Detection of air target based on multi-fractal analysis in a laser radar

A novel approach for detecting laser radar target based on multi-fraction dimension (MFD) of laser clutters has been proposed. The fractal dimension (FD) of laser clutter and the echoes from the plan are estimated using box-counting dimension algorithm. The intrinsic difference in the spectrum of FD Dq between them is extracted and used to detect targets. Experimental results show that the method based on MFD is more reliable than that on FD and can improve the accuracy of detection.     

锥形光纤模场直径极小值特征研究

模场直径是光纤波导基模的特征参量。单模光纤中的功率密度与模场直径成反比。随着光纤激光器单模输出功率的不断攀升,纤芯中的功率密度不断增加,过高的功率密度会使光纤波导产生光学损伤和热损伤。基于锥形光纤模场分布近似模型,研究发现不同参数的拉锥光纤模场直径最小值位置对应的归一化频点具有规律性。采用有限差分波束传播法（FD-BPM）对不同波长、不同数值孔径下纤芯直径和模场直径的对应关系进行模拟,结果表明:模场直径最小值位置在归一化频率1.8附近,与光纤参数和波长的选取没有直接关系。这个特征参数可为高功率激光在光纤中功率密度最大值位置的快速确定提供依据,也为光纤光学理论增加了新的内涵。     

Study of bending loss and mode field diameter in depressed inner core triple-clad single-mode optical fibers

In this paper, the bending loss and the mode field diameter (MFD) of the R-type depressed inner core triple clad single-mode optical fibers are investigated. The effects of the optical and geometrical parameters on the bending loss and the MFD are examined in these fibers. The simulation results indicate that with increasing of the core radius (a), which is desired from manufacturing point of view, the bending loss and MFD coefficients are decreased. Consequently, the large core radius can be used to optimize the bending loss in the foregoing fibers. In the meantime, simulation outcomes show that the Δ and Q have considerable impact on the bending loss in the RI and RII fibers, respectively. The MFD and bending loss is decreased with increasing of Δ, but the case is inversed for Q. Based on the presented simulations, it is found out that the bending loss strongly depends on the distribution profile of the electric field in the cladding region for a given MFD. In other words, the field amplitude and damping rate in the cladding region determine the fiber bending loss.     

Analysis of the monotonicity conditions in the mimetic finite difference method for elliptic problems

The maximum principle is one of the most important properties of solutions of partial differential equations. Its numerical analog, the discrete maximum principle (DMP), is one of the most difficult properties to achieve in numerical methods, especially when the computational mesh is distorted to adapt and conform to the physical domain or the problem coefficients are highly heterogeneous and anisotropic. Violation of the DMP may lead to numerical instabilities such as oscillations and to unphysical solutions such as heat flow from a cold material to a hot one. In this work, we investigate sufficient conditions to ensure the monotonicity of the mimetic finite difference (MFD) method on two- and three-dimensional meshes. These conditions result in a set of general inequalities for the elements of the mass matrix of every mesh element. Efficient solutions are devised for meshes consisting of simplexes, parallelograms and parallelepipeds, and orthogonal locally refined elements as those used in the AMR methodology. On simplicial meshes, it turns out that the MFD method coincides with the mixed-hybrid finite element methods based on the low-order Raviart–Thomas vector space. Thus, in this case we recover the well-established conventional angle conditions of such approximations. Instead, in the other cases a suitable design of the MFD method allows us to formulate a monotone discretization for which the existence of a DMP can be theoretically proved. Moreover, on meshes of parallelograms we establish a connection with a similar monotonicity condition proposed for the Multi-Point Flux Approximation (MPFA) methods. Numerical experiments confirm the effectiveness of the considered monotonicity conditions.     

Mode field diameter preserving fiber tapers

An approach for preserving the mode field diameter (MFD) in fiber tapers is demonstrated. The approach utilizes concentric dual-core fibers, which couple light from an inner core to an outer core through a taper. Fibers with a 6 μm MFD feedthrough and a 15 μm polarization maintaining feedthrough are demonstrated experimentally. Simulations of the MFD in the tapered dual-core fibers are also presented.     

Induced losses in strained loose-tube fiber cables

Measurements of induced losses on primary coated single-mode fibers strained against the inner wall of loose tube cable materials are reported. Fibers have been chosen from current types such as match and depressed clad with mode field diameters (MFD) and cut-off wavelengths spanning the expected range of production tolerances. The loss dependence on MFD and cut-off is characterized, and the combination high MFD/low cut-off turns out to be the most bend sensitive parameter combination while the opposite case with low MFD/high cut-off gives the best performance. Different tube materials provide different bend loss performance depending on the microstructure of the tube and the amount and type of filling jelly.     

Induced losses in strained loose-tube fiber cables

Measurements of induced losses on primary coated single-mode fibers strained against the inner wall of loose tube cable materials are reported. Fibers have been chosen from current types such as match and depressed clad with mode field diameters (MFD) and cut-off wavelengths spanning the expected range of production tolerances. The loss dependence on MFD and cut-off is characterized, and the combination high MFD/low cut-off turns out to be the most bend sensitive parameter combination while the opposite case with low MFD/high cut-off gives the best performance. Different tube materials provide different bend loss performance depending on the microstructure of the tube and the amount and type of filling jelly.     

