KEYWORDS: Signal to noise ratio, Spatial resolution, Fiber Bragg gratings, Data acquisition, Signal processing, Composites, Reflectometry, Signal detection, Optical fibers, Structural health monitoring
We demonstrate a distributed localization of an impact source in the composite cylinder using a home-made phase sensitive optical time domain reflectometer (Φ-OTDR). The system shows the advantages of simple configuration using semiconductor optical amplifier to improve the spectral response of the pump pulses, high spatial resolution and high signal-to-noise ratio (SNR). With a spatial resolution of 10 m, the SNR could be obtained as high as 6.5 dB. A composite cylinder is longitudinally divided in multiple sections of equal lengths by bending the sensing fibers around it at different locations, and the section of a source impact is localized using the time-delay of the signal detected by the sensing fibers.
KEYWORDS: Acoustics, Single mode fibers, Signal processing, Signal to noise ratio, Data acquisition, Signal detection, Reflectometry, Semiconductor lasers, Semiconductor optical amplifiers, Fiber Bragg gratings
We analyze the relations of parameters in moving average method to enhance the event detectability of phase sensitive optical time domain reflectometer (OTDR). If the external events have unique frequency of vibration, then the control parameters of moving average method should be optimized in order to detect these events efficiently. A phase sensitive OTDR was implemented by a pulsed light source, which is composed of a laser diode, a semiconductor optical amplifier, an erbium-doped fiber amplifier, a fiber Bragg grating filter, and a light receiving part, which has a photo-detector and high speed data acquisition system. The moving average method is operated with the control parameters: total number of raw traces, M, number of averaged traces, N, and step size of moving, n. The raw traces are obtained by the phase sensitive OTDR with sound signals generated by a speaker. Using these trace data, the relation of the control parameters is analyzed. In the result, if the event signal has one frequency, then the optimal values of N, n are existed to detect the event efficiently.
The wavelength swept laser (WSL) is a promising optical source in optical coherence
tomography, optical fiber sensor, and optical beat source generation. It is demonstrated by employing a
narrowband wavelength-scanning filter, such as a fast rotating polygonal-scanner-filter, a diffraction
grating on a galvo-scan mirror, or a fiber Fabry-Perot tunable filter (FFP-TF). In this manuscript, we
present our researches on the dynamic fiber-optic sensors. Two kinds of WSLs are used to demonstrate
the dynamic measurement in the fiber-optic sensors. One is the WSL using a polygon-scanner-based
wavelength filter and the other is the Fourier domain mode-locked (FDML) WSL using a FFP-TF. The
dynamic fiber Bragg grating (FBG) sensor interrogation up to 2 kHz by using the WSL with a polygonscanner-
based wavelength filter is reported. And by using the FDML WSL with a FFP-TF, we
demonstrate a resonance FBG sensor interrogation. As another application of the WSL, we successfully
measure a dynamic modulation frequency of the applied electric field using a nematic liquid crystal
Fabry-Perot etalon.
A dual-wavelength tunable fiber laser is demonstrated using two polymer waveguide Bragg gratings (PWBGs) as a wavelength tunable filter. The wavelength tuning in the laser could be achieved by changing the applied electrical power to the PWBG. The interval of dual wavelength can be tuned by 18.9 nm for the maximum applied electrical power of 100 mW. The corresponding frequency range is 2.375 THz. The variation of the lasing wavelength is linearly proportional to the applied electrical power. The side mode suppression ratio of the dual wavelength laser has more than 35 dB.
We report a performance comparison of fiber Bragg gratings (FBGs) sensor interrogation using two kinds of
wavelength swept lasers. In a wavelength swept laser with a fiber Fabry-Perot tunable filter (FFP-TF), the reflected
signal from the FBGs in temporal domain has a nonlinear response with respect to the sinusoidal modulation signal. The
output of the time delay for the strained FBGs sensor array in the wavelength swept laser with FFP-TF has different
slope coefficient for each FBGs, while the reflected output of the strained FBGs in the wavelength swept laser with
polygon scanning filter has same slope coefficient for each FBGs.
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