In this study it is proposed the new measurement method of diameter for a microsphere based on whispering gallery mode (WGM) resonances. WGMs are the light propagation mode, which is occurred inside a spherical shape. Diameter is estimated based on measurement of a circumference that is equivalent to optical path length of WGMs, for which resonant wavelengths and the mode numbers are necessary to be determined. Resonant wavelengths are measured experimentally. In this paper, how to determine the mode numbers are mainly discussed. There are three kinds of modes; radial, azimuthal and angular mode. Resonant wavelengths are discrete number with these mode numbers. Free spectrum ranges (FSRs) of the resonant wavelengths include information about the mode, so focused on FSRs, the radial mode number and the azimuthal mode numbers can be determined. The determination methods were experimentally confirmed. Angular mode numbers are estimated using different two resonant wavelengths. Firstly, certain angular mode numbers are allocated to the resonant wavelengths, and then the diameter is estimated. For proper angular mode numbers, the estimated diameters are agreed with each other. After such mode analysis, the diameter measurement was performed. Variation of the measured diameters was within 30 nm, which validates a feasibility of the proposed method.
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