Determination of the position of a localized heat source within a chirped fibre Bragg grating using a Fourier transform technique

Abstract

The ability to determine the centre position of a localized temperature change within a chirped fibre Bragg grating (CFBG) has been investigated as a function of grating strength. The intragrating sensor is based on the analysis of reflected power spectra arising from a CFBG. The technique uses a discrete Fourier transform (FFT) in which the measured spectrum of the CFBG due to a localized temperature change (heat source) was simulated using a FFT grating design model. The model operated on the reference spectrum and hypothesis temperature distributions, T(z), to generate a spectrum of a CFBG subjected to a hypothesis temperature disturbance. The simulated spectrum was fitted to the measured spectrum using a three-parameter automatic disturbance function fitting algorithm operating on position, width and amplitude of temperature change. RMS deviations to within 0.03 mm of applied values of position have been obtained.

Localized strain measurements using an integration method to process intensity reflection spectra from a chirped FBG

“…The occurrence of two dips and humps suggests that there are two regions of peak strain on either side of the notch centre. The modified peak reflectance is approaching 80%, which justifies the use of a grating with low reflectance for measurements involving large gradients [8]. Fig.…”

“…The sensor was annealed at 330 °C for 150 s to prevent subsequent shift in the spectra due to out-diffusion of H 2 , resulting in a maximum reflectance of 23%, and was calibrated for temperature and strain. Reflected spectra were normalized to 14.6 dB above an average measured reflection spectrum of an FC/PC connector, through a 3-dB, 1550-nm coupler [8].…”

“…Fig. 1(b) shows the schematic of the mechanical and optical arrangements, the latter being similar to that used previously [8][9][10]. The CFBG sensor was monitored in reflection using an Er 3+ broadband light source via a 3-dB 1550-nm coupler.…”

“…Chirped gratings enable non-monotonic field profiles and peak localised strain and temperature to be measured and also increase the spatial resolution of the measurement [2]. Prior work on determining intragrating temperature profiles using a hypothesis profile and employing transfer matrix or Fourier grating models require the shape of the temperature profile to be conjectured and programmed [7][8]. Recently, we applied an integration method to operate only on reflectance spectra from a chirped fiber Bragg grating (CFBG) without the requirement for prior knowledge of the shape of a hypothesis temperature profile [9].…”

Comparison of FBG wavelengths in the regions of 2/3 of the Bragg wavelength and the Bragg wavelength by piecewise irradiation of a chirped phase mask

“…Chirped FBGs have been reported for applications in telecommunications and sensor technology, such as dispersion compensation [1] and to determine the position of a localized heat source using a Fourier transform technique of the responses from the reflection spectrum [2].…”