Phase compensation of laser beam modes: II. Optimization techniques

Phase compensation can lead to a significant improvement in the intensity characteristics of high order laser beam modes. Optimal design procedures for phase compensation structures are described, and the behaviour of the higher order spatial harmonics is considered in detail. The effects of possible phase errors are also derived analytically.     

Phase plates for laser beam compensation

A phase plate inserted in the output beam of a multimode laser oscillator can yield a substantial improvement in the beam characteristics. Experiments with helium-neon and carbon-dioxide lasers are in agreement with the theoretical considerations.     

CAE-based compensation methods of non-uniform pulse laser beam profiles

Applying the pulse laser to speckle methods, non-uniformities of the laser beam profiles and the intensities between each laser pulse have unpleasant consequences on the intensity distribution of the recorded images and following on the assigned fringes of the corresponding subtractive result. This contribution introduces a computer-based technique for compensating this technical and physical problem, so that the fringe quality is improved, even if the homogeneity of the laser beam profiles is on such low level, that the conventional (subtractive) technique fails. The solution is based on algorithms, which refines each intensity distribution and is comparable with the known shading correction.     

Automatic compensation of laser beam focusing parameters for flying optics

Flying optics technologies are used for a range of applications such as a large workpiece processing and laser texturing. It is essential to compensate the variations of laser beam focus parameters while the focus head is moving. A flying optics automatic compensation approach is proposed to achieve invariable laser focal size, focus depth and focus position through computer controlled objective lens and focus lens position. A simple mechanical control method is also presented for the realization of constant beam parameters for flying optics. Numerical simulation is illustrated for a CO₂ laser texturing application. The flying optics parameters compensation is simple and easy to control.     

共孔径偏振耦合分光的旋转四分之一波片相位补偿技术

在地平式折轴望远镜中开展自适应光学激光导星实验,研究了共孔径发射接收信标激光束偏振耦合分光效率随望远镜方位角和天顶角变化的补偿技术。提出了一种由四分之一波片和法拉第旋光器构成的相位补偿器,通过旋转四分之一波片以实时补偿由于望远镜旋转导致的光路相位延迟量的变化。数值计算表明,望远镜处于任意方位角和天顶角位置时,通过1°步长旋转四分之一波片,可使补偿后的偏振分光效率理论上达到99.90％以上。实验从原理上定性地验证了该方法的有效性。只要测量出镜面的相位延迟,便可计算得到望远镜处于不同方位角和天顶角情况下有效补偿所需的四分之一波片旋转角度,据此可建立实用的旋转波片偏振补偿装置。     

A novel laser straightness measurement method with beam bend compensation

In this paper, a novel method to measure straightness errors with beam bend compensation based on Finite Element thermal analysis is proposed. According to the Edlen formula, the main factor that causes beam bend is thermal gradation in the air. Finite Element thermal analysis is applied to the thermal field around the beam. It is evident from the experimental results that high precision straightness errors in horizontal and vertical directions can be obtained after compensation. Hence, the feasibility of straightness errors measuring system with compensation for beam bend measurement is proved.     

共孔径偏振耦合分光的旋转四分之一波片相位补偿技术

 在地平式折轴望远镜中开展自适应光学激光导星实验,研究了共孔径发射接收信标激光束偏振耦合分光效率随望远镜方位角和天顶角变化的补偿技术。提出了一种由四分之一波片和法拉第旋光器构成的相位补偿器,通过旋转四分之一波片以实时补偿由于望远镜旋转导致的光路相位延迟量的变化。数值计算表明,望远镜处于任意方位角和天顶角位置时,通过1°步长旋转四分之一波片,可使补偿后的偏振分光效率理论上达到99.90％以上。实验从原理上定性地验证了该方法的有效性。只要测量出镜面的相位延迟,便可计算得到望远镜处于不同方位角和天顶角情况下有效补偿所需的四分之一波片旋转角度,据此可建立实用的旋转波片偏振补偿装置。     

Efficient angular dispersion compensation in holographic generation of intense ultrashort paraxial beam modes

We experimentally demonstrate that small misalignments of the pulse stretcher or compressor of our chirped-pulse-amplification laser can precompensate for angular chirp when producing ultrashort paraxial beam modes with holographic gratings. Using this approach we can eliminate one of the two gratings needed in our 2f-2f setup [Mariyenko, Opt. Express 13, 7599 (2005)]. This allows for up to an order of magnitude more output power. We see our method as the next step in the production of intense exotic forms of ultrashort pulses, which can be used in the investigation of intense laser-matter interactions. In addition, we produce the first femtosecond (helical-)Ince-Gaussian beams.     

Effect of the laser axial modes on the coherence length of the laser beam

The effect of the presence of the axial modes within the spectral halfwidth of a laser spectral line on the visibility of its interference fringes is represented. It is found in case of axial modes with equal spectral halfwidths that the dominant factor in determining the coherence length of the laser beam is the spectral halfwidth of its axial mode and not the spectral halfwidth of the laser line. For the case of a laser beam with axial modes of different spectral halfwidths, the calculation shows a coherence length, closer to the mode with the smaller halfwidth.     

