Microrefractometer Based on Off-Center Polymer Caps Bonded Onto Optical Fiber Tips

In this study, we report on a microscopic refractometer built with off-center polymer spherical caps bonded on the end facet of a standard single-mode fiber (SMF). To achieve such microcaps, sub nanoliter amounts of UV-curable polymer were deposited onto the tip of SMFs that were cleaved with small angles. The working mechanism of our devices is explained as the interference of two Gaussian beams. High-contrast (visibility) interference patterns were achieved by optimizing the size of the polymer microcaps. The microrefractometers introduced here can operate over a broad wavelength range from 800 to 1600 nm approximately. The measuring refractive index range of our devices goes from 1 up to the index of the polymer. The refractive index sensitivity is high (~10^-4) over the whole measuring range. Temperature and refractive index cause two distinct effects on the interference patterns. Thus, our microrefractometers can be temperature self-compensated. Owing to the microscopic size of the refractometers, they can be ideal candidates to monitor refractive index in small spaces such as microfluidic channels. Other microsensors can be developed with suitable polymers or other materials that can be bonded on optical fiber tips.

I. Introduction

Refractive index (RI) is a unique characteristic of substances, thus, its measurement or monitoring is crucial in a myriad of industrial or scientific applications. For more than a century, the most successful technique to measure the RI of liquids consists of determining the critical angle in an internal reflection configuration [1]–[3]. To do so, sophisticated opto-mechanical or digital instruments are necessary. Such instruments (refractometers), typically use a fixed wavelength, involve a high-RI prism, and a system to heat or cool the sample.