Analysis of the Polarization Influence on the Erbium-Doped Fiber Laser Transient Behavior

Abstract

We studied the erbium-doped fiber laser transient behavior dependence on the pump polarization state or on the cavity polarization eigenvalues. Several series of measurements are shown in which the changes in the transient laser response can be appreciated as the pump polarization state, or the cavity polarization eigenvalues are modified by means of polarization controllers. Results are fitted to a model considering polarization hole burning. It is shown that including this effect is fundamental for a correct erbium-doped fiber laser transient model. On the other hand, the validity of the model's fitting parameters under pump or cavity polarization changes is analyzed.     

The mimetic finite difference method for the 3D magnetostatic field problems on polyhedral meshes

We extend the mimetic finite difference (MFD) method to the numerical treatment of magnetostatic fields problems in mixed div–curl form for the divergence-free magnetic vector potential. To accomplish this task, we introduce three sets of degrees of freedom that are attached to the vertices, the edges, and the faces of the mesh, and two discrete operators mimicking the curl and the gradient operator of the differential setting. Then, we present the construction of two suitable quadrature rules for the numerical discretization of the domain integrals of the div–curl variational formulation of the magnetostatic equations. This construction is based on an algebraic consistency condition that generalizes the usual construction of the inner products of the MFD method. We also discuss the linear algebraic form of the resulting MFD scheme, its practical implementation, and discuss existence and uniqueness of the numerical solution by generalizing the concept of logically rectangular or cubic meshes by Hyman and Shashkov to the case of unstructured polyhedral meshes. The accuracy of the method is illustrated by solving numerically a set of academic problems and a realistic engineering problem.     

Separation of intra- and extramyocellular lipid signals in proton MR spectra by determination of their magnetic field distribution

In skeletal musculature intramyocellular (IMCL) and extramyocellular lipids (EMCL) are stored in compartments of different geometry and experience different magnetic field strengths due to geometrical susceptibility effects. The effect is strong enough to---at least partly---separate IMCL and EMCL contributions in (1)H MR spectroscopy, despite IMCL and EMCL consisting of the same substances. The assessment of intramyocellular lipid stores in skeletal musculature by (1)H MR spectroscopy plays an important role for studying physiological and pathological aspects of lipid metabolism. Therefore, a method using mathematical tools of Fourier analysis is developed to obtain the magnetic field distribution (MFD) from the measured spectra by deconvolution. A reference lipid spectrum is required which was recorded in tibial yellow bone marrow. It is shown that the separation of IMCL contributions can be performed more precisely---compared to other methods---based on the MFD. Examples of deconvolution in model systems elucidate the principle. Applications of the proposed approach on in vivo examinations in m. soleus and m. tibialis anterior are presented. Fitting the IMCL part of the MFD by a Gaussian lineshape with a linewidth kept fixed with respect to the linewidth of creatine and with the assumption of a smooth but not necessarily symmetrical shape for the EMCL part, the only free fit parameter, the amplitude of the IMCL part, is definite and subtraction leads to the EMCL part in the MFD. This procedure is especially justified for the soleus muscle showing a severely asymmetrical distribution which might lead to a marked overestimation of IMCL using common line fitting procedures.     

Decoding the electron dynamics in high-order harmonic generation from asymmetric molecular ions in elliptically polarized laser fields

The high-order harmonic generation from an asymmetric molecular ion is theoretically investigated based on the Born-Oppenheimer model with two-dimensional electron dynamics. It is shown that the harmonic intensity changes periodically in elliptically polarized laser fields. The periodical character is ellipticity-dependent. By establishing the physical image, the periodicity of the harmonic intensity can be ascribed to the contributions of the ground state and the excited state. Furthermore, the electron dynamics from different electronic states can be selected via combining the elliptically polarized laser field with a static electric field. The harmonics dominated either by ground state or excited state are emitted once in an optical cycle in the combined laser field.     

Mimetic finite difference method for the Stokes problem on polygonal meshes

Various approaches to extend finite element methods to non-traditional elements (general polygons, pyramids, polyhedra, etc.) have been developed over the last decade. The construction of basis functions for such elements is a challenging task and may require extensive geometrical analysis. The mimetic finite difference (MFD) method works on general polygonal meshes and has many similarities with low-order finite element methods. Both schemes try to preserve the fundamental properties of the underlying physical and mathematical models. The essential difference between the two schemes is that the MFD method uses only the surface representation of discrete unknowns to build the stiffness and mass matrices. Since no extension of basis functions inside the mesh elements is required, practical implementation of the MFD method is simple for polygonal meshes that may include degenerate and non-convex elements. In this article, we present a new MFD method for the Stokes problem on arbitrary polygonal meshes and analyze its stability. The method is developed for the general case of tensor coefficients, which allows us to apply it to a linear elasticity problem, as well. Numerical experiments show, for the velocity variable, second-order convergence in a discrete L² norm and first-order convergence in a discrete H¹ norm. For the pressure variable, first-order convergence is shown in the L² norm.     

