High energy negative feedback controlled passively mode-locked Nd: YAG laser

High energy picosecond pulse trains from a passively mode-locked Nd: YAG laser are obtained. The negative feedback controlled oscillator delivers 10–30 s trains with energies of up to 30 mJ and single pulse duration of less than 25 ps. The laser is operated with a repetition rate of 10 Hz. An active Q-control of the cavity generates a short pulse train with duration of 30–40 ns. The long pulse train energy reproducibility is better than ± 1.5%.     

Nd:YAP laser pulse compression by three-stage transient stimulated Brillouin and Raman scattering

There is a continuous effort to generate stable, powerful picosecond laser pulses for application in spectroscopy, nonlinear optics and parametric light generation, as well. One of the possible methods is the compression of longer nanosecond laser pulses by stimulated Brillouin and stimulated Raman scattering. The advantages of such a technique, in comparison to the used mode locked picosecond lasers, are as follows: the absence of the active and/or passive mode lockers used to generate a train of picosecond pulses, and the absence of a fast electrooptical shutter used to select a single pulse from a train of pulses. The application of stimulated Brillouin and stimulated Raman scattering permits to generate picosecond pulses in the wavelength regions not covered by mode locked lasers. Of special interest is the wavelength region of 0·8 m, which may be amplified by the attractive Titanium Sapphire lasers. In this paper we are summarizing our results in theoretical modelling and in real generation of picosecond pulses by means of cascaded stimulated Brillouin and Raman scattering. The models of scattering processes have been investigated. The stable generation of 5, 7, 3 picosecond pulses have been optimized for the wavelengths of 0·8, 0·64 and 0·54 m, respectively. In all these cases, the pulses exhibited the far field pattern close to Gaussian, with the pulse energy ranging from 0·2 to 1 mJ.     

High-power source of coherent picosecond light pulses tunable from 0.41 to 12.9 μm

We report a high-power source of coherent picosecond light pulses based on optical parametric generation and amplification in LiB₃O₅ and AgGaS₂ crystals. The spectral range of this continuously tunable source covers the visible, near-infrared and medium-infrared spectrum from 0.41 to 12.9 m. An optical parametric generator and amplifier, consisting of two type-I phase-matched LiB₃O₅ crystals and a diffraction grating, is pumped by the third harmonic of a picosecond Nd:YAG laser and provides spectrally narrow, high-power pulses from 0.41 to 2.4 m. Energy conversion efficiencies up to 16 percent are achieved. The pulse duration is about 14 ps, the bandwidth between 10 and 30 cm^–1. The tuning range is extended to 12.9 m by mixing the infrared output between 1.16 and 2.13 m with the fundamental of the Nd:YAG laser in type-I-phase-matched AgGaS₂ crystals. Up to 25 percent of the pulse energy at 1.064 m is converted into parametric infrared pulses. Bandwidths between 3 and 8 cm^–1 and a pulse duration of approximately 19 ps are measured for these pulses. We also observe a retracing behaviour in the tuning curve of AgGaS₂ not reported before.     

Generation of short light pulses under the stimulated Brillouin scattering in fibers with an optimal feedback

The generation of short light pulses (1 ns) in single mode fibers under pumping by wide laser pulses (of a microsecond duration) due to the backward stimulated Brillouin scattering (SBS) is numerically investigated. The influence of the acoustic diffraction is taken into account. The cases of acoustic waveguide and anti-waveguide fibers are considered. For an acoustic anti-waveguide fiber, a dependence of overlap integral S on the acoustic mode number n has a sharp peak in the region of n 100. Computer simulations have demonstrated the energy conversion of the pump wave into short pulses of the signal (Stokes) wave in the case of synchronous pumping. The optimal length of the fiber should be approximately equal to the half-length of the pump pulse. The bypass time of the Stokes pulse of the optimal circuit fiber and the feedback loop must be equal to the repetition period of the pump pulse. An importance of acoustic mode structure of the fiber for the process of forming pulse train in shown. We have found that the acoustic anti-waveguide fibers with a small core (a < 3 m) can be preferable for obtaining the stable train of compressed pulses.     

