全固态激光中的光谱合成技术

对多种全固态激光中的光谱合成技术进行了探讨和研究，包括光纤激光、Yb:YAG板条激光和半导体激光。对于光纤激光，探讨了基于单个多层介质膜（MLD）光栅、一对MLD光栅、多个体布拉格光栅三种衍射光学元件的光谱合成技术中色散造成的光束质量退化问题，指出子束光谱线型的二阶矩全宽决定了光束质量的退化量，但所允许的光谱宽度又依赖于具体的技术选择途径。进而比较了三种光谱合成方案的优缺点。对于固体激光，实验演示了基于Yb:YAG晶体的板条激光实现光谱合成的原理可行性。通过设计一个基于MLD光栅的振荡器内的光谱合成装置，实现了7束子激光最高241 W的光谱合成输出，合成后光束质量β因子约4.1，表明大功率Yb:YAG板条激光具有通过光谱合束技术实现功率进一步提升的潜力。对于半导体激光，提出并设计了大模场外腔半导体激光+快轴光谱合成的技术。实验演示了9个1 mm宽LD芯片沿快轴方向的光谱合成，用β因子评价合成后的光束质量，在慢轴方向β≈6.3，在快轴方向β≈1.6，表明快轴光谱合成造成的光束质量退化是完全可控的。     

基于多层介质膜光栅的谱合成系统光束特性分析

针对多层介质膜光栅在光束谱合成系统中的应用, 利用光线追迹方法, 建立了基于多层介质膜光栅的谱合成系统光传输模型. 多层介质膜光栅引入的相位调制包括浮雕表面上光程差与浮雕结构光程差两部分, 且受到光栅槽深、占空比和光束入射角等因素的影响. 利用衍射积分方法和光束非相干叠加原理, 计算模拟了基于多层介质膜光栅的谱合成系统的合成光束光强分布. 在此基础上, 利用强度二阶矩方法分析了合成光束的光束质量, 并讨论了多层介质膜光栅的槽深、占空比和制作误差等因素对合成光束特性的影响. 结果表明: 改变多层介质膜光栅的槽深和占空比以及中心光束入射角会影响合成光束能量, 但不会影响合成光束的光束质量, 合成光束的光束质量始终保持与单个子光束的光束质量相当; 多层介质膜光栅的制作误差对合成光束的光束质量和能量均存在明显影响.     

Analysis of beam quality for the laser beams after spectral beam combining

The beam quality of the laser beams after spectral beam combining (SBC) has been analyzed in this paper. Based on the propagation model of the spectral beam combining system, the expression for the output field of an individual beam passing through the spectral beam combining system with lens aberration has been given and the intensity distribution of the combined beam has been studied by the principle of incoherent superposition. Consequently, according to the intensity second-order moments method, the M²-factor of the combined beam has been calculated and the effect of lens aberration on the characteristics of the combined beam has been analyzed quantitatively. It can be shown that the SBC system can improve the beam quality of laser array significantly, and the M²-factor of the combined beam passing through the ideal spectral beam combining system is almost the same as that of an individual beam. For the spectral beam combining system with lens aberration, the spherical aberration has greater effect on the off-axis beam than on the on-axis beam, and the beam quality of combined beam degrades with increase in lens aberration.     

光纤激光共孔径光谱合成实现5kW高效优质输出

采用多层介质膜衍射光栅实现多路高功率光纤激光共孔径光谱合成有望成为光纤激光同时实现高功率、高效率和高光束质量的最具发展潜力的技术途径。搭建了一套基于双光栅色散补偿设计的5kW共孔径光谱合成系统。采用国产多层介质膜衍射光栅实现了5路kW级窄谱子束激光的高效优质共孔径光谱合成,最大输出功率达5.07kW,光束质量因子(M2)小于3,合成效率达到91.2%。初步研究表明:多层介质膜衍射光栅在较高功率水平、较宽光谱范围内均能保持较高衍射效率,是实现高功率光纤激光高效率光谱合成的重要器件;参与合成的子束自身的光束质量水平和线宽是影响合成输出光束质量的重要因素,光谱合成系统的输出功率主要受限于窄谱子束的输出功率和合成路数,增加窄谱子束的功率或合成路数均可进一步提升系统的输出功率。     

