Single-pass high-harmonic generation at 20.8 MHz repetition rate

We report on single-pass high-harmonic generation (HHG) with amplified driving laser pulses at a repetition rate of 20.8?MHz. An Yb:YAG Innoslab amplifier system provides 35?fs pulses with 20?W average power at 1030?nm after external pulse compression. Following tight focusing into a xenon gas jet, we observe the generation of high-harmonic radiation of up to the seventeenth order. Our results show that state-of-the-art amplifier systems have become a promising alternative to cavity-assisted HHG for applications that require high repetition rates, such as frequency comb spectroscopy in the extreme UV.     

Pulsed pumped Yb-doped fiber amplifier at low repetition rate

A pulsed pumped Yb-doped double-clad fiber (DCF) master-oscillator power amplifier (MOPA) at low repetition rate is reported. Seeded by a passive Q-switched Nd:YAG microchip laser, the fiber amplifier can generate 167-kW peak-power and 0.83-ns duration pulses at 200-Hz repetition rate. Because of the pulsed pump approach, the amplified spontaneous emission (ASE) and the spurious lasing between pulses are well avoided, and the repetition rate can be set freely from single-shot to 1 kHz. Peak power scaling limitations that arise from the fiber facet damage are discussed.     

High-harmonic generation from plasma mirrors at kilohertz repetition rate

We report the first demonstration of high-harmonic generation from plasma mirrors at a 1 kHz repetition rate. Harmonics up to nineteenth order are generated at peak intensities close to 101? W/cm2 by focusing 1 mJ, 25 fs laser pulses down to 1.7 μm FWHM spot size without any prior wavefront correction onto a moving target. We minimize target surface motion with respect to the laser focus using online interferometry to ensure reproducible interaction conditions for every shot and record data at 1 kHz with unprecedented statistics. This allows us to unambiguously identify coherent wake emission as the main generation mechanism.     

Generation of relativistic intensity pulses at a kilohertz repetition rate

By using adaptive optics to correct the wave-front distortion of a 21-fs, 0.7-mJ, 1-kHz laser, we are able to focus the pulses to a 1-mum spot with an f/1 off-axis parabolic mirror. The peak intensity at the focal position is 1.5x10(18) W/cm(2) , which is to the authors' knowledge the first demonstration of generating relativistic intensity at a kilohertz repetition rate.     

高重复率猝发高压多脉冲加速组元

详细介绍了基于形成线并联和传输线延时两种不同技术路线的MHz重复率猝发多脉冲加速组元的工作原理、应用背景和技术特点,针对现有传输线延时双脉冲加速组元的主要问题提出了一种新的猝发高压双脉冲加速组元设计思路并进行了实验验证,使双脉冲加速组元在兼顾更高稳定性和更好波形品质的同时,解决了双脉冲电压独立调节、间隔调节及单双脉冲模式转换的问题。     

Adaptive slit beam shaping for direct laser written waveguides

We demonstrate an improved method for fabricating optical waveguides in bulk materials by means of femtosecond laser writing. We use an LC spatial light modulator (SLM) to shape the beam focus by generating adaptive slit illumination in the pupil of the objective lens. A diffraction grating is applied in a strip across the SLM to simulate a slit, with the first diffracted order mapped onto the pupil plane of the objective lens while the zeroth order is blocked. This technique enables real-time control of the beam-shaping parameters during writing, facilitating the fabrication of more complicated structures than is possible using nonadaptive methods. Waveguides are demonstrated in fused silica with a coupling loss to single-mode fibers in the range of 0.2 to 0.5 dB and propagation loss <0.4 dB/cm.     

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     

Coherent quasi-cw 153 nm light source at 33 MHz repetition rate

We present a quasi-cw laser in a vacuum ultraviolet region at megahertz repetition rate. The narrowband pulses generated from an ytterbium-fiber laser system at 33 MHz repetition rate at the central wavelength of 1074 nm are frequency-converted by successive stages of LiB(3)O(5) crystals and KBe(2)BO(3)F(2) crystals. The generated radiation at 153 nm has the shortest wavelength achieved through phase-matched frequency conversion processes in nonlinear optical crystals to our knowledge.     

Linear picosecond electron-optical chronoscopy at a repetition rate of 140 MHz

A picosecond streak camera operating in synchronism with C.W. mode-locked lasers provides real-time integrated measurement of repetitive luminous phenomena with a temporal resolution of ～ 10 ps. The durations of pulses from an actively mode-locked argon ion laser (514.5 nm) have been measured to be ～ 90 ps and the system is particularly suitable for the measurement of dye fluorescence lifetimes.     

Optical loss measurements in femtosecond laser written waveguides in glass

The optical loss is an important parameter for waveguides used in integrated optics. We measured the optical loss in waveguides written in silicate glass slides with high repetition-rate (MHz) femtosecond laser pulses. The average transmission loss of straight waveguides is about 0.3 dB/mm at a wavelength of 633 nm and 0.05 dB/mm at a wavelength of 1.55 μm. The loss is not polarization dependent and the waveguides allow a minimum bending radius of 36 mm without additional loss. The average numerical aperture of the waveguides is 0.065 at a wavelength of 633 nm and 0.045 at a wavelength of 1.55 μm. In straight waveguides more than 90% of the transmission loss is due to scattering.     

