# Single-Multi-Single Fibre Optic Structure Based Water Depth Sensor

## TL;DR

This paper introduces a simple, easy-to-fabricate fibre optic water depth sensor based on selective mode excitation in an SMS structure, achieving high extinction ratio and wavelength tuning through optimized core offset and fibre geometry.

## Contribution

It presents a novel SMS fibre optic sensor design with optimized mode excitation and wavelength tuning, supported by theoretical analysis and experimental validation.

## Key findings

- High extinction ratio achieved
- Wavelength shifting demonstrated by fibre geometry adjustment
- Excellent agreement between theory and experiment

## Abstract

We propose and demonstrate a technique easy to fabricate and measure water depth based on selective mode excitation using single-multi-single (SMS) mode structure. The high extinction ratio has been achieved by equally exciting preselected modes of the multi-mode fibre (MMF) by optimizing the core offset at both the input/output splices of the SMS structure, and a wavelength shifting has been achieved by varying the ratio of major to minor axis of the MMF loop. A theoretical analysis of the observed behaviour is also presented showing an excellent agreement between the theoretical and experimental results.