Ultrashort Laser System Research Articles

High power bessel ultrashort pulses directly output from a fiber laser system

High power Bessel pulses directly output from a fiber-based amplifier system are demonstrated. A compact solution based on the inverse micro-axicon (IMAX) on fiber end is proposed for the conventional ultrashort pulse fiber laser system to enable the direct generation of high power Bessel pulses from lasers without any additional exhausting alignments. The IMAX is fabricated on one facet of a ytterbium-doped large mode area fiber by focusing ion beam technique and constitutes an integrated beam shaper in combination with an inherent collimating lens in the fiber laser system. The experimental results accord qualitatively with the simulations. The system can directly generate chirped Bessel pulses with diffraction-free propagation in meter-scaled free space. The highest average power of such a wavepacket can reach 10.1 W, correspongding to 178 nJ, and the pulse duration can be dechirped to 140 fs.

A new method for measuring the pulse-front distortion of arbitrary shapes in high-power ultrashort laser systems

We present a new method that can be used to calculate pulse-front distortion by measuring the spectral interference of two point-diffraction fields in their overlapped district. We demonstrate, for the first time, the measurement of the pulse-front distortion of the pulse from a complex multi-pass amplification system, which exists in almost all high-power laser systems, and obtain the irregular pulse-front distribution. The method presented does not need any reference light or assumption about the pulse-front distribution, and has an accuracy of several femtoseconds.

Time-domain near-infrared spectroscopy using a wavelength-tunable narrow-linewidth source by spectral compression of ultrashort soliton pulses

Time-domain absorption spectroscopy was demonstrated using a wideband, rapid wavelength-tunable, narrow-linewidth source based on an Er-doped ultrashort pulse fiber laser system. The spectrum of the Raman-shifted ultrashort soliton pulse was compressed using a comb-profile dispersion increasing fiber. Rapid wavelength sweeping was demonstrated using an electro-optical intensity modulator. The absorption spectrum of CH(2)Cl(2) liquid at 1625-1780 nm was observed in a 10 μs time-domain measurement.

Improvement of temporal contrast for ultrashort laser pulses by cross-polarized wave generation

Temporal contrast is a key factor influencing the performance of high-field physics experiments for ultraintense and ultrashort laser systems. The purpose of this paper is to improve the temporal contrast for ultraintense and ultrashort pulses in a laser system by cross-polarized wave (XPW) generation, a new method developed recently to enhance the temporal contrast effectively. Experiments were carried out to study the XPW generation efficiency and the enhancement of temporal contrast at the SILEX-I laser facility with a wavelength of 800nm. The results show that the relationships between the efficiency of cross-polarized wave generation and crystal orientation/input intensity are in good agreement with theoretical predictions. Both the prepulse and the amplified spontaneous emission (ASE) pedestal are suppressed significantly and the temporal contrast is improved by two orders of magnitude by the method. With the amplification of the temporally cleaner pulses in the future, great opportunities will be created for high-field physics experiments at the laser facility.

Carrier-envelope phase control using linear electro-optic effect

We present a new method to control the Carrier-Envelope Phase of ultra-short laser pulses by using the linear Electro-Optic Effect. Experimental demonstration is carried out on a Chirped Pulse Amplification based laser. Phase shifts greater than π radian can be obtained by applying moderate voltage on a LiNbO3 crystal with practically no changes to all other parameters of the pulse with the exception of its group delay. Time response of the Electro-Optic effect makes possible shaping at a high repetition rate or stabilization of the CEP of ultra short CPA laser systems.

Shaper-assisted removal of the direction-of-time ambiguity in second-harmonic generation frequency-resolved optical gating

We report on a practical method where a pulse shaper in combination with second-harmonic generation frequency-resolved optical gating (SHG-FROG) is used to fully characterize ultrashort laser pulses. The original pulse is modified by adding a small satellite, which allows us to determine the direction of time of the retrieved pulse. The setup extends the work of Zeek et al (2002 Appl. Phys. B 74 S265) to more arbitrary pulse structures. Moreover, it is demonstrated that the method is applicable also in high-power ultrashort laser systems, where the pulse former has to be placed before the chirped-pulse amplifier. We found that the amplitude of the satellite must be smaller than 20% of that of the original pulse to avoid nonlinear distortions.

