CPA Laser Research Articles

Thin-disk ring amplifier for high pulse energy.

A CPA laser system for high pulse energy at high average power has been developed. The system is based on Yb:YAG thin-disk technology. It provides two laser beams with more than 500 mJ pulse energy each at 100 Hz repetition rate and 2 ps pulse duration. The system consists of a common oscillator, a grating stretcher and compressor and two identical amplifier chains that are both equipped with a regenerative amplifier and a ring amplifier. The compressor supports an individual alignment of the dispersion for the two laser channels.

Regenerative thin-disk amplifier for 300 mJ pulse energy

A regenerative amplifier based on thin-disk technology has been upgraded and optimized. Within a CPA laser system chirped 1 ns pulses are amplified to more than 300 mJ pulse energy. In addition to the high pulse energy the amplifier shows a very good energy stability with 0.25% (rms) fluctuation as well as an excellent beam quality of M(2) = 1.04. The regenerative amplifier is equipped with an Yb:YAG thin-disk of 17 mm in diameter pumped with 1.75 kW peak power. It is operated at a repetition rate of 100 Hz. The optical-to-optical efficiency is better than 18%. The laser pulses are compressed to 1.8 ps pulse duration.

The generation of amplified spontaneous emission in high-power CPA laser systems.

An analytical model is presented describing the temporal intensity contrast determined by amplified spontaneous emission in high‐intensity laser systems which are based on the principle of chirped pulse amplification. The model describes both the generation and the amplification of the amplified spontaneous emission for each type of laser amplifier. This model is applied to different solid state laser materials which can support the amplification of pulse durations ≤350 fs . The results are compared to intensity and fluence thresholds, e.g. determined by damage thresholds of a certain target material to be used in high‐intensity applications. This allows determining if additional means for contrast improvement, e.g. plasma mirrors, are required for a certain type of laser system and application. Using this model, the requirements for an optimized high‐contrast front‐end design are derived regarding the necessary contrast improvement and the amplified “clean” output energy for a desired focussed peak intensity. Finally, the model is compared to measurements at three different high‐intensity laser systems based on Ti:Sapphire and Yb:glass. These measurements show an excellent agreement with the model.

Multiple THz pulse generation with variable energy ratio and delay

Two methods for multiple high energetic THz pulse generation by two-color filamentation in air with controllable energy ratio and delay ranging from one to hundreds of ps were investigated. In the first method the laser pulse is split into two inside the optical stretcher of a CPA laser system, the resulting consecutive filaments occur in the same region and allows the study of the influence of the first plasma filament on the THz emission of the delayed filament. Based on a polarization sensitive thin film beam splitter placed in front of a 45° mirror, the second method produces multiple parallel consecutive filaments. Above a certain total pump level the THz energy delivered by multiple pulses exceeds the value given by a single filament for the same pump energy, thereby overcoming the THz emission saturation of the single filament.

Welding of PMMA by a femtosecond fiber laser.

Developing versatile joining techniques to weld transparent materials on a micrometer scale is of great importance in a growing number of applications, especially for the fabrication and assembly of biomedical devices. In this paper, we report on fs-laser microwelding of two transparent layers of polymethyl methacrylate (PMMA) based on nonlinear absorption and localized heat accumulation at high repetition rates. A fiber CPA laser system was used delivering 650-fs pulses at 1030 nm with repetition rates in the MHz regime. The laser-induced modifications produced by the focused beam into the bulk PMMA were firstly investigated, trying to find a suitable set of process parameters generating continuous and localized melting. Results have been evaluated based on existing heat accumulation models. Then, we have successfully laser welded two 1-mm-thick PMMA layers in a lap-joint configuration. Sealing of the sample was demonstrated through static and dynamic leakage tests. This fs-laser micro-welding process does not need any pre-processing of the samples or any intermediate absorbing layer. Furthermore, it offers several advantages compared to other joining techniques, because it prevents contamination and thermal distortion of the samples, thus being extremely interesting for application in direct laser fabrication of microfluidic devices.

