Intensity modulation in single-mode microchip Nd：YAG lasers with asymmetric external cavity

Intensity modulation induced by the asymmetric external cavity in single-mode microchip Nd:YAG lasers is presented. Two kinds of experimental results are discussed based on multiple feedback effects. In one case, the intensity modulation curve is a normal sine wave, whose fringe frequency is four times higher than that of a conventional optical feedback system, caused by multiple feedback effects. In the other case, the intensity modulation curve is the overlapping of the above quadruple-frequency signal and conventional optical feedback signal, which is determined by the additional phase difference induced by the asymmetric external cavity. The theoretical analyses are in good agreement with the experimental results. The quadruple-frequency modulation of the laser output intensity can greatly increase the resolution of displacement measurement of an optical feedback system.     

吸入性氚化粒子在人体肺部组织中吸收剂量的研究

涉氚核设施在检修及退役时,其产生的氚化粒子容易被工作人员误吸入,并在呼吸系统的不同部位停留,部分尺寸较小的粒子可能在肺泡间质区(AI 区) 停留超过1 年的时间。氚粒子通过 衰变释放电子,在释放电子的同时以及释放的电子与周围原子相互作用还会分别产生出内、外轫致辐射,这些都会对人体肺部组织造成辐射损伤。本文采用蒙特卡罗程序PENELOPE 对多种氚化粒子在人体肺部组织中的吸收剂量进行了模拟计算,特别是讨论了氚 衰变的内轫致辐射的贡献。研究发现,电子辐射对人体肺泡产生的吸收剂量大于外、内轫致辐射,但后两者作用距离较长,对人体造成的影响重要;电子辐射吸收剂量随着粒子尺寸的增大以及金属原子序数的增大而减小;外轫致辐射吸收剂量随着粒子尺寸的增大而减小,随着金属原子序数的增大而增大;内轫致辐射吸收剂量随着粒子尺寸的增大以及金属原子序数的增大而减小。Some tritium relevant nuclear facilities, such as Tokamak, can produce tritiated particles. There is the possibility for the staffs to inhale the tritiated particles by accident in the process of maintenance or decommission of these facilities. Tritium decays to 3He, during which the beta electron and the internal bremsstrahlung are released. Meanwhile, the released electrons will interact with the surrounding atoms, and the external bremsstrahlung will be generated. All the electrons, internal bremsstrahlung and external bremsstrahlung will deposit the radiation energies to lung tissues and generate the radiation damage. In this paper we studied the radiation doses by inhaled tritiated particles in lung tissues by Monte Carlo code PENELOPE, in particular, internal bremsstrahlung contribution was included. Our results demonstrated that (1) the dose caused by electron radiation is far higher than those of external and internal bremsstrahlung, which however shall not be negligible due to their long-distance effects; (2) the dose caused by electron radiation decreases as the particle size and metal atomic number Z increase; (3) the dose caused by external bremsstrahlung decreases as the particle size increases, but increases as the metal Z increases; (4) the dose caused by internal bremsstrahlung decreases as the particle size and metal Z increase.     

Radiation force caused by scattering, absorption, and emission of light by nonspherical particles

General formulas for computing the radiation force exerted on arbitrarily oriented and arbitrarily shaped nonspherical particles due to scattering, absorption, and emission of electromagnetic radiation are derived. For randomly oriented particles with a plane of symmetry, the formula for the average radiation force caused by the particle response to external illumination reduces to the standard Debye formula derived from the Lorenz–Mie theory, whereas the average radiation force caused by emission vanishes.     

Application of optoelectronic negative feedback to ordering of the temporal structure of the diode-pumped Nd:YLF laser radiation

The conditions of the generation of radiation with ordered temporal structure are determined for a solid-state laser with external negative feedback. Experiments are performed based on the Nd:YLF laser longitudinally pumped by a high-power diode array. Lasing was controlled using an intracavity electro-optic modulator based on a lithium tantalate crystal. Intensity smoothing and self-mode-locking regimes with generation of several short pulses in the axial interval are implemented by varying the delay in the feedback circuit. Under conditions of amplification above the stability boundary, the secondary electro-optic effect manifestation was detected in the lithium tantalate crystal.     

Generation of coherent electromagnetic radiation by superconducting films at liquid-nitrogen temperatures

Generation of coherent electromagnetic radiation by GdBa₂Cu₃O_7?x superconducting films is observed in the frequency range 1–10 MHz at the temperature of boiling liquid nitrogen. The generation is caused by the feedback synchronization of the jumps of Abrikosov vortices produced by a low-frequency external magnetic field. This offers new possibilities for the application of high-T _c superconductors in superconducting electronics.     

