Ultrashort Pulse Laser System Research Articles

New Development in Nonlinear Optical Microscopy

With the development of ultrashort pulse laser systems, various nonlinear optical effects have been applied to laser scanning microscopy. These microscopes have unique imaging properties that are not available in conventional optical microscopes. In this article, laser-scanning microscopes that utilize nonlinear optical phenomena, such as second-, third-harmonic generation and coherent anti-Stokes Raman scattering, are introduced. The principle and contrast mechanism of the microscopes, and their biological applications are described.

Development of high-repetition-rate LD pumped Nd: YAG laser and its application

High-average-power and high-energy lasers with good beam quality are fundamental tools for pumping high-peak and high-average-power ultrafast Ti:sapphire laser systems. Laser-diode (LD) pumped solid slate lasers with phase conjugation offer a unique combination of efficiency, reliability, and compactness. We present the design details and performance characteristics of a 360-W 1-kHz LD pumped Nd:YAG phase-conjugated laser. We also discuss the basic design aspects and present our preliminary experimental investigations on ultrashort-pulse laser systems.

Compensation of gain narrowing by using AOPDF in high-power ultra-short pulse laser systems

Laser pulse durations are dominated by the spectral bandwidths in chirped pulse amplification systems.Therefore,gain narrowing during amplification is one of the most important factors that limits the minimal pulse durations of compressed pulses in high_power ultrashort_pulse laser systems.The gain narrowing can be compensated by applying an acousto_optic programmable dispersive filter(AOPDF) to the laser system.Here we present the experiments and results with an AOPDF for the 200_TW Ti:sapphire laser.The spectral bandwidth is increased from the original 27?nm to 44?nm.The pulse duration correspondingly is decreased from 60?fs to 30?fs and thus the output pulse power is doubled.

A pulsed source of continuous variable polarization entanglement

We have experimentally demonstrated polarization entanglement using continuous variables in an ultra-short pulsed laser system at telecommunication wavelengths. Exploiting the Kerr-nonlinearity of a glass fibre we generated a polarization squeezed pulse with S2 the only non-zero Stokes parameter thus S1 and S3 being the conjugate pair. Polarization entanglement was generated by interference of the polarization squeezed field with a vacuum on a 50:50 beam splitter. The two resultant beams exhibit strong quantum noise correlations in S1 and S3. The sum noise signal of S3 was at the respective shot noise level and the difference noise signal of S1 fell 2.9dB below this value.

Dynamics of space-time self-focusing of a femtosecond relativistic laser pulse in an underdense plasma

The propagation of femtosecond, multiterawatt, relativistic laser pulses in a transparent plasma is studied. The spatio-temporal dynamics of ultrashort, high-power laser pulses in underdense plasmas differs dramatically from that of long laser beams. We present the results of numerical studies of these dynamics within a model which systematically incorporates finite pulse length effects (i.e., dispersion) along with diffraction and nonlinear refraction in a strongly nonlinear, relativistic regime. New space-time patterns of self-compression, self-focusing and self-phase-modulation, typical of ultrashort, high-intensity laser pulses, are analyzed. The parameters of our numerical simulations correspond to a new class of high-peak-power (> 100 TW), ultrashort-pulsed laser systems, producing pulses with a duration in the 10 - 20 femtosecond range. Spatiotemporal dynamics of these self-effects and underlying physical mechanisms are discussed.

Growth, optical characterization, and laser operation of a stoichiometric crystalKYb(WO4)2

We present our recent achievements in the growing and optical characterization of $\mathrm{KYb}({\mathrm{WO}}_{4}{)}_{2}$ (hereafter KYbW) crystals and demonstrate laser operation in this stoichiometric material. Single crystals of KYbW with optimal crystalline quality have been grown by the top-seeded-solution growth slow-cooling method. The optical anisotropy of this monoclinic crystal has been characterized, locating the tensor of the optical indicatrix and measuring the dispersion of the principal values of the refractive indices as well as the thermo-optic coefficients. Sellmeier equations have been constructed valid in the visible and near-IR spectral range. Raman scattering has been used to determine the phonon energies of KYbW and a simple physical model is applied for classification of the lattice vibration modes. Spectroscopic studies (absorption and emission measurements at room and low temperature) have been carried out in the spectral region near $1 \ensuremath{\mu}\mathrm{m}$ characteristic for the ytterbium transition. Energy positions of the Stark sublevels of the ground and the excited state manifolds have been determined and the vibronic substructure has been identified. The intrinsic lifetime of the upper laser level has been measured taking care to suppress the effect of reabsorption and the intrinsic quantum efficiency has been estimated. Lasing has been demonstrated near 1074 nm with $41%$ slope efficiency at room temperature using a 0.5 mm thin plate of KYbW. This laser material holds great promise for diode pumped high-power lasers, thin disk and waveguide designs as well as for ultrashort (ps/fs) pulse laser systems.

