[Show abstract][Hide abstract] ABSTRACT: The effect of asymmetry caused by oblique line-shaped laser ablation on the generation of ultrasonic waves in metal, especially the effect of transverse component of the ablation force source on the ultrasonic waves is analyzed. Due to the oblique force source, the displacements of shear wave increase obviously by the enhanced shear force, the energy concentration area of longitudinal wave deflects to the small range centered on the incident direction while that of shear wave is approximately perpendicular to incident direction. In addition, surface wave enhances in the direction of transverse power flow. Furthermore, some ultrasonic characteristics under vortex laser ablation condition are inferred.
[Show abstract][Hide abstract] ABSTRACT: A magnetic field sensor based on a tilted fiber Bragg grating (TFBG) coated with magnetic fluid is proposed and demonstrated experimentally. The sensing element is made by injecting the magnetic fluid into a capillary tube which contains a TFBG. The resonant wavelengths of the cladding modes of TFBG shift by varying the magnetic field which is perpendicular to the axis of TFBG. The results indicate that the resonant wavelength shifts of the cladding modes show a nonlinear dependence on the magnetic field. As the magnetic field increases to 32 mT, the largest resonant wavelength shift reaches to 106 pm. Moreover, this sensor shows good repeatability when it is used for magnetic field sensing.
[Show abstract][Hide abstract] ABSTRACT: Wavelength-swept laser technique is an active demodulation method which integrates laser source and detecting circuit together to achieve compact size. The method also has the advantages such as large demodulation range, high accuracy, and comparatively high speed. In this paper, we present a FBG interrogation method based on wavelength-swept Laser, in which an erbium-doped fiber is used as gain medium and connected by a WDM to form a ring cavity, a fiber FP tunable filter is inserted in the loop for choosing the laser frequency and a gas absorption cell is adopted as a frequency reference. The laser wavelength is swept by driving the FP filter. If the laser wavelength matches with that of FBG sensors, there will be some strong reflection peak signals. Detecting such signals with the transmittance signal after the gas absorption cell synchronously and analyzing them, the center wavelengths of the FBG sensors are calculated out at last. Here, we discuss the data processing method based on the frequency reference, and experimentally study the swept laser characteristics. Finally, we adopt this interrogator to demodulate FBG stress sensors. The results show that, the demodulation range almost covers C+L band, the resolution and accuracy can reach about 1pm or less and 5pm respectively. So it is very suitable for most FBG measurements.
International Conference on Optics in Precision Engineering and Nanotechnology (icOPEN2013); 06/2013
[Show abstract][Hide abstract] ABSTRACT: Numerical models are established to investigate the influence of transparent coating hardness on the laser-generated thermoelastic force source and ultrasonic waves in coating-substrate systems by using finite element method. With the increase of coating hardness, the characteristic of longitudinal wave in substrate is more obvious due to the gradual increase of reactive force produced by coating constraint; the directivity patterns of longitudinal wave show that the energy concentration area transfers from bilateral area to the axial direction area gradually. Therefore, the directivity pattern can be regulated to obtain the better ultrasonic signals by coating different hardness materials. It is significant for further development of the experiment in composite evaluation and in extreme condition.
Journal of Applied Physics 01/2013; 113(2). · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Sound velocity is important to study the elastic behaviors of materials in high pressure physics and the accuracy acquisition of travel time for velocity calculation is very essential. Thermoelastic finite element models of ultrasonic displacement field induced by a subnanosecond pulsed laser in metal films with a diamond anvil cell are established and the processes of laser ultrasonic generation are analyzed in detail. By confirming the initial time, we improved the accuracy of calculating the longitudinal wave velocity in the aluminum and copper thin films. The formulas of delay time are obtained by a set of simulations, and the overall errors of modified longitudinal velocity are reduced by 10-60 times of the unmodified ones.
