Biomedical subjects

Xiaoyi Bao

Publications and source records attributed to Xiaoyi Bao.

At least 19 recordsLinked to original sources

PubMed2006

Effect of Brillouin slow light on distributed Brillouin fiber sensors.

The effect of Brillouin slow light on distributed Brillouin fiber sensors (DBFSs) is studied. We demonstrate Brillouin slow light for a 1.2 ns pulse with peak powers (PS) from 3.3 to 56.2 mW on depletion of the pump power (PP) ranging from 1.3 to 83.2 mW in conventional optical fibers (SMF-28). Experiments show that, when pump power depletion is not negligible, for a given PP the Brillouin gain and delay time of a pulse decrease when PS increases in a long (> or =10 km) sensing fiber. The optimum pump beam depletion resulting from strong interaction of the pump and the probe in the fiber provides accurate temperature and strain information at a high spatial resolution. Our study reveals that at low PP the spatial resolution error caused by the pulse delay for a DBFS with centimeter spatial resolution is less than 5% of the pulse length.

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PubMed2006

Picosecond-pulse wavelength conversion based on cascaded second-harmonic generation-difference frequency generation in a periodically poled lithium niobate waveguide.

The wavelength conversion of picosecond optical pulses based on the cascaded second-harmonic generation-difference-frequency generation process in a MgO-doped periodically poled lithium niobate waveguide is studied both experimentally and theoretically. In the experiments, the picosecond pulses are generated from a 40 GHz mode-locked fiber laser and two tunable filters, with which the lasing wavelength can be tuned from 1530 to 1570 nm, and the pulse width can be tuned from 2 to 7 ps. New-frequency pulses, i.e., converted pulses, are generated when the picosecond pulse train and a cw wave interact in the waveguide. The conversion characteristics are systematically investigated when the pulsed and cw waves are alternatively taken as the pump at the quasi-phase-matching wavelength of the device. In particular, the conversion dependences on input pulse width, average power, and pump wavelength are examined quantitatively. Based on the temporal and spectral characteristics of wavelength conversion, a comprehensive analysis on conversion efficiency is presented. The simulation results are in good agreement with the measured data.

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PubMed2006

Development and performance comparison of two different approaches for stabilizing a harmonic mode-locked fiber laser at 40 GHz.

What we believe to be a new and simple approach was developed for stabilizing a harmonic mode-locked fiber laser at 40 GHz. It uses a computer to tune the modulation frequency in a 100 kHz band near 40 GHz to follow variations in the length of the optical cavity. A second approach was also developed to compare with the new approach and to draw conclusions on its performance. Results for the pulse characteristics, side-mode suppression ratio, and timing jitter show that both approaches provide an efficient way of stabilizing a harmonic FM mode-locked fiber laser.

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PubMed2006

Distributed Brillouin fiber sensor for detecting pipeline buckling in an energy pipe under internal pressure.

A distributed Brillouin fiber sensor has been employed to detect localized pipe-wall buckling in an energy pipe by measuring the longitudinal and hoop strain distributions along the outer surface of the pipe for the first time. The locations of the localized pipe-wall buckling are found and distinguished using their corresponding strain-load data. The formation of the buckling process for the compression and tension characters is studied in the longitudinal and hoop directions. For the pipe with internal pressure, concentric load, and bending load, a localized pipe-wall buckling takes place away from the middle of the pipe on the compressive side and a strain peak with an overall buckling occurs on the tensile side according to the longitudinal strain distributions along the pipe. Different strains on two neutral lines are also observed in the hoop strain distribution, which should be caused by the pipe weld joint.

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PubMed2006

Generating a high-extinction-ratio pulse from a phase-modulated optical signal with a dispersion-imbalanced nonlinear loop mirror.

We demonstrate a method for generating ultrashort pulses from a phase-modulated optical signal by using a dispersion-imbalanced nonlinear loop mirror instead of the traditional linear dispersion medium. The extinction ratio of the pulses is greatly improved at the same time. By controlling the bandwidth of the phase-modulated signal and the dispersion map of the dispersion-imbalanced nonlinear loop mirror, we can control the pulse width from several picoseconds to hundreds of femtoseconds.

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PubMed2006

Influence of transient phonon relaxation on the Brillouin loss spectrum of nanosecond pulses.

For pump-probe stimulated Brillouin scattering with a probe pulse of a few nanoseconds duration and with a finite DC level, the acoustic wave relaxation time varies with the pump power and the DC level. For a pump power of 1-6 mW, the acoustic wave relaxation changes from approximately 9 to 90 ns for polarization-maintaining fiber at a temperature of -40 degrees C for a 2 ns pulse width. When the pulse DC ratio of the probe varies from 10 to 20 dB, the acoustic relaxation time changes from 24 to 45 ns for single-mode fiber at 25 degrees C. This induces a power-increment spectral feature in the detected AC pump signal in the Brillouin loss spectrum of two temperature or strain sections, where both spectral components appeared at positions far from those related to the natural phonon relaxation time (approximately 10 ns) equivalent length. The theoretical calculations confirm the prolonged phonon relaxation.