Laser operation of the new stoichiometric crystal KYb(WO 4 ) 2

We demonstrate for the first time laser operation with the monoclinic stoichiometric crystal KYb(WO4)2 (hereafter KYbW). Lasing on the 2 F 5/2 ?2 F 7/2 transition of Yb3+ at room temperature has been achieved near 1074 nm with >41% slope efficiency (>26% maximum conversion efficiency) using a 0.5-mm-thick plate of KYbW. This new laser material holds great promise for diode-pumped high-power lasers, thin-disk and waveguide designs as well as for ultrashort (ps/fs) pulse laser systems.

Investigations on femtosecond-pulse-driven soft X-ray lasers using a gas puff target irradiated with a Ti:Sapphire laser

Experimental investigations on soft x-ray lasers based on optical field ionization of gases with an ultrashort-pulse terawatt laser system are presented. The x-ray laser active medium was created using an elongated gas puff target formed by pulsed injection of a small amount of gas from a high-pressure electromagnetic valve through a nozzle in the form of a slit The target was irradiated with a laser pulses from a 20-fs. 50-mJ Ti: Sapphire (Ti:S) laser system Both recombination and collisional soft x-ray laser scheme have been studied. Soft x-ray spectra from nitrogen, oxygen, and xenon targets are presented and discussed.

High-voltage electrical discharges induced by an ultrashort-pulse UV laser system

Using a femtosecond UV laser source (248.6 nm), we have conducted studies on large-gap laser-induced electrical discharges. Parameters such as pressure, lens selection, focal position and laser energy were investigated. We also present an experimental technique that allows the measurement of ionization cross-sections at atmospheric pressures. Spectroscopic analysis of the laser plasma and discharge are also presented. Prospects and conditions leading to a discharge over tens of metres, with dual UV and IR laser pulses, are discussed.

Ce/sup 3+/:LiCaAlF/sub 6/ crystal for high-gain or high-peak-power amplification of ultraviolet femtosecond pulses and new potential ultraviolet gain medium: Ce/sup 3+/:LiSr/sub 0.8/Ca/sub 0.2/AlF/sub 6/

To develop high-peak-power ultrashort pulse laser systems in the ultraviolet region, a large Ce/sup 3+/:LiCaAlF/sub 6/ (Ce:LiCAF) crystal, a tunable ultraviolet laser medium with large saturation fluence and broad gain spectrum width, was grown successfully with a diameter of more than 70 mm. To demonstrate high small signal gain, a four-pass confocal amplifier with 60 dB gain and 54 /spl mu/J output energy was constructed. Chirped pulse amplification (CPA) in the ultraviolet region was demonstrated using Ce:LiCAF for higher energy extraction. A modified bow-tie-style four-pass amplifier pumped by 100-mJ 266-nm 10-Hz pulses from a Q-switched Nd:YAG laser had 370-times gain and delivered 6-mJ 290-nm pulses. After dispersion compensation, the output pulses can be compressed down to 115 fs. This is the first ultraviolet, all-solid-state high-peak-power CPA laser system using ultraviolet gain media, and this demonstration shows further scalability of the Ce:LiCAF CPA system. Additionally, a new gain medium, Ce/sup 3+/:LiSr/sub 0.8/Ca/sub 0.2/AlF/sub 6/, with longer fluorescence lifetime and sufficient gain spectrum width over 18 nm was grown to upgrade this system as a candidate for a final power amplifier gain module.

High efficiency second-harmonic generation in four-pass quadrature frequency conversion scheme

A four-pass quadrature frequency conversion scheme was developed to generate green output with high efficiency for pumping an ultrashort pulse laser system. With this scheme, an efficiency from fundamental energy into total second harmonic energy in excess of 80% was achieved for frequency doubling of 1064 nm in KTP with a low input fundamental laser intensity of 76 MW/cm 2. A total second-harmonic output of 486 mJ was obtained with 607 mJ of the input 1064-nm fundamental laser at 10 Hz.