X-ray emission from short-pulse laser plasmas

Highly intense picosecond and subpicosecond laser pulses interacting with solids can create hot and dense plasmas which emit x-ray pulses in a broad spectral range from 100 eV up to MeV. The duration of these x-ray pulses depends on the transient behaviour of the relaxation and recombination mechanisms, as well as on the lifetime of energetic electrons produced via nonlinear processes in the plasma. This paper reports experiments using a 1.5-ps laser pulse with high constrast ratio (up to 10¹⁰) and intensities up to 10¹⁸ W cm^-2 irradiating solid targets. Both the line spectrum characteristics of a magnesium plasma, recorded using crystal spectrometers with high spectral resolution, and kinetic calculations have allowed the deduction of plasma parameters in the process of plasma evolution. In addition, hard x-ray pulses from a tantalum plasma were measured and their scaling was explained as bremsstrahlung emission from energetic electrons. Absolute dose values of x-ray pulses are given.     

Parametric oscillation of high-power 3-THz pulse by synchronously pumped ZnGeP<Subscript>2</Subscript> crystal: Computer simulation

Possible parametric oscillation of 3-THz pulse at synchronous pumping of the ZnGeP₂ crystal by a train of short second-harmonic pulses from the CO₂ laser has been analyzed. Calculation shows that at changing laser pulse duration τ between 4 and 500 ps and correspondingly pumping energy density (0.5–3.5 J cm⁻²) THz pulse peak power varies from 3 to 70MW with maximum at τ =9 ps.     

Generation of short high-power pulses in excimer lasers by fast pulseQ-switching

The possibility of generation of short high-power pulses by pulse electroopticQ-switching in a high-gain excimer system was investigated numerically. Two methods of short-pulse generation were considered. In the first the electroopticQ-switch is controlled by two half-wave voltage gates. The method normally enables pulses from several hundreds of picoseconds to a few nanoseconds duration to be obtained. In the second method the Q-switch is controlled by a train of voltage gates of amplitude equal to the double half-wave voltage. In this case, even at short times of laser pumping ( 50 ns), it is possible to generate high-power pulses of duration from tens to several hundreds of picoseconds. The influence of various system parameters on the generation process and the parameters of the generated pulses was analysed in detail.     

Picosecond laser ablation of thin copper films

The ablation process of thin copper films on fused silica by picosecond laser pulses is investigated. The ablation area is characterized using optical and scanning electron microscopy. The single-shot ablation threshold fluence for 40 ps laser pulses at 1053 nm has been determinated toF _thres = 172 mJ/cm². The ablation rate per pulse is measured as a function of intensity in the range of 5 × 10⁹ to 2 × 10¹¹ W/cm² and changes from 80 to 250 nm with increasing intensity. The experimental ablation rate per pulse is compared to heat-flow calculations based on the two-temperature model for ultrafast laser heating. Possible applications of picosecond laser radiation for microstructuring of different materials are discussed.     

On the possibility of obtaining incoherent femtosecond X-ray pulses from a laser plasma

It is proposed to use a high rate of collisional ionization in a superdense laser plasma to generate incoherent femtosecond X-ray pulses. The calculations indicate that the use of picosecond laser pulses with a contrast of about 10¹⁰ will allow the generation of an X-ray pulse with a duration of about 10 fs. The adequacy of the proposed model of the excitation of linear X-ray radiation from the plasma has been tested in the experiments with a picosecond laser of a moderately high contrast.     

Pulse properties of a synchronously mode-locked,cavity dumped,picosecond dye laser system

A synchronously mode-locked, cavity-dumped picosecond dye laser is described. The structure and intensity of the picosecond pulses measured under different conditions are reported. It was found that the structure of the pulses from the synchronously pumped dye laser depends critically on the length of the Ar⁺ laser pulses. At the shortest Ar⁺ laser pulses of about 70 ps the dye pulses are as short as 1.1 ps. With Ar⁺ laser pulses of 200 ps the dye laser pulses contains a broad satellite pulse which contains a large fraction of the total intensity. When a cavity dumper is added to the system one gets dye laser pulses 15–20 ps long with a substructure, which indicates incomplete mode-locking. Well mode-locked 1.5–2.0 ps pulses were obtained in the red part of the dye laser action spectrum, i.e. 620–650 nm for R6G, 595–608 nm for R 110 and 657–662 nm for RB, respectively. Addition of mode-locking dyes also improved the pulse quality at some wavelengths.     

