Compression Of Laser Pulses Research Articles

On the estimation of absolute grating groove density and inter-grating groove density errors of laser pulse compression gratings

In this paper, experimental studies on estimation of absolute groove density of gratings and inter-grating groove density errors are reported with typical detector limited accuracies of ±0.23 lines mm−1 and ±0.005 lines mm−1, respectively at groove density of ~1740 lines mm−1 of holographic laser pulse compression gratings. A simple single detector based optical set-up with fixed optical elements to avoid mechanical eccentric errors, if any, due to goniometric movement of a rotatory stage, has been proposed to estimate absolute groove density of gratings. A modified Fizeau or a modified Michelson interferometer based optical set-up has been used to estimate inter-grating groove density errors of gratings. Various gratings from different manufacturers were examined for their absolute groove densities and inter-grating groove density errors.

Kagome-fiber prism compressor combination for Yb:KGW laser pulse compression to sub-40 fs

Spectral broadening in hollow-core fibers is an important tool for pulse compression of low-peak power laser pulses, especially for Yb-based lasers. Here, we present a pulse compression scheme to reduce the pulse duration of a commercial Yb:KGW laser operating at 100 kHz repetition rate and 40 μJ pulse energy from 390 to 38 fs. The spectral broadening is accomplished using a krypton-filled Kagome-type fiber. We report broadened spectra for variable Kr-pressures and input powers. At optimal settings of 8 bar Kr-pressure and 3.3 W input power, the bandwidth of the pulse at the −10 dB level increased from 9.5 to 85 nm corresponding to a Fourier limit of 26 fs. A simple SF10 prism compressor is used to reduce the accumulated chirp and shortens the fiber output from about 500 to 38 fs. In addition to the spectral broadening, a pressure dependent change of the polarization is observed.

339 J high-energy Ti:sapphire chirped-pulse amplifier for 10 PW laser facility.

We report on the laser pulse output of 339J centered at 800nm from a chirped-pulse amplification (CPA) Ti:sapphire laser system at the Shanghai Superintense Ultrafast Laser Facility. The experimental results demonstrated that the parasitic lasing as well as the transverse amplified spontaneous emission of the homemade 235-mm-diameter Ti:sapphire final amplifier were suppressed successfully via the temporal dual-pulse pumped scheme and the index-matching liquid cladding technique. The maximum pump-to-signal conversion efficiency of 32.1% was measured for the final amplifier. With a compressor transmission efficiency of 64% and a compressed pulse duration of 21fs obtained for the sample light at a lower energy level, this laser system could potentially generate a compressed laser pulse with a peak power of 10.3PW. The experimental results represent significant progress with respect to the CPA laser.

Experimental study of variable thermal lens in a two-stage chirped-pulse high-power Ti:sapphire amplifier at 1kHz

In general, misalignment, surface quality of optical components, thermal and nonlinear properties, doping level and inhomogeneity in the gain material can induce unexpected pulse propagation effects on intnese femtosecond laser pulses. Among them, the thermal lens in a thermally loaded material is more serious since it limits power scaling and degrades maximum achievable intensity at the focus by introducing distortion into the wavefront. Thereby, an understanding of thermally distortion wavefront is of great importance for optimum design of high power laser amplifiers and efficient serving femtosecond lasers in various specific applications. In this paper, we experimentally studied beam quality and wavefront distortion in a longitudinally pumped two-stage chirped pulse high power Ti:sapphire amplifier laser at 1 kHz operation in terms of pump power. The amplifier crystals are cooled thermoelectrically to below −10 °C. A simple formula for coupling the thermal lens in a two-stage amplifier consists of a regenerative amplifier followed by a single-pass amplifier pumped equally by the Nd:YLF laser at 527 nm with maximum average output power of 44 W and 130 ns pulse duration at 1 kHz is presented. The wavefront curvature of the compressed laser pulses with ∼40 fs pulse duration and center wavelength of 800 nm is evaluated directly by Shack-Hartmann Wavefront Sensor (SHWS) with help of the Zernike polynomials. While the beam radius and spatial beam quality M2 factor are estimated by knife-edge testing. Also, the optical wavefront distortion caused by an iris inserted into the beam path of a high power laser for quick spot size adjustment has been presented. When diameter of the iris is about 75% of the laser beam size (corresponding to 67% fraction transmission), laser beam distortion associated with the iris is negligible.

