Formation of conical emission of supercontinuum during filamentation of femtosecond laser radiation in fused silica

The formation of conical emission of supercontinuum during filamentation of femtosecond laser pulses with central wavelengths in a wide range is studied experimentally, numerically, and analytically. The frequency-angular intensity distribution of the spectral components of conical emission is determined by the interference of supercontinuum emission in a filament of a femtosecond laser pulse. The interference of supercontinuum emission has a general character, exists at different regimes of group velocity dispersion, gives rise to the fine spectral structure after the pulse splitting into subpulses and the formation of a distributed supercontinuum source in an extended filament, and causes the decomposition of the continuous spectrum of conical emission into many high-contrast maxima after pulse refocusing in the filament. In spectroscopic studies with a tunable femtosecond radiation source based on a TOPAS parametric amplifier, we used an original scheme with a wedge fused silica sample. Numerical simulations have been performed using a system of equations of nonlinear-optical interaction of laser radiation under conditions of diffraction, wave nonstationarity, and material dispersion in fused silica. The analytic study is based on the interference model of formation of conical emission by supercontinuum sources moving in a filament.     

整形飞秒激光脉冲的成丝超连续辐射控制

飞秒激光成丝超连续辐射具有高强度和高时空相干性等优点,作为一种超宽带光源在很多领域都具有广泛的应用前景.本文提出一种结合微透镜阵列的空间调制和基于液晶空间光调制器的时域整形的飞秒激光脉冲整形方式,利用基于遗传算法的反馈优化控制,实现了飞秒激光在熔融石英中成丝产生的超连续辐射强度的调制,得到了在一定范围内光谱强度可控的超连续辐射光谱;光谱的能量密度可以从0.03μJ/nm调制到0.09μJ/nm,其能量密度变化达到了初始值的3倍.计算了典型迭代代数对应的整形脉冲时域包络,分析了超连续光谱随迭代代数的演化趋势,结果表明,脉冲包络的峰值强度和波形分布是影响超连续光谱展宽和强度的主要物理原因.     

温度对飞秒激光脉冲在NaCl溶液中成丝产生的超连续谱的影响

利用飞秒激光脉冲在NaCl溶液中成丝,研究了溶液温度对飞秒激光成丝过程中产生的超连续谱的影响.发现在激光脉冲能量较低时,溶液温度对超连续谱的影响几乎可以忽略;而在激光脉冲能量较高时,随着NaCl溶液温度的升高,超连续谱呈现出被压缩的趋势.此外,在激光脉冲能量较高的情况下, NaCl中会产生大量的气泡.通过分析,得出了飞秒激光在溶液中成丝产生的气泡是影响超连续谱发射的主要因素.     

Spatio-temporal evolution scenarios of femtosecond laser pulse filamentation in fused silica

We investigated the evolution of femtosecond laser pulses at different wavelengths corresponding to normal, zero, and anomalous regimes of group velocity dispersion (GVD) in fused silica. The laser pulse filamentation in different GVD regimes under the same similarity parameters was first considered. It was established numerically that the scenario of the pulse filamentation depends both on temporal factors, which are determined by pulse GVD and self-phase modulation, and spatial factors associated with Kerr self-focusing and plasma defocusing. In presence of strong normal GVD the dispersive stretching causes, a pulse power decrease followed by lowering of the intensity in filament, electron density reduction in plasma channel, and suppressing of the refocusing. For zero GVD the multipeak regime of radiation propagation is realized in the filament as a result of recurring self-focusings of powerful pulse tail, which was defocused in laser plasma. When GVD is anomalous a sequence of ??light bullets?? with duration about 10 fs forms in the filament. And the peak intensity in ??light bullet?? stays the same ?? 5 × 10¹³ W/cm². In the regime of anomalous GVD power is transferred from the pulse edges to its center, where the repeated self-focusings occur and form a ??light bullet?? sequence.     

