an optical fiber. A change in the index of refraction of the medium surrounding the sphere induces a shif t in the optical modes of the sphere. Therefore, by monitoring the WGM shifts, one can determine the concentration of a given species in the medium surrounding the sphere. In the theoretical analysis, we used a simple, heuristic approach to model the WGMs in the sphere that is based on geometric optics and the basic electromagnetic equations of wave reflection. The resulting equations describe the dependence of WGMs on the refraction index of the surrounding medium. These equations indicate that the WGMs ar e not sensitive enough to index of refraction changes in the case of gas media. However, they can be sufficiently sensitive for the development of micro -sensors in liquid media. Experiments were carried out to validate the theoretical analysis and to provi de a first order assessment of this sensor concept. The concentration of two types of salts, potassium carbonate and sodium bicarbonate were measured in water.