光学补偿LD侧向非均匀泵浦全固化激光器

采用定向棱镜谐振腔、半圆柱面LD阵列交错侧向泵浦两根串接NdYAG激光棒、热传导冷却结构,研制出了一台工程实用的全固化激光器.定向棱镜腔内振荡光线平行于光轴的对称位置交换穿行,匀化了增益,降低了模序数;同时,定向棱镜准相位共轭特性和自准直作用在一定程度上修正波前畸变、热光楔以及热致双折射,保证了大的动态范围内热稳定性,改善了光束质量.激光器采用Cr4+YAG被动调Q,在室温条件下,工作电流为53 A、频率10 Hz时,1.06 μm单脉冲激光能量高于150 mJ、脉宽9 ns、输出稳定度±3%、束散度2.5 mrad且光轴指向稳定,远场分布近似相干平顶高斯光束.     

外腔中半导体二极管激光阵列的高阶侧模锁定

理论分析了外腔较短情况下,宽条半导体二极管激光阵列（LDA）高阶侧模相位锁定的可能性,观察了包含主、旁瓣结构的多侧模远场光强分布。从实验记录结果可以看出,在旁瓣中出现了标志锁相的峰、谷结构,而且该结构的调制度明显高于主瓣中的峰、谷结构的调制度,结果表明：在外腔较短情况下,LDA高阶侧模相位锁定的现象是存在的。     

外腔中半导体二极管激光阵列的高阶侧模锁定

 理论分析了外腔较短情况下,宽条半导体二极管激光阵列（LDA）高阶侧模相位锁定的可能性,观察了包含主、旁瓣结构的多侧模远场光强分布。从实验记录结果可以看出,在旁瓣中出现了标志锁相的峰、谷结构,而且该结构的调制度明显高于主瓣中的峰、谷结构的调制度,结果表明：在外腔较短情况下,LDA高阶侧模相位锁定的现象是存在的。     

光泵气体THz激光器用CO2泵浦源腔内光场模式

CO2泵浦源是光泵气体太赫兹激光器的核心器件之一,其性能直接关系到太赫兹激光器的工作稳定性。CO2泵浦源一般由闪耀光栅作为全反射尾镜,构成特殊的光栅腔实现波长的选支输出。采用传输矩阵的特征向量法对光栅谐振腔的腔内光场模式进行了理论分析与数值模拟,计算了光栅腔的一系列本征模式及相应的衍射损耗。结果表明光栅腔的腔内模式特性等效于一个平凹腔,而对于大菲涅尔数的光栅腔, Littrow波长的附近支线也可能具有衍射损耗较低的低阶模,可能优先于Littrow波长的高阶模起振,导致光栅腔的波长选择性降低。     

Phase measurements in surface nonlinear optics: The effect of laser beam quality

When the relative phase of a surface nonlinear polarizability is measured by interference techniques, the resultant interferogram may be of lower contrast than that expected for ideal waves. It is shown, both theoretically and experimentally that this degradation arises from the multimode character of the pulsed lasers used. Differences between longitudinal and transverse mode effects are demonstrated. The laser coherence time may be deduced from the fringe degradation caused by longitudinal modes.     

Talbot外腔锁相相位补偿元件的理论计算

 在菲涅耳衍射公式和模耦合理论的基础上，对有相位补偿的 Talbot外腔锁相在腔内场的分布进行了数值计算。计算结果表明：理想的相位补偿元件模式选择镜能够消除由于边缘效应引起的近场幅度不均匀，而且同相模的衍射损耗非常小，能有效地起到选择同相模的作用。实用的相位补偿元件相位型光栅对近场的幅度没有影响，也能够明显地减小同相模的衍射损耗，起到选择同相模的作用。     

Measurements of the waist and the power distribution across the transverse modes of a laser beam

We describe a novel apparatus which measures the waist, its position and the amplitudes of the transverse modes in a laser system starting from intensity measurements. The validity of our approach is confirmed by many numerical simulations and by preliminary experimental results.     

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.     

Nonlinear interaction of laser modes

The interaction of N atoms each with 3 levels at random lattice sites with a set of cavity modes is considered. The optical transition between the lowest two atomic levels is taken into account explicitely assuming a Lorentzian line shape, whereas the third level just serves for the pumping process. If homogeneous inversion of the atoms is assumed, only one coherent mode oscillates in the steady state. It is the one being closest to the atomic resonance and having highestQ. If, however, in the next approximation a mode-dependent depletion of the excited atomic states is taken into account, with increasing pumping rate several modes may oscillate simultaneously. The behaviour of two such modes is treated in detail and it is shown, that one obtains a stable configuration. Using a higher approximation the nonlinear interaction between these two modes brought about by the amplifying material is studied in detail. As a special result one obtains a repulsion of the frequencies of the modes as a function of pumping power in accordance with gaslaser experiments. Quantum noise effects are neglected throughout the present paper.