CO2激光器虚共焦腔凸面镜失调的实验研究

高功率激光器工作时，由于谐振腔反射镜上的光强分布不均匀，会导致镜面温度不均匀，从而造成镜面的角向偏转，致使激光器光轴发生偏转，严重影响输出光束质量，甚至造成不出光。文中主要从实验研究了CO2激光器虚共焦腔凸面镜失调对激光器输出光束质量的影响，实验结果表明，当凸面镜失调角接近于由理论计算而得的失调临界角时，输出光斑约为半个圆环，与理论分析非常符合，当凸面镜失调角达到约为失调临界角的4．5倍时，完全停止振荡。     

On the Global Efficiency of the Laser Optical Excitation of a Fast Atomic Beam

Abstract

The interaction of a fast atomic beam and a laser beam that crosses at right angles has been considered. We have studied the competition between the Doppler effect, due to the angular divergence of the atomic beam and the effect of the laser light intensity distribution. For low laser power values, an optimum waist size can be determined. For higher laser power values, the conditions for a maximum global efficiency are outlined.     

A comparative study of standard and elegant Hermite–Gaussian beams propagating through turbulent atmosphere

Based on the extended Huygens–Fresnel principle and the definition of the second-order moments of the Wigner distribution function (WDF), the analytical expressions for the root-mean-square (rms) beam width and far-field divergence angle, curvature radius and M ²-factor of standard Hermite–Gaussian (SHG) and elegant Hermite–Gaussian (EHG) beams passing through turbulent atmosphere are derived and compared. It is shown that in turbulent atmosphere the far-field divergence angle of SHG and EHG beams is equal under the same conditions, but the rms beam width, curvature radius and the M ²-factor of SHG and EHG beams are different except for beam orders m?=?0 and m?=?1. The relative rms beam width, relative curvature radius and relative M ²-factor of SHG beams are less than those of EHG beams. Therefore, the conclusion that SHG beams are less influenced by turbulence than EHG beams can be drawn if we examine one of the above three relative beam parameters.     

弹体冲击下测量光束的偏摆修正研究

光栅作为瞬态高速激光测速仪测量过程中的合作目标,其表面的技术状态决定了激光多普勒信号的质量.特别是在弹体高速冲击条件下,极易造成光栅内部结构发生损坏,导致反射光无法入射至探测器敏感部.因此,本文采用LS-DYNA仿真软件对砧体结构与光栅的连接处展开分析,并获取表面处的应力应变分布,从而为光栅结构的布置方式提供参考依据....     

Numerical simulation of a laser resonator with an in-phase doughnut-like output beam

A numerical model of the toric concave mirror laser resonator is founded by using the eigenvector method. Numerical calculation shows that an in-phase doughnut-like beam mode with high beam quality can be obtained in this resonator, whose diffraction loss is the lowest, and whose intensity distribution covers the whole resonator mirror. Systematical simulations indicate that, different from the spherical stable resonator, the effects of Fresnel number of the resonator and curvature radius of the toric concave total reflector on output beam quality is not very obvious. Under the condition of curvature radius of 15 m, the M ² factor of the output beam of this resonator with a large range of Fresnel number from 6.2 to 12.6 is from 1.3 to 1.9. Furthermore, the diffraction loss is close to 1.0% or less than 1.0%. Under the condition of Fresnel number of 7.07, the M ² factor of the output beam of this resonator with curvature radius from 6 to 30 m is from 1.60 to 1.24, and the diffraction loss is close to 1.0% or less than 1.0%.     

Experiments on the impact bending of continuous and notched steel beams

The results of quasi-static four-point bend tests and dynamic bend tests free-free beams, both continuous and notched, are reported. Angles of bend and rotation are measured as a function of time after impact, where the angle of bend is defined as the angle of deflection of one part of the beam relative to a neighbouring segment of the beam, and the angle of rotation is the angle of deflection relative to a datum line through the initial axis of the beam. The effect of notches is to change the strain profile in the beams, localize plastic formation and provides sites for fracture initiation. The influence of the notch depends on its position and depth; however, the responses of beams in terms of angles of bend and rotation are similar for notched and unnotched cases and are also insensitive to notch depth, being governed primarily by inertia of the rotating sections rather than the strength of the joint. Elastic deformation and wave propagation are shown to be important. Comparisons of angles of bend and rotation as functions of time are made with a rigid-plastic solution and with two-dimensional finite-element computations, both of which slightly underestimate the angle of bend at any time after impact.     

Influence of beam polarisation in laser bending process

Abstract

In this paper, the influence of beam polarisation in the laser bending process was experimentally studied. Based on the laser beam polarisation, the laser absorption of the metallic specimen could be enhanced by increasing the incident angle. The bending angles are achieved in stainless foil specimens without absorptive coatings, and larger bending angles can be produced by single laser scanning with higher incident angles. Multipass laser bending experiments were also carried out on the same material with graphite coatings. The results show that the bending angle per pass is affected obviously by the beam polarisation at lower laser line energy; and the bending angle per pass increases significantly when the total bending angle is large enough. When using polarised light, the laser beam distortion and laser absorption change induced by the variation of the absorptive coatings and laser incident angle are the two main factors that affect the coupled laser energy. The bending rule is the synthesis effect of the two factors.     

