多模光纤作可饱和吸收体的锁模光纤激光器

本文报道了一种新兴的锁模方式-多模干涉锁模。这种锁模方式结构简单,搭建方便。在单模光纤激光器中熔接二段短的渐变折射率多模光纤,利用这种单模-多模-单模(SMS)结构的模式干涉效应实现可饱和吸收机制,从而实现锁模脉冲输出。SMS结构实现锁模需要对多模光纤的长度进行精确控制,本文提出将SMS结构缠绕进偏振控制器中,通过理论推导偏振控制器对多模光纤中传输光相位的调控,以实现可饱和吸收效应。在263 mW泵浦功率下实现了24.83 MHz重复频率的传统孤子脉冲输出,其脉冲间隔为40.12 ns,信噪比为50.8 dB,中心波长为1881.7 nm。通过调节偏振控制器和泵浦功率实现孤子分子与传统孤子脉冲的转换。在410 mW的泵浦阈值下实现了25 MHz重复频率的孤子分子脉冲输出,其脉冲间隔为40.3 ns,信噪比为54.4 dB,中心波长为1887.60 nm。     

Current passage in a vacuum diode with field-and laser-controlled ferroelectric cathode

We present the results of experimental investigation of a planar vacuum diode with field-and laser-controlled ferroelectric cathode based on prepolarized ceramics of the TsTS-23 (TsTS-19) grade. The diode current passes when the spontaneous polarization vector is oriented both toward the anode and in the opposite direction. It is established that an increase in the laser power density on the ceramic surface leads to a decrease in the diode current pulse duration. The maximum currents achieved in a field-controlled diode exceed the Langmuir current values by two to three orders of magnitude. The current pulse characteristics are determined by the mutual orientation of the polarization vector and the control electric field.     

Using a Mach-Zehnder-interference-based passive configuration to eliminate the polarization noise in Brillouin optical time domain reflectometry

We propose a passive polarization depolarizer to overcome the polarization noise in Brillouin optical time domain reflectometry (BOTDR). We also give a theoretical analysis of the depolarizer. The depolarizer is highly stable and has small insertion loss. It is based on a Mach-Zehnder interference scheme and mainly consists of two polarization beam splitters and a piece of delay fiber. Experimental results show that the polarization noise can be reduced by 96% by the depolarizer, and the Brillouin frequency shift induced by heat can be accurately detected by BOTDR with this depolarizer. The pulse width is 30 ns and the length of the sensing fiber is 24 km, which is the longest reported distance achieved with such a short pulse for BOTDR, to the best of our knowledge.     

Mode-locked diode-pumped self-frequency-doubling neodymium yttrium aluminum borate laser

As far as we know this is the first report on picosecond green-light generation from an actively mode-locked diode-pumped self-doubling neodymium yttrium aluminum borate (NYAB) laser. The pulse width is approximately 100 ps and is limited by imperfect antireflection coating and the intracavity doubling effect. The green average output power of this laser for mode-locked operation was more than 12 times higher than that for the continuous wave NYAB laser. The polarization state of the laser can be switched from ordinary to extraordinary rays by displacement of the pump spot. The polarization ratio can be better than 200 without any intracavity polarization element.     

Cavity-length optimization for high energy pulse generation in a long cavity passively mode-locked all-fiber ring laser

In order to achieve higher pulse energy in a passively mode-locked fiber ring laser, a long cavity length is commonly implemented. However, a long cavity operating in the anomalous dispersion regime also leads to pulse broadening, which reduces the average pulse power. In this paper, the trade-off between cavity length and average pulse power is investigated with the aim of optimizing the cavity length to achieve maximum pulse energy. Numerical simulation results, presented here, indicate that there exists an optimum cavity length for which the pulse energy is maximum and the optimum length shifts as the pump power changes. The simulation results for a pump power of 500 mW are verified by measurements carried out on a long cavity nonlinear polarization rotation mode-locked all-fiber ring laser operating in the anomalous dispersion regime. With a repetition rate of 266 kHz for the dissipative solitons, we achieve a pulse energy of 139.1 nJ for a cavity length of 700 m. Higher pulse energy can be expected by using a pump laser diode with higher pump power.     