Modeling the terahertz radiation generation process,obtained by filtration of spectral supercontinuum,formed during propagation of femtosecond laser pulse in GaAs crystal

We present the results of theoretical studies of the process of generation of terahertz radiation arising via interaction of few-cycle laser pulses propagating in an isotropic nonlinear medium. Numerical time-integration, by the finite-difference method, of the system of nonlinear Maxwell equations has been performed. We consider the interaction of mutually-orthogonal linearly polarized pulses, both having the central wavelength of 1.98 μm, durations of 30 fs, and the energies of 30 nJ, propagating along the normal to the 〈110〉 plane in a 1 mm-thickness GaAs crystal. In the nonlinear part of the medium polarization the without-inertia nonlinear second-order susceptibility is taken into account. The process of formation of a terahertz pulse arising via spectral filtration of supercontinuum formed in the spectra of pump pulses at the output of nonlinear crystal is studied. The dependences of both the current frequency of the pump pulses on time for different lengths of nonlinear crystal and of pump pulse durations on the crystal length are obtained. Also the dependences of the current frequency of the terahertz pulse on time at different crystal lengths, as well as of the efficiency of generation of terahertz radiation on the length of nonlinear crystal and on the energy of pump pulses are obtained.     

Processing materials in the mode of multiple filamentation of femtosecond laser radiation

Results are presented from experimental studies of the filamentation of femtosecond laser radiation in a transparent medium. Schemes for registering plasma channels of filaments, conical emission, and the spatial distribution of radiation intensity are provided. The results from laser action on a target of stainless steel in the multiple filamentation mode.     

Infrared extension of femtosecond supercontinuum generated by filamentation in solid-state media

We carried out experimental and numerical study of the ultrafast supercontinuum generation by filamentation of 120 fs, 800 nm laser pulses in sapphire under variable focusing conditions. We demonstrate that the supercontinuum spectra produced in loose focusing conditions have much larger infrared extent, while the spectral blueshift remains fairly the same. Our numerical simulations reproduce the experimental results in great detail and disclose that the input beam focusing conditions affect only the nonlinear propagation of the leading sub-pulse, which emerges after the pulse splitting event, and which is responsible for the redshifted spectral broadening.     

Polarization-dependent femtosecond laser filamentation in air

We report on a systematic study of the laser polarization effect on a femtosecond laser filamentation in air.By changing the laser's ellipticity from linear polarization to circular polarization, the onset position of laser filament formation becomes farther from the focusing optics, the filament length is shorter, and less laser energy is deposited. The laser polarization effect on air filaments is supported by a simulation and analysis of the polarization-dependent critical power and ionization rates in air.     

Multiple filamentation of femtosecond laser pulses

The formation of fluorescent channels with color centers in LiF crystals under the action of the multiple filamentation of femtosecond laser pulses is studied experimentally and theoretically for pulse powers around four orders of magnitude higher than the critical self-focusing value.     

超连续谱注入飞秒光参量放大实验研究

提出了一种很有特色的基于超连续谱注入的飞秒光参量放大技术,该种技术方法不仅克服了常规1 053 nm脉冲在前端小能量放大时的增益窄化现象,而且可实现800 nm激光和1 053 nm激光的零时间同步。通过系统的实验研究,获得了大于4 mJ的1 053 nm宽带信号光能量     

超连续谱注入飞秒光参量放大实验研究

 提出了一种很有特色的基于超连续谱注入的飞秒光参量放大技术,该种技术方法不仅克服了常规1 053 nm脉冲在前端小能量放大时的增益窄化现象,而且可实现800 nm激光和1 053 nm激光的零时间同步。通过系统的实验研究,获得了大于4 mJ的1 053 nm宽带信号光能量     

Near-field modification of femtosecond laser beam to enhance single-shot pulse filamentation in glass medium

In this paper, a study of the effect of a transparent dielectric medium (soda-lime glass placed after an objective lens) on femtosecond pulse filamentation in Corning Eagle²⁰⁰⁰ glass is presented. The dielectric medium placed after the objective lens increases the effective numerical aperture of the focusing lens and tightly focuses the beam to elevate the power density in a tiny focal volume. A single-shot narrow filamentary void of diameter 3.8 μm is observed to propagate as long as 700 μm in Corning Eagle²⁰⁰⁰ glass. Elevated optical intensity confined in the small focal volume and low ionization losses per unit length in the narrow plasma column collectively enhance the filamentary propagation of the ultra-short pulse in bulk glass. The applied technique also offers an approach to control the length and width of a filamentary void structure in a transparent dielectric medium.     

