Refractive-index-sensing radio-frequency comb with intracavity multi-mode interference fibre sensor

TL;DR

This paper introduces a novel fibre sensing method that uses an optical frequency comb to measure the refractive index of liquids with high precision by converting optical signals into radio-frequency signals for accurate measurement.

Contribution

It presents a new application of optical frequency combs in fibre sensing, enabling high-precision refractive index measurement through a coherent optical-radio frequency link.

Findings

Refractive index changes are encoded into radio-frequency combs.

Repetition frequency measurement allows precise refractive index determination.

The method demonstrates potential for advanced fibre sensing applications.

Abstract

Optical frequency combs have attracted attention as optical frequency rulers due to their tooth-like discrete spectra together with their inherent mode-locking nature and phase-locking control to a frequency standard. Based on this concept, their applications until now have been demonstrated in the fields of optical frequency metrology and optical distance metrology. However, if the utility of optical combs can be further expanded beyond their optical-frequency-ruler-based application by exploiting new aspects of optical combs, this will lead to new developments in optical metrology and instrumentation. Here, we report a fibre sensing application of optical combs based on a coherent frequency link between the optical and radio-frequency regions, enabling high-precision refractive index measurement of a liquid sample based on frequency measurement in radio-frequency region. Our technique encodes a refractive index change of a liquid sample into a radio-frequency comb by a combination of an intracavity multi-mode-interference fibre sensor and wavelength dispersion of a cavity fibre. Then, the change in refractive index is read out by measuring the repetition frequency of the radio-frequency comb with a frequency counter and a frequency standard. Use of an optical comb as a photonic radio-frequency converter will lead to the development of new applications in high-precision fibre sensing with the help of functional fibre sensors and precise radio-frequency measurement.