[Show abstract][Hide abstract] ABSTRACT: Thermoelastic finite element models are established to study the bulk ultrasonic waves of an aluminum film generated by ring-shaped laser illumination in a diamond anvil cell. By analyzing the amplitudes of bulk ultrasonic waves arrived at the rear surface of film in detail, it shows that there exists strong enhancement effects on the central axis of the ring due to the constructive interference among the waves created by different parts of the ring source. The displacement distributions along the central axis indicate that the focal depth of shear wave is mainly determined by its directivity induced by a point-like laser source in a DAC system while it is more complicated to determine the focal depth of longitudinal wave. In particular, through changing the ring radius, we quantitatively demonstrate that the signal amplitudes generated by a ring source are far greater than those generated by a point-like source.
[Show abstract][Hide abstract] ABSTRACT: Based on the thermoelastic theory, a numerical model of ultrasonic displacement field induced by a vertical incident pulsed laser in an aluminum film in a diamond anvil cell (DAC) is established by using the finite element method (FEM). After precisely calculating the transient temperature field distributions, the bulk ultrasonic waveforms on the rear surface of the film and the characteristics of ultrasonic displacement field with time are obtained. Then directivity patterns of laser-generated longitudinal and shear ultrasonic waves are analyzed in details. The numerical results indicate that the thermoelastic force source and the characteristics of ultrasonic directivity are strongly affected by the diamond window. The energy of longitudinal wave is concentrated near the laser incident direction, and the one of shear wave is concentrated between 30° and 60° that deflected from the laser incident direction to the excited source. These characteristics in DAC system are different from the results of free surface in thermoelastic effect, while are similar to the results of free surface in ablation effect.
Journal of Applied Physics 01/2012; 111(1). · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The general response of a 1-D position-sensitive detector (PSD) is derived by solving the Lucovsky equation in a fully general way. A general light source is divided into many sections in space domain and pulse elements in time domain. Each section is regarded as a plane-wave element and each pulse as a square-wave pulse. By virtue of the linear superposition of the Green function, the general response is regarded as the summation of respective responses to those elements. The response indicates that a PSD is a decaying photocurrent accumulator, both in space and time domains. Results discussed in this paper are quite in agreement with the facts and the conclusions from the previous papers. In addition, analysis on the alternating current (ac) response characteristics by virtue of the general response shows that there is a hysteretic oscillation in accord with the light intensity variation and that the response is insensitive to high-frequency ac light signals.
IEEE Transactions on Electron Devices 01/2012; 59(3):835-840. · 2.06 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: It is very important to monitor the lateral and axial strains of
drilling riser for evaluation its health in deepwater. An optical fiber
strain sensing system based on optical fiber Bragg gratings (FBGs) used
for monitoring the strain of drilling riser is presented. The optical
fiber strain sensors are made by embedding FBGs into thin columned fiber
reinforced polymer which protect FBGs from seawater corrosion. Four
optical fiber strain sensors are installed parallel to the riser axis
and arranged at 90° angles around the riser by a home-made metal
belt, at the same time, twelve resistance strain gauges are pasted near
the sensors around the drilling riser at 30° angles as reference
sensors. A scaled drilling riser about 1 meter long and 0.245m diameter
is pressed in the lateral and axial direction in the range of 0-400KN,
the experimental results show that the relative error between optical
fiber strain sensors and resistance strain gauges is less than 6%.
[Show abstract][Hide abstract] ABSTRACT: A kind of optical beam with controllable rotating intensity blades is generated by coaxially superposing two optical vortex beams with frequency difference (Δω), different topological charges (m1 and m2) and equal amplitude. It is shown theoretically that the number of the blades is determined by the subtraction of topological charges (m1−m2) and the angular velocity of the rotating pattern is equal to Δω/(m1−m2). In our experiment, the rotating beams were generated by two optical vortex beams with opposite topological charges, where the frequency difference is acquired by a rotating plate glass. The results are quite in accordance with the simulations.
