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.     

High power supercontinuum generation by dual-color femtosecond laser pulses in fused silica

High power supercontinuum (SC) is generated by focusing 800 nm and 400 nm femtosecond laser pulses in fused silica with a microlens array. It is found that the spectrum of the SC is getting broader compared with the case of a single laser pulse, and the spectral energy density between the two fundamental laser wavelengths is getting significantly higher by optimizing the phase matching angle of the BBO. It exceeds μJ/nm over 490 nm range which is from 380 nm to 870 nm, overcoming the disadvantage of relative lower power in the ranges far from the fundamental wavelength.     

Manifestations of sub- and superluminality in filamented femtosecond laser pulse in fused silica

Filamented propagation of femtosecond laser pulses in fused silica is studied using microscopy of Femtosecond Time-resolved Optical Polarigraphy (FTOP). Both super- and subluminal translation of axial intensity maxima in a 100 fs time scale has been directly recorded. The observed spatio-temporal periodicity of the pulse shape in longitudinal and transversal directions is tentatively interpreted as two projections on X and Y axes of the same fringe pattern formed by the interference of the axial part of the beam and spontaneously generated conical beam.     

紧聚焦飞秒脉冲与石英玻璃相互作用过程中的电子动量弛豫时间研究

本文通过数值模拟(3+1)维扩展的广义非线性薛定谔方程,研究了紧聚焦飞秒激光脉冲在诱导石英玻璃的非线性电离过程中电子动量弛豫时间对于该电离过程的影响.计算结果证明电子动量弛豫时间会直接影响入射脉冲在焦点区域所形成的峰值场强、自由电子态密度和能流等参量的分布态势,因此在与实验结果相比较后发现适合于相互作用过程的电子动量弛豫时间的理论值约为1.27 fs.进一步的研究表明,电子动量弛豫时间与逆韧致吸收效应、雪崩电离的概率、等离子体密度、等离子体的自散焦效果以及间接引起的焦平面位置的移动都有着密切的联系.当前的研究结果表明电子动量弛豫时间在飞秒激光脉冲与物质相互作用的过程中发挥着重要作用.     

Micro-channel etching characteristics enhancement by femtosecond laser processing high-temperature lattice in fused silica glass

A fused silica glass micro-channel can be formed by chemical etching after femtosecond laser irradiation, and the successful etching probability is only 48%. In order to improve the micro-channel fabrication success probability,the method of processing a high-temperature lattice by a femtosecond laser pulse train is provided. With the same pulse energy and scanning speed, the success probability can be increased to 98% by optimizing pulse delay.The enhancement is mainly caused by the nanostructure, which changes from a periodic slabs structure to some intensive and loose pore structures. In this Letter, the optimum pulse energy distribution ratio to the etching is also investigated.     

Ultrafast time-resolved photography of femtosecond laser induced modifications in BK7 glass and fused silica

The dynamics of absorption after excitation of fused silica and BK7 glass with femtosecond laser radiation are visualized by transient absorption spectroscopy. Focusing laser radiation with pulse durations in the picosecond time regime in BK7 glass generates free electrons with relaxation by emission of radiation or by formation of defects. The temporal and spatial emission characteristics are observed by high-speed photography in the streak mode. The beam waist moves within the pulse duration towards the incoming laser radiation by self-focusing and with the laser radiation absorbed by multi-photon processes. The dynamics of the long lasting stress formation is visualized by time-resolved Nomarski-Photography. The modification of the glass is investigated during and after irradiation with ultra-short pulsed laser radiation (100 fs<t_p<3 ps) at the wavelength =810 nm. The formation of a sound wave in fused silica and BK7 glass is observed and the mechanical stress, depending on the excitation pulse duration, is measured. PACS 06.60.Jn; 42.50.Md; 78.47.-p; 81.16.-c; 82.53.-k     

飞秒激光离焦抽运熔融石英产生超连续白光的实验研究

利用飞秒激光在熔融石英介质中传输产生能量达数毫焦、波长范围覆盖400-900 nm 且光谱分布较为均匀的超连续白光,实验过程中将熔融石英介质离焦放置以避免被击穿. 研究了入射激光能量以及介质离焦距离对超连续白光特性的影响. 结果表明采用高能量的入射激光脉冲离焦抽运介质的方法能够有效避免介质 击穿损伤并提高超连续白光脉冲的能量输出.     

