Improvement of Laser-Beam Irradiation-Intensity Distribution Using Multi Lens Array and Edge-Shaped Plates

A new concept for improving the laser intensity distribution on an inertial confinement fusion target using a phase-controlled multi lens array has been proposed. Circular and hexagonal element lens apertures have been examined, and the circular aperture which eliminated azimuthally asymmetric intensity distribution was chosen. The diffraction fringe of the elementary lens was mitigated in a one-dimensional lens array using edge-shaped plates of a super-Gaussian phase profile. Uniform beam profiles were obtained using a one-dimensional array with 7 spherical lenses and a two-dimensional array with 37 lenses. An approximately flat-top intensity distribution was realized with the lens array with 37 lenses.     

Generation of a hollow Gaussian beam and its anomalous behavior

I attempted to produce a laser beam with no light in the central line of the beam, by Fresnel diffraction of the Gaussian beam by a spiral zone plate (SZP). The study imparts an arbitrary-order phase singularity on the light field. The experimental results show that the optical vortex radius depends on the singularity’s integer order n (also termed topological charge, or order of the dislocation). Also, the radius of maximum intensity is shown to depend on the singularity number. Anomalous behavior of the spectra at phase singularity produced by various zone plates near the focus of a converging Gaussian beam is also studied using a simple experimental technique. It is found that the spectrum of the beam on the SZP (spiral zone plate) consisting of a single spectral profile shows almost vanishing intensity at all frequencies with a tendency of splitting into two peaks at the on-axis point and shows redshift and blueshift around the phase singularity. A comparative study of the anomalous behavior of the spectra at phase singularity produced by various SZPs, with varying phase singularity order n, near the focus of a converging Gaussian laser beam is studied.     

High-accuracy long distance alignment using single-mode optical fiber and phase plate

A long distance alignment technique is developed using the combination of a single-mode optical fiber and a phase plate. The characteristics of the diffraction pattern of a Gaussian beam passing through a π step phase plate are analyzed theoretically. The feasibility of the method is validated in theory and with computer-simulation, and the factors affecting the alignment precision are discussed. The reliability of the alignment technique is demonstrated when it is used in the measurement of the straightness of a 12 m-track, in which measured results using the new technique coincide very well with those using an HP5528 dual frequency laser interferometer.     

相位片角向衍射产生拉盖尔-高斯光束的实验研究

设计了一种角向分布的相位片,利用离子束刻蚀技术加工成0和π二级的相位片.利用角向衍射理论对相位片的衍射场分析表明,衍射场为拓扑指数相反的两束拉盖尔-高斯光束的叠加场.用直径为4 mm的近平行光照射相位片,获得径向指数为零,拓扑荷相反的叠加拉盖尔-高斯光场.采用较大孔径的光束入射时,仍为拓扑荷相反的两束光的叠加,但径向指数会发生变化. 关键词： 信息光学 拉盖尔-高斯光束 相位片 离子束刻蚀     

Dynamic thermometry of a solid via optical diffraction method at pulsed irradiation

A method for the fast measurement of the temperature of solids under the action of a high-power light pulse is proposed and demonstrated. This method is based on the application of the Fraunhofer diffraction and is implemented for silicon samples heated to melting temperatures T _melt = 1412°C by a high-power light pulse. The current silicon temperature was determined by measuring the varying diffraction angle of the probing laser beam. The diffraction angle was varied over time because the period of the diffraction grating increased as a result of the dynamic thermal expansion of the crystal. An initial grating was formed on the surface of the silicon plate with a period of d = 4 μm. The radiation beam of a He-Ne laser with λ = 0.6328 μm was used as the probing beam; the measured signal was recorded in the pair of symmetric fifth-order diffraction maxima.     

合肥国家同步辐射光源软X射线束空间相干性实验研究

本文从实验角度对合肥国家同步辐射实验室软X射线束的空间相干性进行了研究.利用放置在物面上的高分辨正性光刻胶,记录了中心波长为3.2nm的波带片和针孔直径为30μm组合的光场分布(光斑),获得相应条件下的爱里斑及空间相干直径实验数据;完成了一个双缝干涉实验.所得的这些结果与理论计算相一致.     

Near-diffraction-limited laser beam shaping with diamond-turned aspheric optics

We used a pair of diamond-turned CaF(2) aspheres to convert the pure TEM(00) Gaussian spatial profile output of a diode-pumped Nd:YAG laser oscillator into a super-Gaussian intensity profile with a nearly flat phase front. The resulting super-Gaussian beam was nearly diffraction limited with an M(2) of 1.75; in the near field the 5-mm diameter beam retained a nominally flat-top intensity distribution without significant diffraction peaks for an excellent working distance of more than 50cm. A 10% improvement in amplifier-energy extraction obtained by use of the reshaped beam is demonstrated.     