High peak-power single-frequency pulses using multiple stage, large core phosphate fibers and preshaped pulses

We have developed a monolithic high power pulsed fiber laser in a master oscillator power amplifier (MOPA) configuration, which is capable of reaching 0.38 mJ pulse energy and 128 kW peak power for 3 ns pulses at ~1550 nm while maintaining transform-limited linewidth. The fiber laser pulse seed was achieved by directly modulating a CW single-frequency fiber laser using an electro-optic modulator. We used an arbitrary waveform generator to preshape the fiber laser pulses before amplification to avoid pulse steepening and dynamic gain saturation. Single-mode, polarization maintaining highly Er/Yb codoped large core phosphate fibers were used in the power amplifier stages to scale the transform-limited fiber laser pulses, avoiding any nonlinearities.     

脉冲摩擦喷射电沉积纳米晶镍的电化学腐蚀行为

为进一步研究脉冲电源对脉冲摩擦喷射电沉积的影响,本文分别采用脉冲摩擦喷射电沉积工艺和传统喷射电沉积工艺制备镍沉积层,利用TEM和XRD对比分析了占空比和频率对纳米晶镍微观组织结构的影响,采用电化学极化法研究了各沉积层在3.5%氯化钠溶液中的耐腐蚀性能,以期为拓展纳米晶材料的应用提供理论依据.研究表明:脉冲摩擦电沉积的应用,使电沉积结晶过程更加均匀,晶粒得到极大细化,最小平均晶粒尺寸可达9.12 nm;在氯化钠溶液中,脉冲摩擦喷射电沉积具有更小的腐蚀电流密度和更宽的钝化区,制备的镍沉积层电化学腐蚀性能均优于传统喷射电沉积;占空比和频率对沉积层腐蚀性能的影响与其各自对晶粒大小的影响基本吻合,过小或过大的占空比及频率均会导致耐腐蚀性能的降低.     

Relativistic doppler effect in quantum communication

Abstract

When an electromagnetic signal propagates in vacuo, a polarization detector cannot be rigorously perpendicular to the wave-vector because of diffraction effects. The vacuum behaves as a noisy channel, even if the detectors are perfect. The ‘noise’ can, however, be reduced and nearly cancelled by a relative motion of the observer towards the source. The standard definition of a reduced density matrix fails for photon polarization, because the transversality condition behaves like a superselection rule. We can, however, define an effective reduced density matrix which corresponds to a restricted class of positive operator-valued measures. There are no pure photon qubits and no exactly orthogonal qubit states.     

Nontrivial polarization shaping of femtosecond pulses by reference to the results of dual-channel spectral interferometry

Adaptive shaping of time-dependent polarization pulses is performed by reference to the analyzed results of dual-channel spectral interferometry. The desired pulses can be generated only by use of such a polarization-characterization technique. We demonstrate the generation of shaped femtosecond pulses whose ellipticity increases at a constant rate. The relative error between the shaped pulse and the target pulse is less than 6% over the main part of the pulse. Shaped time-dependent polarization pulses have many potential applications.     

Evolution of an N-level system via automated vectorization of the Liouville equations and application to optically controlled polarization rotation

The Liouville equation governing the evolution of the density matrix for an atomic/molecular system is expressed in terms of a commutator between the density matrix and the Hamiltonian, along with terms that account for decay and redistribution. To find solutions of this equation, it is convenient first to reformulate the Liouville equation by defining a vector corresponding to the elements of the density operator, and determining the corresponding time-evolution matrix. For a system of N energy levels, the size of the evolution matrix is N^2?×^?N². When N is very large, evaluating the elements of these matrices becomes very cumbersome. We describe a novel algorithm that can produce the evolution matrix in an automated fashion for an arbitrary value of N. As a non-trivial example, we apply this algorithm to a 15-level atomic system used for producing optically controlled polarization rotation. We also point out how such a code can be extended for use in an atomic system with arbitrary number of energy levels.     