522 W average power, spectrally beam-combined fiber laser with near-diffraction-limited beam quality

We report a three-channel, spectrally beam-combined (SBC), 1 mum fiber laser that produces 522 W of average power with near-diffraction-limited (M2 ~ 1.2) beam quality. The laser features a SBC power combining efficiency of 93%, versatile master-oscillator, power-amplifier fiber channels with up to 260 W of narrow-band, polarized, and near-diffraction-limited output that is tunable over nearly the entire 1 micro m Yb(3+) gain bandwidth, and excellent prospects for significant power scaling. To our knowledge, these results represent the highest beam quality and average power achieved to date for a beam-combined fiber laser system.     

Expansion of the channel number in spectral beam combining of fiber lasers array based on cascaded gratings

Aiming at the problem that the channel number in spectral beam combining (SBC) of fiber lasers array is limited by the narrow spectral selectivity of grating, a new scheme based on cascaded volume Bragg gratings (VBG) is proposed to expand the channel number. A system cascaded by two VBGs is presented and analyzed. A formula used to accurately determine the angle of incidence of all the channels is derived using rigorous coupled-wave analysis (RCWA). The channel number can be multiplied with increasing the number of VBGs. Numerical results show that the system based on cascaded VBGs is a good SBC combiner that has the ability to combine a large number of channels.     

Modified hollow Gaussian beam and its paraxial propagation

A model named modified hollow Gaussian beam (HGB) is proposed to describe a dark hollow beam with adjustable beam spot size, central dark size and darkness factor. In this modified model, both the beam spot size and the central dark size will be convergent to finite constants as the beam order approaches infinity, which are much different from that of the previous unmodified model, where the beam spot size and the central dark size will not be convergent as the beam order approaches infinity. The dependences of the propagation factor of modified and unmodified HGBs on the beam order are found to be the same. Based on the Collins integral, analytical formulas for the modified HGB propagating through aligned and misaligned optical system are derived. Some numerical examples are given.     

DLA光源质量对光栅-外腔谱合成系统光束质量的影响

在二极管激光阵列(DLA)光栅-外腔谱合成系统中,由于DLA存在子单元光束发散角、“smile”效应的位置偏差及指向性偏差等因素的综合作用,将导致合成光束的光束质量降低。综合考虑DLA子单元光束发散角、“smile”效应等因素对谱合成系统中光束传输特性的影响,建立了DLA光栅-外腔谱合成系统的光传输模型,进而对谱合成系统中DLA子单元光束发散角、“smile”效应的位置偏差及指向性偏差等因素对合成光束的光束质量影响进行了定量分析。结果表明,DLA光源质量会明显影响合成光束的光束质量：DLA子单元光束发散角和“smile”效应引入的指向性偏差越大,合成光束的光束质量就越差;“smile”效应引入的位置偏差在合束方向上对合成光束的光束质量没有影响,而在非合束方向上引入的位置偏差将会明显降低合成光束的光束质量。在实际工作应用中,需要采取措施提高DLA光源质量,以减小对合成光束的光束质量影响。     

Degradation of beam quality and depolarization of the laser beam in a step-index multimode optical fiber

The characteristics of a laser beam are altered during propagating through large-core multimode optical fibers. The distribution of modes excited by the input laser beam is modified by means of mode coupling on transmission through the fiber, leading to the degradation of beam quality and the depolarization of the delivered beam. The relationship between the beam quality factor (M²) of output beam from a large-core multimode fiber and the fiber length, as well as the relationship between the degree of polarization (V) of output beam from such a fiber and the fiber length, are introduced in this paper. When a laser beam was well launched into a large-core step-index multimode fiber, M² of the output beam was a compound tanh function of the fiber length. A linear polarization beam that well launched into such fiber suffered depolarization. The V of the output beam was an exponent function of fiber length. And the misalignment between beam axis and fiber axis made the beam quality degrade faster but made no difference of the utmost M² in the aligned and misaligned conditions. Also, the misalignment condition made the polarization of output beam degrade faster.     