外腔半导体激光器随机数熵源的腔长分析

利用光反馈半导体激光器产生的混沌激光作为随机数熵源,详细研究了混沌源外腔长度对500Mbit/s随机数特性的影响.研究表明:在单路混沌源情况下,外腔反馈引起的谐振会使产生的随机序列具有弱周期性,且当外腔反馈时间与采样时间的比值为整数时,产生序列的随机性最差,仅能通过NIST统计测试2,3项;在两路混沌源情况下,当混沌源的外腔长不相等且不成比例时,通过两路异或处理可消除由外腔反馈引起的弱周期性,产生的随机序列能够通过NIST的全部统计测试项.     

双反馈半导体激光器的混沌特性研究

实验利用双反馈半导体激光器获得了关联维数为3.8的高维混沌光.同时对比分析了双反馈与单反馈两种不同模式产生混沌的区别.结果表明：在反馈强度均为-26 dB时,双反馈产生混沌的关联维数高于单反馈产生混沌的关联维数2.6;双反馈可获得带宽为11 GHz的混沌光,为单反馈产生混沌带宽5.5 GHz的两倍.当双反馈的两个外腔长度不相等时,混沌的自相关曲线能很好的隐藏外腔长度信息,可提高混沌通信的保密性. 关键词： 双反馈 半导体激光器 混沌 带宽     

A mirror based on stimulated brillouin scattering with a double feedback loop in a laser cavity

The possibility of switching an external mirror based on stimulated Brillouin scattering (SBS) with a double feedback loop to a laser cavity is shown experimentally. A significant increase in the output laser power is obtained with conservation of the diffraction quality of the initial radiation. Experimental comparison with an ordinary SBS mirror, excited in a single pass through a nonlinear medium, is performed and the advantage in the energy parameters of the scheme with a double feedback loop is shown. The threshold characteristics of the SBS mirror under study are calculated and good agreement with the experimental data is obtained.     

Suppression of Chaotic Generation Caused by the External Optical Feedback in a Laser Diode

It is shown by computer simulation and in experiments that chaotic generation caused by the external optical feedback in a laser diode can be suppressed by means of a delayed optoelectronic feedback controlled by the injection current. The suppression of chaotic generation is realized for an AlGaAs/GaAs laser of the quantum wall structure. In the presence of the external optical feedback provided by the mirror placed at a distance of 1~mm the noise of optical intensity is of the oder of 10 W, the single mode dominates in the spectrum, and its spectral width exceeds 600 MHz. The optoelectronic feedback controlled by the injection current reduces the noise by several times and narrows the generation line width down to 80 MHz.     

Influence of radiation fluctuations on the magnitude of optoelectronic signal in semiconductor lasers with external optical feedback

Numerical modeling of dynamical processes in semiconductor lasers with external delayed optical feedback has been performed on the basis of the rate equations taking into account the quantum fluctuations of radiative transition rates. It has been shown that the nature of the optoelectronic signal as a function of pump current is determined by the magnitude of the optoelectronic coupling and corresponds to the behavior of the autocorrelation function of the radiation. It has been found that the radiation coherence decreases substantially with strong optical feedback. Experimental measurements confirming the theoretical results have been performed. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 6, pp. 809–813, November–December, 2008.     

Light-induced kinetic effects in gas with spatial nonuniformity of radiation and external force fields

The influence of spatial nonuniformity of radiation and external force fields on the macroscopic gas state with selective velocity excitation of it by resonance radiation is considered. A sharp change in the character of manifestation of the symmetry properties for the spectral distribution of the exciting radiation is found. Temperature and density nonuniformities and macroscopic flows in gas, caused by the above factors, are analyzed.     

Effect of external actions on the characteristics of vortex sound

The dependence of the characteristics of vortex sound caused by an air flow around a rigid cylinder on various factors, including the turbulence of the incident flow, the inhomogeneity of the cylinder surface, and the sound radiation of an external source, is experimentally investigated. Measurements have made it possible to specify the mechanism of vortex sound radiation under the action of external factors, to relate the radiation intensity and the drag to the type of air flow around the body, and to propose possible ways of reducing the vortex sound radiation intensity.     

锶原子光晶格钟黑体辐射频移评估

在中性原子光晶格钟的系统不确定度评估中,通常黑体辐射引起的频移是最大的一项.黑体辐射频移主要受周围环境温度的影响.针对国家授时中心的锶原子光晶格钟实验系统,通过理论分析、腔体表面温度的测量和软件模拟相结合的方法,评估了锶原子光晶格钟黑体辐射频移的修正量和不确定度.其中主要分析了锶原子炉、蓝宝石加热窗口、透过窗口片进入到真空腔体内的室温以及Zeeman减速装置对原子团处的热辐射引起的黑体辐射频移.在真空腔体外表面设置了5个测温点,利用校准过的铂电阻温度传感器监测真空腔体外表面的温度变化,用SolidWorks绘图软件建立腔体模型,通过有限元分析软件模拟出在真空腔体温度变化0.72 K时,原子团所处位置温度的波动为0.34 K.最终得到黑体辐射频移总的修正量为-2.13(1) Hz,不确定度为2.4×10~(-17).     

