Study on the possibilities of controlling the laser output beam properties by an intracavity deformable mirror

The possibilities of controlling the laser beam properties by a deformable mirror introduced into the laser optical cavity were studied theoretically and experimentally. The experiments were performed under conditions of an industrial high power transverse flow cw CO₂ laser operating with a stable resonator of a folded configuration. A deformable bimorph mirror of a surface profile controlled by the voltage applied to the mirror electrodes is implemented to the laser system as a back cavity mirror or as a one of the inner folding mirrors. The near-and far-field characteristics of the laser beam versus the resonator configuration controlled by the changes of the focal length of the deformable mirror are discussed in the paper. The analysis reveals that the resonator with an inner deformable mirror is much more sensitive to the mirror curvature variations than the resonator in which the deformable mirror is used as a back cavity mirror. The presented results show that dynamic and controllable changes in the resonator properties result in the controlled modification and optimisation of the laser output power and spatial parameters of the laser radiation.     

Verfahren zur hochgenauen Bestimmung des Reflexionsvermögens von Laserspiegeln in einer Resonatoranordnung

Method for Accurate Determination of the Reflectivity of Laser Mirrors in an Optical Cavity If a high frequency intensity chopped laser beam is passing a passive optical resonator a phase lag between its output and input signal results. Due to the limited photon lifetime inside the resonator the measurable phase lag is determined by resonator length and mirror reflectivities. Changing the resonator length in a time-proportional regime a time dependent phase lag results from which the product of mirror reflectivities is calculable unambiquously.     

Hybrid optical resonator type laser proposed for direct coupling into optical fiber

Combining one stable resonator with a concave mirror and a coupling-hole plane mirror, and another unstable resonator having a concave mirror and a convex mirror, a new hybrid optical resonator type laser has been proposed by overlapping the concave mirror as a common one and fitting the convex mirror exactly to the coupling-hole. Using theABCD law, the so-called g-parameters are first found for beam confinement. Then, based on the eigen-equation, the field distribution on the plano-convex mirror is obtained with the g-parameters above. Knowing this field distribution results in an effective transmittanceG _{e e}. Finally, the ratio of the inner convex to outer plane mirror radii can be determined for an optimum coupling in both the limiting cases of homogeneously and inhomogeneously broadened gain media. Some numerical examples will also be given for a CO₂ laser having the proposed hybrid resonator.     

高功率激光器抗失调谐振腔的实验研究

为简化高功率激光器谐振腔结构,提高激光谐振腔的稳定性,增加基模体积,改善光束质量,提出了两种新型激光谐振腔结构,分别是带角隅反射镜的激光谐振腔和直角内圆锥面反射镜激光谐振腔。使用高功率脉冲CO2激光器,研究了这两种激光谐振腔的输出特性和腔镜失调的关系,并结合平凹稳定腔,对三种激光谐振腔的抗失调稳定性进行了对比。实验结果表明,随着全反镜或平面输出镜失调角的增加,角隅腔激光器的单脉冲输出能量下降速度小于直角内圆锥面全反镜腔。两种新型谐振腔全反镜的抗失调稳定性都超过平面输出镜。角隅全反镜谐振腔和直角内圆锥面全反镜谐振腔的抗失调稳定性均大大超过平凹稳定腔。     

Properties of a laser beam generated from an axicon resonator

A high power cw CO₂ laser beam is generated from a resonator configuration having an axicon mirror as its end reflector. The properties of this beam are evaluated in terms of its polarization, focusing capability and temporal stability and the resulting effects on processing materials are presented. A comparison is made between a folded and a straight resonator having a spherical focusing mirror as its end reflector.     

Mode profiles in resonators with a near 90° roof-top mirror

The transverse modes of a laser resonator comprising a near 90° roof-top mirror and a feedback mirror are studied theoretically. We relate this configuration to an almost plane roof-top resonator and apply the Fox and Li technique to compute the lowest-order symmetric and antisymmetric modes. The effects of perturbations to the vertex angle of the roof-top mirror, misalignments of the feedback mirror and curved feedback mirrors are examined.     

A New Resonator With A Non─misaligned FourMirorReflectorAndItsEfectOnBeamQualityImprovementOfHighAveragePowerNd：YAGLasers

ＡＮｅｗＲｅｓｏｎａｔｏｒＷｉｔｈＡＮｏｎ─ｍｉｓａｌｉｇｎｅｄＦｏｕｒＭｉｒｏｒＲｅｆｌｅｃｔｏｒＡｎｄＩｔｓＥｆｅｃｔＯｎＢｅａｍＱｕａｌｉｔｙＩｍｐｒｏｖｅｍｅｎｔＯｆＨｉｇｈＡｖｅｒａｇｅＰｏｗｅｒＮｄ：ＹＡＧＬａｓｅｒｓＣ．Ｍ．Ｚｈａｏ，Ｇ...     

