Relativistic cross-phase modulation driven compression of Terawatt laser pulses to Sub-7 fs Regime: A scalable approach to petawatt systems

With advent of chirped-pulse amplification, the peak power of femtosecond laser pulse was reached to petawatt (PW) or hundreds of terawatt (TW). Many progresses of high-field physics and ultrafast dynamics in matter are achieved using TW or PW laser. Pre-amplifier is an exponential growth amplifier which is also a bridge between oscillator and power amplifier. The best choice of pre-amplifier is amplification in regenerative cavity, due to its high stability and beam quality. The quality of pre-amplified laser pulse is significant to efficiency and beam quality of the successive power amplifier. High energy pre-amplifier with high beam quality will reduce the requirement of pump laser in final power amplifier. But typical regenerative amplifier only support low output energy of few millijoule. Higher energy from only one regenerative amplifier is crucial to whole laser system. High energy regenerative amplifier can be achieved by increasing the size of TEM00 in cavity. A new femtosecond Ti:sapphire regenerative amplifier with large mode size was demonstrated in this letter. The regenerative cavity is designed as stable linear resonator in which end mirrors are planar, the diameter of beam waist in Ti:sapphire crystal is larger than 2 mm, which can support high energy pulse amplified in cavity. By matching the focal spot of pump laser with the size of mode and optimization of cavity, the output laser energy up to 17.4 mJ was achieved under the pump energy of 60 mJ at repetition rate of 10 Hz, which corresponds to the efficiency of 29%. The amplified laser pulse from regenerative amplifier was compressed in a grating-pair compressor. By carefully alignment of incident angle and distance between the two gratings of compressor, the shortest pulses duration of 40.6 fs and energy of 13.9 mJ are obtained, which is a little bit longer than Fourier-transform limit based on spectrum of laser. The dispersion in the CPA laser system was also analyzed, after optimization of compressor, there are still high order dispersions uncompensated, which results in the duration of compressed pulses longer than Fourier-transform limit. Based on this large mode size regenerative amplifier, peak power of 1.9 TW laser pulses which compressed pulse energy of 81.4 mJ in 43 fs were also further realized by following only one stage of multipass amplifier. The beam quality (M2) was measured to be 1.6 and 1.5 in X and Y directions respectively, and the energy stability is 2.15% (rms). The results show that this large mode size regenerative amplifier is an ideal choice of pre-amplifier in TW laser system.

The compression of a relativistic Gaussian laser pulse in a magnetized plasma is investigated. By considering relativistic nonlinearity and using non-linear Schrödinger equation with paraxial approximation, a second-order differential equation is obtained for the pulse width parameter (in time) to demonstrate the longitudinal pulse compression. The compression of laser pulse in a magnetized plasma can be observed by the numerical solution of the equation for the pulse width parameter. The effects of magnetic field and chirping are investigated. It is shown that in the presence of magnetic field and negative initial chirp, compression of pulse is significantly enhanced.

This article presents a preliminary study of the longitudinal self-compression of ultra-intense Gaussian laser pulse in a magnetized plasma, when relativistic nonlinearity is active. This study has been carried out in 1D geometry under a nonlinear Schrodinger equation and higher-order paraxial (nonparaxial) approximation. The nonlinear differential equations for self-compression and self-focusing have been derived and solved by the analytical and numerical methods. The dielectric function and the eikonal have been expanded up to the fourth power of r (radial distance). The effect of initial parameters, namely incident laser intensity, magnetic field, and initial pulse duration on the compression of a relativistic Gaussian laser pulse have been explored. The results are compared with paraxial-ray approximation. It is found that the compression of pulse and pulse intensity of the compressed pulse is significantly enhanced in the nonparaxial region. It is observed that the compression of the high-intensity laser pulse depends on the intensity of laser beam (a0), magnetic field (ωc), and initial pulse width (τ0). The preliminary results show that the pulse is more compressed by increasing the values of a0, ωc, and τ0.

Compression of pulsed Nd : glass laser radiation under stimulated Brillouin scattering (SBS) in perfluorooctane is investigated. Compression of 16-ns pulses at a beam diameter of 30 mm is implemented. The maximum compression coefficient is 28 in the optimal range of laser pulse energies from 2 to 4 J. The Stokes pulse power exceeds that of the initial laser pulse by a factor of about 11.5. The Stokes pulse jitter (fluctuations of the Stokes pulse exit time from the compressor) is studied. The rms spread of these fluctuations is found to be 0.85 ns.

A simple theoretical model is proposed for describing the simultaneous effects of phase self-modulation and nonlinear birefringence in the course of propagation of laser pulses along a fiber waveguide. It is shown that the use of these two effects provides a satisfactory method for compressing high-energy laser pulses of energy three orders of magnitude higher than the energy attainable by compression of pulses in traditional systems. A report is given of experiments which support satisfactorily the proposed model.

