An embedded fiber optic sensor based on the photonic crystal fiber is proposed to measure the transverse stress in composited material for structural health monitoring. The sensing principle has been analyzed and validated by experiments. The 0.1%F.S high precision calibration device has been designed to indicated that linear correlation between the wavelength shift of Sagnac loop and pressure, and the good performances of sensor, such as the sensitivity coefficient is 0.1285nm/N, linearity is accessible to 1.02%F.S, maximum error is 3%F.S. Other experiments have been done by a material testing machine on the sensor, which have shown that the sensitivity coefficient is increasing with sensing length but immune with the shape of sensor. The sensor will be suitable for solving the problem of high precision online pressure monitoring in some complex structure health monitoring.
Pressure method using polarization-maintaining photonic crystal fiber (PM-PCF) as sensing element based on Sagnac interferometer is proposed to monitor inter layer pressure in especial compact structure. Sensing model is analyzed and test system is set up, which is validated by experiment. The birefringence can be modified by the deformation of PM-PCF under transverse pressure, realizing pressure measurement by detecting the wavelength shift of one specific valley from output of the Sagnac interferometer. The experiment results show that the output interference fringes were shifted linearly with pressure. The dynamic range of 0 kN ~10kN, sensing precision of 2.6%, and pressure sensitivity of 0.4414nm/kN are achieved, and the strain relaxation phenomenon of cushion can be observed obviously. The sensor has better engineering practicability and capability to restrain interference brought up by fluctuation of environment temperature, which temperature sensitivity is -11.8pm/°C.
A miniaturized fiber tip Fabry-Perot interferometer (FPI) with Pt/WO3 coating was demonstrated as a hydrogen sensor.
The air-cavity of FPI was formed by inserting a single mode fiber (SMF) into a partially polymer-filled glass capillary,
which was highly sensitive to temperature. A layer of Pt/WO3 was coated on the glass capillary of the FPI serving as a reaction heater upon hydrogen exposure. The heat locally raised the FPI temperature, which led to the wavelength shift of interference spectrum. The average wavelength shift of two interference dips was -20.3 nm upon exposure to ~20000 ppm hydrogen concentration. The hydrogen sensitivity of the sensor was estimated to be better than 1 pm/ppm.
A fiber optic relative-humidity sensor with a low-fineness Fabry-Perot cavity is proposed and demonstrated. Manufactured by e-beam evaporation, the Fabry–Perot cavity composed of five-layer dielectric oxides has columnar and porous structures, which is sensitive to relative-humidity change of environment. Experimental investigation shows that the proposed sensor exhibits a sensitivity of 0.42 nm/%RH when the ambient relative humidity changes from 15.2%RH to 74.3%RH.
A novel method was proposed to improve hydrogen sensitive performance of fiber Bragg grating (FBG) hydrogen sensor. The hydrogen performance of the sensor can be greatly improved by combing Pt/WO3 composite film annealed under 315°C for 1 h and temperature sensitive FBG. At room temperature of 25°C, FBG coated with Pt/WO3 composite film has 536 pm wavelength shift towards 10000 ppm hydrogen, and hydrogen detection threshold of FBG hydrogen sensor can reach to 200 ppm. The hydrogen performance of FBG hydrogen sensor was studied under different humidity, and ambient humidity has little effect on the hydrogen performance of FBG hydrogen sensor.
A Mach-Zehnder-like interferometer based on cascaded long period gratings with palladium silver (Pd-Ag) film coating
is developed for monitoring the hydrogen concentration. The visibility of fringes is measured using Fourier analysis. The
basic theory was given and preliminary experiment had been proved that this sensor can used to monitor the hydrogen
concentration. The sensor showed a good response.
The principle of an extrinsic Fabry-Perot interferometric (EFPI) optical fiber hydrogen sensor is introduced in the paper.
In order to carry out the mathematic model of the relationship between the change of Fabry-Perot gap length and
hydrogen concentration, the stress transfer model of the sensor is analyzed. Two channels temperature compensation
method is used to avoid the interference caused by the variation of the environmental temperature, so the stability of the
system is enhanced. The validity of theory has been proved by means of investigating the responsibility of the sensor in a
function of the hydrogen concentration, and the analytic data was close to the experiment results.The feasibility of the
sensor has been validated by the temperature experiment.
Theoretical and experimental aspects of the extrinsic Fabry-Perot(FP) interferometric(EFPI) optical fiber sensor are
studied. For a low-finesse EFPI sensor, the change of cavity length will influence the visibility of the interference fringes
reflected back into the fiber. In this paper, an analysis on the fringe visibility of EFPI sensors based on the power
distribution is presented. The effect that the cavity length and the source bandwidth play in the fringe visibility of EFPI
sensors interrogated with a broadband light source which makes the sensor immune to source fluctuations is explored.
The analysis may provide useful guidance for sensor design.
An extrinsic Fabry-Perot interferometric (EFPI) optical fiber hydrogen sensor based on palladium silver (Pd-Ag) film is
designed for hydrogen leakage detection. The sensing mechanism of such a sensor is based on the mechanical stress that
is induced in the Pd-Ag film when it absorbs hydrogen. The sensor system which is portable and suitable for field
detection is formed by a conventional coupler, a low-power LED operating at 850 nm, and a high resolution miniature
spectrometer. To obtain the absolute length of the EFPI air gap, a cross-correlation signal processing method is
introduced.The sensor is suitable for monitoring concentrations of hydrogen below the lower explosive limit.
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