In order to satisfy the sensing requirement for low-concentration or even trace detection using Terahertz (THz) spectroscopy, a new metamaterial sensor (MS) based on a double-opening elliptical ring array is designed by electromagnetic simulations. After optimization of the MS structure, the proposed MS has a strong resonant peak absorption peak around 2.853 THz and is entitled with high-Q and high-sensitivity simultaneously. The Q-value of the designed MS can reach to 385.0 and the sensitivity can reach to 371.5 GHz/RIU for a dielectric analyte with a thickness of 30 μm and it still can remain 40.0 GHz/RIU for a very thin analyte with a thickness of 1 μm. These results indicate that the designed MS has good sensing performance and can potentially be applied to high-sensitive detection of low-concentration or even trace samples.
This paper proposes the deconvolution image restoration algorithm, which uses the point spread function constructed by the physical characteristics of the terahertz beam to reconstruct the terahertz image and improve the image resolution[1]. At the same time, by constructing a point spread function with different penetration depths in the sample, the effect of image chromatogram on the sample can also be achieved. The integrated circuit (IC) electronic package terahertz imaging results clearly show the spatial position of the pins, internal chips and defects of the electronic packages, and analyze a variety of failure defect types, which are <1% more than the actual size.
Optical coherence tomography (OCT) with advantages of high resolution and high sensitivity is an emerging optical imaging technology that performs cross-sectional tomographic imaging of microstructure in biological system. In spite of the improvement of the longitudinal resolution by using low coherence light source, the dispersion effect aggravates with the increase of spectral bandwidth, which result in serious image blur and loss of image features. We demonstrate a double-path rapid scanning optical delay line (D-RSOD) structure in reference arm that produce the additional phase delay and group delay independent in OCT. The phase delay determined by grating defocus offset (GDO) of D-RSOD is controllable and the system dispersion is compensated by accurate adjustment of GDO in experiment. The full width at half maximum (FWHM) of the interference peak was measured to be 23.2μm and 15.2μm respectively before and after compensation. The experimental results indicate that the interference signal envelops shrink and the tomographic image clarify after dispersion compensation, D-RSOD is an effective means of dispersion compensation in OCT as well.
A simulated annealing method for material parameter extraction with terahertz time-domain spectroscopy is introduced to improve the common extraction method. The simulated annealing method seeks the global minimum of the error function to obtain material constants where the characterized material is not constrained by a certain boundary condition. It is shown from the research results that more accurate material parameters can be acquired to meet the actual requirements of material analysis by use of the simulated annealing method.
A theory is presented to show the analytic expression of ultra-short pulses existing in a non-instantaneous-response optical fiber with complex parameters. The analytical investigations show that the explicit solitary solutions can be found in form of Jacobi elliptic functions when the imaginary parts of the parameters fulfill a linear relationship. It is found that the single Jacobi elliptic function solutions have two free parameters while hybrid Jacobi elliptic function solutions have only one free parameter.
Because the Fox-Li diffraction integral iteration method is not suitable to calculate
transverse-mode fields of a resonator with big Fresnel number, the tansfer matrix method is used
for analysis of the eigen modes of an optical resonator. The process of this method is as follows:
from the Collins Formula, the diffraction integral equation of a resonator is obtained and
transformed to the finite-sum matrix equation. Finally, the transverse-mode distribution and loss
of the resonator can be calculated by use of the eigen function and eigen value of the matrix. In
this paper, the transfer matrix method is discussed, and the precision of the method is control. It
is shown from the simulated results of a general confocal resonator that numberical results by
use of the transfer matrix method are accord with that of the intergral iterative method, and the
convergence problem of the intergral iterative method can be overcome and the analysis of laser
resonators can be met by the transfer matrix mothod, as long as the calculation precision is
reasonably controlled.
To analyze mode fields in a seven-folded stable resonator with circular diaphragms, we introduce a new method combining the traditional self-reproducing theory with diffraction theory in the form of the ray matrix. Since the iterative method presented by Fox and Li is complex for the seven-folded resonator with diaphragms, in this paper, by means of Collins's formula in polar coordinates, diffraction integral equations expressed by ray matrices are converted to finite-sum matrix equations along diffraction interfaces. Moreover, diaphragms and reflective areas in the seven-folded resonator are simplified to apertures given by the pupil function. Finally, using the self-reproducing principle, we describe mode fields and their losses in the folded resonator as eigenvectors and eigenvalues of a transfer matrix. By calculating the eigenvectors and eigenvalues of the transfer matrix, we obtain eigenmode field distributions and their losses in the resonator. It is shown from simulation results that the seven-folded resonator with circular diaphragms can easily yield the fundamental mode, so it can output laser beams of good quality.
The experiment of laser heat treatment of workpieces with a sharp board milled to be 90°, 55°, 45° and 35° has been finished to investigate the influences of the sharp board on the temperature field and the heat-affected zone. The spot of laser has been sampled using the thermal paper in two planes of different height h1 and h2, and the power distribution of laser has been simulated numerically. A finite element model of numerical calculation is developed to simulate the transient three-dimensional temperature field, and the shade plot and isotherm can be output in any longitudinal and transverse sections and then the hardened zones are determined. The several cross sections of treated workpieces are chosen to make the metallographical analysis and the comparisons with the numerical results. It has proved that the numerical results are in a good agreement with the experimental data and the sharp boards have a great influence on the temperature and the heat-affected zone when laser approaches to the sharp board of workpiece.
In this paper, a finite element model has been developed to numerically calculate the temperature field and velocity field of fluid in laser material remelting process. The effects of fluid flow on the temperature field and the shape of laser pool are investigated carefully and presented graphically to display the subtle changes of temperature field. The numerical results show that (1) in general, there are two opposite vortices in the pool and one is greater than other due to the translation of workpiece; (2) the mainly driven force in the pool is the thermocapillary force compared with the nature convection; (3) the shapes of the molten pools, due to the convection of flow, become flat compared with that without the effect of fluid flow.
A general method to analyse an optical resonator is based on the equivalent resonator theory namely the equivalent confocal system theory, but it can not accurately calculate the field distributions ofthe steady-state modes. In this paper, according to the self-reproducing theory and the angular spectra theory for diffraction, the seven-folded stable resonator with circular diaphragms is numerically simulated, and by FFT (Fast Fourier Transform) laser mode-fields in the folded resonator are spatially traced to obtain the stable laser fields. The computed results indicate that the seven-folded resonator with circular diaphragms can easily obtain the fundamental mode, so it can output laser beams of good quality.
In terms of Collins' formuJa, the diffraction integral equations for a piano-concave resonator are presented and transformed to the finite-sum matrix equations. The mode-field distributions and losses of a common piano-concave resonator and that with a Gaussian-reflectivity mirror are calculated. It is shown that the Gaussian-reflectivity piano-concave resonator has good choosing capability for transverse modes, which can be employed in the transverse-flow high-power CO2 laser to improve quality of laser beam.
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