This paper designs an all fiber-optics disc accelerometer, which uses an online prepared weak fiber Bragg grating (WFBG) array to form a matching interferometer. By utilizing the linear relationship between acceleration and adjacent grating phase changes, 3 × 3 demodulation technology is used to demodulate the phase signal. Through finite element simulation, theoretical analysis is applied on sensitivity and resonance frequency of the accelerometer. Through the tests on vibration exciter, the frequency response range of the sensor array can reach 5Hz-1kHz. The experimental results confirm that the accelerometer features a wide frequency response and excellent linearity.
Using phase mask technology, the method of online preparation of wire-drawing tower grating (DTG) under single pulse laser was studied. the self-prepared DTG separately tested its breaking strength, the consistency of hundreds of identical gratings (center wavelength and reflectivity), and the temperature response of thousands of gratings in a single fiber. The results show that under the optimal process, the mechanical strength of DTG is not only consistent with the original fiber, but also relatively 6% higher than that of FBG; the isotropic grating at different positions, the center wavelength is stable within 80pm, and the reflectance difference is less than 3dB, and the temperature response consistency is very high. Therefore, the proposed single-pulse laser based on the phase mask to prepare the drawing tower grating can fully meet the requirements of the internet of things for large-capacity, fast response, long-distance, densely distributed fiber sensor network technology.
The continuous three-dimensional curved track measurement system (CTMS) based on vehicular strapdown inertial navigation system (SINS) and four odometers is firstly presented and validated in order to achieve rapidly the long span curve measurement for bridge health monitoring. The line-shape measurement is conducted on a viaduct. Experimental results show that the Z-axis and Y-axis error in comparison with total station for 180 m curved viaduct are 6.39 cm and 37.16 cm respectively. The system which will be further improved has potential applications for bridge health monitoring in the future.
A distributed acoustic sensing system (DAS) with low-coherence ASE and Michelson interferometer based on continuous width-band ultra-weak fiber Bragg grating (UW-FBG) array is proposed and experimentally demonstrated. The experimental result shows that the proposed system has better performance in detecting acoustic waves than the conventional hydrophone.
KEYWORDS: Spatial resolution, Signal detection, Temperature metrology, Signal to noise ratio, Fiber optics sensors, Analytical research, Scattering, Acoustics, Signal processing, Roads
A differential double-pulse pair Brillouin optical time-domain analysis (DDP-BOTDA) combined with the double-pulsed technique and the differential pulse-width pair technique is proposed and simulated to detect small temperature and strain changes. Using a symmetrical double-pulse pair, the system can detect small Brillouin shift with high spatial resolution and large dynamic range. Sub-meter spatial resolution is decided by the difference between the pulse-width and the peak and valley of the spectrum which is derived from differential pump depletion signals, and detectable Brillouin shift is less than 1 MHz.
Phase-shifted chirped fiber Bragg grating (PS-CFBG) with dual π phase shifts was studied theoretically and designed by
classic transfer matrix approach. The transmission characteristics of the PS-CFBG with two transmission peaks are only
directly dependent on the position of dual π phase shifts. The linewidth of two peaks increase with the chirp rate of the
grating, and is not related with the wavelength difference of the peaks. A dual PS-CFBG was fabricated by ultraviolet
point-by-point scanning with a ~2.4 nm/cm chirped phase mask. Phase shifts were introduced by controlling half period
movement of grating accurately through the piezoelectric ceramic (PZT). The wavelength spacing, extinction ratio, and
3dB linewidth of two transmission peaks are 8nm, 20 dB and 0.08 nm, respectively, which agree with the theoretical
design.
A quantitative method in the fine grinding stage on the base of self-made test system is presented and is investigated by
the experiments is presented in the paper. The contact mechanical equipment measures aspheric vector heights directly
by a long rail and high precision grating probe. After calibrating system error and preprocessing data, the surface shape
error and wave-front aberration coefficients of aspheric mirror are obtained by Zernike polynomials wave-front fitting
the collected discrete three-dimensional data. Comparison with Hartmann sensor test method through polishing the
aspheric mirror, the accuracy of wave-front error is in approximate agreement with each other, and the accuracy can
meet the requirement of lager aperture aspheric mirror in the fine grinding stage. By experiment investigation in the
process of manufacture and test, the system can guide machining in the fine grinding stage. The measurement system
shows the advantages of low costs, operating convenience, as well as high-speed calculation.
According to the third order aberration theory, the calculating formulae of original geometrical structural parameters of Dall compensator for null testing of large aperture aspherical surface and convex surface of lens are derived. The computer optimum design method is discussed, and some design examples are given.
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