Super-Gaussian resonators for long-pulse XeCl lasers

Near-diffraction-limited laser beams of high energy have been achieved with a long-pulse (110-ns) XeCl laser fitted with a super-Gaussian unstable cavity of magnification M = 2.7. It has been found that when the super-Gaussian output coupler is replaced with a hard-edge aluminized mirror of radius equal to the super-Gaussian mirror spot size, the output laser energy is reduced by 10%, whereas the beam divergence increases more than 50%.     

Single-mode operation of a broad-area semiconductor laser with an anamorphic external cavity: experimental and numerical results

The emission of high-power broad-area semiconductor lasers inherently contains many lateral modes that increase the beam divergence and reduce the spatial coherence. Elimination of higher-order lateral modes from the output beams of commercially available broad-area lasers will be beneficial in many applications of these lasers. Experimental results obtained with a broad-area laser coupled to an anamorphic external cavity are presented and are compared with the predictions from our numerical model. We have predicted and observed with the anamorphic external cavity a greatly improved discrimination against high-order lateral modes. The measurement of the spectrally resolved near-field intensity patterns provides much more comprehensive information on their longitudinal- and lateral-mode content than do observations of near-field and far-field beam intensity profiles. With a broad-area laser of 100-mW nominal power, it has been possible to extract 40% of the maximal power in a stable single-lateral and single-longitudinal mode regime.     

Generation of high quality Airy beams with blazed micro-optical cubic phase plates

We design and fabricate a hybrid refractive-diffractive cubic phase plate (CPP) with a combined conventional blazed grating for generating high quality Airy beams. The grating enables elimination of direct incident illumination in the reconstructed beam. The CPP is fabricated in a negative photoresist on a substrate by laser direct writing lithography with precise exposure control of gray scales. Experimentally measured intensity distribution of the Airy beam is found in good agreement with the theoretical predictions. Furthermore, self-healing and nondiffraction properties of the Airy beam are verified experimentally. The proposed method gives rise to a simple, reliable, and low-cost micro-optical element solution for the generation of high quality Airy beams.     

Propagation properties of cylindrical sinc Gaussian beam

Abstract

We investigate the propagation properties of cylindrical sinc Gaussian beam in turbulent atmosphere. Since an analytic solution is hardly derivable, the study is carried out with the aid of random phase screens. Evolutions of the beam intensity profile, beam size and kurtosis parameter are analysed. It is found that on the source plane, cylindrical sinc Gaussian beam has a dark hollow appearance, where the side lobes also start to emerge with increase in width parameter and Gaussian source size. During propagation, beams with small width and Gaussian source size exhibit off-axis behaviour, losing the dark hollow shape, accumulating the intensity asymmetrically on one side, whereas those with large width and Gaussian source size retain dark hollow appearance even at long propagation distances. It is seen that the beams with large widths expand more in beam size than the ones with small widths. The structure constant values chosen do not seem to alter this situation. The kurtosis parameters of the beams having small widths are seen to be larger than the ones with the small widths. Again the choice of the structure constant does not change this trend.     

Experimental study of generalized self-filtering unstable resonators in an ablative-wall flash-lamp-pumped dye laser

The performance of a generalized self-filtering unstable resonator (GSFUR) that consists of two curved mirrors in a nonconfocal scheme with a low magnification of M = -1.62 in an ablative-wall flash-lamp dye laser is reported. The objective was to study the near- and far-field intensity distribution and the divergence of the laser beam. It was found that the output beam has a nearly Gaussian distribution with a pulse duration of ~400 ns FWHM, almost independent of the diameter of the field-limiting aperture, but increases slightly with the pumping rate. A diffraction-limited laser beam of 1.1 mrad was obtained from this laser cavity. The output energy was ~1 mJ when an intracavity glass plate was used as an output coupler. The required relations needed for the GSFUR design were also derived.     

Laser beam propagation in atmospheric turbulence

The optical effects of atmospheric turbulence on the propagation of low power laser beams are reviewed in this paper. The optical effects are produced by the temperature fluctuations which result in fluctuations of the refractive index of air. The commonly-used models of index-of-refraction fluctuations are presented. Laser beams experience fluctuations of beam size, beam position, and intensity distribution within the beam due to refractive turbulence. Some of the observed effects are qualitatively explained by treating the turbulent atmosphere as a collection of moving gaseous lenses of various sizes. Analytical results and experimental verifications of the variance, covariance and probability distribution of intensity fluctuations in weak turbulence are presented. For stronger turbulence, a saturation of the optical scintillations is observed. The saturation of scintillations involves a progressive break-up of the beam into multiple patches; the beam loses some of its lateral coherence. Heterodyne systems operating in a turbulent atmosphere experience a loss of heterodyne signal due to the destruction of coherence.     

