Abstract
New dual temperature and strain sensor has been designed using eccentric second-order fiber Bragg gratings produced in standard single-mode optical fiber by point-by-point direct writing technique with tight focusing of 800 nm femtosecond laser pulses. With thin gold coating at the grating location, we experimentally show that such gratings exhibit a transmitted amplitude spectrum composed by the Bragg and cladding modes resonances that extend in a wide spectral range exceeding one octave. An overlapping of the first order and second order spectrum is then observed. High-order cladding modes belonging to the first order Bragg resonance coupling are close to the second order Bragg resonance, they show a negative axial strain sensitivity (−0.55 pm/με) compared to the Bragg resonance (1.20 pm/με) and the same temperature sensitivity (10.6 pm/°C). With this well conditioned system, temperature and strain can be determined independently with high sensitivity, in a wavelength range limited to a few nanometers.
Highlights
The use of femtosecond laser pulses has emerged as a competitive technology for Fiber Bragg Gratings (FBGs) inscription
To decouple between temperature and strain effects, we propose to use here the second order Bragg resonance (Peak 1 at λ1) and the nearest high order cladding mode resonance (Peak[2] at λ2) in the gold-coated eccentric second order FBG3
Eccentric FBGs were produced by point-by-point direct writing technique with femtosecond pulses
Summary
The production of eccentric FBGs is carried out with the direct PbP inscription technique and 800 nm femtosecond pulses delivered by Spitfire pro Spectra-Physics amplifier of 120 fs duration and 1 kHz repetition rate. For silica fiber neff,core = 1.4467, the cut-off wavelength in air occurs around 1345 nm for FBG1, at 1545 nm for FBG2 and at 1340.7 nm for the second order grating FBG3 These values are in good agreement with the reported experimental results (Fig. 2). The advantage of having these high order cladding modes spectrally close to the Bragg resonance can be exploited to study simultaneously their sensitivities to axial strain and temperature. Different families of cladding modes with different polarisation dependence compose the transmission amplitude spectrum of eccentric FBGs10 These families of modes may undergo different sensitivities as it was demonstrated in standard tilted FBGs for radially and azimuthally polarized modes[24]. We will study axial train and temperature sensitivities of this eccentric second-order grating
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