We present an intrinsic Fabry-Perot interferometric fiber sensor for high density quasi-distributed temperature and strain
measurement. The two internal partial reflection mirrors in such a sensor are formed by a pair of ultra-short fiber Bragg
gratings. We experimentally demonstrate the multiplexing of 56 such sensors in a single fiber using a frequency-division
multiplexing scheme. We show that theoretically as many as 500 sensors can be multiplexed. We demonstrated the experimental results of the multiplexed sensors for quasi-distributed temperature and strain measurement and the temperature compensation of strain sensors.
We present UV-induced intrinsic Fabry-Perot interferometric (IFPI) fiber sensors and a frequency-division-multiplexing
(FDM) scheme for quasi-distributed temperature and strain sensing. We present a spectrum-based measurement system
with a swept laser source to measure the fringe patterns of IFPI sensors serially arranged along a single fiber. The FDM
scheme is based on the multiplexing of sub-carrier frequencies generated by the frequency-modulation of a continuouswave
light source. IFPI sensors with different optical path differences (OPD) will have different sub-carrier frequencies.
We use band pass filters to select individual frequency component and use frequency-estimation based signal processing
algorithms to determine the OPD of each sensor. Experimental results for multiplexed temperature and strain sensing
are demonstrated. The performance of the multiplexing system is discussed.
We present the quasi-distributed temperature measurement results in a selective catalytic reduction unit of a power plant by using a frequency-division-multiplexing optical fiber measurement system with eight intrinsic Fabry-Perot interferometric fiber sensors along a single fiber. The sensor was constructed by splicing a section of multimode fiber between single mode fibers. A high resolution swept laser interrogator was used to measure the spectrogram of the reflected light from the sensors, which contains multiple frequency components in wave number domain corresponding to sensors with different cavity lengths. The temperatures were measured by estimating the optical path length of each Fabry-Perot interferometer. Field test results show that the proposed technology can potentially be used in applications of multi-point high temperature sensing.
In this paper, we present the experimental results of a research investigation for a hollow waveguide based fiber optic sensing device for simultaneous multiple gas detection. Gas molecules that exhibit characteristic vibrational absorption lines in the near-IR region that correspond to the transmission window for silica optical fiber have been detected by this device. An experimental setup was constructed, a fiber optic sensing device was designed and demonstrated which can monitor multiple gases simultaneously. The experimental results clearly demonstrate the characteristic absorptions in the optical spectra corresponding to the narrow molecular absorption lines of the gases tested that included acetylene and carbon monoxide.
In this paper, we present a novel point-wise laser writing method that utilizes a focused ultraviolet (UV) laser beam and metallic masks to write local Fresnel reflectors and intrinsic Fabry-Perot interferometric (IFPI) sensors in photosensitive fibers. These UV-induced IFPI sensors have features of low reflectance and low power loss and have the potential to be densely multiplexed. We also present a sweeping laser based measurement system that measures the interference spectra and estimates the optical path distances (OPD) of IFPI sensors. We also demonstrated IFPI sensors for temperature, strain and pressure measurement. Laboratory test results show that these UV-induced IFPI sensors can have a resolution of 0.1°C for temperature measurement and 0.5 micro-strain for strain measurement, and can be used in a temperature environment as high as 600°C.
Sapphire (single crystal alumina) has superior optical and mechanical properties. With a very high melting point of about 2050°C, sapphire fiber is an excellent candidate in optical fiber sensing area for high-temperature measurements. This paper presents a new type of sapphire-fiber-based extrinsic Fabry-Perot interferometric (EFPI) temperature sensor. The spectral interference pattern is generated by a sapphire diaphragm placed in front of the sapphire fiber. The sensing element is interrogated by a white-light source. Temperature is demodulated from the spectral change of interference pattern. Prototype sensor is tested at high temperature up to 1545°C. Both theoretical and experimental analysis are presented. Preliminary data shows the sensor is very promising for measuring ultra-high temperature.
A novel Intrinsic Fabry-Perot fiber-optic sensor is presented in this paper. The sensors were made through two simple steps: wet chemical etch and fusion splice. Micro air-gaps were generated inside the fibers and functioned as reflective mirrors. This procedure not only provides a simple and cost effective technology for fabricating intrinsic Fabry-Perot Interferometric (IFPI) fiber sensors, but also provides two possible IFPI structures. Both of the fiber cavity between the air-gaps or the air-gap and cleaved fiber end can be used as sensing elements. With these two structures, this sensor can be used to measure the temperature, strain, pressure, refractive index of chemicals and the thin film thickness by itself. Multi-point measurements can also be achieved by multiplexing. Furthermore, it also can be multiplexed with other sensors such as Long Period Gratings (LPG) to provide compensations for other perturbation sensing. Theoretical and experimental studies of two sensor structures are described. Experimental results show that high resolution and high sensitivity can be obtained with appropriate signal processing.
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