Generalized Approximation of Strong Interaction of Waves in the Theory of High-Power Radiation Frequency Doubling

The approximation of strong interaction of waves is developed to analyze the generation of the second harmonic under the conditions of self-action. Based on this method, an approximate expression is obtained for the generation efficiency of the second harmonic with allowance for the effect of higher nonlinearities, depletion of pumping radiation, and linear phase detuning. The effect of phase detuning and change in the spatial distribution of the amplitude of the fundamental radiation on the efficiency of second-harmonic generation has been analyzed. Good agreement of the results of the developed approximate method with experimental data and numerical calculations is shown. The optimum conditions of the second-harmonic generation in a wide range of the laser system radiation intensities are determined.     

Strong-interaction approximation in the theory of second harmonic generation of intense short laser pulses

The method of strong interaction of nonlinear waves has been developed to analyse the second harmonic generation of intense laser radiation in the transient regime. Analytic solution for frequency conversion efficiency – taking into account the pump radiation depletion, influence of phase mismatch, dispersion of group velocities and higher nonlinearities – has been first obtained. A comparison of theoretically derived results and known experimental data has been conducted, and a general agreement of these results has been obtained. The optimal conditions of the second harmonic generation of neodymium laser radiation into femtosecond pulse duration range have been determined.     

基于群速度失配的超声兰姆波二次谐波的时域测量方法

考虑到发射和接收换能器对超声兰姆波时域二次谐波信号所带来的不可避免的影响,提出一种基于基频与二倍频兰姆波群速度失配的超声兰姆波二次谐波的时域测量方法。当基频与二倍频超声兰姆波的相速度匹配而群速度失配时,在超声兰姆波传播过程中所发生的二次谐波信号,在时域上可与源于斜劈换能器的二次谐波信号相分离。采用仅源自于基频兰姆波的时域二次谐波的积分振幅,定量描述兰姆波二次谐波的发生效率。以铝板中传播的兰姆波为例,给出了时域二次谐波的具体测量过程。本文提出的测量方法放宽了超声兰姆波二次谐波的测量条件,且扣除了换能器对二次谐波信号所带来的影响,所测得的二次谐波信号完全来自于基频兰姆波时域信号的二次谐波发生效应。      

Multiphoton ionization of atoms by a two-color laser pulse

The probability of multiphoton ionization of atoms in a laser radiation field containing an additional second harmonic is calculated by the imaginary time method [9, 10]. The conditions are found when the second-harmonic contribution to the ionization of atoms dominates over that of the first harmonic. It is shown that the average momentum of photoelectrons ejected from atoms depends on the phase shift between the first and second harmonics and their mutual polarization. The obtained asymptotic expressions can be used for the qualitative explanation of experiments on generation of terahertz radiation from the optical breakdown region in gas in the focus of a femtosecond laser.     

Peculiarities of self-action in second harmonic generation of laser radiation in nonlinear crystals

By using the generalized method of strong interaction of nonlinear waves and computer simulation, an analysis of the influence of self-action effects on second harmonic generation of an intense diverging picosecond pulse is performed. Using the approximation of strong wave interaction, an analytic solution is obtained for frequency conversion of the laser radiation taking into account the pump intensity depletion, the influence of the angular dispersion effect (ADE), and the higher nonlinearities. It is shown that the self-action effects, the ADE, and the linear phase mismatch can compensate for each other, increasing the efficiency of second harmonic generation. Optimum conditions for second harmonic generation in converging and diverging laser beams were found. An asymmetry of the angular dependence of the second harmonic intensity was experimentally observed and theoretically explained. The asymmetry is caused by the reverse energy conversion of the second harmonic into the pump by the influence of the ADE and Kerr nonlinearities.     

Highly efficient SHG of a sequence of laser pulses with a random peak intensity and duration

One of the methods of obtaining a highly efficient second-harmonic generation (SHG) for laser pulses with random amplitudes and durations is mathematically modeled. The essence of the method proposed consists of the use of quadratic nonlinearity of the medium under conditions of a large mismatch between the wave numbers of the fundamental wave and the second harmonic. In the literature, the SHG under these conditions is referred to as the cascade SHG. Under certain conditions, this process is equivalent to the action similar to that of the cubic nonlinearity. This allows one to implement the both compression and decompression of laser pulses due to the cascade generation at a fairly low intensity of the pulses such that the Kerr nonlinearity does not manifest itself. The cascade generation is shown to enable one to implement tenfold or higher suppression of fluctuations of the input peak intensity of the laser radiation in the regime of free generation, which potentially makes it possible to implement the SHG under optimum conditions for all pulses in the sequence.     

