Generation of 16-fs, 10-TW pulses at a 10-Hz repetition rate with efficient Ti:sapphire amplifiers

We describe a two-stage Ti:sapphire amplifier laser system that is capable of producing 16-fs pulses of 10-TW peak power at a 10-Hz repetition rate. Thin solid etalons were used to control gain narrowing and gain saturation during amplification. A cylindrical mirror expander was used to permit compensation of the dispersion of the system. An efficiency greater than 90% of the theoretical maximum for conversion of 532-nm pump light to 790-nm radiation is demonstrated.     

Generation of higher harmonics of laser radiation in plasma formed by pulses with a 1-kHz repetition rate

Studies of higher-order nonlinearities in plasma plumes by the method of higher-harmonic generation (HHG) upon laser ablation of different materials by pulses with a repetition rate of 1 kHz are reviewed. It is shown that an increase in the pulse repetition rate compared to the previous works on HHG in plasma using lasers with a pulse repetition rate of 10 KHz considerably increases the average power of converted radiation in the far-ultraviolet (FUV) region. The review considers the results of works on the interference of quantum trajectories of accelerated electrons, two-color pumping, HHG in graphite plasma, HHG in gases compared to HHG in plasma plumes, resonant enhancement of harmonics, and stabilization of the output parameters of radiation converted in plasma. It is shown that the new methods of optimization of the HHG process considerably increase the potential of the use of coherent FUV radiation owing to the improvement of some characteristics of harmonics in plasma generated by pulses with a high repetition rate.     

Higher harmonic generation in laser plasma upon interaction of femtosecond laser pulses with doubly and singly charged ions under conditions of resonance frequency conversion of laser radiation

The higher harmonic generation (HHG) in plasmas formed on the surface of different solid targets (Pb, Ag, Ge, In, Au, Cr, Pt, V, Mn, Ga, Bi, Sb, Cu, Al, Ca, and Sn) by laser ablation is studied. The possibility of HHG upon interaction of femtosecond radiation with doubly charged ions is studied on the example of Mn plasma. This process allowed generation of higher harmonics of up to the 101st order. The results of investigation of individual enhanced harmonics in Sn, Sb, In, and Cr plasmas are presented. Comparison of the conditions of HHG in weakly and strongly excited plasmas showed that the latter is preferable for efficient frequency conversion. It is demonstrated that the use of laser plasma spectroscopy with high time resolution allows one to determine the optimal conditions for generation of harmonics with a plateaulike distribution in the far-UV spectral region.     

Generation of millijoule-level soft-x-ray laser pulses at a 4-Hz repetition rate in a highly saturated tabletop capillary discharge amplifier

Laser pulses with energies of as much as 1 mJ were generated at a wavelength of 46.9 nm by single-pass amplification in a 34.5 cm-long Ne-like Ar capillary discharge plasma. The large gain-length product of this plasma column allows for soft-x-ray amplification in a highly saturated regime, resulting in efficient energy extraction. Average laser output pulse energy of 0.88 mJ and peak power of 0.6 MW were obtained at a repetition rate of 4 Hz. With an estimated peak spectral brightness of approximately 1x10(23)photons /(s mm(2)mrad> (2)0.01% bandwidth) this tabletop laser is one of the brightest soft-x-ray sources to date.     

Laser action in CdS by optical pumping with radiation from a CO2 laser-produced plasma

Radiation from a CO₂ laser-produced plasma has been used to produce laser action in CdS at 5260 Å. This technique compares favorably with optical pumping of CdS by 3371 Å radiation from a pulsed N₂ laser and its potential as a new source for the optical pumping of solids at short wavelengths is strongly indicated.     

