Intensity-referenced and temperature-independent high accuracy displacement measurement is demonstrated by using a
Gaussian-chirped tilted fiber Bragg grating (TFBG). A specially designed bending cantilever beam is used to provide a
displacement-induced Gaussian-strain-gradient and internal tilt angle modulation along the sensing TFBG. Compared
with a straight FBG, the peak intensity of the Bragg resonance in the TFBG shows an improved sensitivity to the
displacement but is immune from the spatially uniform temperature changes. A displacement resolution up to 1×10-3 mm
and thermal fluctuation less than 2% full-scale for a temperature range of 0~80°C are experimentally achieved.
Fiber grating based techniques have proven themselves as low cost, small size and low weight solutions for sensing
strain and/or temperature in various applications, including structural health monitoring of aircrafts, ships and other man-made
structures. However, normal fiber gratings are sensitive to both strain and temperature in a manner that is
impossible to distinguish from the sensor response. Methods devised to circumvent this problem rely on combinations of
gratings with different sensitivity to these two perturbations. Simultaneous measurements on two gratings then provide
the necessary information to decode strain and temperature values but this requires special grating configurations and
packaging to maximize the differential sensitivity. We will present experimental results of an alternative approach where
we use a single very weakly tilted fiber Bragg grating (TFBG), to achieve the same effect. The grating couples light from
the fundamental mode guided in the core to a large number of cladding modes, depending on the wavelength of
interrogation. We propose and demonstrate a novel configuration in which many high order cladding mode resonances
are removed by bonding the TFBG in a pre-bent state on a metal plate. After bonding, only a few low order mode
resonances are left and occupy less than 5 nm of bandwidth (thereby allowing multiplexing). These resonances all have
the same temperature sensitivity but very different behavior when the plate vibrates, bends or stretches statically.
Differential measurements of the resonance power levels and shifts then provide valuable information about the
mechanical state of the sensor.
Since their introduction in the mid seventies, a variety of fibre optic sensor configurations have been developed for the
measurement of strain, deformation, temperature, vibration, pressure, etc. Variation of these parameters alters the
refractive index and the geometric properties of the optical fibre, which in turn perturbs the intensity, phase, or
polarization of the light wave propagating in the waveguide. Only in the past decade that Bragg grating-based fibre
optic sensors emerged as the non-disputed champion in multiplexing and dual parameter sensing with increased potential
for smart structure applications. Stringent requirements for single point discrete or distributed simultaneous strain and
temperature measurements prompted this characterization study which has the objective of developing a detailed
understanding of grating characteristics and response under external stimuli. Collocated and serially placed gratings
were evaluated and tested for their effective sensitivity to strain and temperature and to coating materials variation such
as polyamide and acrylite. Experimental sensitivity results correlated well with theoretical estimation for strain in single
gratings. Whereas, significant wavelength differential is required for simultaneous temperature and strain measurement
if collocated gratings are used.
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