Diode-pumped 250-W zigzag slab Nd:YAG oscillator–amplifier system

A laser-diode-pumped zigzag slab Nd:YAG master oscillator power amplifier (MOPA) system featuring high pulse energy and high average power was developed for pumping of an ultrashort-pulse laser system. The MOPA system consists of an oscillator, a preamplifier, two postamplifiers, and image-relay telescopes. The postamplifiers have an angle-multiplexed ring-type double-pass configuration. A pulse energy of 1.26 J and an average power of 251 W were obtained at a repetition rate of 200 Hz. The frequency-doubled power when a LiB(3)O(5) crystal was used was 105 W at a repetition rate of 170 Hz. The intensity profiles of the fundamental and the second harmonic are nearly top-hat shaped and are suitable for pumping.

Contrast-enhanced radiography by differential absorption, using a laser-produced x-ray source.

The authors evaluate the feasibility of differential imaging of contrast media, with division of individual pixel values obtained from digital images generated by characteristic radiation from a laser-produced plasma, bridging the K-absorption edge of the contrast agent. Laser pulses from an ultrashort-pulse terawatt laser system were focused onto gadolinium and tantalum targets, creating a plasma from which characteristic radiation and Bremsstrahlung was emitted. The elements of the target were selected so the characteristic emission lines of one of the elements were below the K edge of the contrast agent and the emission lines of the other element above. A phantom with gadolinium and other elements in various concentrations was examined. One radiographic exposure was made using a gadolinium target source and a subsequent exposure using a tantalum source. Both images were recorded digitally and the transmission ratios calculated by division of the individual pixel values. When viewed separately, the two images of the test phantom appeared similar. In the differential image, only the gadolinium solutions were bright, reflecting a difference in attenuation between the two exposures. Element-specific radiographs can be obtained by differential imaging. When fully explored, the technique may allow for contrast-enhanced radiography with increased sensitivity and decreased contrast dose.

Research and Development on Compact Ultra-Short and Highly-Intense Pulse Lasers in Europe.

The activity of research and development on ultra-short and highly-intense pulse laser systems in Europe is reviewed. The new field of highly-intense laser-matter interaction physics motivates major institutes owing conventional nsec- or p-sec laser facilities to improve theirs to compact f-sec laser systems which can generate TW-level power. The elemental techniques of mode-locked oscillators to generate ultra-short pulses and chirped-pulse amplification are also intensively studied.

ATOMIC AND MOLECULAR DYNAMICS IN INTENSE OPTICAL FIELDS

▪ Abstract The unprecedented high intensities made accessible by the development of ultrashort pulse laser systems have uncovered a host of new phenomena and changed the way that we think about atomic and molecular dynamics. We review these developments in the context of some of the basic models that have evolved to explain them. The successes and limitations of simple semiclassical models of strong-field atomic phenomena are discussed. Photodissociation dynamics have shown a rich complexity in strong fields—bond softening, above-threshold dissociation, coulomb explosion—which we review. The promise of the new laser systems in the field of coherent control of molecular dynamics is also discussed.

11-J, 120-fs laser system based on Nd:glass-pumped Ti:sapphire

We have developed an ultrashort-pulse laser system in which the final Ti:sapphire amplifier stage is pumped by the frequency-doubled output of a Nd:glass laser. The laser produces pulses with an energy in excess of 1 J on target and an estimated peak focused irradiance of 5 x 10(19) W/cm(2).

Femtosecond-pulse-driven, electron-excited XUV lasers in eight-times-ionized noble gases

We propose three XUV laser schemes in the 30-50-nm wavelength region that can be driven by 10-Hz ultrashort-pulse terawatt laser systems. Tunneling ionization by circularly polarized radiation produces both the ions and hot electrons necessary to excite the upper laser level.

Coherence effects in hybridly mode-locked dye lasers: theory and experiment

A semiclassical model is presented for a synchronously-pumped mode-locked dye laser with a saturable absorber included in the cavity. The model is verified through comparison with experiments performed on a commercially available hybridly mode-locked dye laser operating with separate jets of rhodamine 6G and DODCI/DQOCI. The effects of varying the absorber molarity and the saturation parameter are mapped theoretically for two different saturable absorber dyes, DODCI and DQOCI. The model predicts a pulse shape and therefore allows more detailed interpretation of measured autocorrelations. A better understanding of this important ultrashort pulse laser system is gained, and possible ways to improve performance are suggested. >

Investigation of controlled ultrashort pulse dye laser for femtosecond nonlinear spectroscopy

Characteristics of a simple but practical ultrashort pulse laser system for transient nonlinear spectroscopy around and below 100 fs are described. The system is based on a hybridly mode-locked, cavitydumped cw dye laser, with high stability, broad tunability and specific phase properties, producing both clean transform-limited and chirped pulses. The laser system is applied to demonstrate the feasibility of using chirped pulses in high time-resolution measurement of relaxation times in condensed matter by means of degenerate transient four-wave mixing.