机载激光波束扫描回波脉冲特征分析

本文介绍了脉冲激光束远区场的时空分布，导出了机载脉冲激光对地面扫描探测时回波脉冲的时域表达关系，通过数值计算得到了回波脉冲波形及其有关参数随扫描倾角的变化规律，结果表明不同波束倾角时回波脉冲特征发生了明显的变化。     

不同离焦条件对骨硬组织激光消融的影响

评估了不同离焦辐照条件对脉冲CO2激光骨硬组织消融的影响。实验样品为新鲜离体牛胫骨组织,置于由计算机自动控制的一维电动平移台上。调节工作距离,分别在光束聚焦平面前后光斑尺寸约为510μm处进行非接触式垂直照射。脉冲CO2激光波长为10.64μm,脉冲频率为60 Hz,能量密度范围5~45 J/cm2。平移台移动速度为20 mm/s,重复扫描6次。肉眼和显微镜观察组织样品形态学变化,常规组织病理切片处理,苏木精-伊红(HE)染色,共焦扫描显微镜观察并摄取图像。运用测量软件测量骨样品消融凹陷的几何尺寸,获得切口宽度、深度和切口截面积随辐射曝光量的变化关系。结果表明,脉冲CO2激光可以应用于骨头等硬组织的切割,不同的离焦辐照条件对组织消融效果具有重要影响;在临床上,为获得窄且深的消融切口和高的消融率,可以稍微将光束聚焦在组织表面下方。     

A new theory for the treatment of a pulsed beam propagating througha grating pair

A new theoretical model, without the first-order approximation of grating diffraction, has been developed for studying the propagation of a pulsed beam through a grating pair. By using our model, the astigmatic aberration of the grating pair has been analyzed in detail. It was shown that the grating pair may be a good optical element for astigmatic compensation of monochromatic optical beams. But for grating pulse compression, the astigmatic aberration affects the pulse fronts and the effects of finite beam size (FBS). These effects on the compression of ultra-broadband pulses have been studied. It was found that, due to these effects, the space-time shapes of the compressed pulse are severely distorted. The magnitude of this distortion depends not only on the astigmatic aberration, but also on the spectral bandwidth and spatial divergence of the pulse. When the input pulsed beam is collimated, the waveform distortion due to the effects of FBS can be eliminated, but the pulse front distortion remains. In addition, the spatial and temporal properties of the compressed pulse for a single-pass compressor have been studied. An analytical expression without the well-collimated condition was obtained to describe the effect of lateral frequency shift. Also, the effect of third-order dispersion on a single-pass compressor is discussed     

有限束宽下超高斯光束经光栅对压缩的研究

利用有限束宽下入射脉冲通过光栅对压缩的理论模型,从脉冲光束的傅里叶角谱入手,分析了脉冲超高斯光束单次及两次通过光栅对时的变换特性,以及高阶色散对脉冲压缩的影响。结果表明,脉冲超高斯光束单次通过光栅对压缩器时,横向谱移动使得出射光束产生时间相关的空间移动而两次通过压缩器时则会消除此种影响。高阶色散不仅影响输出脉冲的时间特性同时还影响其空间特性,导致输出脉冲波形扭曲,产生时空畸变。     

准直脉冲激光大气传输热晕数值分析

大气通道上介质对激光部分能量的吸收,导致大气折射率的改变,形成自散焦负透镜,产生热晕．使激光发生畸变,限制了激光能量在大气中的有效传输．以准直脉冲高斯激光束大气传输为例,数值分析了热晕对由光束的声传递时间（即流体动力学时间）定义的两种脉冲长、短脉冲大气传输的影响：长脉冲受到非线性热晕效应影响很大,光束截面上光强发生了重新分布,轴上光强凹陷,峰值光强向周围扩展;短脉冲主要受到线性吸收的作用,受到的非线性热晕效应影响很小．提出了采用脉冲间隔适当的序列短脉冲传输来提高激光传输能量的方法．     

不同脉宽的啁啾脉冲高斯光束载波相位漂移研究

为了研究不同脉宽情况的线性啁啾脉冲高斯光束在传播方向和横截面内的载波相位变化分布,采用数值研究方法,对载波相位在传播过程中的变化进行了研究。由数值研究可知,在传播方向上,不同脉宽的脉冲光束,从负无穷远到正无穷远将经历π相位变化,并在正无穷远处渐尽于-π/2;在传播距离越远的平面,其载波相位变化量越小,且其变化量为0的位置r越小。结果表明,载波位相变化的特点主要由波前和相位曲率半径的关系决定。     