Nonlinear discrete wavefront shaping for spatiotemporal pulse compression with multicore fibers

In this work, we apply an effective discrete phase front focusing method to control the coherent combining and temporal compression of laser pulses by means of nonlinear multicore fibers. We numerically demonstrate the possibility of combining almost entirely the input optical pulses injected in all cores, into the central core of a hexagonal lattice multicore fiber. We investigate the optimal operational conditions for the most effective pulse combining and analyze the influence of a positive input pulse temporal chirp and spatial phase modulation. We demonstrate that a pulse, which has been focused into an arbitrary core of the fiber, also undergoes temporal compression.

Effect of temporally modified ultra-short laser pulses on ion acceleration from thin foil targets

We demonstrate the significant enhancement of ion energy from the interaction of intense, chirped pulse amplification based ultrashort laser pulse with thin foil targets by adjusting the laser temporal characteristics with a simple adjustment of grating pair separation inside the pulse compressor. The laser pulse was focused to a maximum intensity of 3.2 × 1019 W/cm2 on to thin metallic foil targets of submicron thickness. The pulse duration was varied from 25 to 500 fs by adjusting the laser pulse compressor grating separation for fixed laser pulse energy (2.1 J). Contrary to the conventional belief, we found that the ion acceleration is not optimum at the shortest laser pulse duration and, thus, at highest intensity as predicted by the previous intensity scaling data. Instead, stretched pulses are better suited for efficient ion acceleration. In addition to this, we found a contrasting effect with respect to the sign of the laser chirp for almost the same laser pulse duration. In particular, we show 70% enhancement in maximum ion energy (from 6 MeV to 10 MeV) with positively chirped 250–350 fs stretched pulses. On the contrary, negatively chirped pulses show gradual monotonic reduction in ion energy and flux. Independent electron energy spectra measurement along the laser forward direction exhibits a strong correlation with proton data. The temporal pulse skewness arising due to displaced grating separation is believed to be responsible for this asymmetric proton acceleration behaviour. Efficient absorption of positively modified skewed pulses (shallow rise time and sharp fall) along with the characteristic ion acceleration time can qualitatively explain the experimental result. The present study can be very much important for efficient ion acceleration based on modern day compact, ultra-short pulse 100 TW to PW class lasers.

Simple technique for the compression of nanojoule pulses from few-cycle laser oscillator to 1.7-cycle duration via nonlinear spectral broadening in diamond.

We report on a simple approach for the compression of few-cycle laser pulses generated in an ultrafast laser oscillator to a duration corresponding to 1.7 cycles of near-infrared light (compression factor of 1.44) by nonlinear spectral broadening in diamond and subsequent dispersion compensation using chirped mirrors. After the spectral broadening, the pulse spectrum spans over almost an octave (580-1000nm at the -10 dB level). The pulses are compressed by broadband-chirped mirrors and a wedge pair to a duration of 4.5fs measured by spectral phase interferometry for direct electric-field reconstruction (SPIDER). The properties of the broadened spectrum and their modelling by numerical solution of a 1D nonlinear Schrödinger equation show that the main source of spectral broadening is self-phase modulation, whereas stimulated Raman scattering does not play a significant role.

Enhancement in self-compression due to co-propagating laser pulse in plasma

The study proposes enhancement in self-compression of the laser pulse with relativistic nonlinearity in a plasma. It is presented that, a low-intensity laser pulse can tune the pulse duration and peak intensity of the high-intensity laser pulse. The coupled modified nonlinear Schrodinger equations (MNLS) have been established for both laser pulses in plasma. The enhancement in compression of the high-intensity laser pulse is solved by numerical methods. The nonlinearity of plasma, self-phase modulation (SPM), and group velocity dispersion (GVD) compress the laser pulse to the extremely short duration high peak intensity laser pulse. As a result, it is observed that the compression of the high-intensity laser pulse is highly sensitive to the combined intensity of both laser pulses, plasma density and even a small walkoff between two pulses can affect the high-intensity laser pulse compression.

Influence of nonlinear detuning at plasma wavebreaking threshold on backward Raman compression of non-relativistic laser pulses

Taking into account the nonlinear dispersion of the plasma wave, the fluid equations for the three-wave (Raman) interaction in plasmas are derived. It is found that, in some parameter regimes, the nonlinear detuning resulting from the plasma wave dispersion during Raman compression limits the plasma wave amplitude to noticeably below the generally recognized wavebreaking threshold. Particle-in-cell simulations confirm the theoretical estimates. For weakly nonlinear dispersion, the detuning effect can be counteracted by pump chirping or, equivalently, by upshifting slightly the pump frequency, so that the frequency-upshifted pump interacts with the seed at the point where the plasma wave enters the nonlinear stage.