[Show abstract][Hide abstract] ABSTRACT: Process of laser ultrasonic generation was simulated in detail for confirming the exact time and location ultrasonic produced. The particular generating information in thin-film could help to accurately analyze the ultrasonic characteristics, especially in high pressure physics. Numerical models based on the two-dimensional axis-symmetry was built in cylinder coordinate system and calculated by finite element method (FEM). The duration and penetration-depth of pulse laser were considered instead of point approximation. In addition, parameters of material were set as functions related with temperature. According to the results, strain energy accumulated below the incidence point, so that the maximum amplitude of ultrasonic wave appeared few distances inside of the sample. Non-negligible errors were caused by that distances in velocity calculation. Without amendment, the errors increased with the broadening of pulse width and decreased with the growth of propagation distance. Therefore, the time ultrasonic generated should be fixed a head of the time that laser peak arrived. Furthermore, wave shape were easy to distinguish but inaccurately while the sample were covered with a transparent window.
[Show abstract][Hide abstract] ABSTRACT: A kind of bare fiber Bragg grating (FBG) gas pressure sensor based on fiber loop ring-down is presented, in which there are no sensitizing means or assistant structure on the bare FBG. The pressure sensitivity is analyzed theoretically via discussing the relationship between the ring-down time and the gas pressure. By detecting the ring-down time at different gas pressure levels, the gas pressure characteristics of the bare FBG are investigated in the time domain. The experimental results demonstrate that there exists a linear dependence of the pressure applied to the bare FBG on the ring-down time. A high sensitivity of −0.384μs/MPa (being equivalent to −240pm/MPa) could be achieved, which is 79 times larger than that of the optical spectral analyzer interrogated. This kind of pressure sensor could be used for gas or liquid pressure measurement.
Optics and Lasers in Engineering 01/2010; 48(12):1262-1265. · 1.92 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A transverse pressure sensor with high-sensitivity based on a mechanically induced long-period fiber grating (MLPFG) and fiber loop ring-down technique is presented. When a MLPFG is spliced into a fiber loop, an extra loss is introduced, which leads to a decrease of the ring-down time. The results demonstrate that the difference between the reciprocals of the ring-down time with and without pressure increases exponentially with increasing the pressure in the range of 0–23.4MPa. This sensor shows good repeatability, and the least detectable pressure is only 0.0068MPa which is about 18 times less than detecting the output light intensity directly.
[Show abstract][Hide abstract] ABSTRACT: We present a theoretical and experimental study on sensitivity enhancement of a fiber-loop cavity ring-down pressure sensor. The cladding of the sensing fiber is etched in hydrofluoric acid solution to enhance its sensitivity. The experimental results demonstrate that the pressure applied on the sensing fiber is linearly proportional to the difference between the reciprocals of the ring-down time with and without pressure, and the relative sensitivity exponentially increases with decreasing the cladding diameter. When the sensing fiber is etched to 41.15 microm, its sensitivity is about 36 times that of nonetched fiber in the range of 0 to 32.5 MPa. The measured relative standard deviation of the ring-down time is about 0.15% and, correspondingly, the least detectable loss is about 0.00069 dB.
[Show abstract][Hide abstract] ABSTRACT: In digital speckle interferometry, subtracted fringe patterns are always influenced by inhomogeneous light that is reflected from the tested object and received by the CCD. In this paper, by analyzing speckle's statistic property, we propose a numerical processing method to correct this nonuniform light intensity distribution within adaptive windows. This method includes estimating light intensity distribution of the tested object, constructing an adaptive window for every pixel, and correcting the intensity in the adaptive windows. By applying this method to our experiment, we find it is valid for intensity correction without changing necessary phase information.
[Show abstract][Hide abstract] ABSTRACT: The diffraction and refraction of light beam in optical periodic structures can be determined by the photonic band-gap structures
of spatial frequency. In this paper, by employing the equation governing the nonlinear light propagations in photorefractive
crystals, we study the photonic band-gap structures, Bloch modes, and light transmission properties of optically induced planar
waveguide arrays. The relationship between the photonic band-gap structures and the light diffraction characteristics is discussed
in detail. Then the influence of the parameters of planar waveguide arrays on the band-gaps structures, Bloch modes, and linear
light transmissions is analyzed. It is revealed that the linear light transmission properties of waveguide arrays are tightly
related to the diffraction relationships determined by band-gap structures. And the Bloch modes corresponding to different
transmission bands can be excited by different excitation schemes. Both the increases of the intensity and the period of the
array writing beam will lead to the broadening of the forbidden gaps and the concentration of the energy of the Bloch modes
to the high-index regions. Furthermore, the broadening of the forbidden gaps will lead to separation and transition between
the Bloch modes of neighboring bands around the Bragg angle. Additionally, with the increase of the intensity of the array
writing beams, the influences from light intensity will tend to be steady due to the saturation of the photorefractive effect.