Unstable resonator and output-coupling design study for diffraction radiation generator near-mm sources

Configurations of unstable resonators that could be used for diffraction radiation generators (DRG's) are presented. The output coupling of a negative branch unstable resonator exhibits an extreme sensitivity to mirror radius and appears to be unsuited to DRG use. The positive branch resonator does not exhibit this sensitivity to mirror radius, and the mirror radii that are needed to give output couplings of a few percent appear practical.Work conducted at the Harry Diamond Laboratories.     

Initial operation of a higher-power quasi-optical gyrotron

Results from the initial operation of a high-power quasi-optical gyrotron based on the 90-kV, 50-A Varian VUW-8144 electron gun are reported. The output power and efficiency have been measured for a resonator mirror separation of 19.4 cm with a magnetic field of 4.95 T, corresponding to resonator output coupling of 1.9%, and for a resonator mirror separation of 21.4 cm with a magnetic field of 4.7 T, corresponding to a resonator output coupling of 3.1%. Operation was multimoded with 3-6 modes excited in the range of 125-130 GHz for the 4.95-T magnetic field. A peak efficiency of 15% at an output power of 161 kW was obtained for a gun voltage of 93 kV and a current of 12 A. A peak-output power of 364 kW at an efficiency of 10% was obtained at a voltage of 95.6 kV and 37.5 A     

空间谐振式陀螺谐振腔的失调分析

为了克服平面谐振腔调节精度高、损耗大的困难,提出了一种新型空间谐振式陀螺的谐振腔结构。并利用光学变换矩阵和菲涅尔-基尔霍夫衍射积分公式建立数学模型,通过MATLAB仿真计算谐振腔各腔镜因失调引起的衍射损耗。计算结果表明当谐振腔边长为4cm,入射光束束腰半径为0.25mm时,凹面镜的曲率半径为0.397m,反射镜失调引起的衍射损耗近似为0,即失调对陀螺的性能没有影响。得到陀螺的极限灵敏度为1.54°/h。     

微变形镜内腔补偿激光模式畸变研究

 采用Fox-Li迭代方法，对畸变相差以Zernike多项式展开，计算了不同菲涅耳数的谐振腔在畸变发生时的低阶输出模式，分析了激光器谐振腔内畸变对激光输出模式的影响。对微变形镜的面形以Zernike多项式进行拟合，计算了中心电极的影响函数，并建立了微变形镜控制模型。实验上将微变形镜作为激光器的一个腔镜，验证了微变形镜对激光输出模式的补偿作用。结果表明，内腔微变形镜对激光输出模式具有校正作用，激光基本上可以达到基模输出。     

Design and Cold Tests of Asymmetric Quasioptical Resonators

This paper presents the design and test of asymmetric quasioptical resonators for gyrotron applications, where most of the diffraction output coupling comes from the smaller of two mirrors. A computer code based on scalar diffraction theory is used to calculate the resonator losses and the electric field near the mirrors. Resonators with mirror separations approaching confocal have reduced side lobes in the diffraction patterns, which should allow for better power transmission and focusing. Cold tests have been performed at 94 and 141 GHz to measure the quality factor of the asymmetric resonators as a function of mirror separation and mirror misalignment. Typical resonator parameters include 65 cm mirror separation, 1-3% output coupling and 85% of the losses from the small mirror. A half-symmetric resonator with one flat and one curved mirror has also been investigated. The calculated values of Q are in reasonably good agreement with the cold test measurements.     

一种新颖的用于光腔模式及光束传输模拟的特征向量法

根据有限元法单元划分的思想，提出了一种新颖的模拟光腔模式及光束传输的特征向量法. 该方法的关键之处在于基于衍射积分理论构造了一种新的光束传输矩阵，通过求解特征矩阵方程可一次性得到谐振腔的一系列特征向量，每一列特征向量即代表了腔镜上光场的一个确定模式的振幅及相位分布. 并可采用该方法模拟光场传输到腔内或腔外任意地方的场分布. 该方法将传统方法中大量的迭代过程转化成为本征积分方程特征向量的求解过程，并与初值取值无关，且可一次性求得多个模式分布，从而可方便地分析谐振腔的模式鉴别能力. 特征向量法对圆形镜共焦 关键词： 谐振腔 特征向量法 模式分布     

