We demonstrate for the first time to our knowledge a dynamic measurement for an electric field sensor using a nematic liquid crystal (NLC) Fabry-Perot etalon and a wavelength-swept laser. It is well known that the wavelengths of the transmitted peaks of the NLC Fabry-Perot etalon depend on the applied electric field. The change in the effective refractive index of the NLC is measured according to the applied static electric field. The effective refractive index decreases from 1.67 to 1.51 as the applied the electric field intensity is increased. In addition, we measure the frequency of the dynamic variation in the electric field using a high-speed wavelength-swept laser. By measuring the modulation frequency of the transmitted peaks in the temporal domain, the frequency of the amplitude-modulated electric field can be estimated. The frequencies of the measured dynamic variations show a close agreement with the amplitude modulation frequencies of the induced electric field.
We carried out experiments to detect impact locations on a composite plate using two types of composite plates, a composite flat plate with a constant thickness of 5 mm and a composite stiffened panel with stringers. Four multiplexed FBG sensors were attached to the bottom surface of the composite plates to acquire impact signals. The FBG sensor wavelength shift data were collected at a sampling frequency of 40 kHz using a high-speed FBG interrogator (SFI-710, Fiberpro Inc., Korea). The arrival times of the impact signals at each FBG sensor were obtained using a signal processing procedure. The arrival times were affected by noise level and signal-to-noise ratio. In order to overcome this weakness, signal processing techniques such as wavelet decomposition, normalization using each noise level and filtering with a moving average were adopted. To calculate the impact locations of the composite plate, a neural network algorithm was applied.
We report significant performance improvement of a mode-locked fiber laser gyroscope. A Fabry-Perot cavity GaAlAs laser diode whose front facet was antireflection coated was used as a gain medium. The rms noise equivalent rotation rate measured with a time-interval counter was improved to 0.06deg/ radicalh. The long-term drift of the gyroscope signal was reduced to 100deg/ radicalh after a polarizer was inserted into the laser cavity formed with a polarization-maintaining fiber.
It is highly desirable to increase the sampling rate of a fiber Bragg grating (FBG) interrogator in other to sense dynamic strains caused by impulsive acoustic wave. We have developed a wavelength interrogator featuring 100k samplings per second that consists of a solid-state spectrometer, a photodiode array and fully parallel read-out circuits. Central wavelengths on the reflected partial spectra corresponding to FBGs are calculated by the centroid method with the selected groups of the consecutive photodiodes at which each FBG spectrum is imaged. The centroid calculation is simple to be implemented in a field-programmable gate array (FPGA) and fast enough to capture impulsive strain waveforms in real time. Short-term noise on the interrogated wavelengths is estimated to be around 0.5 in terms of stain within the sampling bandwidth.
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