Dispersion compensation by two-stage stretching in a sub-400 fs, 1.2 mJ Yb:CaF2 amplifier.

We generate 1.24 mJ, 390 fs pulses at 1035 nm in a CPA laser system featuring a 2.8 mJ Yb:CaF(2) regenerative amplifier, stretcher/compressor based on a single chirped volume Bragg grating and a compact, low-dispersion grating compressor. The auxiliary compressor is used to effectively pre-compensate the intra-cavity dispersion of the amplifier.

High precision elimination of angular chirp in CPA laser systems with large stretching factors or high bandwidth

We present a novel and highly sensitive method to determine the residual angular dispersion of high-power laser pulses after stretching, amplification, and re-compression of the pulses in a chirped-pulse amplification laser system. This method is based on the intentional deflection of a part of the the spectrum within the compressor and aligning the centers of gravity of the two resulting and separated foci with largest possible spectral separation in the far field. Using this technique, we were able to reduce the residual angular dispersion on pulses to less than 0.05 μrad/nm in the vertical plane and less than 0.03 μrad/nm in the horizontal plane, respectively. With this method, it is possible to minimize the deviation of the actual peak intensity for the focused laser pulses to less than 2 % of its theoretical limit.

Femtosecond laser ablation of TiO2 films for two-dimensional photonic crystals

Femtosecond laser ablation of TiO2 thin films was studied as a function of laser pulse energy and number of pulses. The ablated holes were characterized by atomic force microscopy and the optimum irradiation conditions were chosen for producing a periodic structure of triangular lattice of air holes in the TiO2 films. A photonic structure with period of 1.5μm was designed to show photonic band gap in the near-infrared, at the telecommunication wavelengths. The plane wave expansion method was used to compute the photonic band gap of the laser ablated structure. The impact of limited laser processing accuracy on the photonic band gap has been studied as variation of the radius of the holes. The structure was produced by tightly focused femtosecond laser beam in multi-pulses ablation regime, using a Ti:Sapphire CPA laser system with pulse duration of 200fs and energy per pulse of tens of nanojoule for a focusing optics with numerical aperture NA=0.5. The optimum irradiation conditions were found as following: number of pulse N=15 and laser beam energy E=58nJ. The resulting periodic structure has an estimated photonic band gap centered at 1.53μm with a bandwidth of about 42nm.

Conserved correlation in [formula omitted]-symmetric systems: Scattering and bound states

For one-dimensional PT-symmetric systems, it is observed that the non-local product ψ∗(−x,t)ψ(x,t), obtained from the continuity equation can be interpreted as a conserved correlation function. This leads to physical conclusions regarding both discrete and continuum states of such systems. Asymptotic states are shown to have necessarily broken PT-symmetry, leading to modified scattering and transfer matrices. This yields restricted boundary conditions, e.g., incidence from both sides, analogous to that of the proposed PT CPA laser (Longhi, 2010) [4]. The interpretation of ‘left’ and ‘right’ states leads to a HermitianS-matrix, resulting in the non-conservation of the ‘flux’. This further satisfies a ‘duality’ condition, identical to the optical analogues (Paasschens et al., 1996) [17]. However, the non-local conserved scalar implements alternate boundary conditions in terms of ‘in’ and ‘out’ states, leading to the pseudo-Hermiticity condition in terms of the scattering matrix. Interestingly, when PT-symmetry is preserved, it leads to stationary states with real energy, naturally interpretable as bound states. The broken PT-symmetric phase is also captured by this correlation, with complex-conjugate pair of energies, interpreted as resonances.

Alignment of chirped-pulse compressor

An original method of alignment of grating compressors for ultrahigh-power CPA laser systems is proposed. The use of this method for adjustment of the grating compressor of a PEARL subpetawattlaser complex made it possible to align the diffraction gratings with a second accuracy in all three angular degrees of freedom, including alignment of the grooves, and to adjust the angles of beam incidence on the grating with a high accuracy. A simple method for measuring the difference in the groove densities of gratings with accuracy better than 0.005 lines mm-1 is proposed and tested.