Generation of picosecond and subpicosecond light pulses with saturable absorbers

Single light pulses, generated by a mode-locked Nd-glass laser, were shortened with saturable absorbers of low initial transmissionT ₀. The pulse duration was reduced from 8 to 2.6 ps after a single pass through a dye cell ofT ₀=10^–7. Light pulses as short as 0.5 ps were observed after five transits through an absorberamplifier system. Detailed calculations of the stationary and the transient situation (with respect to the dye relaxation time) are presented to demonstrate optimum conditions for the pulse shortening.     

Angular distributions of CH3I fragment ions under the irradiation of single pulse and trains of ultrashort laser pulses

The angular distribution of CH₃I is investigated experimentally using a single Fourier transform-limited laser pulse and a pulse train, where a 90-fs 800-nm linearly polarized laser field with a moderate intensity of 2.8×10¹³ W/cm² is used. The dynamic alignment is demonstrated in a single pulse experiment. Moreover, a pulse train is used to optimize the molecular alignment, and the alignment degree is almost identical to that with the single pulse. The results are analysed by using chirped femtosecond laser pulses, and it demonstrates that the structure of pulse train rather than its effective duration is crucial to the molecular alignment.     

Sum frequency beam analysis for the determination of the temporal characteristics of ultrashort light pulses

An extension of the single-shot second harmonic beam method proposed earlier for picosecond pulse duration measurements is presented for the case of two incident pulses of differing frequencies, durations and transverse sizes. The solution of the wave equation for noncollinear sum frequency generation in a nonlinear crystal by two Gaussian, spatially limited ultrashort pulses is given. It is shown that the width ( ₁ ² + ₂ ² )^1/2 of the temporal cross-correlation function of the two pulses can be deduced from the spatial energy distribution of the sum frequency beam. The method can be used e.g. in the case of a relatively weak secondary pulse obtained in some nonlinear processes. Preliminary experimental results demonstrating the possibilities offered by the method are presented.     

Comparison of simulated X-ray conversion efficiency of laser produced iron plasma with experimental results

X-ray resonance lines between 11 Å and 17 Å emitted from iron plasmas created by a modest KrF laser have been simulated by modifying the atomic and hydrodynamic code EHYBRID. Free–free and free–bound emission from the Si-, Al-, Mg-, Na-, Ne- and F-like ions is calculated in the simulation. In the original experiments, a KrF laser (249 nm wavelength) with focused irradiances between 1×10¹² W/cm² and 1×10¹⁵ W/cm² was focused on iron targets. The laser pulse duration was varied between 10 ps and 20 ns. We have calculated X-ray conversion efficiencies to be, for example, 0.5% over 2 sr for 2×10¹³ W/cm² and 20 ns pulse duration, in good agreement with experimental measurements. The simulation of X-ray emission is also presented for an experiment where a train of eight 7 ps KrF laser pulses is incident onto an iron target. PACS 52.50.Jm; 52.38.Ph; 52.65.Kj; 52.30.Ex; 32.30.Rj     

Plasma-mediated ablation of brain tissue with picosecond laser pulses

Plasma-mediated ablations of brain tissue have been performed using picosecond laser pulses obtained from a Nd:YLF oscillator/regenerative amplifier system. The laser pulses had a pulse duration of 35 ps at a wavelength of 1.053 µm. The pulse energy varied from 90 µJ to 550 µJ at a repetition rate of 400 Hz. The energy density at the ablation threshold was measured to be 20 J/cm². Comparisons have been made to 19 ps laser pulses at 1.68 µm and 2.92 µm from an OPG/OPA system and to microsecond pulse trains at 2.94 µm from a free running Er:YAG laser. Light microscopy and scanning electron microscopy were performed to judge the depth and the quality of the ablated cavities. No thermal damage was induced by either of the picosecond laser systems. The Er:YAG laser, on the other hand, showed 20 µm wide lateral damage zones due to the longer pulse durations and the higher pulse energies.     