脉冲激光大气传输热晕数值分析

由于大气通道上介质对激光能量的吸收,改变了大气折射率,形成自散焦负透镜,从而产生热晕。热晕使激光发生畸变,限制了激光能量在大气中的有效传输。以准直高斯光束为例,模拟单脉冲和序列脉冲激光大气传输,对均匀风场和无风两种情况下激光在不同靶程光束截面的光强分布进行了数值分析。结果表明：长脉冲比短脉冲受到非线性热晕效应影响大;序列脉冲存在一个最优化脉冲间隔,使上靶光强存在20%~30%的涨幅。最后,在没有采用自适应光学系统对畸变进行补偿的情况下,提出了采用脉冲间隔适当的序列短脉冲传输来提高激光传输能量的方法。     

Pulsed field diffraction by a perfectly conducting wedge: aspectral theory of transients analysis

The canonical problem of pulsed field diffraction by a perfectly conducting wedge is analyzed via the spectral theory of transients (STT). In this approach the field is expressed directly in the time domain as a spectral integral of pulsed plane waves. Closed-form expressions are obtained by analytic evaluation of this integral, thereby explaining explicitly in the time domain how spectral contributions add up to construct the field. For impulsive excitation the final results are identical with those obtained previously via time-harmonic spectral integral techniques. Via the STT, the authors also derive new solutions for a finite (i.e., nonimpulsive) incident pulse. Approximate uniform diffraction functions are derived to explain the field structure near the wavefront and in various transition zones. They are the time-domain counterparts of the diffraction coefficients of the geometrical theory of diffraction (GTD) and the uniform theory of diffraction (UTD). An important feature of the STT technique is that it can-be extended to solve the problem of wedge diffraction of pulsed beam fields (i.e., space-time wavepackets)     

LOW COHERENCE PULSED DOPPLER LIDAR WITH AMPLIFIED REFERENCE BEAM STORAGE

In this paper, we have successfully developed a new low coherence pulsed Doppler lidar concept for wind speed measurements, in which a pulsed laser is used as the source for measurement and reference beam. A fraction of the transmitted pulse is stored in a fiber optic ring resonator with a path length longer than the pulse. The output of the resonator is a pulse train that is used as the reference beam and can be mixed with the Doppler shifted measurement signal. Because this reference has traveled a distance equivalent to the measurement beam’s path length, low coherence sources can be used. Inserting an erbium-doped fiber amplifier into the resonator ensures that the stored pulses do not decay in amplitude. Careful control of gain and amplified spontaneous emission is required to prevent laser oscillation while maintaining sufficient gain for the signal. Experiments prove that 16 reference pulses of sufficient amplitude and stability can be generated. Computer simulations suggest that 70 pulses should be achievable, which would be equivalent to a Doppler lidar measurement range of 2,550 m.     

Pulsed beam reflection and transmission at a dielectric interface:two-dimensional fields

Highly focused pulsed fields in vacuum can be generated analytically by assigning complex values to the space-time source coordinates of the conventional transient free-space Green's function. These new wave objects have been named complex-source pulsed beams (CSPB). Their utility can be extended to generating new solutions for pulsed beam propagation and diffraction in a perturbed environment by making the space-time source coordinates in the corresponding Green's function complex. The analytic extension required in this process is performed systematically via the spectral theory of transients. A canonical test case for reflection, including critical angle, lateral (head) wave and evanescent transmission effects, is provided by a dielectric half-space. The exact solution for CSPB scattering is derived in spectral integral form, evaluated in terms of the time-dependent spatial spectrum singularities in the complex plane, and interpreted physically. Numerical evaluation reveals the detailed space-time behavior of these physical constituents and their role in establishing the total scattered field. To simplify the analysis, a two-dimensional problem is considered wherein the pulsed beam is generated by a complex pulsed line source     

单级静态高峰值功率灯抽运脉冲Nd∶YAG固体激光器

报道了采用对称平面平行腔结构实现单级静态输出30 kW高峰值功率灯抽运Nd∶YAG固体激光器的研究结果。从速率方程出发,推导出脉冲Nd∶YAG固体激光器的单脉冲能量表达式,模拟出输出镜最佳透过率及最大输出能量。通过实验选取激光器工作的最佳参数,研制出一台高峰值功率灯抽运脉冲Nd∶YAG激光器,理论模拟和最佳实验结果基本一致。激光器在最大输入电压为800 V,脉宽为2 ms时,输出最大单脉冲能量60 J,最大峰值功率30 kW,光束质量M2为5.9,总体电光转换效率3.3%。在最大输入电压为800 V,脉宽为1.5 ms时最大平均功率405 W。采用该激光器切割6 mm低碳钢和4 mm不锈钢,在脉宽为2 ms,频率为6 Hz,峰值功率为30 kW时,切割4 mm不锈钢速度为1 mm/s,切割6 mm低碳钢速度为1.5~2 mm/s。     