Development of ultra-short pulse VUV laser system for nanoscale processing

We have developed intense vacuum ultraviolet (VUV) radiation sources for advanced material processing, such as photochemical surface reactions and precise processing on a nanometer scale. We have constructed a new VUV laser system to generate sub-picosecond pulses at the wavelength of 126 nm. A seed VUV pulse was generated in Xe as the 7th harmonic of a 882-nm Ti:sapphire laser. The optimum conversion was achieved at the pressure of 1.2 Torr. The seed pulse will be amplified by the $\mathrm{Ar}_{2}^{*}$ media generated by an optical-field-induced ionization Ar plasma produced by the Ti:sapphire laser. We have obtained a gain coefficient of g=0.16 cm−1. Our developing system will provide VUV ultra-short pulses with sub-μJ energy at a repetition rate of 1 kHz.

Polarized spectral properties of Nd3+ ions in CaYAlO4 crystal

Nd:CaYAlO4 single crystal has been grown by the Czochralski technique. The cell parameters were analyzed with X-ray diffraction (XRD). The polarized absorption spectra, the polarized fluorescence spectra, and the fluorescence decay curve of the crystal were measured at room temperature. The spectroscopic parameters were determined by Judd–Ofelt theory and Fuchtbauer–Ladenburg formula. The effective intensity parameters Ω 2, Ω 4, and Ω 6 were obtained to be 2.19, 8.16, and 8.57×10−20 cm2, respectively. The calculated radiative probabilities, branching ratios, and radiative lifetime were also evaluated for the 4F3/2 excited state using the calculated intensity parameters. The results indicate Nd:CaYAlO4 has potential as a laser gain medium for ultrashort laser system.

Crystal growth and spectroscopic properties of Yb:CaYAlO 4 single crystal

A Yb 3+-doped CaYAlO 4 laser crystal has been grown by the Czochralski technique. The segregation coefficient was measured by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). The cell parameters were analyzed with X-ray diffraction experiments. Color defects in Yb:CaYAlO 4 have been evidenced to be similar to those in undoped CaYAlO 4. The polarized absorption spectra and the fluorescence spectrum of the Yb:CaYAlO 4 crystal were measured at room temperature. The fluorescence decay time of the Yb 3+ ion was investigated. The results show that Yb:CaYAlO 4 has potential as a laser gain medium for an ultrashort laser system.

Relativistic electron diffraction at the UCLA Pegasus photoinjector laboratory

Electron diffraction holds the promise to yield real-time resolution of atomic motion in an easily accessible environment like a university laboratory at a fraction of the cost of fourth-generation X-ray sources. Currently the limit in time-resolution for conventional electron diffraction is set by how short an electron pulse can be made. A very promising solution to maintain the highest possible beam intensity without excessive pulse broadening from space charge effects is to increase the electron energy to the MeV level where relativistic effects significantly reduce the space charge forces. Rf photoinjectors can in principle deliver up to 10 7–10 8 electrons packed in bunches of ∼100-fs length, allowing an unprecedented time resolution and enabling the study of irreversible phenomena by single-shot diffraction patterns. The use of rf photoinjectors as sources for ultrafast electron diffraction has been recently at the center of various theoretical and experimental studies. The UCLA Pegasus laboratory, commissioned in early 2007 as an advanced photoinjector facility, is the only operating system in the country, which has recently demonstrated electron diffraction using a relativistic beam from an rf photoinjector. Due to the use of a state-of-the-art ultrashort photoinjector driver laser system, the beam has been measured to be sub-100-fs long, at least a factor of 5 better than what measured in previous relativistic electron diffraction setups. Moreover, diffraction patterns from various metal targets (titanium and aluminum) have been obtained using the Pegasus beam. One of the main laboratory goals in the near future is to fully develop the rf photoinjector-based ultrafast electron diffraction technique with particular attention to the optimization of the working point of the photoinjector in a low-charge ultrashort pulse regime, and to the development of suitable beam diagnostics.