Generation of 220 mJ nanosecond pulses at a 10 Hz repetition rate with excellent beam quality in a diode-pumped Yb:YAG MOPA system

A novel all-diode-pumped master oscillator power amplifier system based on Yb:YAG crystal rods has been developed. It consists of a Q-switched oscillator delivering 3 mJ, 6.4 ns pulses at a 10 Hz repetition rate and an additional four-pass amplifier, which boosts the output energy to 220 mJ, while a close to TEM(00) beam quality could be observed. Additionally a simulation of the amplification was written that allows for further scaling considerations.     

CuBr-Ne-HBr laser with a high repetition frequency of the lasing pulses at a reduced energy deposition in the discharge

Experimental and computer studies of the CuBr laser with periodic-pulse pumping are performed with and without addition of HBr. The analysis based on the numerical simulation reveals how additions improve the lasing characteristics of this laser at both typical (10 to 20 kHz) and higher (up to 100 kHz) excitation pulse repetition frequencies during the operation with a low energy deposition in the discharge. This allows the lasing pulse repetition frequency to be increased in the experiment to 750 kHz, which does not seem to be a limit as yet.     

Optical breakdown and filamentation of femtosecond laser pulses propagating in air at a kHz repetition rate

We report the experiments on the optical breakdown and filamentation of femtosecond laser pulses propagating in air at a kHz repetition rate and with several hundreds micro-joule-energy.A 10m-long filament and its breakup and merging at the nonlinear focal region produced by modulational instability of femtosecond laser pulses in air are observed.A simple model based on the nonlinear Schroedinger equation coupled with multiphoton ionization law is presented to explain the several experimental results.     

Electromagnetic radiation at twice the plasma frequency emitted from the region of interaction of two short laser pulses in a rarefied plasma

We study the electromagnetic radiation at twice the plasma frequency, which emerges because of the interaction of two identical counterpropagating short laser pulses in a rarefied plasma and caused by excitation of small-scale standing plasma waves in the pulse overlap region. The energy, spectral, and angular characteristics of radiation are investigated, and the dependence of these characteristics on the parameters of the laser pulses is analyzed. The possibility of applying this effect for diagnostics of localized plasma oscillations is discussed.     

Coherent X-ray radiation generated by a beam of relativistic electrons in a single crystal under conditions of multiple scattering

The dynamic theory of the coherent X-ray radiation of a divergent beam of relativistic electrons generated in a single-crystal wafer under conditions of the multiple scattering of incident particles is developed. Radiation cross sections are averaged over the divergent beam of rectilinear trajectories of electrons. Expressions describing the spectral-angular characteristics of parametric X-ray radiation and diffracted transition radiation under conditions of multiple scattering are obtained. Conditions under which the contribution of diffracted brehmsstrahlung can be disregarded are shown, and the spectral-angular characteristics of parametric X-ray radiation and diffracted transition radiation are calculated numerically for such conditions.     

Generation of X-ray radiation with a high pulse repetition rate by means of a volume discharge in an open gas diode

The subject of study is ultrashort avalanche-produced electron pulses generated in air under atmospheric pressure. The current amplitude of the pulses behind 45-μm-thick AlBe foil exceeds 100 A, and their FWHM is ≈0.2 ns. The conditions of generation of ultrashort pulses persist at repetition rates as high as 1.5 kHz. A volume discharge initiated in an open coaxial-electrode gas diode by high-voltage nanosecond pulses generates hard (> 60 keV) radiation.     

Degenerate parametric frequency conversion of phase-modulated femtosecond laser pulses in crystals with a regular and chirped domain structure

A systematic theoretical analysis of the degenerate parametric frequency conversion in a LiNbO₃ crystal with a regular domain structure and a linearly varying domain thickness (chirped crystal) is presented for the pulses of a titanium-sapphire laser with a wavelength of 0.8 μm and durations of 100 and 50 fs in the presence and in the absence of phase modulation. The results are obtained with regard to the difference in the group velocities of interacting pulses and the group velocity dispersion. For an effective frequency conversion of the phase-modulated (PM) pump pulse, it is expedient to employ chirped crystals in which the domain thickness decreases from the entrance to the exit of the crystal. The pump energy is effectively converted into subharmonic energy when the pump carrier frequency decreases with time. It is demonstrated that the efficiency of the energy conversion to a subharmonic of 80% can be realized for PM pumping with a pulse duration of 100 fs in the chirped LiNbO₃ crystal. The efficiency of the parametric frequency conversion depends on the pump intensity as well as on the phase modulation of the pulse and the chirp of the crystal. Note that a variation in one of these parameters causes variations in the remaining parameters needed for the maximum efficiency of the parametric frequency conversion.     

