Fiber Bragg Grating Sensors for Temperature and Strain Monitoring: Simulation-Based Analysis of Quasi-Distributed Configurations

Abstract

Fiber Bragg Grating (FBG) sensor is one of the most mature wavelength encoded precision measurement transducers, which has the advantage of immunity to electromagnetic disturbance, millimetre-level sensing scale and the ability to mount many independent sensing nodes on a single fibre. In this paper, a simulation study of the FBG sensors and their quasi-distributed configuration has been performed in OptiSystem, in which the sensors are used for both temperature and axial strain monitoring. The first FBG is a uniform FBG with a grating length of 10 mm, and its Bragg wavelength, total output power, noise power and signal to noise ratio (SNR) are measured as the temperature and axial strain are sequentially applied in the range of 30 °C to 210 °C and the range of 1 με to 9 με, respectively. This model is then replicated in a four-element quasi-distributed network consisting of four FBGs positioned at 1545, 1550, 1555 and 1560 nm, and multiplexed and interrogated serially with wavelength division multiplexing (WDM) across three separate sensing locations, with a co-located temperature–strain grating pair included at the third sensing location to enable cross-sensitivity discrimination. The point sensor has a linear thermal response of ~14.2 pm/°C and a linear strain response of ~1.2 pm/με, which is in good agreement with the thermo-optic and elasto-optic models. The quasi-distributed network generates four completely separated spectral channels (no measurable inter-channel overlap) at a channel spacing of 5 nm, and ensures wavelength traceability at each node. The four gratings are represented on a three-span girder bridge to show how the simulated channel plan is translated into an instrumentation layout for structural health monitoring, and how the extracted sensitivities are compared with experimental and numerical results reported.