Multi-terawatt laser system generating 12-fs pulses at 100 Hz repetition rate

We have developed a laser system generating high peak-power ultrashort pulses based on the chirped pulse amplification of a Ti:sapphire laser. The pulse duration of the laser is reduced to 12 fs, owing to the successful compensation of gain narrowing during amplification with novel optics. The pulse energy and the repetition rate are 40 mJ and 100 Hz, respectively, as a consequence of sufficient energy extraction from a laser medium in the final stage of a multipass amplifier chain, which is designed to balance focusing with a thermal lens against defocusing with convex mirrors without cryogenic cooling for Ti:sapphire crystals. The laser pulses delivered from this laser system are suitable for generating an intense attosecond pulse train in a vacuum ultraviolet or extreme ultraviolet region.     

Interference microscopy of femtosecond laser written waveguides in phosphate glass

By focusing fs-laser radiation in the volume of a transparent material the refractive index can be changed locally, leading to 3-dimensional waveguiding structures. Waveguides are written in phosphate glass (IOG from Schott) at a depth of 100 μm below the surface. The pulse energy and the scan velocity are varied. For the first time the optical path difference caused by the waveguides and therefore the refractive index distribution of the waveguides and their cross sections are determined using interference microscopy. The optical path difference measured in the written structures and their cross sections is analyzed by a phase-shift algorithm. Thus, the refractive index distribution both along a line perpendicular to the waveguide and in the plane of a cross section is determined. The results are visualized as 2-dimensional graphics. Several regions of opposite sign of the refractive index change are observed in the cross sections of waveguides generated by femtosecond laser pulses. The number and the size of these regions are increasing with increasing pulse energy and decreasing scan velocity.     

Laser heating and ablation at high repetition rate in thermal confinement regime

Laser heating and ablation of materials with low absorption and thermal conductivity (paint and cement) were under experimental and theoretical investigations. The experiments were made with a high repetition rate Q-switched Nd:YAG laser (10 kHz, 90 ns pulse duration and λ = 532 nm). High repetition rate laser heating resulted in pulse per pulse heat accumulation. A theoretical model of laser heating was developed and demonstrated a good agreement between the experimental temperatures measured with the infrared pyrometer and the calculated ones. With the fixed wavelength and laser pulse duration, the ablation threshold fluence of paint was found to depend on the repetition rate and the number of applied pulses. With a high repetition rate, the threshold fluence decreased significantly when the number of applied pulses was increasing. The experimentally obtained thresholds were well described by the developed theoretical model. Some specific features of paint heating and ablation with high repetition rate lasers are discussed.     

Ultra-short efficient laser-driven hard X-ray source operated at a kHz repetition rate

Ultra-short bursts of hard X-ray radiation are generated by interaction of high-contrast femtosecond laser pulses with a jet of liquid Ga. The X-ray emission shows a strong dependence on the angle of incidence and polarization of the laser beam, consistent with the processes of resonant absorption and vacuum heating. As much as 60% of the total X-ray emission consists of [_Ga]K radiation (9.22–9.25 keV) with a photon flux of 6×10⁹ photons/(ssrad). Using this novel X-ray source, static diffraction patterns from a GaAs(111) crystal were recorded with an acquisition time of 2 s. PACS 52.38.Ph; 52.59.Px; 52.50.Jm     

kHz激光与固体靶相互作用产生的X射线源

利用kHz激光与固体靶相互作用产生了平均流强为1.3×10⁷ photons·sr^-1·s^-1的X射线源, 研究了激光的对比度和能量对激光与固体靶相互作用产生的X射线能谱及K_α 产额的影响, 使用刀边成像技术测量了X射线源的源尺寸, 并进行了初步的成像实验. 实验中观察到对于非相对论级别的激光脉冲, 降低激光的对比度有利于提高K_α 产额, 而使用高对比度高强度激光, 更有利于获得高通量高信噪比X射线源. 关键词： kHz激光 固体靶 X射线     

Optical pulse compression to 5 fs at a 1-MHz repetition rate

We report on the characterization and compression of the white-light continuum produced by injection of a 13-fs pulse from a cavity-dumped self-mode-locked Ti:sapphire laser into a single-mode fiber. Pulses as short as 5 fs were generated at repetition rates up to 1 MHz.     

MHz重复频率脉冲功率技术

采用串联单传输线、并联Blumlein脉冲形成线和高重复频率固体开关等技术路线开展了MHz重复频率脉冲功率技术研究。利用串联单传输线获得了幅度约200 kV,时间间隔约500 ns的双脉冲。利用并联使用的Blumlein系统和特殊设计的汇流/隔离网络获得了幅度约275 kV,时间间隔约500 ns的三脉冲。利用并联MOSFET和感应叠加原理研制了6 kV/2.5 MHz固体调制器。结果表明：3种方式均可以猝发MHz的方式输出高品质的高压脉冲串,可根据实际的需求选择合适技术路线。     

MHz重复频率脉冲功率技术

采用串联单传输线、并联Blumlein脉冲形成线和高重复频率固体开关等技术路线开展了MHz重复频率脉冲功率技术研究。利用串联单传输线获得了幅度约200 kV,时间间隔约500 ns的双脉冲。利用并联使用的Blumlein系统和特殊设计的汇流/隔离网络获得了幅度约275 kV,时间间隔约500 ns的三脉冲。利用并联MOSFET和感应叠加原理研制了6 kV/2.5 MHz固体调制器。结果表明:3种方式均可以猝发MHz的方式输出高品质的高压脉冲串,可根据实际的需求选择合适技术路线。