Photonic crystal beam splitters

In this work, the beam splitter with two input ports and two output ports in two-dimensional photonic crystals is studied through the finite-difference time-domain method. The beam splitter consists of two orthogonally cross line defects. The diameter of the two diagonal air holes at the intersection of the two line defects was modified. The input light can be identically divided into the two output ports. The beam splitters can be applied in the photonic crystal Mach-Zehnder interferometers or photonic crystal optical switches.     

特殊函数在湍流大气传输光束漂移中的应用

研究特殊函数在激光湍流大气传输光束漂移效应中的应用,为提高跟瞄系统的瞄准精度提供理论依据。基于特殊函数和修正Von Karman谱,对斜程湍流大气传输光束漂移方差表达式进行了数值仿真。数值模拟表明,特殊函数高次项的近似舍去对激光湍流大气传输光束指向精度影响甚微。在外尺度较小时,光束漂移方差变化较快;随着外尺度的增加,光束漂移增加缓慢且趋于饱和。在相同的外尺度下,发射孔径（初始光束束径）与发射天顶角的增大对光束漂移具有抑制作用,且天顶角接近π/2,光束漂移角方差快速减小。     

Cylindrical-sinc beam

Propagation of diffracted beams in free space has already been identified. One possible solution is derived from the Helmholtz wave equation and this solution is named as cylindrical-sinc beam. Therefore, cylindrical-sinc beam is a new beam type which can be obtained from Helmholtz equation. Diffraction properties of this new beam type were observed from an opaque aperture screen. Additionally, geometrical theory of diffraction is used to determine numerical values of diffracted fields. This new beam type which does not exist in the literature was observed by using these methods passing through an opaque aperture. The obtained expression was analyzed numerically. Simulation results of the beam depending on the length of aperture and distance to the observation point were added.     

Effects of a modified spectral model on the spatial coherence of a laser beam

The influence of a modified (bump) spectrum of refractive index fluctuations on the spatial coherence of an optical wave is studied here and compared with that based on a von Karman spectrum. Analytical expressions are derived for the mutual coherence function (MCF) and wave structure function (WSF) of a lowest-order Gaussian beam wave from which the beam spot size and degree of coherence are deduced. The qualitative behaviour of beam spreading and coherence length is basically the same for both spectral models. Also, when the radius of the Fresnel zone and initial beam radius are of comparable size, the presence of a spectral bump appears to have minimal effect on spatial coherence for all beams. However, the choice of spectral model is important for certain ranges of parameters. In particular, the implied spatial coherence length for a collimated beam based on the modified spectrum is significantly smaller than that based on the von Karman spectrum whenever the Fresnel zone is either much larger or much smaller than the initial beam radius, whereas for a focused beam the predicted coherence length based on the modified spectrum is slightly larger when the Fresnel zone size is much smaller than the initial beam radius.     

Suppression of synchrotron radiation due to beam crystallization

With respect to a “hot”, non-crystallized beam the synchrotron radiation of a cold crystallized beam is considerably modified. We predict suppression of synchrotron radiation emitted by a crystallized beam in a storage ring. We also propose experiments to detect this effect. Received: 19 June 1998 / Revised version: 10 August 1998     

基于Zemax的He-Ne激光光束聚焦物镜的设计

 为了得到一个合理的He-Ne激光光束聚焦物镜,采用以正前凸型为基础的高折射率双片结构,应用Zemax软件进行优化设计,获得了弥散斑直径为0.002mm的He-Ne激光聚焦物镜,该镜头只需校正轴上点球差。实验结果表明：设计的镜头比低折射率单片透镜得到的弥散圆直径更小,达到0.0019mm,球差被控制在-0.05mm～+0.05mm范围内,MTF曲线所围面积变大,中心点亮度增高,符合实际需要。     

Multifunctional and compact spectrometers based on cylindrical beam volume holograms

We propose a new class of slitless spectrometers using cylindrical beam volume holograms. These holograms disperse an input beam in one direction in an output plane while they do not affect the beam in the perpendicular direction. We show that the spectral mapping of the input beam can be obtained in one direction and the beam can be independently modified in the perpendicular direction. Using this unique property, we demonstrate a spectral wrapping technique to considerably increase the operation spectral range of the slitless spectrometers, without sacrificing their resolution.     