Modification of the fused silica glass network associated with waveguide fabrication using femtosecond laser pulses

Atomic-scale structural changes have been observed in the glass network of fused silica after modification by tightly focused 800-nm, 130-fs laser pulses at fluences between 5 and 200 J cm^-2. Raman spectroscopy of the modified glass shows an increase in the 490 and 605-cm^-1 peaks, indicating an increase in the number of 4- and 3-membered ring structures in the silica network. These results provide evidence that densification of the glass occurs after exposure to fs pulses. Fluorescence spectroscopy of the modified glass shows a broad fluorescence band at 630 nm, indicating the formation of non-bridging oxygen hole centers (NBOHC) by fs pulses. Waveguides that support the fundamental mode at 633 nm have been fabricated inside fused silica by scanning the glass along the fs laser beam axis. The index changes are estimated to be approximately 0.07×10^-3. Received: 17 December 2001 / Accepted: 9 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +1-925/423-2463, E-mail: dmkrol@ucdavis.edu     

Nanosecond and femtosecond excimer laser ablation of fused silica

Ablation of fused silica using standard excimer lasers (20–30 ns pulse duration at 193, 248, and 308 nm) and a short pulse laser system (500 fs at 248 nm) is reported. Ablation rates range from several hundred nm/pulse (193 nm or fs-laser) up to about 6 m/pulse (308 nm). The performance of the ablation is found to depend not only on wavelength and pulse duration but also on the existing or laser induced surface quality (e.g., roughness) of the material. Special ablation phenomena are observed. At 193 nm and moderate fluence (3 J/cm²) ablation takes place at the rear side of a plate without affecting the front side, whereas at higher fluence normal ablation at the front side occurs. At 248 nm (standard excimer) the existence of two consecutive ablation phases is observed: smooth ablation at low rate is followed by explosive ablation at high rate. Using fs-pulses smooth shaped holes are formed during the first pulses, whereas high pulse numbers cause the development of a ripple structure in the ablation craters.The results lead to the conclusion that two different ablation mechanisms are involved: the first is based on two photon bulk absorption, the second on controlled surface damage in relation with (partially laser induced) singularity conditions at the surface.Presented at LASERION '91, June 12–14, 1991, München (Germany)     

Intense supercontinuum generation in the near-ultraviolet range from a 400-nm femtosecond laser filament array in fused silica

An intense supercontinuum(SC) in the near-ultraviolet range is generated from filamentation by focusing a 400-nm laser into fused silica with a microlens array(MLA). The spectrum of the SC is shown to be sensitive to the distance between the MLA and fused silica. In our optimal conditions, the near-ultraviolet SC can cover a range of 350-600 nm,where a bandwidth of approximately 55 nm above the 1μJ/nm spectral energy density and 20 nm bandwidth with tens ofμJ/nm are achieved. In addition, the energy conversion efficiency of the 400 nm laser for SC generation is further analyzed.A maximum conversion efficiency of 66% is obtained when the entrance face of fused silica is set around the focus of the MLA.     

Control of femtosecond laser written waveguides in silica glass

Writing conditions for the fabrication of optical waveguides in bulk fused silica glass by use of 1 kHz focused femtosecond laser pulses at 800 nm were systematically determined for different focusing geometries. The results demonstrate that waveguides can be formed based on optical breakdown, filamentation (single or multiple), or a combination of both processes, when using pulse energies lower than the threshold of structural damage. The mechanisms of laser-induced index change are also discussed. PACS 42.65.Jx; 42.70.Ce; 42.79.Gn     

Two-dimensional ‘hat-scratch’ periodic structures induced by a single femtosecond laser pulse on silica glass

Two-dimensional ‘hat-scratch’ structures are fabricated on silica glass by the interference of three non-coplanar beams originating from a single femtosecond laser pulse. The scanning electron microscope (SEM) characterizations show that the as-formed structures are composed of hat holes and scratch marks. The experimental results indicate that the structures are dependent on the intensity of laser beam. The formation of the two-dimensional ‘hat-scratch’ structures is mainly due to the combined laser ablation effects including ionization, shock wave, plasma expansion, and phase explosion.     