Science in China Series G Physics Mechanics and Astronomy 01/2009; 52(5):747-754. · 1.41 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Dense wavelength division multiplexing (DWDM) is an important technology for expanding the capacity of optical network. The optical component based on the superimposed Bragg gratings shows that it can be used as one of advantageous multichannel components because of its excellent angle and wavelength selectivities. An optimized method for recording multiple Bragg gratings for wavelength demultiplexing in optical telecommunication band is proposed to achieve gratings with equal diffraction efficiency. A structure of three layers with twenty four gratings is demonstrated in a LiNbO(3):Fe crystal by employing the optimized recording method. Then an initial wavelength demultiplexing experiment based on the formed gratings is carried out in optical telecommunication C-band. The results obtained by measuring and analyzing the transmitted spectra of the fabricated gratings show that the diffraction efficiencies of the gratings are uniform. It is suggested that this kind of multiple gratings can be used for increasing the number of the demultiplexed wavelengths in recording medium with unit volume for WDM.
[Show abstract][Hide abstract] ABSTRACT: We theoretically predict and experimentally observe the two-dimensional (2-D) bright solitons in a nonconventionally biased strontium barium niobate (SBN) crystal. A theory describing light propagating in an SBN crystal with a bias field along an arbitrary direction is formulated. Then the existence of 2-D bright solitons in such a crystal is numerically verified. By employing digital holography, the index changes induced by Gaussian beams in an SBN crystal under different biasing conditions are visualized. Finally, skewed elliptical solitons are experimentally demonstrated.
[Show abstract][Hide abstract] ABSTRACT: The anisotropic dependence of the formation of one-dimensional (1-D) spatial dark solitons on the orientation of intensity gradients in lithium niobate crystal is numerically specified. Based on this, we propose an approach to fabricate channel waveguides by employing 1-D spatial dark solitons. By exposure of two 1-D dark solitons with different orientations, channel waveguides can be created. The structures of the channel waveguides can be tuned by adjustment of the widths of the solitons and/or the angles between the two exposures. A square channel waveguide is experimentally demonstrated in an iron-doped lithium niobate crystal by exposure of two orthogonal 1-D dark solitons in sequence.
[Show abstract][Hide abstract] ABSTRACT: Light-induced waveguide arrays pave a way to implement massive parallel and adaptive interconnection, and provide a fertile ground for investigating self-localized states, better known as discrete solitons. In this paper, creating planar and channel waveguide arrays by illuminations of two- or four-beam interference fields are investigated both theoretically and experimentally in LiNbO3:Fe, SBN:Cr, and KNSBN:Ce crystals. The distributions of refractive index changes induced by multi-beam interference fields with different orientations in various photorefractive crystals are numerically simulated. Employing illuminations of two-beam interference fields, planar waveguide arrays are experimentally demonstrated in an open circuit LiNbO3:Fe crystal and biased SBN:Cr and KNSBN:Ce crystals. An approach for fabricating channel waveguide arrays by employing illuminations of two-beam interference fields is presented. By fabricating a square and a rectangular channel waveguide arrays in a LiNbO3:Fe crystal this approach is experimentally demonstrated. Creating channel waveguide arrays using four-beam interference fields with different orientations are also performed in the three kinds of crystals. The index profiles of the light-induced waveguide arrays are measured employing the interferometric method or the digital holography. The near field patterns, diffraction patterns and the guiding test results show that the waveguide arrays are successfully fabricated.