Browsing by Author "Tran, Tuan A."

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    Digital spectral analysis and adaptive processing techniques for phase modulated optical fiber sensors
    Tran, Tuan A. (Virginia Tech, 1996)
    The objective of this work is to investigate new signal processing techniques for optical fiber sensors that utilize the phase information of the electromagnetic field. Research concentrated on Fourier transform spectroscopy as a means for capturing wavelength encoded information from the fiber sensor. Classical spectral analysis utilizing the Fourier transform as a mathematical foundation for relating a time or space signal to its frequency-domain representation was shown to be inadequate for mitigating the bias errors caused by harmonic distortions. A modified spectral estimation algorithm is presented to overcome some of the practical issues while maintaining the high spectral resolution characteristic of the classical technique. This research also showed that unlike in free-space propagation, an optical signal propagating through a fiber waveguide, even over short distances, can experience significant phase modulation noise. A number of chromatic distortion mechanisms including modal interference, mode coupling due to periodic perturbations such as microdeformation and macrobends, and mode field diameter variations are addressed. We treated these issues by employing both theoretical simulation and experimental data. Coupled-mode formalism based upon approximated field solutions is used in the theoretical analysis. An extensive error analysis was also performed to determine how waveguide and noise distortion affect the performance of the spectral estimation algorithm.
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    Extrinsic Fabry-Perot interferometer for surface acoustic wave measurement
    Tran, Tuan A. (Virginia Tech, 1991)
    A surface acoustic wave sensor based on an extrinsic Fabry-Perot interferometer is described. A single-mode fiber, used as the input/output fiber, and a multimode fiber, used mainly as a reflector, form an air-gap that acts as a low-finesse Fabry-Perot cavity. The Fresnel reference reflection from the glass/air interface at the front of the air-gap interferes with the sensing reflection from the air/glass interface at the far end of the air-gap in the input/output fiber. Strains in the silica tube housing the two fibers change the air-gap length, thereby altering the phase difference between the reference and sensing reflections and modulating the output intensity. A theoretical analysis of the interaction between the strain induced by the acoustic fields and the fiber sensor is presented. Because signal drifting in interferometric sensors is common, a dual optical wavelength stabilization technique is also incorporated into the sensor to minimize the effect. Signal to noise ratios (SNR’s) on the order of 39 dB are obtained with a strain sensitivity of 4°/ μstrain cm⁻¹.
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    Microbend loss fiber optic direction and amplitude sensors for underwater applications
    Vengsarkar, Ashish M.; Murphy, Kent A.; Tran, Tuan A.; Claus, Richard O. (Acoustical Society of America, 1990-07-01)
    Dual purpose fiber optic microbend loss sensors have been developed for measurement of underwater acousticwave amplitudes and for detection of the direction of wave propagation. Three different construction schemes for cylindrical sensing elements are considered. The dual purpose hydrophones have been characterized for frequencies ranging from 15 to 75 kHz. They exhibit sensitivities in the range of -175 to -200 dB r e:1 V/uPa and directionality sensitivity limited by geometrical construction. 1990 Acoustical Society of America
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    Miniaturized Fiber-Optic Michelson-Type Interferometric Sensors
    Murphy, Kent A.; Miller, W. V.; Tran, Tuan A.; Vengsarkar, Ashish M.; Claus, Richard O. (Optical Society of America, 1991-03-01)
    We present a novel, miniaturized Michelson-type fiber-optic interferometric sensor that is relatively insensitive to temperature drifts. A fused-biconical tapered coupler is cleaved immediately after the coupled length and polished down to the region of the fused cladding, but short of the interaction region. The end of one core is selectively coated with a reflective surface and is used as the reference arm; the other core serves as the sensing arm. We report the detection of surface acoustic waves, microdisplacements, and magnetic fields. The sensor is shown to be highly stable in comparison to a classic homodyne, uncompensated Michelson interferometer, and signal-to-noise ratios of 65 dB have been obtained.