大口径薄片激光器失调输出特性

对大口径的薄片激光器的失调输出特性进行了理论和实验研究,V-型腔由平面输出耦合镜和凹面反射镜及孔径光阑组成。基于矩阵光学和失调衍射积分方程得到了激光器的功率失调曲线。实验测量了在200 Hz泵浦频率下加入不同孔径光阑后谐振腔失调对输出功率的影响和谐振腔失调对光束质量的影响,以及在300 Hz泵浦频率下输出功率分别与平面输出耦合镜和凹面反射镜失调的关系。实验结果表明:失调输出功率和失调角度成二次函数关系,失调容限和腔镜口径成正比,光束质量因子随失调角度的增大而变小。     

非稳腔模式计算中Colins公式适用性的分析

Ｃｏｌｉｎｓ公式将几何光学和波动光学联系在一起，能以简单的形式描述相干光在复杂光学系统中的传输，但忽略了光学系统中一些光学元件的衍射损耗。当Ｃｏｌｉｎｓ公式用于激光谐振腔的本征模式计算时，其适用程度与被忽略腔镜的有效菲涅尔数有关。本文用数值方法求解了两组谐振腔本征积分方程，一组应用Ｃｏｌｉｎｓ公司推导，另一组用菲涅尔－惠更斯衍射积分推导，结果表明：对稳定腔Ｃｏｌｉｎｓ公式基本适用；对非稳腔，当被忽略腔镜的有效菲涅尔数＞１时，原则上可以用，当被忽略腔镜的有效菲涅尔数＜１时不适用；对有源非稳腔基本适用。     

大功率高亮度固体激光腔的选择

 分析了大功率激光器可供选择的几种腔型的动力稳定区。并对最近报道的两种固体激光腔型进行了评价，指出了存在的问题，提出采用小曲率半径的凹面镜和一凸面镜构成谐振腔是设计大功率高亮度固体激光器的较好方案。     

Diffraction losses of an asymmetric stable laser resonator using an equivalent resonator

An asymmetric stable laser resonator with one large mirror and the other mirror apertured is considered. An equivalent symmetric resonator is derived which enables the determination of the theoretical diffraction losses from Li's computations. Experiments are done to test the validity of the predictions. The experimental diffraction losses agree very well with the theoretical values and the usefulness of the concept of equivalent resonator is therefore confirmed.     

输出环状光束的新型激光谐振腔

提出一种输出环状光束的新型激光谐振腔,即在稳定激光谐振腔中采用外环耦合方式得到环状光束的输出。利用For-Li数值迭代算法求解此类激光谐振腔的积分方程,通过计算机编程计算了平面一凹球面稳定谐振腔的输出端平面镜在不同半径的情况下,谐振腔镜面上和衍射耦合输出场基模的振幅和相位分布。计算结果预示了当输出端平面镜半径小于通常的稳定腔的基模半径时,衍射耦合输出场有稳定的环状光束输出。实验中TEA CO2激光器采用印刷电路板预电离结构,增益长度90cm,腔长5m,球面全反射镜曲率半径20m,输出端平面镜半径4．5mm,得到了内环半径为4．5mm、外环半径为5．5mm的基模环形光斑输出。从而在理论上和实验上证实了该方案产生环状光束的可行性。     

Effective mass in quantum effects of radiation pressure

We study the quantum effects of radiation pressure in a high-finesse cavity with a mirror coated on a mechanical resonator. We show that the optomechanical coupling can be described by an effective susceptibility which takes into account every acoustic modes of the resonator and their coupling to the light. At low frequency this effective response is similar to a harmonic response with an effective mass smaller than the total mass of the mirror. For a plano-convex resonator the effective mass is related to the light spot size and becomes very small for small optical waists, thus enhancing the quantum effects of optomechanical coupling.     

Intracavity control of a 200-W continuous-wave Nd:YAG laser by a micromachined deformable mirror

A silicon micromachined membrane deformable mirror with a diameter of 10 mm, reflectivity of better than 99.8%, and a surface deflection range of 3mum has been used for intracavity control of an industrial 200-W cw Nd:YAG laser. When it was placed in the resonator, the mirror demonstrated continuous (more than 40-h) stable operation under an ~550-W cw optical load, with beam diameters in the range 3-6 mm. Periodic modulation of the curvature of the deformable mirror with a frequency of 250 Hz produced quick switching between stable and unstable resonator configurations, which resulted in pulse-period Q -switched generation with average power in the range 50-200 W, modulation depth from 95% to 10%, and an M(2) parameter of 6.5 to 30.