Powder diffraction from a continuous microjet of submicrometer protein crystals

Atomic‐resolution structures from small proteins have recently been determined from high‐quality powder diffraction patterns using a combination of stereochemical restraints and Rietveld refinement [Von Dreele (2007), J. Appl. Cryst. 40 , 133–143; Margiolaki et al. (2007), J. Am. Chem. Soc. 129 , 11865–11871]. While powder diffraction data have been obtained from batch samples of small crystal‐suspensions, which are exposed to X‐rays for long periods of time and undergo significant radiation damage, the proof‐of‐concept that protein powder diffraction data from nanocrystals of a membrane protein can be obtained using a continuous microjet is shown. This flow‐focusing aerojet has been developed to deliver a solution of hydrated protein nanocrystals to an X‐ray beam for diffraction analysis. This method requires neither the crushing of larger polycrystalline samples nor any techniques to avoid radiation damage such as cryocooling. Apparatus to record protein powder diffraction in this manner has been commissioned, and in this paper the first powder diffraction patterns from a membrane protein, photosystem I, with crystallite sizes of less than 500 nm are presented. These preliminary patterns show the lowest‐order reflections, which agree quantitatively with theoretical calculations of the powder profile. The results also serve to test our aerojet injector system, with future application to femtosecond diffraction in free‐electron X‐ray laser schemes, and for serial crystallography using a single‐file beam of aligned hydrated molecules.     

Investigating the QED vacuum with ultra-intense laser fields

In view of the increasingly stronger available laser fields it is becoming feasible to employ them to probe the nonlinear dielectric properties of the vacuum as predicted by quantum electrodynamics (QED) and to test QED in the presence of intense laser beams. First, we discuss vacuum-polarization effects that arise in the collision of a high-energy proton beam with a strong laser field. In addition, we investigate the process of light-by-light diffraction mediated by the virtual electron-positrons of the vacuum. A strong laser beam “diffracts” a probe laser field due to vacuum polarization effects, and changes its polarization. This change of the polarization is shown to be in principle measurable. Also, the possibility of generating harmonics by exploiting vacuum-polarization effects in the collision in vacuum of two ultra-strong laser beams is discussed. Moreover, when two strong parallel laser beams collide with a probe electromagnetic field, each photon of the probe may interact through the “polarized” quantum vacuum with the photons of the other two fields. Analogously to “ordinary” double-slit set-ups involving matter, the vacuum-scattered probe photons produce a diffraction pattern, which is the envisaged observable to measure the quantum interaction between the probe and strong field photons. We have shown that the diffraction pattern becomes visible in a few operating hours, if the strong fields have an intensity exceeding 10²⁴W/cm².     

激光直写制作高阶螺旋相位板及其性能

将拓扑荷为4的螺旋相位光束与平面光干涉的计算全息图输入到空间光调制器中,得到含有多个衍射级次的高阶涡旋光束。为提高衍射效率,利用激光直写技术制作拓扑荷为4的高阶螺旋相位板,经测定,相位板深度理论数值为1.073 m,测量数值为1.082 m,相位板制作误差在0.83%以内。平行光束通过此相位板时,在夫琅和费衍射场获得一个高质量的高阶光学涡旋,光强分布与理论数值基本吻合,衍射效率达到86%。     

Gaussian laser beam tailoring using acoustooptic cell

Profile shaping of a Gaussian laser beam by an acoustic wave is well described using Collins integral and ABCD matrix formalism. It is shown by a numerical simulation that the relative width of the laser beam to the ultrasonic wavelength and the acoustic pressure inside the acoustooptic cell act on the light intensity diffraction pattern.Obtained results show that the output intensity profile differs from the incident Gaussian beam shape, and it is more broadened with an increase in the acoustic pressure. The intensity of a focused laser beam is transformed in a flat form in the central region if the acoustic pressure is proprely controlled.On the other hand the intensity longitudinal range (ILR) of the flat shape is discussed along the propagation axes, we have found the ILR is about 2 mm for a focal length distance f=100 mm.     

高功率二极管泵浦腔内倍频激光器

 成功研制了一台高功率二极管泵浦声光Q开关腔内倍频Nd:YAG激光器。当泵浦功率约为350W时，采用透过率为30%的输出耦合镜，调Q激光输出功率32.5W，脉宽约200ns，重复频率7kHz；采用Ⅱ类匹配KTP晶体腔内倍频，获得了32.5W的绿光输出，脉宽约120ns。输出光束平滑，远场为类高斯分布，测得的光束质量因子为3.6。     

Method for measuring elastooptical constants of solids

The spherical aberration observed in the far-field diffraction pattern of a He−Ne laser beam traversing a silica plate locally heated by a CO₂-laser is investigated. This leads to a thermooptical method suitable to evaluate the effective derivative of the refractive index and elastooptical method suitable to evaluate the effective derivative of the refractive index and elastooptical coefficient.     