瞬态标量平面波的平行三维复源脉冲波束展开

本文从理论上较全面地分析了入射瞬态平面波的三维复源脉冲波束展开,出发点是从频域球面波严格的格林函数导出球面复源点表达式,由此得到频域平面波的复源点展开公式,     

窄带和宽带双曲正割脉冲光束的解析传输式

从近轴波动方程出发推导了窄带和宽带双曲正割脉冲光束的解析传输公式。通过数值计算比较了分别采用复振幅包络(CAE)表示式和复解析信号(CAS)表示式得到的脉冲光束，得出了选择脉冲光束研究方法的条件。结果表明复振幅包络表示式得到的解会存在空间奇异性，使光束出现不符合物理意义的非光束行为。对于宽带光束，复振幅包络解的奇异点的位置距离轴中心较近，使复包络解不能正确表示脉冲光束，而对于奇异点位置远离轴中心的窄带光束，对脉冲光束产生的影响可以忽略。因此，窄带脉冲光束可以采用复振幅包络和复解析信号两种表示式来研究，而对于宽带脉冲光束，必须采用严格的复解析信号表示式。     

Power output of a pulsed Raman laser with saturable excitation

A theoretical expression is obtained for the time dependence of the Stokes power output of a pulsed Raman laser, the pump pulse duration being comparable to the relaxation time of the Raman excitation and the output saturation limited. A rate-equation approach is employed, and the pump beam is assumed to have a Gaussian intensity distribution. The theory is developed in terms of the practical example, stimulated spin-flip Raman scattering in InSb.     

中心波长及脉宽对能量叠加光束远场的影响

多束激光的远场叠加对于提高远场靶面能量是一种有效的方法,单元光束的参数的变化对于远场合成光束都会产生影响。为了分析脉冲激光参数控制对远场能量分布的影响,利用夫琅禾费衍射积分公式导出了4束普通脉冲高斯光束合成的远场光强的解析式,并整理了激光中心波长和脉宽的改变对远场叠加后能量分布的影响之间的具体关系。通过MATLAB的数值模拟,结果表明:当光束的中心波长减小或者脉冲宽度减小时,远场能量向中心轴上汇聚,远场轴上光强增加。当轴上光强相对于理想中心波长和脉宽的远场轴上光强的变化率小于5%,中心波长变化率最大不能超过7.89%,或者脉宽变化率最大不能超过91%,因此,中心波长的变化对于远场光强分布的影响比脉宽更大。     

全固态调Q激光器产生脉冲Bessel光束

使用连续式输出的砷化镓半导体激光器泵浦的掺钕钒酸钇激光器作为光源,被动式通过轴棱锥产生Bessel光束.在激光谐振腔内加入Cr4+:YAG晶体调Q产生脉冲Bessel光束.理论模拟了1 064 nm波长光束经过轴棱锥之后的三维传播图和截面光强分布.通过实验得到了1 064 nm波长脉冲Bessel光束的脉冲宽度和脉冲重复率,并计算得到了单脉冲Bessel光束的能量和峰值功率.利用光束分析仪记录了1 064 nm波长Bessel光束的截面光强分布,并测量了中心光斑的直径,得到的数据与理论计算基本吻合.     

Synthesis of Current–Voltage Characteristics of 670 GHz Gyrotron Magnetron Injection Gun and Calculation of the Helical Electron Beam Parameters at the Leading Edge of a High-Voltage Pulse

A method of synthesis of current–voltage characteristics (CVC) and calculation of the parameters of a helical electron beam (HEB) at the leading edge of the accelerating voltage pulse for gyrotron electron guns is proposed. These data can be used for a study of the gyrotron startup scenario with the mode competition taken into account. As an example, the results of calculations for a pulsed gyrotron with a frequency of 670 GHz are presented.     

可用于高功率脉冲激光的面阵CCD模式测量装置

本文介绍了一种用面阵ＣＣＤ摄像机作探测器的激光光束测量装置。整个装置包括光束衰减器、单脉冲选取及脉冲同步、光束直径匹配、ＣＣＤ摄像机、图像卡、计算机及监示器以及系统软件。软件根据激光混合模的二阶矩阵理论［１］编制，可测量高功率、高重复频率脉冲激光的空间能量分布，并可对所测结果进行模拟。由软件给出所测的光束参数，可对光束质量作出正确评价。     

高功率激光通过多程放大系统传输特性的计算模拟

从Frantz-Nodvik方程出发,采用脉冲分割和介质薄片损耗模型,详细研究在不同的注入脉冲能量密度、放大器的增益恢复系数、增益分布、程间损耗和光束口径放大倍数的情况下,激光脉冲通过多程脉冲放大系统的传输特性。     

超短脉冲光束传输的时间分辨功率谱

利用小波变换（WT）对超短高斯脉冲光束作时频分析，获得了光脉冲的时间分辨功率谱，给出了在1个脉冲周期内频随时间变化的关系。对光脉冲在自由空间传输的时频谱特性进行了研究，结果表明：在离轴点上，时频谱相对光束中心出现明显的时间延迟，而频谱分布不变。