Statistical distribution of the optical intensity obtained using a Gaussian Schell model for space-to-ground link laser communications

Abstract

Based on the characteristics of the laser device and the inevitable error of the processing technique, a laser beam emitted from a communication terminal can be represented by the Gaussian Schell model (GSM). In space-to-ground link laser communications, the optical intensity is affected by the source coherence parameter and the zenith angle. With full consideration of these two parameters, the statistical distribution model of the optical intensity with a GSM laser in both downlink and uplink is derived. The simulation results indicate that increasing the source coherence parameter has an effect on the statistical distribution of the optical intensity; this effect is highly similar to the effect of a larger zenith angle. The optical intensity invariably degrades with increasing source coherence parameter or zenith angle. The results of this work can promote the improvement of the redundancy design of a laser communication receiver system.     

Annular resonator with a Cassegrain configuration

An advanced annular resonator is studied both theoretically and experimentally. This resonator has a Cassegrain mirror configuration and is designed to extract circular and high-quality beams from annular gain media. We carried out beam propagation in this resonator, calculating the intensity distributions of the laser beam. We also proved the performance experimentally by applying the resonator to a CO2 laser. The operation of the new resonator is demonstrated successfully. The quality of the output beam is in good agreement with theoretical calculations of laser output power of 20 W.     

Generation of partially coherent laser beam directly from spatial-temporal phase modulated optical resonators

Abstract

The generation of a partially coherent laser beam directly from a spatial-temporal phase modulated optical resonator is investigated both experimentally and theoretically. The laser material used in the experiment is Nd:YAG rod pumped by Krypton lamps working in continuous wave mode. The phase modulation is fulfilled by an intra-cavity LiNbO₃ electro-optic crystal driven by high voltage. The experimental results show that intracavity phase modulation is an effective way to generate partially coherent laser beams. The theoretical analysis and numerical simulation shows that the output beam can be characterized by Gaussian Schell-model (GSM) beams. The two-slit interference experiment confirms that the output beam is partially coherent.     

Influence of the wavefront mismatch,spatial chirp and multimode pump on the beam quality of OPCPA

The beam quality and wavefront distribution of the optical parametric chirped‐pulse amplification (OPCPA) front‐end system of the petawatt laser has become a focus for research. The influence of wavefront mismatch, spatial chirp and multimode pump on the beam quality of OPCPA is numerically simulated and analyzed. It is shown that the beam quality and wavefront distribution becomes worse as the mismatch angle or the order of the spatial chirp increases. Meanwhile, it is also found that the beam quality and conversion efficiency of the amplified signal becomes worse as the transverse distribution of the pump laser descends.     

多束相干光叠加的计算机模拟

研究了具有空间不对称分布的多束相干光的叠加问题。对参考平面上相干光产生的干涉光强分布进行理论推导和数值计算,从理论上定量地分析了多束相干光的波矢量、偏振态对干涉光强分布的影响。模拟结果表明波矢量的改变主要影响干涉光强的空间分布,激光束偏振态的改变主要影响干涉场的对比度。     

弯曲梁柱结构平面外失稳的研究

研究了两节梁组成的弯曲梁在受竖向集中载荷作用下的平面外稳定问题,通过建立每节梁临界状态下弯曲和扭转变形微分方程,根据变形协调关系,得出了一端固定一端悬臂的弯曲梁平面外失稳的特征方程,并以等截面等长度梁构成的弯曲梁为对象,探讨了弯曲梁上拱和上翘对其平面外稳定性的影响,以及抗扭刚度对稳定性的影响。结果表明,同样的载荷作用幅度和梁高度,采用上拱的弯曲梁比直梁具有更高的侧向稳定性,并且存在一个最佳的弯曲角度,而上翘弯曲梁的侧向稳定性要低于直梁的侧向稳定性。当弯曲梁上翘时,增大抗扭刚度可以提高侧向稳定性,弯曲梁上拱时,增大抗扭刚度却会降低其侧向稳定性。     

Saturable-absorber-based spatial filtering of high-power laser beams

A saturable-absorber-based technique for spatial filtering of high-average-power laser beams is described. For a focused, radially symmetric beam having its highest intensity at the center, this saturable absorber behaves like a soft aperture with gradually increasing attenuation toward the beam edges, thus selectively transmitting the low divergence components that are confined close to the central axis of the propagating laser beam. This technique has been successfully used to reduce the divergence of a high-power, high-repetition-rate, tunable, narrowband, pulsed dye laser. Our results demonstrate how a judicious choice of operating parameters allows spatial filtering to be achieved with the introduction of a minimum absorption loss of the laser beam in the saturable absorber. Following a time-dependent analysis of a rate equation model describing the propagation and interaction of the laser beam with the saturable absorber, we have also obtained theoretical estimates for the extent of spatial filtering. Our theoretical estimates have been found to be in good agreement with our experimental observations.