一种评价固体板材表面性质的非线性超声兰姆波方法

在基频与二倍频兰姆波相速度匹配但群速度失配的条件下, 通过选择适当的兰姆波二次谐波时域信号的测量起止时间, 可完全扣除换能器对二次谐波积分振幅测量所带来的影响。本文提出采用兰姆波二次谐波的积分振幅作为评价参量, 以实现对板材表面性质的准确评价。当板材表面性质发生改变时, 原本在理想表面条件下成立的基频与二倍频兰姆波相速度匹配的条件不再严格满足, 这将显著地影响到兰姆波的二次谐波发生效率, 相应的二次谐波积分振幅随表面性质的改变也将发生非常敏感的单调变化。实验结果表明,利用扣除换能器影响之后所测得的兰姆波二次谐波的积分振幅,可对板材表面性质的变化情况进行准确评价。     

Phase effects at second harmonic generation of powerful laser radiation in noncentrosymmetrical media

There has been developed the theory of second harmonic generation of the intensive laser fields in the existence of both quadratic and cubic polarization in medium in the constant-intensity approximation accounting for the reverse effect of the excited wave on the exciting one and simultaneously allowing us to take into account phase mismatch and the damping of all the interacting waves. It is shown that the changes of pump intensity through self-phase and cross-phase modulation processes effect optimum phase relationship between interacting waves, the change of the spatial beating period. The conditions of compensating undesirable phase shifts between interacting waves have been determined, the analytical expressions for calculation of optimum values of phase mismatch, length of noncentrosymmetrical medium and the spatial beating period are offered. It is shown that in the absence of linear phase mismatch with an increase of basic radiation intensity the spatial beating period is being reduced. The numerous analysis has been made of frequency doubling process efficiency for KDP and LiNbO₃ crystals.     

Second-harmonic generation of a Gaussian laser beam in a self created magnetized plasma channel

The second-harmonic generation of an intense self-guided right circularly polarized laser beam in a magnetized plasma is investigated. The laser imparts oscillatory velocity to electrons and exerts a radial ponderomotive force on them to create a depleted density channel. The critical power for self-focusing shows huge reduction as electron cyclotron frequency approaches the laser frequency (/spl omega//sub c/ /spl rarr/ /spl omega/). In the presence of the self-created radial density gradient, the laser drives a density perturbation at the fundamental frequency. The density perturbation beats with the oscillatory velocity to produce a second harmonic current density, driving second harmonic radiation copropagating with the laser. The second harmonic, however, is azimuthally asymmetric with /spl theta/-variation as exp(i/spl theta/). Its amplitude shows resonant enhancement as /spl omega//sub c/ /spl rarr/ /spl omega/.     

101 W of average green beam from diode-side-pumped Nd:YAG/LBO-based system in a relay imaged cavity

A 52-W green laser at 532 nm by extra-cavity second-harmonic generation in a coupled-cavity configuration is demonstrated. The fundamental laser is a diode-side-pumped acousto-optic (AO) Q-switched Nd:YAG rod laser producing 84 W of average power at 1064 nm at 8 kHz repetition rate. Type-II phase-matched polished KTP crystal is used as the nonlinear crystal for second-harmonic generation. The individual green pulse width is 50 ns and the fundamental to second harmonic conversion efficiency is 61.8%.     

Efficient and high-power green beam generation by frequency doubling of acousto-optic Q-switched diode-side pumped Nd:YAG rod laser in a coupled cavity

A 52-W green laser at 532 nm by extra-cavity second-harmonic generation in a coupled-cavity configuration is demonstrated. The fundamental laser is a diode-side-pumped acousto-optic (AO) Q-switched Nd:YAG rod laser producing 84 W of average power at 1064 nm at 8 kHz repetition rate. Type-II phase-matched polished KTP crystal is used as the nonlinear crystal for second-harmonic generation. The individual green pulse width is 50 ns and the fundamental to second harmonic conversion efficiency is 61.8%.     

KNbO_3高效倍频产生427～470nm蓝紫光

厚1.5 mm的KNbO_ 3晶体在室温下角度相位匹配倍频脉冲参量激光获得427～470nm蓝紫光,能量转换效率超过40％,输出单脉冲蓝紫光的峰功率达60kW.对相位匹配曲线、倍频效率与基波光强的关系和相应匹配角允差作了计算和讨论.     

基于角色散的非共线匹配宽带三倍频技术

宽带光打靶可以有效降低激光等离子体相互作用过程中非线性效应。提出一种基于角色散的非共线匹配宽带三倍频方案,利用宽带基频与窄带二倍频的非共线和频产生宽带三倍频,和频过程中通过特殊设计的渐变光栅实现不同频率的基频光束以特定角度入射,补偿了波长差异引入的位相失配使得全波段满足位相匹配条件。理论模拟表明,采用KDP晶体Ⅱ类位相匹配,将中心波长为1058 nm、带宽10 nm的宽带基频光与526.5 nm的二倍频光进行非共线匹配和频,可以实现高效宽带三倍频转换。     

Methods of enhancement of the efficiency of generation of the second optical harmonic of spatially limited laser beams

Frequency doubling of spatially limited laser beams in nonlinear optical crystals is analyzed. Conditions are found under which the diaphragm aperture effect limiting the efficiency of the process of second optical harmonic generation can be reduced. It is shown that a spatial shift of mutually orthogonally polarized beams of fundamental radiation in the direction perpendicular to the direction of propagation of the waves, as well as the optimization of the ratio of the spatial dimensions and the intensity at the entrance of the nonlinear medium, gives rise to an increase in the efficiency of second harmonic generation.     