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.

Pulse compression below 40fs at 1μm: The first step towards a short-pulse, high-energy beam line at LULI

We present the upgrading project ELFIE (Equipement Laser de Forte Intensité et Energie) based on the "100TW" mixed Nd:glass CPA laser system at 1μm at LULI, which includes an energy enhancement and the development of a short-pulse, high-energy, good temporal contrast beam line (50fs/5J). We report the first experimental step towards the short-pulse, high-energy beam line: spectral broadening above 60nm from 7nm and temporal pulse compression below 40fs from 300fs at 1μm through a Krypton-filled hollow fiber compressor.

Amplified spontaneous emission contrast of CPA laser

The influence on the level of amplified spontaneous emission (ASE) contrast resulting from the different seed pulse energies has been experimentally studied in a 10-TW-level femtosecond Ti:sapphire chirped pulse amplification laser. It is found that, with the seed pulse energy increasing, the ASE pedestal is suppressed more efficiently, and the ASE contrast is improved with a saturable tendency. The measurement of ~2×10-8 ASE contrast in the range of more than 50 ps before the main pulse is achieved in the laser.

Gain-saturated Ni-like antimony laser at 11.4nm in grazing-incidence pumping geometry

Abstract We report on gain-saturated operation of the 4d → 4p, J = 0–1, 11.4 nm soft-X-ray laser line in Ni-like antimony (Sb) at a pump energy of only 2.5 J. The driving laser used was a 1054 nm Nd:glass CPA laser system with a pulse duration of 7 ps (FWHM). The pump beam was focused with a tilted on-axis parabolic mirror in a grazing-incidence (GRIP) pumping configuration at an incidence angle of 45°. A fraction of 2.8% of the pump energy (∼70 mJ) was used for the prepulse, which was propagated along the same beam line as the main pulse and arrived at the target 4.4 ns before the main pulse.

High-Conversion-Efficiency and Broadband Tunable Femtosecond Noncollinear Optical Parametric Amplifier

We build a compact high-conversion-efficiency and broadband tunable noncollinear optical parametric amplifier (OPA) in the infra-red (IR) pumped by a femtosecond Ti:sapphire CPA laser. The OPA consists of an internal seed of white-light continuum generator (WLG) and two noncollinear optical parametric amplifiers. The tunable wavelength range is from 1.2 μm to 2.4 μm for both signal and idle pulses. The total OPA efficiency in the last OPA stage reaches about 40% in a wider tunable spectral range (from 1.3 μm to 1.7 μm for signal pulse, from 1.5 μm to 2.0 μm for idle pulse respectively).

Enhanced energy localization and heating in high contrast ultra-intense laser produced plasmas via novel conical micro-target design

We report new experiments showing enhanced laser-target coupling and energy localization using nano-fabricated micro-conical Cu targets performed at the 100 TW CPA laser at LULI. A comparison was made between 1ω (λ = 1.057 μm, I = 1019 W/cm2) and 2ω (λ = 0.53 μm, I = 4-8 × 1018 W/cm2) irradiation to determine the effect of ASE induced preformed plasma filling the cone, using as principal diagnostics 2D Cu Kα imaging (transverse and rear-side), and high-resolution conical crystal spectroscopy of the Cu Kα bands. The 2ω irradiation exhibits laser absorption up to 50 μm deeper into the cone tip (versus at 1 ω), with a commensurately smaller Kα emission zone. Spectroscopy indicates a higher average charge state for the Cu emission at 2ω, with some shots exhibiting up to at least O-like emission. We deduce that micro-cone targets have similar performance in terms of material heating as a 50 μm diameter reduced mass target, despite a 900-fold larger mass. The observed enhancement in energy localization and heating in the cone geometry is supported by 2D collisional PIC simulations which indicate the presence of self-generated resistive magnetic field structures (≥ 10 MG) which confine the energetic electrons to the tip region