Deformation of the Focal Spot of an Ultrashort High-Power Laser Pulse due to Chromatic Aberration by a Beam Expander

Deformation of the Focal Spot of an Ultrashort High-Power Laser Pulse due to Chromatic Aberration by a Beam Expander Tae Moon Jeong, Do-Kyeong Ko and Jongmin Leey Advanced Photonics Research Institute, Gwangju Institute of Science and Technology, Gwangju 500-712 (Received 4 February 2008) The spatial and temporal intensity distribution of a focal spot is investigated when an ultrashort high-power laser pulse is focused with chromatic aberration induced by a simple beam expander. The chromatic aberration due to the simple beam expander is calculated and measured for the 100-TW Ti:sapphire laser system at the Advanced Photonics Research Institute. The chromatic aberration reduces the peak intensity of the focal spot and broadens the temporal pro le of the focal spot. By calculating the spatial and temporal intensity distribution of the focal spot, we can accurately estimate the peak intensity of the focal spot for the ultrashort high-power laser pulse with chromatic aberration. To avoid chromatic aberration, an air-spaced achromatic beam expander is designed and characterized for application to the ultrashort high-power laser system. PACS numbers: 42.55.Rz, 42.15.Fr, 42.60.Jf

超短脉冲激光系统中空间滤波器透镜的优化设计

超短脉冲激光系统中空间滤波器透镜的优化设计

Electron wave-packet dynamics in a relativistic electromagnetic field: 3-D analytical approximation

A solution to the Klein Gordon equation for a laser-driven electron is constructed from a superposition of Volkov states. The time-and space-dependent three-dimensional superposition integral can be evaluated analytically for an initial Gaussian momentum distribution when the expression for relativistic energy is expanded in a Taylor series over the scaled initial momenta. The solution preserves many complicated wave-packet dynamics in a strong field, including so-called wave-packet shearing and the formation of multiple peaks when the wave packet spreads to the scale of the driving-field wavelength. The range of applicability of the solution applies to much of the parameter space accessible by current intense ultra-short laser systems.

Performance of multilayer dielectric grating irradiated by ultrashort optical pulse

In the ultrashort pulse high-energy laser system based on chirped pulse amplification technique, multilayer dielectric grating with good output waveform and high laser induced damage threshold plays an important role. Based on Fourier spectral transform and rigorous modal method, optical characteristic of multilayer dielectric grating under irradiation of ultrashort optical pulse is analyzed. The results show that the reflected pulse at the -1 order exhibits asymmetrical Gaussian shape with oscillatory tail in the front edge and decreasing energy when the reflection bandwidth is narrower than the frequency bandwidth of the incident pulse with Guassian shape. The effect of structure parameters of multilayer dielectric grating on the reflection bandwidth are discussed in detail, which provides guidance to the design of multilayer dielectric grating with desired reflection bandwidth. Finally, near-field distribution of multilayer dielectric grating is analyzed to study its damages induced by ultrashort pulses.

New laser crystals for the generation of ultrashort pulses

Since the beginning of the 1990s, titanium sapphire has become the crystal of choice for the development of ultrashort laser systems producing very short and powerful pulses using the Chirped Pulse Amplification technique. In parallel to these developments leading to commercial products, new laser crystals have been studied in order to reach directly other wavelength ranges and to overcome the need to develop cw or pulsed green laser to pump the titanium sapphire crystal. In order to be able to directly pump the crystals with a very efficient and high-power semiconductor laser, new crystals doped with chromium or ytterbium ions have been developed. This article will review the latest development in this research field with the best performances obtained in terms of pulse duration. To cite this article: F. Druon et al., C. R. Physique 8 (2007).

An improved configuration of reflective-type electro-optic modulator with high light-induced damage threshold

A high light-induced damage threshold attenuated-total-reflection modulator is fabricated by replacing the metal film with poly(methyl methacrylate) as coupling layer, and the optimum conditions of the device are analyzed by calculating the reflectivity of a prism-waveguide configuration. The light-induced damage thresholds as high as ∼4 W/cm2 and ∼0.2 MW/cm2 for CW laser and ns pulses, respectively, are obtained. The device offers potentials in wireless optical communication for long distance transmission and applications in ultrashort laser systems such as energy balance, mode-locking or Q-switching.