Multiplex frequency conversion of femtosecond laser pulses in trefoil and quatrefoil channels of a holey fiber

We report that trefoil and quatrefoil silica channels with sizes ranging from 0.6 μm to as large as 4–5 μm in a holey fiber can strongly confine the light field and act as highly nonlinear waveguides. Supercontinuum emissions, efficient dispersive-wave generation and parametric four-wave-mixing processes were achieved in these channels with sub-nanojoule femtosecond Ti:sapphire laser pulses. Two dispersion curves corresponding to two orthogonal modes in each channel were simulated and used to explain the experimental results. PACS 42.81.Gs; 42.81.Qb     

Terahertz radiation generation by beating of two chirped laser pulses in a warm collisional magnetized plasma

Analytical equations of terahertz(THz) radiation generation based on beating of two laser beams in a warm collisional magnetized plasma with a ripple density profile are developed. In this regard, the effects of frequency chirp on the field amplitude of the terahertz radiation as well as the temperature and collision parameters are investigated. The ponderomotive force is generated in the frequency chirp of beams. Resonant excitation depends on tuning of the plasma beat frequency,magnetic field frequency, thermal velocity, collisional frequency, and effect of the frequency chirp with the plasma density.For optimum parameters of frequency and temperature the maximum THz amplitude is obtained.     

采用多通腔望远镜谐振腔结构的10MHz重复频率飞秒钛宝石激光器特性研究

通过在钛宝石激光腔内引入望远镜长腔系统增加腔长,实现了重复频率低到10?MHz的飞秒激光振荡,在5?W的抽运功率下获得了平均输出功率200?mW、单脉冲能量20?nJ的稳定输出. 在此基础上分析了腔内不同色散情况下的输出脉冲光谱和脉宽特性,结果表明在腔内存在一定负色散的情况下,锁模脉宽可接近转换极限,最短脉宽可达56?fs. 而在正色散的情况下,锁模输出的脉宽较宽,并且随着腔内正色散的增多,脉宽可到大于600?fs,锁模光谱也呈马鞍形. 关键词： 钛宝石激光器 飞秒脉冲 低重复频率 望远镜腔     

Enhanced frequency conversion of nonadiabatic pulses in a double Λ system driven by two pumps with and without carrier beams

We show that nonadiabatic, resonant amplitude- and phase-modulated pulses can be frequency converted with greater efficiency than adiabatic resonant pulses in a double Λ system, interacting with two strong cw beams on one side of the system, and a weak pulsed probe on the other. Indeed, in this double EIT (electromagnetically induced transparency) configuration, conversion efficiencies close to unity, similar to those achieved using highly detuned pulses, can be obtained using highly nonadiabatic resonant pulses. The distance at which the maximum conversion occurs is shorter than in a coherently-prepared Λ system. This counteracts the increased absorption that occurs in the double EIT configuration, so that both produce similar conversion efficiencies. The absorption experienced by matched nonadiabatic pulses in the double EIT system, at all propagation distances, can be overcome by superimposing the nonadiabatic pulses as amplitude modulations on carrier fields. Thus we demonstrate the formation of adiabatons in the double EIT system, and of diabatons in both the coherently-prepared Λ system and the double EIT system. Both the diabatons and adiabatons satisfy pulse-matching conditions. In addition, the asymptotic amplitude of the complementary amplitude modulations is proportional to the ratio of the pump to probe carrier Rabi frequencies, and is the same in each of the configurations.