Two-dimensional point spread matrix of layered metal-dielectric imaging elements

We describe the change of the spatial distribution of the state of polarization occurring during two-dimensional (2D) imaging through a multilayer and in particular through a layered metallic flat lens. Linear or circular polarization of incident light is not preserved due to the difference in the amplitude transfer functions for the TM and TE polarizations. In effect, the transfer function and the point spread function (PSF) that characterize 2D imaging through a multilayer both have a matrix form, and cross-polarization coupling is observed for spatially modulated beams with a linear or circular incident polarization. The PSF in a matrix form is used to characterize the resolution of the superlens for different polarization states. We demonstrate how the 2D PSF may be used to design a simple diffractive nanoelement consisting of two radial slits. The structure assures the separation of nondiffracting radial beams originating from two slits in the mask and exhibits an interesting property of a backward power flow in between the two rings.     

Theory of the adiabatic passage in two-level quantum systems with superpositional initial states

Abstract

An analytic theory of the adiabatic passage (AP) regime of interaction of short frequency chirped laser pulses with a two-level quantum system (QS) being initially in the superpositional quantum state is presented. We show that the initial value of the non-diagonal elements of the density matrix can influence the dynamics of the population transfer during the action of the laser pulse, but do not influence essentially the final populations of the states of the QS obtained at the end of the interaction near the AP regime. A novel solution to the Bloch equations is obtained in the form of a converging power series generalizing the ordinary AP solution.     

理论研究垂直腔半导体光放大器脉冲放大特性

从速率方程和薄膜光学理论出发,对垂直腔半导体光放大器(Vertical Cavity Semiconductor Optical Amplifiers,简称VCSOAs)在脉冲工作情况下的动态增益进行了数值模拟.在计算中考虑了载流子和光强沿光轴方向的不均匀性,以及腔内介质折射率的不连续性对光波传输的影响.详细分析了反射模式下VCSOA在脉冲通过时载流子密度和瞬时增益的变化、输出光脉冲的畸变以及抽运光功率、分布布拉格反射镜(Distributed Bragger Reflector,简称DBR)周期数、输入脉冲能量以及脉冲宽度等参量对脉冲放大中的能量增益的影响.结果表明,能量增益随抽运光功率增大而增加;在峰值功率一定时输入脉冲宽度的增加将减小能量增益;输入脉冲能量的增加也会引起能量增益的下降,而适当减少顶部DBR的周期数可以改善这种脉冲放大的能量增益饱和特性.     

Tuning magnetic domain structure in nanoscale La0.7Sr0.3MnO3 islands

The realization of spin-based devices requires high density, ordered arrays of magnetic materials with a high degree of spin polarization at surfaces. We have synthesized, for the first time, highly spin polarized complex magnetic oxide nanostructures embedded in a paramagnetic matrix by electron beam lithography and ion implantation. Imaging the magnetic domains with X-ray photoemission electron microscopy and magnetic force microscopy reveals a delicate balance between magnetocrystalline, magnetoelastic, and magnetostatic energies that can be tuned by the choice of SrTiO3 substrate orientation, film thickness, island size, and island shape.     

Miniaturized, high-power diode-pumped, Q-switched Nd:YAG laser oscillator-amplifier

A miniaturized, passively Q-switched Nd:YAG laser oscillator-power amplifier is reported, which is axially pumped by a compact, fiber-coupled, high-power, quasi-cw diode laser module. The pumping intensity of the oscillator crystal can be adjusted independently of the pumping intensity of the amplifier. This ensures that the oscillator pulse enters the amplifier when its maximum population density is reached. Furthermore, pulse bursts can be generated with a definite, adjustable number of single pulses. Maximum pulse energies of 8.4?and 22?mJ were achieved for a single pulse and for a pulse burst, respectively, at a pumping power of 470?W. The pulse widths were 2?ns, whereas the beam quality corresponded to M2<1.5. The laser is appropriate for scaling the power to the 10?MW range. Operation by using a 100?m pumping fiber was demonstrated.     

