# Non-Adiabatically Tapered Optical Fiber Humidity Sensor with High Sensitivity and Temperature Compensation

## TL;DR

This paper presents a high-sensitivity optical fiber sensor for measuring humidity and temperature simultaneously with good stability and fast response.

## Contribution

A novel dual-parameter optical fiber sensor design with high humidity sensitivity and temperature compensation is introduced.

## Key findings

- The sensor achieves a humidity sensitivity of −286 pm/%RH without moisture-sensitive materials.
- The sensor demonstrates a response time of 0.26 s and good humidity stability.
- Cascading a fiber Bragg grating enables dual-parameter measurement by reducing crosstalk.

## Abstract

We demonstrate an all-fiber, high-sensitivity, dual-parameter sensor for humidity and temperature. The sensor consists of a symmetrical, non-adiabatic, tapered, single-mode optical fiber, operating at the wavelength near the dispersion turning point, and a cascaded fiber Bragg grating (FBG) for temperature compensation. At one end of the fiber’s tapered region, part of the fundamental mode is coupled to a higher-order mode, and vice versa at the other end. Under the circumstances that the two modes have the same group index, the transmission spectrum would show an interference fringe with uneven dips. In the tapered region of the sensor, some of the light transmits to the air, so it is sensitive to changes in the refractive index caused by the ambient humidity. In the absence of moisture-sensitive materials, the humidity sensitivity of our sensor sample can reach −286 pm/%RH. In order to address the temperature and humidity crosstalk and achieve a dual-parameter measurement, we cascaded a humidity-insensitive FBG. In addition, the sensor has a good humidity stability and a response time of 0.26 s, which shows its potential in fields such as medical respiratory dynamic monitoring.