光子晶体光纤模场直径增加方法

通过加热光子晶体光纤,其包层中空气孔由于表面张力的作用而塌缩减小。理论和实验结果表明,空气孔的塌缩在满足波导的渐变条件下,引入的能量损耗非常小。空气孔的塌缩减小,可以有效地增加光子晶体光纤的模场直径,从而不仅可以提高光耦合的效率和光纤端面的损伤阈值,而且可以降低与其它模场直径不匹配的普通光纤的熔接损耗。     

腔内偏振元件对激光特性的影响

Nd:YAG固体激光器的偏振特性在许多实际应用中非常重要.特别是在非线性光学频率变换中,无论是实现Ⅰ类相位匹配还是Ⅱ类相位匹配的非线性光学晶体,对基频光的偏振特性都有一定的要求.研究了半导体激光侧向泵浦固体Nd:YAG激光器的偏振特性.实验测量了腔内镀膜偏振片和它们在腔内不同位置时对激光偏振特性的影响.消光比测量表明,...     

Period-control and chaos-anti-control of a semiconductor laser using the twisted fiber

A novel semiconductor laser system is presented based on a twisted fiber.To study the period-control and chaos-anticontrol of the laser system,we design a type of optic path as a control setup using the combination of the twisted fiber and the polarization controller while we present a physical dynamics model of the delayed dual-feedback laser containing the twisted fiber effect.We give an analysis of the effect of the twisted fiber on the laser.We use the effects of the delayed phase and the rotation angle of the twisted fiber and the characteristics of the system to achieve control of the laser.The laser is deduced to a stable state,a double-periodic state,a period-6 state,a period-8 state,a period-9 state,a multi-period state,beat phenomenon,and so on.The periodic laser can be anti-controlled to chaos.Some chaos-anti-control area is found.The laser system is very useful for the study of chaos-control of the laser setup and the applications of some physics effects.     

Optimization of dye-laser output coupling by consideration of the spatial gain distribution

The problem of output-coupling optimization of a transversely or longitudinally pumped dye laser is treated in terms of a rate equation approach under steady state considerations. The theoretical study considers the spatial variation of the gain distribution and such typical features of a dye laser as reabsorption, triplet effects and excited state absorption. Numerical results obtained with realistic values of typical laser parameters (cross-sections for stimulated emission and reabsorption, number density of active molecules etc.) indicate that a higher degree of output coupling and consequently higher output powers are feasible upon a complete consideration of the spatial gain distribution     

Squeezing of an atom laser originating from atomic Bose-Einstein condensate interacting with light field

The quadrature squeezing properties of an atom laser originating from atomic Bose-Einstein condensate interacting with light field are studied. We find that the squeezing properties of the atom laser are dependent on the initial light field interacting with the atomic Bose-Einstein condensate. If the initial light field cannot be squeezed, such as number state or coherent state light field, the atom laser as well cannot be squeezed. However, if the initial light field interacting with the atomic Bose-Einstein condensate being in squeezed state, the atom laser can be squeezed periodically, and its squeezing depth is dependent on the intensity of interatoms interaction.     

Laser properties of a conjugate polymer (MEH-PPV) in the liquid-excimeric state

We have studied the amplified spontaneous-emission characteristics (ASE) of conjugated poly[2-methoxy-5-(2-ethylhexyloxy)1,4-phenylenevinylene] MEH-PPV in a few organic solutions. The ASE of MEH-PPV appears to arise from its excimeric state. This is perhaps the first report, to the best of our knowledge, on an excimeric-liquid state laser. MEH-PPV in solutions of benzene or tetrahydrofuran (THF) was pumped by the second and third harmonics of an Nd:YAG (532 and 355 nm) for different pump-pulse energies. The ASE of MEH-PPV was compared with the conventional laser dyes Rhodamine B (Rh B) and Rhodamine 6G (Rh 6G). The most important and distinguishing features are: (a) MEH-PPV has a four-times better photochemical stability than Rh B or Rh 6G; (b) the threshold and concentration for the laser action in MEH-PPV is far less than Rh B or Rh 6G.     

半导体激光器混沌光电延时负反馈控制方法研究

提出半导体激光器混沌光电延时反馈控制方法.通过附加一个光电延时电路控制系统,建立了三个有光电延时反馈条件下的注入激光混沌控制物理模型.进行了有控制时的最大Lyapunov指数分析.数字仿真发现,当调节延时时间和反馈光电流时,能控制激光混沌到3周期态、5周期态等;当周期键控光电流时,能控制激光混沌到8周期态、9周期态等;最后,通过组合应用光电延时反馈控制电路系统与附加周期调制驱动电流时,能有效地控制激光混沌到单周期态以及其他周期态. 关键词： 混沌 控制 激光器 光电转换