Cylindrical optical microcavities: Basic properties and sensor applications

A detailed theoretical analysis of the basic properties of whispering-gallery modes (WGM) of thin capillaries is presented. The sensitivity of the resonances, as well as the Q factor is analyzed as a function of the structural parameters of the capillary and the refractive index inside the capillary. A practical application where thin capillaries are used for the measurement of glucose concentration in water is presented as a chemical sensor demonstrator. A best sensitivity of 1.24 nm/% of glucose concentration in water is reported, which corresponds to more than 850 nm/RIU. Additionally, different experimental approaches are discussed to improve the sensitivity and the detection limit that can be achieved using thin capillaries. Using thin and large diameter microcapillaries, and using a tapered fiber with the proper diameter for the excitation of the WGM, we estimate that refractive index changes smaller than 10−6 can be detected, maintaining an optimum compatibility with microfluidic systems.