External-cavity birefringence feedback effects of microchip Nd:YAG laser and its application in angle measurement

External-cavity birefringence feedback effects of the microchip Nd:YAG laser are presented. When a birefringence element is placed in the external feedback cavity of the laser, two orthogonally polarized laser beams with a phase difference are output. The phase difference is twice as large as the phase retardation in the external cavity along the two orthogonal directions. The variable extra-cavity birefringence, caused by rotation of the external-cavity birefringence element, results in tunable phase difference between the two orthogonally polarized beams. This means that the roll angle information has been translated to phase difference of two output laser beams. A theoretical analysis based on the Fabry--Perot cavity equivalent model and refractive index ellipsoid is presented, which is in good agreement with the experimental results. This phenomenon has potential applications for roll angle measurement.     

Light-induced grating formation in dye solutions by two independent laser beams

The distributed feedback laser effect was used for the detection of light-induced gratings created in organic dye solutions due to the interference of actively mode-locked ruby laser radiation and that of electrooptically Q-switched ruby laser. Radiation kinetics in different regions of the dye superfluorescence spectrum was studied. The distributed feedback effect was shown to be caused by the amplification coefficient grating in the conditions under discussion.     

Line-Width Reduction of a Laser Diode Array Using an External Cavity with Two Feedback Mirrors

A novel Littman-Metcalf external cavity laser diode array with two feedback mirrors is introduced. The linewidth broadening effect caused by smile can be reduced by the novel external cavity. At the drive current of 16A, the line-width is narrowed to 0.1 nm from free-running width of 1.6 nm with output efficiency of 84%.     

Quasi-common-path microchip laser feedback interferometry with a high stability and accuracy

We report on an improved scheme of quasi-common-path microchip laser feedback interferometry, which demonstrates a high stability and accuracy. An additional beam splitter is used to divide the external cavity into a measuring feedback cavity and a reference feedback cavity. With this scheme, the optical path lengths of the measuring and reference feedback light can be made nearly the same, thus greatly reducing the long-period phase fluctuation caused by the laser frequency drift. The final system performances are evaluated as followed: the short-term displacement resolution is better than 2 nm, the output fluctuation is less than 10 nm within a 40-min-long stability test, and the maximum error within the 100-μm range is 25 nm when calibrated with the Agilent 5529A dual-frequency laser interferometer.     

分布反馈光纤激光器阵列中外腔反馈特性研究

以半导体激光器中的复合腔模型和实测的分布反馈光纤激光器的外腔端面反射率为基础,对不同反射率条件下分布反馈光纤激光器的输出功率进行了仿真,同时搭建了二基元分布反馈光纤激光器阵列实验平台,对仿真结果进行了验证.仿真与实验结果表明:外腔反馈光重新注入分布反馈光纤激光器会增加激光器的输出功率,降低了阵列的输出功率平坦性.并且外腔端面反射率越大,这种平坦性降低的效果越明显.在构建分布反馈光纤激光器阵列时,应考虑外腔反馈对阵列输出功率平坦性造成的影响,尽量选择外腔端面反射率较小的激光器进行组阵.     

Distortion of optical feedback signals in microchip Nd:YAG lasers subjected to external multi-beam interference feedback

This paper proposes a theoretical analysis for the characteristics of an external cavity Nd:YAG laser with feedback of multiple-beam interference, which is induced by the multi-reentrance of the light from the external Fabry--Perot cavity. The theoretical model considers the multiple beam interference of the external Fabry--Perot cavity. It is found that the optical feedback signals are distorted to pulse waveforms instead of the sinusoidal ones in conventional feedback. The experimental results are in good agreement with the theoretical analysis. The obtained theoretical and experimental results can advance the development of a laser feedback interferometer.     

半导体激光自混频干涉技术对振幅的测量

激光自混频干涉是一种非常特别的现象 ,在许多应用中 ,通常都尽可能地避开来自各种各样的光器件反馈到激光器腔内的光 ,把它看成是有害的 ,例如 :来自光纤端面、或大型光学系统中的各个小光学镜面表面的反射光 ,这些光都能增加噪声及改变半导体激光的一些特性 ,这种性质的惊人影响已由Lenstra[1] 等人详尽描述过。然而 ,激光自混频干涉技术正是应用这种外反馈光携带的可被提取的信息对振动振幅和频率进行测量。