Compression of femtosecond ytterbium fibre laser pulses using nonlinear processes in silica fibre

The temporal compression of 100-fs nanojoule laser pulses using nonlinear processes in silica fibre has been studied by numerical simulation and experimentally. The simulation results demonstrate that, in the case of spectral broadening in a normal dispersion fibre, followed by the temporal compression of the pulse in a compressor with a negative second-order dispersion, up to 90 % of the compressed pulse energy can be concentrated in the main peak, whose duration is a factor of 8 shorter than the initial laser pulse duration. In our experiments, 100-fs ytterbium fibre laser pulses have been compressed to pulses with an ∼13-fs main peak accounting for 87 % of the compressed pulse energy. At an average ytterbium laser output power of 4.2 W, the average power at the compressor output has been 3.1 W.

Optimisation of parameters of Raman laser pulse compression in a plasma for its implementation using the PEARL laser facility (IAP RAS)

We consider Raman compression of laser pulses in a plasma under the conditions of an experiment planned at the Institute of Applied Physics of the Russian Academy of Sciences on the PEARL laser facility. The analysis is based on the equations describing, among other things, the effect of plasma dispersion and relativistic nonlinearity, as well as the dynamics of the field near the plasma wave breaking threshold. It is shown that the main limiting factors are excessive frequency modulation of the pump pulse and a too low plasma density in which the plasma wave breaking can occur. To reduce the negative influence of these effects, we suggest using an intense and short (on the order of the plasma period) seed laser pulse. Numerical simulation shows the possibility of a hundredfold increase in the intensity of the compressed pulse in comparison with the intensity of the pump pulse at a length of uniform plasma of 2 cm.

Design, fabrication and application of dispersive mirrors with a SiO2 sculptured layer

We report on the design, fabrication and application of dispersive mirrors (DMs) with a SiO2 sculptured layer. The group delay dispersion (GDD) of the DMs are -200 fs2 in the spectral range from 680 to 920 nm. DMs with a multilayer structure consisting of a SiO2 sculptured layer with a low refractive index of ~1.09 fabricated by glancing-angle deposition (GLAD) on top of a conventional chirped film stack fabricated by dual-ion beam sputtering are designed and fabricated. The GDD oscillations of the DMs could be significantly reduced compared to the conventional chirped film stack without the SiO2 sculptured layer while maintaining a large GDD of several hundred femtoseconds squared and a wide dispersion bandwidth. For the first time, a single DM with a SiO2 sculptured layer has been successfully applied in a fiber chirped system without being used in DM-pair, which can realize the compression of laser pulses to 16fs.

Manufacturing of highly-dispersive, high-efficiency transmission gratings by laser interference lithography and dry etching

Diffraction efficiency for transmission gratings with period-to-wavelength ratios smaller than 0.6 e.g. for laser pulse compression is mainly limited due to reflection losses from imperfect effective refractive index matching. In our contribution, different approaches for achieving high efficiencies by reducing these losses are discussed based on 1d and 2d rigorous coupled wave analysis. These include multilayers underneath the corrugation, sub-wavelength gratings and optimized non-binary profiles. Experimental results from the manufacturing of 1700 l/mm gratings for 1030 nm center wavelength using two-beam interference lithography and reactive ion beam etching are presented. Diffraction efficiencies of above 96% were achieved, significantly exceeding the theoretical limit for widely used monolithic binary profiles in this application.

Using a cover layer to improve the damage resistance of gold-coated gratings induced by a picosecond pulsed laser

The chirped pulse amplification (CPA) technology is the main approach to achieve high-intensity short-pulse laser. Diffraction gratings are good candidates for stretching and compressing laser pulses in CPA. In this paper, a kind of gold-coated grating has been prepared and its laser damage experiment has been performed. The results reflect that the gratings laser damage was dominated by thermal ablation due to gold films or inclusions absorption and involved the deformation or eruption of the gold film. Based on these damage phenomena, a method of using a cover layer to prevent gold films from deforming and erupting has been adopted to improve the gold-coated gratings laser damage threshold. Since the addition of a cover layer changes the gratings diffraction efficiency, the gratings structure has been re-optimized. Furthermore, according to the calculated thermal stress distributions in gratings with optimized structures, the cover layer was demonstrated to be helpful for improving the gratings laser damage resistance if it is thick enough.

