# Long-Distance FBG Sensor Networks Multiplexed in Asymmetric Tree Topology

## TL;DR

This paper presents a new method for connecting fiber Bragg grating sensors over long distances using an asymmetric tree structure.

## Contribution

The novel asymmetric tree topology enables simultaneous long-distance and parallel sensor interrogation.

## Key findings

- FBGs were successfully interrogated over distances from 15 m to 80 km.
- Heterodyne detection improved sensitivity for weakened reflection signals.
- Time- and wavelength-division multiplexing enabled efficient sensor network operation.

## Abstract

This article reports on the interrogation of fiber Bragg grating (FBG)-based sensors that are multiplexed in an asymmetric tree topology. At each stage in the topology, FBGs are connected at one output port of a 50:50 coupler with fibers of different lengths. This asymmetric structure allows the simultaneous interrogation of long-distance and parallel sensor networks to be realized. Time- and wavelength-division multiplexing techniques are used to multiplex the FBGs. Using the heterodyne detection technique, high-sensitivity detection of reflection signals that have been weakened by losses induced by a round-trip transmission through the couplers and long-distance propagation is performed. Quasi-distributed FBGs are interrogated simultaneously, over distances ranging from 15 m to 80 km.