Giant refractometric sensitivity by combining extreme optical Vernier effect and modal interference

André D Gomes,Hartmut Bartelt,Manfred Rothhardt,Jens Kobelke,Jörg Bierlich,Jan Dellith,Orlando Frazão

doi:10.1038/s41598-020-76324-7

Abstract

The optical Vernier effect consists of overlapping responses of a sensing and a reference interferometer with slightly shifted interferometric frequencies. The beating modulation thus generated presents high magnified sensitivity and resolution compared to the sensing interferometer, if the two interferometers are slightly out of tune with each other. However, the outcome of such a condition is a large beating modulation, immeasurable by conventional detection systems due to practical limitations of the usable spectral range. We propose a method to surpass this limitation by using a few-mode sensing interferometer instead of a single-mode one. The overlap response of the different modes produces a measurable envelope, whilst preserving an extremely high magnification factor, an order of magnification higher than current state-of-the-art performances. Furthermore, we demonstrate the application of that method in the development of a giant sensitivity fibre refractometer with a sensitivity of around 500 µm/RIU (refractive index unit) and with a magnification factor over 850.

Highlights

The use of the Vernier effect has found great interest in interferometric sensing concepts, and especially in fibre Fabry–Perot elements, as a tool to considerably increase their sensitivity[1,2,3,4]
Two access holes milled near the splice regions of the sensing Fabry–Perot interferometers (FPIs) by means of a focused ion beam enable the cavity to be filled with an aqueous solution
A similar sensing structure was fabricated to generate the first harmonic of the Vernier effect, but with the single-mode sensing FPI previously analysed

Summary

Introduction

The use of the Vernier effect has found great interest in interferometric sensing concepts, and especially in fibre Fabry–Perot elements, as a tool to considerably increase their sensitivity[1,2,3,4]. On the other hand, detuning by a very small amount (as an extreme optical Vernier effect) may result in a beating modulation with long period, which may become undetectable for a limited spectral range available These contradictory requirements present a considerable challenge for the experimental implementation of such sensors with large magnification factors for sensitivity. The method of combining the two modes in the sensing interferometer provides magnification factors an order of magnitude beyond the expected limits for the standard Vernier effect technique We demonstrate such a result with a giant magnification factor by implementing a few-mode fibre-optic Fabry–Perot refractometer in combination with a single-mode Fabry–Perot reference interferometer, achieving a giant magnification factor of over 850 for the measurement of the liquid refractive index of aqueous solutions. The case of a single-mode and a few-mode sensing interferometer in combination with a reference interferometer for the Vernier effect will be analysed and discussed

Methods

Results

Conclusion