Efficient aberrations pre-compensation and wavefront correction with a deformable mirror in the middle of a petawatt-class CPA laser system

AbstractIn this paper, we describe the experimental validation of the technique of correction of wavefront aberration in the middle of the laser amplifying chain. This technique allows the correction of the aberrations from the first part of the laser system, and the pre-compensation of the aberrations built in the second part. This approach will allow an effective aberration management in the laser chain, to protect the optical surfaces and optimize performances, and is the only possible approach for multi-petawatt laser system from the technical and economical point of view. This approach is now possible after the introduction of new deformable mirrors with lower static aberrations and higher dynamic than the standard devices.

The dependence of the Fe K α yield on the chirp of the femtosecond exciting laser pulse

The hard X-ray yield generated with femtosecond laser pulses is studied for differently chirped irradiating laser pulses. The radiation of a Ti:sapphire CPA laser system (29 fs, 750 μJ, 1 kHz) is focused onto an iron containing solid state target producing incoherent hard X-ray radiation, Bremsstrahlung as well as target-specific Kα and Kβ lines. The hard X-ray yield has been optimized by introducing negative and positive group delay dispersion (GDD) and third order dispersion (TOD) to the femtosecond laser pulse. The Kα yield could be enhanced by a factor of 1.7 and reached 1.9×108 Fe Kα photons/s in 4π with the laser pulse positively chirped, and 1.5×108 Fe Kα photons/s with the pulse negatively chirped. When the pulse energy is lowered to about 400 μJ the yield maximum at negative chirp vanishes and only the maximum at positive chirp remains. We explain this behavior with an increased electron temperature caused by the induced GDD and TOD in the pulse.

Heating of buried layer targets by 1 ω and 2 ω pulses using the HELEN CPA laser

Targets of plastic with a buried layer of aluminium were heated using the HELEN CPA laser to irradiate one surface of the plastic. The emission spectra from the aluminium were used to infer the conditions in the target by comparing the measured spectra against synthetic spectra generated by the FLY code. The input to the FLY code was the temperature and density history calculated by a radiation-hydrodynamics code, which was iterated to achieve the best match to the experimental data. Aluminium layers at different depths in the plastic were used to measure how heat was transported into the target. Measurements were taken with the laser at wavelengths of 1.06 μm and wavelength converted to 0.53 μm. The laser irradiance was varied between 2 × 10 17–10 19 W/cm 2 by varying the laser pulse length, energy and wavelength. The data show that the plastic target was heated above 200 eV to a depth of about 4 μm. The FLY comparisons indicate that the buried layers heated with 0.53 μm light remained near solid density for the duration of the X-ray emission pulse and achieved a peak temperature of 450 ± 50 eV. In the case where the target was heated with 1.06 μm radiation, the density was an order of magnitude lower and the peak temperature achieved was also substantially lower, at 320 ± 50 eV. The depth to which the target was heated was similar at the two wavelengths studied and was not a strong function of irradiance. The aluminium data are presented and compared to radiation-hydrodynamic and spectral modelling.

Polychromatic X-ray Beam from the Acceleration of Energetic Electrons in Ultrafast Laser-Produced Plasmas

Polychromatic beams of hard X-rays from ultrafast laser plasma interaction are studied. Just as in a conventional synchrotron, electrons are accelerated and wiggled, but on a much shorter scale of a few millimeters. By focusing a 50 TW CPA laser system (30 fs duration) onto a helium gas jet, we obtained a polychromatic collimated beam (50 mrad) of X-ray radiation in the keV range. In addition, its perfect synchronization with the laser system, its ultrafast duration (≃30 fs) and its brightness (up to 10^8 photons/shot/solid angle at 0.1% BW) will make it applicable to both X-ray science and backlighting to address laboratory astrophysics research issues.