Theory of backward second-harmonic generation of short laser pulses in periodically and aperiodically poled nonlinear crystals

We study numerically and analytically the backward second-harmonic generation in periodically and aperiodically poled nonlinear crystals in the quasiphase-matched regime. In our numerical analysis, we apply the optimum control technique based on the shooting method. Due to the fast and efficient numerical technique, we are able to analyze this frequency-conversion process in detail, taking into account various factors ?C the pump intensity, group-velocity mismatch and dispersion, linear phase mismatch, and ??chirp?? of nonlinear grating. Also we derive several approximate solutions for the backward harmonic efficiency employed in the undepleted pump approximation.     

BBO晶体中飞秒脉冲的二次和三次谐波振荡产生

同时考察了群速色散失配、最低阶群速色散和二阶群速色散。研究了在ＢＢＯ晶体中飞秒激光脉冲的谐产生，计算了ＢＢＯ晶体的作为波长函数的晶体的最低阶和二谐群速色散，借助求解改进的耦合波方程的数值计算，得到了飞秒脉冲的二次及三次谐波辐射，分析了最低工阶及二阶ＧＶＤ对于基波脉冲和谐波脉的影响，最后考虑了三次谐波产生中群速失配的补偿。     

Continuous-wave single-frequency 532 nm laser source emitting 130 W into the fundamental transversal mode

The nonlinear effect of second-harmonic generation is an efficient way to realize high-power green laser sources. But when scaling up the harmonic power, many setups reported in the literature have been limited by conversion efficiency degradation or the fundamental laser power. Here we report on the generation of 134 W of cw laser light at a wavelength of 532 nm from a fundamental power of 149 W by second-harmonic generation in an external optical resonator comprising a lithium triborate crystal. The external conversion efficiency was 90%. The harmonic light consisted of a single spectral line. At least 97% of it was emitted into the fundamental transversal mode.     

Frequency doubling of multi-terawatt femtosecond pulses

We present an experimental and theoretical study of the influence of the spatial beam quality (fluence and phase distributions) on the second-harmonic generation in KDP crystals pumped by 180-fs pulses at 790 nm. Conversion efficiency and beam focusability are investigated experimentally and theoretically by the numerical analysis of the second harmonic, considering effects due to the cubic nonlinearity, beam diffraction, group-velocity walk-off, and dispersion of the pulses. It was found that the uniform intensity and phase distributions of the fundamental beam are essential to obtain a high focal intensity of the second-harmonic beam. PACS 42.65.Ky; 42.65.Re     

Coherent control of THz wave generation in ambient air

Our study of THz wave generation in the pulsed laser induced air plasma with individually controlled phase, polarization, and amplitude of the optical fundamental wave (omega) and its second harmonic (2omega) indicates that the third-order nonlinear optical process mixing the omega and 2omega beams in the ionized plasma is the main mechanism of the efficient THz wave generation. The polarity and the strength of the emitted THz field are completely controlled by the relative phase between the omega and 2omega waves. The measured THz field amplitude is proportional to the pulse energy of the fundamental beam and to the square root of the pulse energy of the second-harmonic beam once the total optical pulse energy exceeds the plasma formation threshold. The optimal THz field is achieved when all waves (omega, 2omega, and THz waves) are at the same polarization in the four-wave-mixing process.     

Vector solitary optical beam control with mixed type I–type II second-harmonic generation

The four vector equations, appropriate to a mixed type I–type II second-harmonic generation, in a thin planar waveguide, made from a second-order non-linear material, are solved, both approximately and exactly. The solutions are then used to generate possible applications. In this investigation, both the fundamental and the second-harmonic waves have two transverse field components. It is shown that, by controlling the ratio of the components of the fundamental wave, one component of the second-harmonic wave can be rigorously controlled, and even switched off. The ratio of the harmonic field components depends strongly upon the polarization angle of the fundamental wave, with the extinction angle depending strongly upon the phase mismatch parameter. In a second application, an aperture is placed at the output and it is shown that if approximate stationary states are used as an input to a quadratically non-linear medium, then varying the angle of incidence of two input beams produces an excellent output control. A numerical demonstration is given in which a switch from 80% of the input energy arriving at the output port to less than 3% is readily achievable.