共查询到20条相似文献,搜索用时 31 毫秒
1.
The spatiotemporal transformation of a focused femtosecond pulse in fused silica at power much less than the critical power for self-focusing was numerically studied. We found that the pulse experiences unusual spatiotemporal reshaping, such as refocusing of the near-axis portion of the defocused pulse tail, even when the self-focusing effect was not taken into the calculation. Further studies revealed that a tubelike spatiotemporal distribution of the plasma is formed during pulse propagation and that it plays a decisive role in the transformations of the pulse. In addition, the tubelike plasma was found to exist universally under different external focusing conditions if the input pulse had sufficient power. 相似文献
2.
Salih H.A. Tripathi V.K. Pandey B.K. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》2003,31(3):324-328
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/. 相似文献
3.
针对脉冲宽度约1 ns、波长为351 nm的三倍频紫外激光,定量分析了熔石英介质中的自聚焦长度、峰值光强与强紫外激光光束质量及环境条件等因素的关系,研究了产生紫外光非线性自聚焦效应的阈值条件。研究结果表明:在强紫外激光光束质量一定的前提下,可将B积分值定义为自聚焦的阈值条件;入射强紫外激光光束质量越差,在熔石英介质中产生自聚焦的阈值条件越低;即使对于空间分布均匀的理想光束,当空气中存在灰尘时,经过一段距离的传输后,在熔石英介质中将导致强紫外激光自聚焦效应的产生,且灰尘尺寸较大时的自聚焦效应较明显,自聚焦的阈值条件也相对较低。 相似文献
4.
Expressions for the critical power for self-focusing, the self-focusing distance and the transverse dimension of the region of the developed instability have been derived for obliquely propagating whistler waves. These expressions have been used to estimate the various self-focusing parameters for ionospheric as well as magnetospheric propagation. 相似文献
5.
We study theoretically wideband modulation instability at combination frequencies in media having cubic nonlinearity of self-focusing
type along with the higher-order defocusing nonlinearity. It is assumed that in a medium with a purely cubic nonlinearity,
the medium dispersion does not permit modulation instability. In this case, a collapse of the wave field exists if the beam
power is higher than the critical power of self-focusing. The higher-order nonlinearity limits the field at the nonlinear
focus, and the instability at combination frequencies becomes possible. It turns out that the field at the nonlinear focus
increases with increasing excess of the beam power over the critical power of self-focusing. The obtained values of the nonlinear
dielectric permittivity are used for determination of the growth rates of instability at combination frequencies. These growth
rates ensure an increase in the combination fields from noise levels up to values comparable with the field of the high-power
beam. Such an increase takes place if the beam power is severalfold higher than the critical one. The developed theory can
be used for explanation of spectrum superbroadening during self-focusing of sufficiently short laser pulses and high-harmonic
generation.
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Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 6, pp. 522–532, June 2007. 相似文献
6.
Theory of self-focusing in a hollow waveguide 总被引:1,自引:0,他引:1
We present a theoretical investigation of self-focusing in a hollow waveguide filled with noble gas. Our analysis was performed for a laser pulse that was predominantly in the fundamental mode and revealed the physical processes involved in self-focusing in a hollow waveguide. A critical power for self-focusing was obtained that was found to be substantially higher than the critical power for self-focusing in a bulk medium. Useful design criteria for pulse-compression systems are presented. We identify the parameter range for which the transverse variation of the pulse phase introduced by the Kerr nonlinearity is small. 相似文献
7.
Malka V. Faure J. Marques J.R. Amiranoff F. Courtois C. Najmudin Z. Krushelnick K. Salvati M.R. Dangor A.E. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》2000,28(4):1078-1083
We report on experimental results regarding the propagation of ultraintense laser pulses in a preformed plasma channel. In this experiment, the long (4-mm) fully ionized plasma channel created by the amplified spontaneous emission (ASE) was measured by interferometry before and after the propagation of the short laser pulse. Forward spectra show a cascade of Raman satellites, which merge with one another when the laser power was increased up to critical power for relativistic self-focusing Pc. The number of filaments measured by interferometry increases when the laser power increases. High conversion efficiency (≈10%) of second harmonic generation was observed in the interaction 相似文献
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9.
地基激光空间碎片清除和利用激光辐射把转换的太阳能从空间轨道输运到地面等应用中,不可避免地遇到高功率激光在非均匀大气中的传输问题。由于激光功率已远远超过大气非线性自聚焦临界功率,大气自聚焦效应是影响光束质量的一个重要物理因素。概述了近年来国内外高功率激光在非均匀大气中上行或下行传输的自聚焦效应研究进展,主要介绍了高功率激光在非均匀大气中的传输模型、理论基础、数值和解析研究方法,着重介绍了自聚焦效应对激光传输特性和光束质量的影响,并总结了优化靶面光束质量的方案。此外,还介绍了大气群速度色散效应和大气湍流效应等物理因素对激光光束质量的影响。最后,还提出了该领域值得进一步深入研究的一些问题。 相似文献
10.