提高能量密度的超衍射极限激光光束相位补偿技术

定义了激光光束衍射远场光斑压缩前后的能量比以及能量密度比来衡量超衍射极限激光光束的质量。通过利用反向传递算法设计了合适的补偿相位板,不但对准直放大的单一横模激光光束进行小于光学衍射极限的发散度的压缩,同时又保证光束能量集中于压缩后的远场衍射主瓣中,使压缩后的远场衍射光斑的能量密度增加。给出了相应的实例。这一结论不但解决了光学超分辨中光束压缩与能量损失不可避免这一矛盾,而且为发散度小且能量密度高的超衍射极限激光光束的实验工作以及该类光束的实际应用提供了理论基础。     

Airy beam laser

A method to design lasers that emit an arbitrary beam profile is studied. In these lasers, output-coupling is performed by a diffraction grating that imposes a phase and amplitude distribution onto the diffracted light. A solid-state laser emitting beams with a two-dimensional Airy intensity profile is demonstrated both theoretically and experimentally. In this case, the diffraction grating adds a transverse cubic phase to the diffracted light. An Airy beam is obtained by performing optical Fourier transform of the out-coupled light. The laser beam profile and power characteristics are shown to agree with theory.     

Generation of millimeter wave Gaussian beams and their propagation in a dielectric waveguide

Linearly polarized fundamental mode Gaussian beams were generated and coupled to a hollow circular oversized dielectric waveguide by placing the waist of the beam at the guide entrance. The transmission properties of the waveguide were characterized as a function of frequency for a variety of coupling conditions. These conditions included changes in the input beam waist radius, angle of incidence, and displacement perpendicular and parallel to the guide axis. It has been found that: 1.) power transmission is maximized when the waist of the input beam is centered at the guide input, injected normally, and has a radius of 0.43 times the waveguide radius, 2.) power transmission decreases rapidly with increasing angle of incidence and the rate of that loss increases with frequency, 3.) the waveguide preserves the linear polarization of the input beam, 4.) power transmission in the fundamental waveguide mode is not greatly affected by moderate displacements in the input beam position, and 5.) upon exit from the waveguide the launched EH₁₁ mode propagates as a fundamental mode Gaussian beam in the quasi-far field. The results compare favorably to the transmission theory of Belland and Crenn and approximately to the near and far field mode pattern theory of Degnan.     

Optical resonator for high-power transverse flow CO2 lasers

A new design of the U-type resonator is described. In this way, a laser beam with symmetrical intensity profile (regarding to a symmetry plane) can be extracted from an active medium that exhibits gain asymmetry along one of the transverse directions. The whole area of the active medium cross-section can be used, and consequently the laser efficiency will be increased. This resonator structure was applied for efficiency power extraction (as a low order TEM modes laser beam) from a DC excited transverse flow CO₂ laser with cylindrical geometry. Although the cross-section area of the discharge was entirely used (including the cathode fall region), a symmetrical intensity profile of the laser beam (regarding to the two orthogonal symmetry planes) was obtained in the near field as well as in the far field; the gain asymmetry along the flow direction was compensated by the gas circulation fluidodynamical circuit with two counterflowing discharge channels. A double-U optical resonator was introduced in order to provide a laser beam with axial symmetry.For the practical construction of these two types of optical resonators we have developed two new types of 90° deflection elements: the first one, which does not reverse the image (and which has the properties of the pentaprism), and the second one, which rotates the image with 90° angle. Both elements exhibit good focusability if they are equipped with two concave mirrors.     

非稳腔输出环状激光束的传输特性

本文从菲涅耳衍射积分公式出发研究了激光器非稳腔输出环状光束在空间的传输规律.由轴上光强的极值点位置给出了划分非稳腔传输空间的近场、中场和远场区域公式.理论计算结果表明,非稳腔输出端环状光束在中场区域激光能量逐渐向轴心方向转移,而在远场变为爱里型圆盘状光束的过程.在非稳腔脉冲TEA CO_2激光器上测得的实验结果与理论计算结果很好地符合.     

用于光束匀滑的连续相位板近场均匀性

 为降低高功率激光系统中连续相位板(CPP)后续元件的强激光损伤风险,综合考虑入射光强调制、干涉及衍射作用等多种影响因素,建立了CPP近场计算分析模型,模拟和分析了这些因素对CPP后的近场均匀性的影响。理论分析结果表明：CPP后的光束近场均匀性主要受入射光调制、CPP表面剩余反射率和衍射传输距离的影响;当入射光束质量较差时,CPP后的近场均匀性主要由入射光束质量决定,CPP剩余反射率和衍射传输距离对近场均匀性影响相对较小;但当光束质量比较理想时,干涉和衍射作用会破坏CPP的近场均匀性,衍射传输距离影响尤为突出。     

Acousto-Optic Control of the Energy Profile of Laser Radiation

An acousto-optic shaper is developed for transformation of the Gaussian beam profile into almost rectangular profile. As distinct from lens devices, an acousto-optic device allows modification of the shape of the output beam in a fixed optical configuration using changes in radio control with relatively short response times of about 10 μs. The method is based on multibeam high-efficiency Bragg diffraction when the diffraction orders are overlapped with respect to angle. It is shown that the beam profile can be transformed into rectangular and more complicated profiles with an efficiency of no less than 80%. The experiments are performed using the TeO₂ crystal.