Reliability of laser safety eye wear in the femtosecond regime

Appropriate eye protection is a prerequisite for the safe operation of ultrashort laser systems in industrial and laboratory environments. In this paper we report on the measurement of the transmission of ion-doped phosphate glass filters for pulses having a center wavelength of 800nm, a duration of 10fs to 1.2ps and a fluence range of 0.01 to 30J/cm(2). The measurements suggest, that the filter material preserves its protective features over the whole range. Saturation of absorption was only observed in the picosecond pulse duration range.

Extreme-ultraviolet high-order harmonic pulses in the microjoule range

We study high-order harmonic generation at a high pumping energy using a long focal length lens. We identify different saturation regimes of the harmonic emission, revealing the interplay between phase matching, absorption, and laser defocusing. In the optimal conditions, high conversion efficiencies are obtained, resulting in an increase of at least one order of magnitude of the harmonic energies compared to previously reported values. In xenon, microjoule energies are reached, opening new perspectives for the applications of this ultrashort coherent radiation. The generation of the high-order harmonics of intense laser pulses in gases @1# has recently opened new perspectives for probing matter in the extreme-ultraviolet ~XUV! pulses on an unprecedented time scale. The ultrashort harmonic pulse duration is used in pump-probe experiments in atomic @2,3# and molecular @ 4‐6 # spectroscopy, as well as in solidstate physics @7,8#. Combined to the high intrinsic beam coherence, it has allowed ultrafast diagnosis of laser-produced plasmas through XUV interferometry @9,10#. However, the harmonic beam energy is still relatively low and many applications would become possible if the number of generated photons were increased: ultrafast XUV holography, diagnosis of dense bright plasmas, or even study of nonlinear processes in the XUV, limited so far to low harmonic orders @11#. Recently a number of studies have demonstrated high conversion efficiencies, using ultrashort laser pulses focused in hollow core fibers @12‐14# or cells @15,16#. However, these efficiencies were obtained at a very low laser energy ~less than 1 mJ in most cases! that imposed a relatively tight focusing geometry in order to reach saturation intensities of the generating rare gases. This resulted in a low harmonic energy, in the nanojoule range. The fact that much larger energies are now available on ultrashort laser systems raises a number of questions: using higher laser energies and looser focusing, is it possible to achieve similar efficiencies, and thus to generate microjoule harmonic pulses? In particular, how will phase matching be affected by these unusual generating conditions? In this Rapid Communication, we report a thorough study of harmonic generation at a high pumping energy using a long focal length lens. Using a pumping energy of 27 mJ and a f 52 m lens, we study the influence of the beam aperture, the medium length, and the atomic density on the harmonic yield produced in a pulsed gas jet. We identify different saturation regimes, thanks to the excellent quantitative agreement obtained with detailed three-dimensional ~3D! simulations. In particular, we clearly observe the interplay between phase matching, absorption, and defocusing, the main limiting factors of the macroscopic emission. Absolute photon number measurements in the optimal conditions give conversion efficiencies as high as those reported using ultrashort (,20 fs) laser pulses, but now with ten times more energy. Using a f 55 m lens, we show that even higher conversion efficiencies can be obtained, resulting in harmonic energies in excess of 1 mJ. The experiments were performed on the LUCA laser facility with an amplified Ti:sapphire system delivering 60 fs pulses at 800 nm, with an energy of up to 100 mJ at 20 Hz. In our experiment, a pumping energy of 27 mJ was focused with either a f 52 m or 5 m lens in a pulsed gas jet. The nozzle was formed by a slit of dimensions 300 mm3 3m m producing a jet at pressure 10‐100 Torr characterized by Mach-Zehnder interferometry. By rotating the jet relative to the laser axis, we can change the length of the generating medium, while keeping the same peak density. Harmonics produced in the jet are analyzed by an XUV spectrometer without entrance slit @17#, and detected with a calibrated XUV photodiode blinded for diffused IR light with two 100 nm Al filters. The absolute spectrometer response~as well as the filter transmission! was measured using the harmonic radiation as a source further monochromatized with another spectrometer, like a synchrotron beam line. Since the total aperture of the laser beam was 40 mm, the