Factorization of the polarization transformation matrix in integrated photoelasticity

In integrated photoelasticity, assessment of stresses in a three-dimensional specimen is based on the measurement of the change of polarization on many light rays that pass the specimen. Since the medium is optically anisotropic and inhomogeneous, the optical phenomena are nonlinear and solution of the inverse problem is complicated. Several methods of solving the inverse problem demand an efficient algorithm for solving the direct problem, i.e., for the calculation of the polarization transformation matrix on the basis of the stress field in the medium. We propose for this use factorization of the transformation matrix. We show that if the transformation of polarization is described by characteristic parameters, the three characteristic parameters can be determined by solving a single third-order differential equation. Since characteristic parameters can be measured experimentally, this approach can be used in practical three-dimensional stress analysis with integrated photoelasticity.     

Generalized Stokes parameters of a stochastic electromagnetic beam propagating through a paraxial ABCD optical system

On the basis of the generalized diffraction integral formula for an ABCD optical system in the spatial domain, a propagation law for the generalized Stokes parameters of a stochastic electromagnetic beam passing through an ABCD optical system is obtained. We describe the Stokes parameters of the source as linear combinations of the elements of the cross-spectral density matrix, and study the changes in the spectral degree of polarization and in the state of the polarization ellipse of a stochastic electromagnetic Gaussian Schell-model beam propagating through a gradient-index fiber with the help of generalized Stokes parameters and the cross-spectral density matrix. The medium has significant effect on the change of the spectral degree of polarization. However, when the correlation coefficients of the source satisfy the relation delta(xx)=delta(yy)=delta(xy)=delta(yx), the medium does not influence the spectral degree of polarization.     

亚波长径向偏振光栅的设计

针对现有电力光学电流传感中法拉第旋转角的非线性测量、解调模式的光强依赖性等问题,本文设计了一种环型亚波长偏振光栅,其光栅矢量径向分布,可将偏振光的偏振分布转化为光斑强度分布并与偏振面同步旋转。应用琼斯矩阵对其偏振特性进行分析,运用严格耦合波理论对光栅进行仿真分析与优化设计,并制备了辐射状的环型铝金属光栅。测试结果表明,光栅TM光的透过率大于80%、整体消光比大于100,可实现对光偏振态的直接检测,并具有线性测量范围大、测量结果不依赖于光的绝对强度等优点,可用于基于图像分析的偏振检测技术。     

Corrosion behavior of pulse electrodeposited nanostructure Ni-SiC composite coatings

Ni-SiC nano-composite coatings with various contents of SiC were prepared by pulse electrodeposition from a modified Watts bath containing SiC nano-particles. The effect of SiC concentration, current density, duty cycle and pulse frequency on the corrosion behavior of the coatings was investigated by means of potentiodynamic polarization and Electrochemical Impedance Spectroscopy (EIS) methods. It has been found that the Ni-SiC composite coatings show better corrosion resistance in a 3.5% NaCI solution than pure nickel. The corrosion resistance of the coatings increases by increasing the amount of embedded SiC particles. This improvement can be attributed to the morphology and the crystallographic texture of the coatings.     

Fiber-based polarization-sensitive Mueller matrix optical coherence tomography with continuous source polarization modulation

We report on a new configuration of fiber-based polarization-sensitive Mueller matrix optical coherence tomography that permits the acquisition of the round-trip Jones matrix of a biological sample using only one light source and a single depth scan. In this new configuration, a polarization modulator is used in the source arm to continuously modulate the incident polarization state for both the reference and the sample arms. The Jones matrix of the sample can be calculated from the two frequency terms in the two detection channels. The first term is modulated by the carrier frequency, which is determined by the longitudinal scanning mechanism, whereas the other term is modulated by the beat frequency between the carrier frequency and the second harmonic of the modulation frequency of the polarization modulator. One important feature of this system is that, for the first time to our knowledge, the Jones matrix of the sample can be calculated with a single detection channel and a single measurement when diattenuation is negligible. The system was successfully tested by imaging both standard polarization elements and biological samples.