Measurement of absolute radius, refractive index and dispersion of a long cylinder

TL;DR

This paper presents a non-destructive optical method to accurately measure the radius, refractive index, and dispersion of long cylindrical fibers using resonance wavelengths of whispering gallery modes, applicable with standard equipment.

Contribution

A novel technique that determines absolute radius, refractive index, and dispersion of long cylinders solely from resonance wavelength measurements, enhancing precision and simplicity.

Findings

Achieved 2 nm accuracy in diameter measurement.

Obtained refractive index with an accuracy of 2×10⁻⁵.

Validated method on optical and borosilicate fibers.

Abstract

Long cylinders, such as optical fibers, are some of the most widely used photonic devices. The radius and refractive index of these fibers are therefore fundamentally important parameters in determining their performance. We have developed a method to determine the absolute radius, refractive index, and chromatic dispersion of a long cylinder using only the resonance wavelengths of the whispering gallery modes around its circumference for two different polarizations. Since this method only requires the measurement of resonance wavelengths, it is non-destructive and it can be performed using standard equipment. As a proof-of-concept, we demonstrate the method on a $125\mu m$ optical fiber and an $80\mu m$ borosilicate capillary fiber with thick walls, obtaining values for the diameter and the refractive index with an accuracy of $2 nm$ and $2\times 10^{-5}$, respectively.