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PubMed2005

Enhancement of stimulated Brillouin scattering of higher-order acoustic modes in single-mode optical fiber.

Solving the elastic wave equation exactly for a GeO2-doped silica fiber with a steplike distribution of the longitudinal and shear velocities and density, we have obtained the dispersion, attenuation, and fields of the leaky acoustic modes supported by the fiber. We have developed a model for stimulated Brillouin scattering of these modes in a pump-probe configuration and provided their Brillouin gains and frequencies for an extended range of core sizes and GeO2 doping. Parameter ranges close to cutoff of the acoustic modes and pump depletion enhance the ratio of higher-order peaks to the main peak in the Brillouin spectrum and are suitable for simultaneous strain-temperature sensing.

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PubMed2005

Simple approach to determining the minimum measurable stress length and stress measurement accuracy in distributed Brillouin sensing.

A simple approach is proposed for quantifying the errors in measuring the Brillouin frequency shifts associated with stresses whose lengths are shorter than the pulse length. The smallest detectable Brillouin frequency shift is thus determined with respect to the size of the stressed sections and the frequency resolution. The lowest detectable frequency shift is found to be approximately 42% of the Brillouin gain natural linewidth. A worst-case iso-error curve that associates the minimum frequency shift to the length of the stressed region is derived. A minimum resolvable frequency shift and minimum detectable stress length are defined with an approach based on a Rayleigh equivalent criterion.

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PubMed2005

Simple method to identify the spatial location better than the pulse length with high strain accuracy.

The second-order partial derivative of the Stokes signal with respect to frequency and position shows a maximum or minimum at the boundary between two different strained sections. This idea is used to locate the boundary of different stress regions. Knowing the boundaries, we then fit the Brillouin spectrum at the middle between them to get the strain value. This allows a location accuracy of between 5 and 10 cm, which is shorter than the pulse length of 20 cm used in the experiment. The lowest detectable Brillouin frequency difference between two strain sections is 1.2 MHz.

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PubMed2005

Subpeaks in the Brillouin loss spectra of distributed fiber-optic sensors.

Subpeaks in the Brillouin loss spectra of distributed fiber-optic sensors were observed for what is believed to be the first time and studied. We discovered that the Fourier spectrum of the pulsed signal and the off-resonance oscillation both contributed to subpeaks. The off-resonance oscillation at frequency /v - vB/ is the oscillation in the Brillouin time domain when beat frequency v of the two counterpropagating laser beams does not match local Brillouin frequency vB. This study is important in differentiating the subpeaks from actual strain-temperature peaks.

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PubMed2005

Effect of optical phase on a distributed Brillouin sensor at centimeter spatial resolution.

Because of the power imbalance between the two arms of an interferometer in an electro-optic modulator (EOM), the output of the EOM is combined amplitude modulation (AM) and phase modulation (PM) for the probe signal consisting of the pulse and the dc component. Because of this PM, the Brillouin gain-loss spectrum becomes asymmetric. The central Brillouin frequency is shifted from that of an AM pulse. The maximum extinction ratio of the EOM is limited to approximately 29 dB for a power-splitting ratio of 51% to 49%. The asymmetric property induced by PM is not pulse shape dependent; for both Gaussian- and super-Gaussian-shaped pulses the Brillouin loss spectrum is symmetric for AM and asymmetric for combined AM and PM (power imbalance).

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PubMed2005

Brillouin spectral deconvolution method for centimeter spatial resolution and high-accuracy strain measurement in Brillouin sensors.

Combining a dc and a short pulse (approximately 1 ns) as the probe beam in the pump-probe configuration of Brillouin-based distributed sensors allows us to represent the Brillouin spectrum as a top Lorentzian-like portion and a bottom Gaussian-like portion. Because of the interaction of these two parts, the Lorentzian-like portion carries spatial information that can be extracted within centimeter spatial resolution. Using this information, we develop a spectrum deconvolution method, which considers the location correlation of the strain distribution, to find the number of Brillouin peaks and their frequencies in the top Lorentzian-like portion and hence achieve accurate strain information. An optimum level of dc to pulse power for the best signal and position detection capability is discussed.

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PubMed2005

Coherent probe-pump-based Brillouin sensor for centimeter-crack detection.