Surface modification of TA2 pure titanium by low energy high current pulsed electron beam treatments

Surface integrity changes of TA2 pure titanium including surface topography, microstructure and nanohardness distribution along surface layer were investigated by different techniques of low energy high current pulsed electron beam treatments (LEHCPEBTs). The surface topography was characterized by SEM. Moreover, the TEM observation and X-ray diffraction analysis were performed to reveal the surface modification mechanism of TA2 pure titanium by LEHCPEBTs. The surface roughness was modified by electron beam treatment and the polishing mechanism was analyzed by studying the cross section microstructure of electron beam treated specimens by SEM and TEM. The results show that the surface finish obtains good polishing quality and there is no phase transformation but the dislocations by LEHCPEBT. Furthermore, the nanohardness in the surface modified layer is improved. The remelt and fine-grain microstructure of surface layer caused by LEHCPEBTs are the main polishing mechanism and the reason of modification of surface topography and the increment in nanohardness is mainly due to the dislocations and fine grains in the modified layer induced by LEHCPEBT.     

Micromechanics-based digitally controlled tunable optical beam shaper

A digitally controlled programmable optical beam shaper that uses a two-dimensional (2-D) small-tilt micromirror device is introduced. The optical intensity profile at any spatial position can be independently modified by use of a macropixel, incorporated with binary pulse-width modulation. An experimental proof-of-concept optical beam shaper with a commercial 2-D digital micromirror device has shown that the spatial profile of a He-Ne laser beam can be digitally manipulated. Investigation of the analog intensity control showed 256-level nonlinear degamma behavior with a measured 24.5-dB optical contrast ratio and a 10-bit spatial resolution. The performance of this tunable optical beam shaper is limited by the dimensions of the micromirror, the intermirror distance, the size of the optical beam, the number of bits used to control the micromirror, the diffraction effect, and the quality of the imaging optical system.     

Theoretical and experimental study on the transverse vibration properties of an axially moving nested cantilever beam

An axially moving nested cantilever beam is a type of time-varying nonlinear system that can be regarded as a cantilever stepped beam. The transverse vibration equation for the axially moving nested cantilever beam with a tip mass is derived by D’Alembert?s principle, and the modified Galerkin?s method is used to solve the partial differential equation. The theoretical model is modified by adjusting the theoretical beam length with the measured results of its first-order vibration frequencies under various beam lengths. It is determined that the length correction value of the second segment of the nested beam increases as the structural length increases, but the corresponding increase in the amplitude becomes smaller. The first-order decay coefficients are identified by the logarithmic decrement method, and the decay coefficient of the beam decreases with an increase in the cantilever length. The calculated responses of the modified model agree well with the experimental results, which verifies the correctness of the proposed calculation model and indicates the effectiveness of the methods of length correction and damping determination. Further studies on non-damping free vibration properties of the axially moving nested cantilever beam during extension and retraction are investigated in the present paper. Furthermore, the extension movement of the beam leads the vibration displacement to increase gradually, and the instantaneous vibration frequency and the vibration speed decrease constantly. Moreover, as the total mechanical energy becomes smaller, the extension movement of the nested beam remains stable. The characteristics for the retraction movement of the beam are the reverse.     

The influence of pump beam polarization on the signal beam, in TWM in photorefractive crystals: Transmission geometry

Two-wave mixing (TWM) in transmission geometry of photorefractive (PR) crystals of 23 symmetry such as BSO, BGO is investigated by solving a set of coupled wave equations and taking their optical activity as one of the effective parameter. Coupled wave equations are solved numerically by fourth order Runge-kutta method and results are presented in graphical form. The effects of various parameters such as crystal thickness, optical activity, coupling coefficient, absorption, and input polarization angle of pump beam has been studied on the input beam polarization components. It has been observed that by varying the behaviour of pump beam polarization, signal beam can be modified. This can be used in various applications such as spatial light modulation and optical information processing, etc.