An FBG Sensing System comprises three discrete sub-systems:
i) A network of fibre Bragg grating sensors or transducers embedded within or attached to the structure being monitored
ii) An FBG Interrogator, an optoelectronic unit which illuminates the sensor network and records the optical reflection returned from each FBG sensor
iii) A Processing Unit, often simply a PC, which takes recorded data from the interrogator and performs further data processing, user interfacing, data transmission and storage functions.
The fibre Bragg grating (FBG) is an optical sensor recorded within the core of a standard, single-mode optical fibre using spatially-varying patterns of intense UV laser light.
Short-wavelength UV photons have sufficient energy to break the highly stable silicon-oxygen bonds, damaging the structure of the fibre and increasing its refractive index slightly. A periodic spatial variation in the intensity of UV light, caused by the interference of two coherent beams or a mask placed over the fibre, gives rise to a corresponding periodic variation in the refractive index of the fibre. The grating formed at this modified region of fibre becomes as a wavelength selective mirror: light travelling down the fibre is partially reflected at each of the tiny index variations, but these reflections interfere destructively at most wavelengths and the light continues to propagate down the fibre uninterrupted. However, at one particular narrow range of wavelengths, constructive interference occurs and light is returned down the fibre.
Maximum reflectivity occurs at the so-called Bragg wavelength λΒ, given by:
λΒ=2neff Λ ...........(1)
where neff is the effective refractive index of the mode propagating in the fibre and Λ is the FBG period.
Equation (1) implies that the reflected wavelength λΒ is affected by any variation in the physical or mechanical properties of the grating region. For example, strain on the fibre alters Λ and neff, via the stress-optic effect. Similarly, changes in temperature lead to changes in neff via the thermo-optic effect and in an unconstrained fibre, Λ is influenced by thermal expansion or contraction. This situation is expressed in Equation 2, where the first term on the RHS gives the effect of strain on λΒ and the second describes the effect of temperature.
ΔλΒ = λΒ(1-ρα)Δε + λΒ(α+ξ)ΔT .......(2)
where ΔλΒ is the change in Bragg wavelength, ρα, α and ξ are respectively the photoelastic, thermal expansion and thermo-optic coefficients of the fibre, Δε is the change of strain and ΔT is the temperature change. For a typical grating written in a silica fibre and with λB ≈ 1550 nm, sensitivities to strain and temperature are approximately 1.2 pm/με and 10 pm/ºC respectively.
Importantly, the two terms of equation (2) are independent, meaning that the FBG can be used to make temperature measurements by isolating the fibre from strain, and temperature compensated strain measurements can be made with knowledge of the temperature, often conveniently derived from a second, strain-isolated FBG.