We provide a theoretical explanation for a coherent probe-pump-based Brillouin sensor system that achieves centimeter spatial resolution with high-frequency resolution. It was recently discovered that, when a combination of cw and pulsed light (the probe beam) interacts with a cw laser (the pump beam), centimeter spatial resolution with high-frequency resolution can be achieved even though the probe-pulse duration is 1.5 ns [Opt. Lett. 29, 1485 (2004)]. Our study reveals that the coherent portion inside the pulse length of these two interactions caused by the same phase is responsible for this behavior. It allows us to detect 1.5-cm outer-layer cracks on an optical ground-wire cable.

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PubMed2004

Strain dependence of Brillouin frequency, intensity, and bandwidth in polarization-maintaining fibers.

The strain dependence of the Brillouin gain-loss spectrum for PANDA, bow-tie, and tiger polarization-maintaining fibers has been studied in the range 0 to 50 degrees C. We found a linear relationship between the strain and the Brillouin frequency, intensity, and bandwidth for PANDA and bow-tie fibers. For PANDA fiber at 20-cm spatial resolution, Brillouin frequency gives 7-micro epsilon uncertainty, which is the highest accuracy among three parameters, whereas the Brillouin bandwidth gives 19-micro epsilon uncertainty.

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PubMed2004

Dependence of the brillouin frequency shift on strain and temperature in a photonic crystal fiber.

The dependence of the Brillouin frequency shift on strain in a photonic crystal fiber (PCF) was measured at a wavelength of 1320 nm for the first time to the authors' knowledge. Together with measurements of the dependence of the Brillouin frequency shift on temperature in the PCF, we demonstrate the feasibility of the highly precise simultaneous measurement of temperature and strain by use of the PCF in a distributed Brillouin sensing system with a spatial resolution of 15 cm.

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PubMed2004

Simultaneous strain and temperature measurements with polarization-maintaining fibers and their error analysis by use of a distributed Brillouin loss system.

Simultaneous temperature and strain measurement with a distributed Brillouin loss system is proposed by use of the parameters Brillouin frequency, power, and bandwidth, for PANDA, bow-tie, and tiger polarization-maintaining fibers for the first time to our knowledge. The expressions for simultaneous temperature and strain sensing and the maximum errors and rms values of temperature and strain measurements are derived with three combinations of the parameters: (1) power and Brillouin frequency, (2) bandwidth and Brillouin frequency, and (3) bandwidth and Brillouin power. Our experiments demonstrate that simultaneous temperature and strain sensing at 20-cm spatial resolution for Brillouin frequency combined with bandwidth the strain/temperature resolutions are 39 microepsilon/2 degrees C (PANDA), 126 microepsilon/3 degrees C (bow tie), and 598 microepsilon/16 degrees C (tiger); for the Brillouin frequency combined with power the strain/temperature resolutions are 153 microepsilon/8 degrees C (PANDA) and 237 microepsilon/4 degrees C (bow tie); and for the bandwidth combined with power the strain/temperature resolutions are 135 microepsilon/38 degrees C (PANDA) and 195 microepsilon/38 degrees C (bow tie).

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PubMed2004

Distributed brillouin scattering sensor for discrimination of wall-thinning defects in steel pipe under internal pressure.

A distributed Brillouin scattering sensor has been employed to identify several inner wall cutouts in an end-capped steel pipe by measuring the axial and hoop strain distributions along the outer surface of the pipe. The locations of structural indentations that constitute 50-60% of the inner pipe wall are found and distinguished by use of their corresponding strain-pressure data. These results are quantified in terms of the fiber orientation, defect size and depth, and behavior relative to those of unperturbed pipe sections.

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PubMed2004

Temperature dependence of Brillouin frequency, power, and bandwidth in panda, bow-tie, and tiger polarization-maintaining fibers.

We report a study of the temperature dependence of the Brillouin gain and loss for three different kinds of commercial polarization-maintaining fibers for the first time to our knowledge. The Brillouin frequency differences between the fast and slow axes are independent of the temperature, varying between 2.9 and 4.3 MHz. Using 2-ns pulses (equivalent to a spatial resolution of 20 cm), we find that the temperature coefficients for the relative Brillouin power at a wavelength of 1310 nm are 0.26%/degrees C (panda fiber), 0.23%/degrees C (bow-tie fiber), and 0.04%/degrees C (tiger fiber); the temperature coefficients for the Brillouin frequency are 1.37 MHz/degrees C (panda), 1.66 MHz/degrees C (tiger), and 2.30 MHz/degrees C (bow-tie). The temperature coefficients for the Brillouin gain bandwidth are 0.15 MHz/degrees C (panda), 0.20 MHz/degrees C (bow-tie), and 0.22 MHz/degrees C (tiger).

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