Laser-absorption effect on pulse-compression under Ohmic and weak-relativistic ponderomotive nonlinearity in plasmas

The inclusion of laser absorption in plasmas plays an important role in laser–plasma interactions. In this work, the laser pulse compression in weakly relativistic plasmas has been revisited by incorporating the collision-based laser absorption effects. By considering the role of laser absorption in plasmas, a set of coupled nonlinear equations is derived to describe the evolution of pulse compression. The laser pulse compression is reduced due to the collisional absorption in the plasmas. Fast dispersion is also observed with increasing the absorption coefficient, which is obviously due to the strong energy attenuation in plasmas. Using our theoretical model, the involvement and importance of a particular absorption mechanism for pulse compression in plasmas is analyzed.

Temperature control of the ultra-short laser pulse compression in a one-dimensional photonic band gap structure with nematic liquid crystal as a defect layer

We investigate numerically the controllable chirped pulse compression in a one-dimensional photonic structure containing a nematic liquid crystal defect layer using the temperature dependent refractive index of the liquid crystal. We consider the structure under irradiation by near-infrared ultra-short laser pulses polarized parallel to the liquid crystal director at a normal angle of incidence. It is found that the dispersion behaviour and consequently the compression ability of the system can be changed in a controlled manner due to the variation in the defect temperature. When the temperature increased from 290 to 305 K, the transmitted pulse duration decreased from 75 to 42 fs in the middle of the structure, correspondingly. As a result, a novel low-loss tunable pulse compressor with a really compact size and high compression factor is achieved. The so-called transfer matrix method is utilized for numerical simulations of the band structure and reflection/transmission spectra of the structure under investigation.

Binary-phase compression of stretched pulses

Pulse stretching and compression are essential for the energy scale-up of ultrafast lasers. Here, we consider a radical approach using spectral binary phases, containing only two values (0 and π) for stretching and compressing laser pulses. We numerically explore different strategies and present results for pulse compression of factors up to a million back to the transform limit and experimentally obtain results for pulse compression of a factor of one hundred, in close agreement with numerical calculations. Imperfections resulting from binary-phase compression are addressed by considering cross-polarized wave generation filtering, and show that this approach leads to compressed pulses with contrast ratios greater than ten orders of magnitude. This new concept of binary-phase stretching and compression, if implemented in a multi-layer optic, could eliminate the need for traditional pulse stretchers and more importantly expensive compressors.

Kinetic simulations of laser parametric amplification in magnetized plasmas

Laser pulse compression using magnetized resonance near the upper-hybrid frequency is promising for achieving higher output intensity in regimes previously thought impossible using unmagnetized plasmas. Using one dimensional particle-in-cell simulations, we verify that, by partially replacing plasma with an external transverse magnetic field of megagauss scale, the output pulse can be intensified by a factor of a few, due to the increased allowable amplification time despite a decreased growth rate. Further improvement is impeded by the generation of an electromagnetic wakefield, to which the amplified pulse loses more energy than it does in the unmagnetized case. This limitation can however be circumvented by the use of a stronger pump. In contrast to unmagnetized compression, the magnetized amplification remains efficient when the pump intensity is well above the wavebreaking threshold, until a higher phase-mixing threshold is exceeded. This surprising resilience to wavebreaking in magnetized plasma is of great benefit for magnetized compression.

X-ray amplification by stimulated Brillouin scattering

Plasma-based parametric amplification using stimulated Brillouin scattering offers a route to coherent x-ray pulses orders of magnitude more intense than those of the brightest available sources. Brillouin amplification permits amplification of shorter wavelengths with lower pump intensities than Raman amplification, which Landau and collisional damping limit in the x-ray regime. Analytic predictions, numerical solutions of the three-wave-coupling equations, and particle-in-cell simulations suggest that Brillouin amplification in solid-density plasmas will allow compression of current x-ray free-electron laser pulses to subfemtosecond durations and unprecedented intensities.

Self-compression of soliton-like laser pulses in the process of self-focusing

We study the possibility of efficient self-compression of femtosecond laser pulses in nonlinear media with anomalous dispersion of group velocity during the self-focusing of wave packets with a power several times greater than the critical self-focusing power. The results of qualitative analysis of the evolution of three-dimensional wave packets with the quasi-soliton field distribution are confirmed by the computer simulation. The simulation proves that the considered regime of compression of high-power laser pulses with initial durations of about ten optical cycles is stable relative to filamentation instability, due to the influence of the nonlinear dispersion. We demonstrate the possibility of self-compression of laser pulses at a multi-millijoule energy level and up to one optical cycle with an energy efficiency of more than 50%.