Picosecond light pulses influenced by the passage through a LiF color center crystal

LiF crystals containing different types of color centers were used for pulse shaping of picosecond (ps) light pulses of a Nd-glass laser. Besides the saturable absorption of the F₂-centers a two-photon one-photon step absorption was found. Inserting the crystal in the laser resonator reduces the duration and steeps the envelope of the ps-pulse train. No remarkable pulse shortening of single ps-light pulse could be achieved by passing these crystals.     

Investigation and spectral analysis of the plasma-induced ablation mechanism of dental hydroxyapatite

Experiments on the ablation of dental substance performed with picosecond laser pulses are reported for the first time. A mode locked Nd:YLF oscillator laser was used to generate 25 ps pulses at a wavelength of 1.053µm. These were seeded and amplified to pulse energies up to 1 mJ in a regenerative amplifier laser at repetition rates up to 1 kHz. Very precise cavities were ablated in the enamel of extracted human teeth by mounting the probes onto a computer controlled 3D translation stage. Scanning electron microscopy and dye penetration tests were performed there-after. In contrast to longer pulse durations, picosecond pulses ablate with no signs of thermal damage, if the laser pulses are spatially distributed over the target. Definitions of the physical mechanisms plasma-induced ablation and photodisruption are given. Furthermore, the generated plasma spark has been spectroscopically analyzed. Excitations of calcium and sodium have been observed. From the spectra, the plasma temperature and free electron density could be estimated. By converting part of the laser energy into the second harmonic using a LiNbO₃ crystal, a reference amplitude was achieved for the spectra. With this reference signal, a clear distinction could be made between spectra obtained from healthy and caries infected teeth, thus enabling a better control of caries removal in the near future.     

Neutron yield upon the irradiation of thin TiD<Subscript>2</Subscript> foils with a superintense laser pulse

The yield of neutrons from the thermonuclear-fusion reaction D(d, n)³He induced in a thin skin layer by the interaction of a high-intensity laser pulse of picosecond duration with thin TiD₂ foils is calculated. A multiple ionization of titanium atoms at the leading edge of the laser pulse is considered. The heating of free electrons proceeds via induced inverse bremsstrahlung in elastic electron scattering on multiply charged titanium ions. The electron temperature is calculated. It proves to be about 10 keV at the laser-pulse intensity of 5×10¹⁸ W/cm² at the peak. The neutron yield is estimated at 10⁴ per laser pulse. These results are in qualitative agreement with experimental data.     

New single-pulse generation technique for distributed feedback dye lasers

The distributed feedback dye laser (DFDL) generates a train of picosecond pulses when pumped well above threshold. This DFDL emission can be quenched by injecting a laser pulse into DFDL. By proper timing of the quencher laser pulse, only the first DFDL pulse is generated while the successive pulses are suppressed. Operational characteristics and practical design considerations of such a quenched DFDL are given. With 2.5 ns long pump pulses from a N₂ laser, a shortest DFDL pulse of 17 ps was obtained at 380 nm.     

High quantum yield of photochemical reactions of protoporphyrin-IX induced by powerful ultrashort laser pulses

The action of powerful pulsed picosecond radiation from a Nd: YAG laser (λ=530 nm, pulse energy: 0.01 J, intensity: 2GW/cm²) and an argon laser (λ=515 nm, power: 50 mW) on protoporphyrin-IX dimethylether in three solvents (trichlormethane, carbon tetrachloride, dioxane) has been studied. Under continuous irradiation the quantum yield and resulting products do not differ materially from the ones produced under mercury lamp irradiation. When irradiation is performed by powerful laser pulses of picosecond duration the quantum yield of photodecomposition of protoporphyrin-IX dimethylether inereases substantially: by 10 in dioxane, by 4 in carbon tetrachloride and by 100 in trichlormethane. It is assumed that a quite different mechanism of multistep excitation is responsible for photodecomposition under powerful picosecond pulses.