3D Raman bullet formed under filamentation of femtosecond laser pulses in air and nitrogen

Complex experimental study of spectral, spatial and temporal behaviors of the IR shifted component observed under filamentation of the collimated femtosecond laser beam (80 GW, 50 fs, 805 nm) in molecular gases showed that this component behaves like a Raman soliton. Namely, it is confined in all domains: (a) it propagates within the filament core, (b) it has a stable duration of 30 fs along the filament, and (c) its spectrum shifts as a whole from 820 to 870 nm on the distance of 2 m from the filament start. A simple model explaining the origin of anomalous group velocity dispersion in the plasma channel of a filament is suggested.     

Investigation of the transformation of the spectrum of femtosecond laser radiation on filamentation in gas medium

The dependence of the parameters of femtosecond laser radiation (55 fs, 3.7 mJ, 800 nm) in a filament on the distance along the filament propagation axis, the diaphragm diameter, and the type and pressure of gas used for filamentation has been studied experimentally.     

激光波前相位因子对飞秒脉冲激光成丝动力学的影响

利用一个全阶色散非线性传输模型采用数值模拟的方法研究了初值条件中描述透镜聚焦效应的相位因子对飞秒脉冲激光成丝动力学的影响.研究发现,利用单色指数相位因子e^{－ik(0)r2/2f}来描述飞秒脉冲的透镜聚焦效应是不准确的,特别是对于周期量级超短激光脉冲的成丝现象,甚至会导致完全不同的成丝动力学过程.为了准确描述飞秒脉冲的透镜聚焦效应,必须采用准确的相位因子e^{－ik(0)r2关键词： 相位因子 成丝 飞秒脉冲 透镜聚焦}     

Self-transformation of a powerful femtosecond laser pulse into a white-light laser pulse in bulk optical media (or supercontinuum generation)

We present experimental and theoretical results on white-light generation in the filamentation of a high-power femtosecond laser pulse in water and atmospheric air. We have shown that the high spatio-temporal localization of the light field in the filament, which enables the supercontinuum generation, is sustained due to the dynamic transformation of the light field on the whole transverse scale of the beam, including its edges. We found that the sources of the supercontinuum blue wing are in the rings, surrounding the filament, as well as at the back of the pulse, where shock-wave formation enhanced by self-steepening takes place. We report on the first observation and demonstration of the interference of the supercontinuum spectral components arising in the course of multiple filamentation in a terawatt laser pulse. We demonstrate that the conversion efficiency of an initially narrow laser pulse spectrum into the supercontinuum depends on the length of the filament with high intensity gradients and can be increased by introducing an initial chirp. PACS 42.65.Jx; 42.65.Re; 42.25.Bs; 42.50.Hz     

Towards a control of multiple filamentation by spatial regularization of a high-power femtosecond laser pulse

We propose a new method for controlling randomly generated multiple filaments during the propagation of femtosecond laser pulses in optical media. The method is based on introducing a periodic amplitude modulation of the transverse beam profile. It is shown both experimentally and numerically that the introduction of a periodic mesh into a propagation path of a femtosecond near-infrared laser pulse leads to a deterministic spatial distribution of multiple filaments in the presence of initial random fluctuations. As a result, the number of filaments is increased as compared to the random case. PACS 42.65.Jx; 42.25.Bs; 42.60.Jf     

Multiple filamentation of intense femtosecond laser pulses in air

The propagation of focused femtosecond laser pulses with supercritical peak powers in air has been investigated by the methods of optical visualization, profilometry, and calorimetry. Laser pulses with supercritical powers create a bundle of submillimeter filaments with a diameter of about 5 μm ahead of the lens focus; the maximum number of filaments in the beam cross section and their length increase linearly and sublinearly, respectively, with the radiation peak power. The optical visualization and calorimetry indicate that the plasma channels of filaments are optical contrast (a plasma density of 10¹⁸–10¹⁹ cm⁻³), ensuring the refraction of laser radiation incident on them.