超短脉冲强激光在空气中传输时由于非线性克尔自聚焦效应会使激光光束聚焦,造成空气的离化而形成等离子体,等离子体对激光光束又会产生散焦作用,这两种过程的动态平衡可以形成很长的等离子体通道,从而产生一系列复杂而有趣的现象.文章对通道形成的机理、锥角辐射、超连续谱和三次谐波的产生进行了阐述,并介绍了目前等离子体通道形成的几种理论模型.文章还对通道内的各种复合和辐射机制进行了分析. 相似文献
11.
In the present paper we have investigated the self-focusing behaviour of radially symmetrical rippled Gaussian laser beam
propagating in a plasma. Considering the nonlinearity to arise from relativistic phenomena and following the approach of Akhmanov
et al, which is based on the WKB and paraxial-ray approximation, the self-focusing behaviour has been investigated in some detail.
The effect of the position and width of the ripple on the self-focusing of laser beam has been studied for arbitrary large
magnitude of nonlinearity. Results indicate that the medium behaves as an oscillatory wave-guide. The self-focusing is found
to depend on the position parameter of ripple as well as on the beam width. Values of critical power has been calculated for
different values of the position parameter of ripple. Effects of axially and radially inhomogeneous plasma on self-focusing
behaviour have been investigated and presented here. 相似文献
12.
Efficient frequency-doubling is experimentally demonstrated in presence of beam self-trapping in congruent lithium niobate crystal. The self-trapping is induced by the generated second harmonic beam via photorefractive effect under an external applied field. The local space charge field distribution, formed by the second harmonic beam, is shown to efficiently trap both wavelengths. The dynamics of self-focusing is studied along with the power evolution of the second harmonic beam. Fast tuning of phase matching conditions in the written waveguide is realized by an externally applied voltage also used for the photorefractive confinement. 相似文献
13.
Farshad Sohbatzadeh Mohsen Rabbani Mahboubeh Ghalandari 《Optics Communications》2012,285(13-14):3191-3194
In this paper, we numerically investigate the relativistic self-focusing of laser beam with LG01 mode in plasmas. An effective potential is introduced to identify the critical power. Numerical methods are employed to solve the governing equations. To verify the numerical procedures, we compare the known analytical solution of the relativistic critical power for TEM00 mode with our numerical method. It is shown that there is a good agreement between our numerical results and the analytical solution. The critical power for relativistic self-focusing of a LG01 laser beam is about 6 times of that for a TEM00 laser beam. 相似文献
14.
We report the results of a study of white light generation in a 7.5 cm long crystal of a high band-gap material, barium fluoride, using ultrashort (<42 fs) laser pulses over a range of values of incident laser power that extend up to more than three orders of magnitude larger than the critical power for self-focusing (Pcr). We explore white light generation and the intensity and spectral distributions within filaments that are formed as a result of the interplay of self-focusing and plasma-induced defocusing. The onset of plasma effects occurs at power levels in excess of 7 GW for ultrashort pulses in BaF2. For incident power levels that are three orders of magnitude larger than the critical power, blue-shifting of the incident laser wavelength is observed in addition to asymmetric continuum generation. The blue shift enables us to estimate the temporal variation of the electron density in the plasma that is generated within the BaF2 crystal. PACS 52.38.Hb; 42.65.Jx; 42.65.Tg; 33.80.Wz; 52.35.Mw 相似文献
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16.
实验研究了超短脉冲激光的空间小尺度自聚焦的动态变化过程.具体实验过程是使经过正交细丝的衍射来实现激光的空间调制,然后通过非线性Kerr介质二硫化碳,随着输入功率的增大,超短脉冲开始出现小尺度自聚焦增长现象.实验发现,每个小尺度自聚焦峰值并不是随着功率的增大而无限地增长下去,而是在不同位置出现了新的调制增长点,这些新出现的增长与原有的增长相互竞争,最终导致了分裂成丝.通过数值模拟相应的传输过程,发现数值模拟结果与实验结果符合很好.
关键词:
小尺度自聚焦
高功率激光
超短脉冲 相似文献
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18.
We analyze the method of moving focus to determine the critical power for self-focusing by means of numerical simulation and a semianalytical model. It is shown that the original interpretation of a moving focus experiment does not hold in general and that inclusion of defocusing effects due to free electrons is necessary to relate the measured data to critical power. 相似文献
19.
M.R. Siegrist 《Optics Communications》1976,16(3):402-407
The propagation of intense laser pulses in a plasma is discussed in terms of a constant shape, paraxial ray approximation. Self-focusing due to ponderomotive forces and relativistic effects is investigated. It is found that the stationary self-focusing behaviour of each mechanism treated separately is similar, with several orders of magnitude difference in critical power. In stationary self-focusing due to the combined mechanisms, complete saturation of ponderomotive self-focusing prevents the occurrence of relativistic effects. Self-focusing lengths and minimum radii are given for a large range of beam powers. A characteristic focal spot radius is found which depends only on the plasma density. 相似文献
20.
We present a theoretical investigation of the self-focusing dynamics of femtosecond pulses in a hollow waveguide. We show that transverse effects play an important role in these dynamics, even for pulses that are significantly below the critical power for self-focusing in free space, and that excitation of higher-order modes of the waveguide results in the spreading of the pulse in time. Inclusion of self-steepening and space-time focusing in our model is necessary for properly capturing the pulse dynamics. 相似文献