Wavelength-tunable optical pulse generation from a fiber ring laser with a reflective semiconductor optical amplifier

This work describes, for the first time, wavelength-tunable optical pulse generation from a fiber ring laser with a reflective semiconductor optical amplifier. The optical signal to noise ratio of this fiber laser is higher than 49 dB over the wavelength-tunable range of 16 nm. Performance of this fiber ring laser operated at different wavelengths is also discussed.     

The polarization-dependence of femtosecond laser damage threshold inside fused silica

The laser damage threshold inside fused silica is dependent not only on the numerical aperture (NA) of the focusing objective, but also on the polarization of the incident femtosecond laser pulses. The damage threshold for circularly polarized beams is higher than that for linearly polarized beams when NA>0.4, but the former was lower than the latter when NA<0.4. The reverse might be due to different damage processes: laser induced damage at high NA and the self-focusing induced breakdown at low NA.     

Optical and ultrasonic monitoring of femtosecond laser filamentation in fused silica

Millimeter-long filaments and accompanying luminous plasma and defect channels created in fused silica (FS) by single focused femtosecond laser pulses with supercritical powers were probed in situ using optical imaging and contact ultrasonic techniques. Above the threshold pulse energy E_opt = 5 μJ corresponding to a few megawatt power levels pulses collapse due to self-focusing, producing channels filled by electron-hole plasma and luminescent defects, and exhibits predominantly compressive pressure transients. Analysis of the optical and ultrasonic response versus the laser pulse energy suggests that filamentary pulse propagation in the channels occurs with considerable dissipation of about ∼10 cm⁻¹. The predominant ionization mechanism is most likely associated with avalanche ionization, while the main mechanism of optical absorption is free-carrier absorption via inverse Bremsstrahlung interaction with the polar lattice.     

Optically encoded second-harmonic generation in germanosilicate glass by a femtosecond laser

Second-harmonic generation in germanosilicate glasses was encoded by coherent superposition of the 810-nm fundamental and the 405-nm second-harmonic light of a femtosecond laser. The difference spectra between the absorption spectra of the glasses before and after preparation were measured. An evident correlation between the induced second-order nonlinearity and the creation of a Ge electron center was observed, suggesting that a band-to-band transition by multiphoton absorption is probably responsible for the photoinduced second-harmonic generation.     

Deep subsurface optical waveguides produced by direct writing with femtosecond laser pulses in fused silica and phosphate glass

In the present work, we have analyzed the use of elliptical beam shaping along with low numerical aperture focusing optics in order to produce circular cross-section waveguides in different materials at large processing depths by direct femtosecond laser writing (100 fs, 800 nm, 1 kHz). A variable slit located before the focusing optics allows to generate a nearly elliptical beam shape and also to reduce the effective numerical aperture of the beam along the shat axis of the ellipse. The focusing optics allows to focus the beam deep inside the sample, which is translated at a constant speed transversely to the writing beam direction. The influence of several experimental parameters (energy per pulse, slit width, processing depth) on the properties of the produced waveguides has been analyzed. The influence of the intrinsic properties of the material (refractive index, composition) has been analyzed by comparing results obtained in fused silica and Er:Yb co-doped phosphate glass. The results obtained show that this approach leads to the successful production of deep subsurface (up to 7 mm) waveguides with circular cross-sections. Preliminary results using chirped pulses in the phosphate glass suggest that temporal pulse shaping can be used as an additional parameter to optimize the guided mode symmetry.     

Multiple foci and a long filament observed with focused femtosecond pulse propagation in fused silica

Multiple foci and a long filament are observed when we focus a femtosecond laser pulse into a fused-silica sample. The dependences of the intensity distribution of the plasma luminescence on the pulse energy and the numerical aperture (NA) of the focusing objective are investigated. Multiple foci are observed when NA of 相似文献   

Direct observation of the space-time transformation of a femtosecond laser pulse in fused quartz

The transformation of a femtosecond laser pulse propagating in fused quartz before and after its transition to a filamentation regime has been investigated by femtosecond time-resolved optical polarigraphy. The spatial periodicity of a light field along and across the propagation axis has been detected and its nature has been attributed to the interference of the conical and plane components of the wave packet of the filament. The “supraluminal” motions of the observed filament intensity maximum are due to the longitudinal transformation of the pulse profile.