腔内倍频调Q激光器中脉冲光场的时间不稳定性研究

以腔内倍频调Q激光器工作原理为基础,研究了多种形式抽运光分布的情况下倍频光的脉冲形状以及脉冲持续过程中光场分布的时间不稳定性,分析了倍频光光束质量随时间的变化规律.发现腔内倍频调Q激光器输出的倍频光光场分布在脉冲的持续过程中并不稳定,呈持续变化.对高斯型抽运的情况研究发现,减小腔长会使脉冲光场分布的不稳定性减小;而增加...     

Determination of spectral width and chirp of ultrashort light pulses by analysis of second harmonic beam divergence

It is shown that in the noncollinear second harmonic generation scheme used for ultrashort light pulse duration measurements the divergence of the second harmonic beam is determined by the spectral width and chirp of the pulse. This enables one to measure simultaneously the spectral width, pulse duration, spectral-time product, chirp and its sign for a single pulse in the same device.     

Non-one-dimensional optical solitons and ideal fluid dynamics

Influence of transverse effects on the propagation of light beams in gradient glass fiber and on the pulsed mode of the second harmonic generation is investigated. Within the approaches under consideration the equations for the pulse or beam field envelopes are reduced to a system of hydrodynamictype equations for amplitudes and eikonals. These equations are used to describe the vortex and non-vortex beam channeling modes and the propagation of light bullets.     

飞秒BBO光参量放大中闲频光二次谐波的产生

由于相位匹配条件和非线性晶体透光范围的限制，400nm蓝光抽运的飞秒β-BaB₂O₄（BBO）光参量放大（OPA）输出的参量光调谐范围有限，很难得到波长小于460nm的蓝光和近紫外光.实验采用1kHz钛宝石九通啁啾脉冲放大器的倍频蓝光作抽运光，超连续白光 作种子光，在Ⅰ类非共线相位匹配条件下，利用宽带的飞秒BBO OPA，在一定的实验参数下 获得了530—810nm放大的信号光，以及810nm—17μm波段范围的闲频光.与此同时 ，还获得了410—700nm连续可调的闲频光的二次谐波，其与闲频光层叠分布，单脉冲能量 为26μJ，转换效率大于5%.仅利用单块晶体的飞秒BBO OPA就可以获得410—810nm连 续可调的飞秒脉冲输出，从而为更多研究和应用的需要提供了重要的光源.对飞秒光参量放 大中闲频光二次谐波产生的条件也进行了理论分析. 关键词： 二次谐波 闲频光 非共线相位匹配 飞秒光参量放大     

Exact solitary-wave solutions of chi(2) Ginzburg-Landau equations

A family of exact temporal solitary-wave solutions (dissipative solitons) to the equations governing second-harmonic generation in quadratically nonlinear optical waveguides, in the presence of linear bandwidth-limited gain at the fundamental harmonic and linear loss at the second harmonic, is found, and the existence domain for the solutions is delineated. Direct numerical simulations of the solitons demonstrate that, as well as the classical pulse solutions to the cubic Ginzburg-Landau equation, the dissipative solitons can propagate robustly over a considerable distance before the model's intrinsic instability leads to onset of "turbulence." Two-soliton bound states are also predicted and then found in the direct simulations. We estimate real values of the physical parameters necessary for the existence of the solitons predicted, and conclude that they can be observed experimentally. A promising application for the solitons is their use in closed-loop cavities.     

Peculiarity of metal drilling with a commercial femtosecond laser

A commercial femtosecond pulse laser was used to study the interaction of ultrashort laser pulses with aluminum. Tests were conducted to measure the average drilling rate over a range of laser pulse energies in both air and vacuum at the wavelengths corresponding to the fundamental and second harmonic of the laser. For the fundamental wavelength, it was observed that the drilling rates in vacuum were significantly higher than that for drilling in atmospheric air. For the laser beam that was converted to second harmonic, the drilling rate in vacuum at the same energy was slightly lower than that for drilling in air. The observed results can be explained by the presence of an energetic nanosecond pedestal in the laser pulse produced by the femtosecond laser system. This nanosecond component provides a major contribution into drilling and it is strongly affected by the optical breakdown plasma that reduces the drilling rate in air. Conversion to second harmonic reduces the relative energy content of the nanosecond component resulting in a higher contrast femtosecond pulse that is not affected by the near surface plasma. The presence of air results in self-focusing of the second harmonic laser beam, causing an increased drilling rate as compared to the interaction in vacuum.     

SECOND HARMONIC GENERATION WITH PHOTON NUMBER SQUEEZED LIGHT AS INPUT

We investigate the quantum statistics of the fields produced in second harmonic generation when photon number squeezed light ia used as input. It is found that the squeezed properties of the pump beam can be transferred to a certain extent to the second harmonic beam. The normalized photon fluctuations, or Fano factors, of the fundamental and second harmonic output fields are derived for various values of the interaction parameters.     

