Browsing by Subject "laser"
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Item Applications of TAP-NDE technique to non-contact ultrasonic inspection in tubulars(Texas A&M University, 2005-02-17) Baltazar-Lopez, Martin EduardoThe possibility and feasibility of experimental detection of localized defects in tubes using laser-induced ultrasonic wave approach through Thermo Acousto Photonic Non Destructive Evaluation (TAP-NDE) and Signal processing through wavelet transform is examined in this research. Guided waves in cylindrical surfaces provide solutions for detection of different defects in the material. Several experiments were conducted to this respect. Wave propagation in both axial and circumferential directions was studied. The dispersive wave propagation of ultrasonic waves in hollow cylinders has been investigated experimentally, primarily for use in non-contact and nondestructive inspections of pipes and tubes. The laser ultrasonic waves propagated in cylindrical waveguides are particularly attractive because of their unique characteristics in the applications of nondestructive evaluation (NDE). Contrary to studies making use of only axially symmetric guided waves in hollow cylinders, here are analyzed also nonaxisymmetric waves. The analysis of data is made by using the Gabor wavelet transform. The capability of modeling the guided wave dispersion in hollow cylinders is used in developing guided wave experimental techniques for flaw detection. Good agreement was obtained when comparing the dispersion spectra between theory and experimentation. Measurement of group velocities of guided waves, which are obtained directly from the wavelet transform coefficients, can be used to determine allocation and sizing of flaws.Item Electronic resonance enhanced coherent anti-Stokes Raman scattering technique for detection of combustion species and biological molecules(Texas A&M University, 2006-10-30) Hanna, Sherif FayezThe application of electronic-resonance enhanced (ERE) coherent anti-Stokes Raman scattering (CARS) for the detection of nitric oxide (NO) and acetylene (C2H2) is experimentally demonstrated and the effects of various parameters on the ERE CARS signal investigated. In addition, the detection of dipicolinic acid (DPA) using ??????normal?????? CARS is demonstrated. For NO detection, the frequency difference between a visible Raman pump beam and Stokes beam is tuned to a vibrational Q-branch Raman resonance of the No molecule to create a Raman polarization in the medium. The second pump beam is tuned into resonance with the rotational transitions in the (1,0) band of the A2????+-X2???? electronic transition at 236 nm, and the CARS signal is thus resonant with transitions in the (0,0) band. A NO gas cell was used for the experiment to detect NO at various pressure levels. A significant resonant enhancement of the NO CARS signal was observed and good agreement between calculated and experimental data was obtained. For C2H2 detection, ERE CARS experiments were performed in a roomtemperature gas cell using mixtures of 5000 ppm C2H2 in N2. Visible pump and Stokes beams were used, with the frequency difference between the pump and Stokes tuned to the 1974 cm-1 ????2 Raman transition of C2H2. An ultraviolet probe beam with the wavelengths ranging from 232 nm to 242 nm is scattered from the induced Raman polarization to generate the ERE CARS signal. The effects of probe wavelength and pressure on signal generation are discussed. CARS was used to detect the 998 cm-1 vibrational Raman transition from a sample of polycrystalline DPA. The transition is the breathing ring vibration in the pyridine ring structure in the DPA molecule. The DPA 998 cm-1 transition is detected with excellent signal-to-noise ratio and the full-width-at-half-maximum is very narrow, approximately 4 cm-1.Item Fiber Fabry-Perot interferometer (FFPI) sensor using vertical cavity surface emitting laser (VCSEL)(Texas A&M University, 2006-10-30) Lee, Kyung-WooThis research represents the first effort to apply vertical cavity surface emitting lasers (VCSELs) to the monitoring of interferometric fiber optic sensors. Modulation of the drive current causes thermal tuning of the laser light frequency. Reflection of this frequency-modulated light from a fiber Fabry-Perot interferometer (FFPI) sensor produces fringe patterns which can be used to measure the optical path difference of the sensor. Spectral characteristics were measured for 850nm VCSELs to determine the combination of dc bias current, modulation current amplitude and modulation frequency for which single mode VCSEL operation and regular fringe patterns are achieved. The response characteristics of FFPI sensors were determined experimentally for square, triangular, saw-tooth waveforms at frequencies from 10kHz to 100kHz. The dependence of VCSEL frequency on the dc bias current was determined from spectral measurements to be ~165GHz/mA. An independent measurement of this quantity based on counting fringes from the FFPI sensor as the laser modulated was in good agreement with this value. The effect of optical feedback into the laser was also studied. By observing the fringe shift as the FFPI sensor was heated, a fractional change in optical length with temperature of 6.95 X 10-6/????C was determined in good agreement with previous measurements on a 1300nm single mode fiber. The performance of 850nm VCSEL/FFPI systems was compared with their counterparts using 1300nm distributed feedback (DFB) lasers. The results of these experiments show that the 850nm VCSEL/FFPI combination gives regular fringe patterns at much lower bias current and modulating current amplitudes than their 1300nm DFB/FFPI counterparts.Item Optical control of nuclear resonant absorption: theory and experiment(Texas A&M University, 2004-09-30) Kolesov, Roman L.Modification of nuclear resonant absorption by means of laser radiation is analyzed both theoretically and experimentally. Theoretical analysis is done on the basis of four-level model of atom. This model includes both electronic and nuclear excitations. It is predicted that under coherent laser driving nuclear resonant Mossbauer absorption can be significantly modified, e.g. new Mossbauer resonances can appear, some of the existing resonances can vanish, both can be Rabi-split, broadened by laser action. In addition, it is predicted that Mossbauer absorption can be completely suppressed due to coherent population trapping. Experimental observation of laser-induced transformations of Mossbauer spectra of 57Fe2+ : MgO is accomplished. New Mossbauer lines appear with laser driving while the existing are broadened. Possible explanations of the observed changes in 57Fe2+ : MgO Mossbauer spectra are population of higher-lying electronic states of iron ion and significant modification of electronic relaxation processes due to modified Jahn-Teller interaction.