Ultrasonic sensing directivity of π-phase-shifted fiber Bragg grating hydrophone

Abstract

π-phase-shifted fiber Bragg grating with a short effective sensing length becomes one of research hotspots in ultrasonic sensing, because light undergoes strong localization centered on its phase shift position. To investigate the directional sensing characteristics of π-phase-shifted fiber Bragg grating as hydrophone, the theory of sound propagation in layered media is used to calculate the strain of fiber core, then the transfer matrix method based on the coupled-mode theory in optics is applied to calculate the shift of central wavelength in optical reflection spectrum. Results of strain and wavelength shift under obliquely incident ultrasonic from 1-10MHz are divided into A, B and C areas, and analyzed by numerical calculation and simulation calculation. Axial strain and elasto-optical strain alter grating period and effective refractive index by the mechanical effect and elasto-optical effect, respectively, which cause wavelength shift. In A area (frequency below 5MHz, incident angle below 15゜), axial strain nearly equals zero, thus elasto-optical effect plays a predominant role on wavelength shift. The maximal response occurs at vertical incidence, and then obviously declines with angle increasing. The maximum is essentially unchanged with grating length. In B and C areas (angle above 15゜), both mechanical effect and elasto-optical effect contribute to wavelength shift. In B area (frequency below 5MHz), the amplitude of strain is the largest in three areas. A peak of wavelength shift appears at the same angle of the peak of strain, where exists the interference of the guided wave in fiber and the direct ultrasonic wave form water. The peak amplitude of wavelength shift decreases when grating length increasing. In C area (frequency below 5MHz), the amplitude of strain is larger than that in A area, but the wavelength shift is smaller, which is correlated to its higher axial wave number. Comparing the results in three areas, it is clear that the wavelength shifts larger at lower frequency and at vertical incidence. Experiments of 3MHz and 5MHz were then performed with a π-phase-shifted fiber Bragg grating. The experiment agrees well with the theory. The research is important for practical application of π-phase-shifted fiber Bragg grating in ultrasonic sensing.

Study on the dynamic mechanical properties of winding‐block axial structure considering short circuit strain rate effect

“…Due to the short duration of the transformer short-circuit test, the temperature rise is only about 2-5 K, so the influence of temperature on the centre wavelength is ignored. The relationship between the central wavelength λ B of the FBG sensor and the strain ε can be described in Equation ( 20) [21]:…”

“…Due to the short duration of the transformer short‐circuit test, the temperature rise is only about 2–5 K, so the influence of temperature on the centre wavelength is ignored. The relationship between the central wavelength λ B of the FBG sensor and the strain ε can be described in Equation () [21]: $$$\frac{{\Delta }{\lambda }_{\mathrm{B}}}{{\lambda }_{\mathrm{B}}}=\left\{1-\frac{{n}_{\text{eff}}^{2}\left[{p}_{12}-{\mu }_{\mathrm{g}}\left({p}_{11}+{p}_{12}\right)\right]}{2}\right\}\varepsilon =\left(1-{P}_{\mathrm{e}}\right)\varepsilon $$$ where p 11 and p 12 are the elasticity coefficients, P e is the effective elasticity coefficient and μ g is the Poisson's ratio of the fibre.…”