Third order harmonic imaging for biological tissues using three phase-coded pulses

Compared to the fundamental and the second harmonic imaging, the third harmonic imaging shows significant improvements in image quality due to the better resolution, but it is degraded by the lower sound pressure and signal-to-noise ratio (SNR). In this study, a phase-coded pulse technique is proposed to selectively enhance the sound pressure of the third harmonic by 9.5 dB whereas the fundamental and the second harmonic components are efficiently suppressed and SNR is also increased by 4.7 dB. Based on the solution of the KZK nonlinear equation, the axial and lateral beam profiles of harmonics radiated from a planar piston transducer were theoretically simulated and experimentally examined. Finally, the third harmonic images using this technique were performed for several biological tissues and compared with the images obtained by the fundamental and the second harmonic imaging. Results demonstrate that the phase-coded pulse technique yields a dramatically cleaner and sharper contrast image.     

Ba1-xB2-y-zO4SixAlyGaz晶体和频可调谐深紫外飞秒激光器

介绍了一种基于新型非线性晶体Ba_1-xB_2-y-zO₄Si_xAl_yGa_z 的可调谐深紫外飞秒激光光源. 从理论上分析了基频光和倍频光在通过非线性晶体时所造成的空间走离和群速度失配, 为了补偿空间走离以及波长调谐过程中晶体折射造成的光束偏离现象, 将两块相同的倍频晶体成镜像放置来产生二次谐波. 并调节延迟线的长度来补偿基频光和倍频光之间的群速度失配, 从而提高和频转换效率. 然后通过和频方式进行三倍频和四倍频来突破晶体相位匹配条件的限制, 产生了波长低于200 nm的深紫外飞秒激光. 利用钛宝石激光器提供基频光光源, 最终在250–300 nm, 192.5–210 nm 范围内获得了高重频、可调谐超短脉冲紫外和深紫外激光. 并在基频光波长为800 nm时, 得到的二倍频、三倍频和四倍频的功率分别为1.28 W, 194 mW和5.8 mW, 相对于前一级的转换效率依次为46.14%, 15.16%和3%. 采用互相关法测量得到266.7 nm紫外激光的脉冲宽度约为640.4 fs.     

High-energy, spatially flat-top green pump laser by beam homogenization for petawatt scale Ti:sapphire laser systems

We have demonstrated that we can homogenize the spatial profile of a high-energy green laser pulse used for pumping a petawatt scale Ti:sapphire amplifier. The second harmonic of a high-energy, large aperture Nd:glass laser system generates laser emission at a green wavelength with 75 J single pulse energy. Using a diffractive optical element for beam homogenization, we have obtained a highly spatially uniform flat-top second harmonic profile.     

Ultrashort pulse propagation and nonlinear frequency conversion in superconducting and magnetic photonic crystal

We study the propagation and second harmonic generation of ultrashort pulse in nonlinear photonic crystal using a combination of Fourier transform and transfer matrix method. The focus is on the reflected and transmitted output of fundamental pump pulse and second harmonic pulse in frequency and time domains, the temperature dependence of the reflection and transmission spectra where the superconducting transition frequency is close to the magnetic resonance. Interesting features include output pump and second harmonic pulses that can be strongly modulated with the transmitted pulses being delayed by slow light effect.     

Evidence of ultrashort electron bunches in laser-plasma interactions at relativistic intensities

The second harmonic of the laser light (2omega(0)) is observed on the rear side of thick solid targets irradiated by a laser beam at relativistic intensities. This emission is explained by the acceleration by the laser pulse in front of the target of short bunches of electrons separated by the period (or half the period) of the laser light. When reaching the rear side of the target, these electron bunches emit coherent transition radiation at 2omega(0). The observations indicate that, in our conditions, the minimum fraction of the laser energy transferred to these electron bunches is of the order of 1%.     

Generation of 0.1-TW 5-fs optical pulses at a 1-kHz repetition rate

A compact all-solid-state femtosecond Ti:sapphire oscillator?amplifier system using no grating-based pulse stretcher produces 20-fs, 1.5-mJ pulses at a 1-kHz repetition rate. The pulses are subsequently compressed in a hollow-fiber chirped-mirror compressor. The system delivers bandwidth-limited 5-fs, 0.5-mJ pulses at 780 nm in a diffraction-limited beam.     

激光二极管侧面泵浦高功率266 nm紫外激光器

 报道了一种半导体激光列阵侧面泵浦Nd：YAG四倍频266 nm全固态紫外激光器,采用Z型腔结构,Ⅰ类临界相位匹配LBO和BBO晶体分别作为二倍频晶体和四倍频晶体。在调制频率为5 kHz时,最终获得了2.1 W的266 nm紫外激光输出,单脉冲能量420 μJ, 绿光到紫外激光的转换率为13.13%,在相同的泵浦功率下利用V型腔结构仅获得305 mW的266 nm紫外激光输出。     

Optical properties of a waveguide written inside a LiTaO3 crystal by irradiation with focused femtosecond laser pulses

Optical waveguides were fabricated by modifying the optical properties inside a LiTaO₃ crystal by irradiation with femtosecond laser pulses, and the performance and optical properties of the waveguide were investigated. On the basis of near-field pattern of the light guided through the waveguides, the optimum writing conditions such as the pulse energy and the pulse duration were determined. The waveguide written inside a LiTaO₃ crystal had a strong polarization dependence of guided light. Observation by a polarized microscope suggested that the stress distribution around the photoexcited region should be responsible for the polarization dependence of guided light. In addition, the second harmonic generation from a laser beam passing through the waveguide was observed, which suggests that the ferroelectric properties of LiTaO₃ were preserved after the photoexcitation.     

Femtosecond third-harmonic generation:. self-phase matching through a transient. Kerr grating and the way to ultrafast computing

The third harmonic of 810-nm 100-fs pulses at 130 μJ is generated very efficiently when ultrashort pulses from two noncollinear beams interfere in an optical medium to create an instantaneous transient grating via the optical Kerr effect. The grating couples two pathways for third-harmonic generation, each taking two photons from one beam and one photon from the other beam, respectively. The coupling enables self-phase matching in the complete process, resulting in a conversion efficiency of ≈3%. Scattering an independent beam at the transient grating confirms a lifetime limited by the pulse duration, with a reaction on the order of one optical cycle. Using the second harmonic of a Ti-sapphire laser at 405 nm, it is shown that the generation of the transient Kerr grating is a general feature, requiring less than 20 μJ/pulse. By introducing a third femtosecond beam we are able to emulate various digital logic units with femtosecond response. Received: 16 October 2001 / Published online: 6 June 2002     

非线性有限束光声效应理论

当一束受正弦调制的激光入射于光声腔中的固体样品上时,由于非线性的光声效应,在光声腔中不仅能接收到基频成份的声信号,还能接收到其二次谐波成份。本文提出一个非线性热波束方程及其相应的非线性边界条件。借助于逐步近似法在光源为高斯径向分布的情形下,求解这一方程。利用Hankel变换获得这一方程的一级与二级近似解。解析结果表明,二次谐波的热波束仍然维持高斯径向分布,而其高斯半径比基频成份小。分析还表明,二次谐波的振幅不仅与线性热参数,而且也与非线性热参数有关。后者或许能提供样品的更多的有意义的信息。综合其各种特点, 关键词：     

Generation of superintense femtosecond pulses by the Cr:forsterite laser system

A repetitively pulsed chromium-forsterite laser system is created. High-power femtosecond light pulses are generated at the fundamental (1.24 μm) and second-harmonic (0.62 μm) wavelengths. Theoretical analysis is performed to optimize the output pulse energy. Laser pulses with a duration of 110 fs, an energy of 1 mJ, and a repetition rate of 1–50 Hz are generated. The intensity of the focused beam is greater than 10¹⁶ W/cm². High-efficiency radiation conversion into the second harmonic is used to increase the energy contrast of the generated pulses. Original Text ? Astro, Ltd., 2006.     

Spectral distributions of harmonic generation from electron oscillation driven by intense femtosecond laser pulses

The characteristics of harmonic radiation due to electron oscillation driven by an intense femtosecond laser pulse are analyzed considering a single electron model. An interesting modulated structure of the spectrum is observed and analyzed for different polarization. Higher order harmonic radiations are possible for a sufficiently intense driving laser pulse. We have shown that for a realistic pulsed photon beam, the spectrum of the radiation is red shifted as well as broadened because of changes in the longitudinal velocity of the electrons during the laser pulse. These effects are more pronounced at higher laser intensities giving rise to higher order harmonics that eventually leads to a continuous spectrum. Numerical simulations have further shown that by increasing the laser pulse width broadening of the high harmonic radiations can be limited.     

高斯光束在偏硼酸钡单晶和孪晶中的倍频现象

理论探讨了高斯光束在非线性单轴晶体中的倍频效应。基于麦克斯韦方程组并考虑晶体的各向异性推导出负单轴I型晶体相位匹配下的倍频耦合波方程组。建立了具体的关于偏硼酸钡（BBO）单晶以及孪晶的物理模型并做数值求解。对求解所得到的单晶以及孪晶的谐波能流分布、晶体出射面探测点处谐波电场时域分布、总输出能量、输出光束频谱和输入光束角谱分别进行分析讨论,从束腰宽度、晶体结构以及材料性质对倍频的影响等方面做出具体合理的解释。研究结果对于单轴晶体的非线性光学性质的研究以及相应的光学晶体器件的研发等具有一定的参考价值。