In this paper, we report on the enhanced pulse compression due to the interaction between the positive third-order dispersion (TOD) and the nonlinear effect (cross-phase modulation effect) in birefringent fibres. Polarization soliton compression along the slow axis can be enhanced in a birefringent fibre with positive third-order dispersion, while the polarization soliton compression along the fast axis can be enhanced in the fibre with negative third-order dispersion. Moreover, there is an optimal third-order dispersion parameter for obtaining the optimal pulse compression. Redshifted initial chirp is helpful to the pulse compression, while blueshifted chirp is detrimental to the pulse compression. There is also an optimal chirp parameter to reach maximum pulse compression. The optimal pulse compression for TOD parameters under different N-order solitons is also found.

Purpose: The interaction of powerful laser pulses with solid‐state targets has recently been demonstrated to produce beams of energetic protons. The generation and characterization of laser accelerated protons is nowadays one of the most rapidly developing fields in accelerator physics. On the application side, the development of a cost‐effective and energy efficient way of producing proton beams represents a major breakthrough in hadron therapy. In this work we present a detailed study of the requirements imposed on the laser system in order to achieve extremely high peak intensities, necessary for proton acceleration. Different target designs are experimentally investigated. Method and Materials: The laser system consists of a chain of commercial lasers and amplifiers, based on Titanium doped sapphire crystal as active medium, followed by a custom‐design power amplifier. The technique of chirped‐pulse amplification is employed as the most efficient amplification scheme for ultra‐short laser pulses developed to date. Sharp focusing is achieved by an off‐axis parabolic mirror designed for low losses and minimal aberrations. Frequency resolved optical gating measurements provide the complete laser pulse characterization. Results: After pulse compression in vacuum we achieved 40 fs pulse duration with 1.1 J in each pulse at 10 Hz repetition rate. Focused beam spot size of 8 μm has been accurately measured. We demonstrate capability of obtaining pulses with characteristics suitable for proton acceleration. All stages of the pulse generation, amplification, compression, and conditioning are discussed in detail in this study. Different regimes of acceleration are discussed along with experimental evidence for their realization. Conclusion: We present a detailed characterization of the current state of our laser system and a novel target chamber design. The implications for proton acceleration along with optimization strategies for enhanced proton yield and collimation are discussed as well.

Separation and compression of pulses from a gain-switched multimode semiconductor laser are achieved by using a dispersion-shifted fiber. The evolution of the pulse waveform is analyzed by an improved expression of diode laser chirp, suitable for large signal modulation. A brief discussion is made on the possible application of this technique to a novel hybrid time/wavelength division multiplexing scheme for fiber-optic signal transmission.

Pulse stretcher based on a single diffraction grating and an Offner telescope is modified using an off-plane (conical) diffraction to reach the Littrow angle of diffraction. We present a high power transmission over a broad spectral bandwidth of ultrashort laser pulses. The slight conical diffraction does not involve significant optical aberrations of system consisting of a pulse stretcher and a pulse compressor. It is verified by ray-tracing calculations and by the experimental pulse stretching of 10-fs laser pulses to ~300 ps and by a back compression of the stretched pulses. The described pulse stretcher and compressor can improve performance of systems for chirped pulse amplification.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Degenerate-cross-phase-modulation is a new class of cross-phase-modulation where two or more laser pulses with the same frequency but different polarizations interact in condensed matter to produce controllable spectral and temporal changes of a weak probe pulse. This process is different from the conventional cross-phase-modulation where pump and probe pulses have different wavelengths. In some cases, such as in isotropic media, or using the modulation instability to compensate the birefringence in a medium, the phasematched degenerate-four-wave-mixing can exist in degenerate-cross-phase-modulation process. We numerically compare the results of the pulse compression and amplification of degenerate-cross-phase-modulation with and without the influence of degenerate-four-wave-mixing. In the case without the influence of degenerate-four-wave-mixing, the probe pulse has been compressed while the pulse energy remains the same as its input value. When the effect of the degenerate-four-wave-mixing is taken into consideration, the weak probe pulse is amplified as well as compressed. The amplification via degenerate-four-wave-mixing improves the effect of pulse compression. In the dual pump pulse compression case with the normalized delay between the probe and pump pulses Td = 2, the pulse duration of the probe pulse has been reduced by a factor of 15 with the energy gain of 4.3 times.

We review the recent progress on ultra-short pulse laser beam delivery and compression using hypocycloid-shaped core-contour (i.e. negative curvature) hollow-core photonic-crystal-fibre.

High-repetition-rate femtosecond "OR" gate design in a differential SOA-MZI

: Compression of ultrafast laser pulses is critical to obtain optimal results from many experiments that use a femtosecond laser system. However, as the pulse lengths decrease and peak pulse power increases, pulse compression using static elements such as prisms may not be sufficient to completely compress the laser pulse. This report details the complete construction of a spatial light modulator (SLM)-based 4 frequency Fourier pulse shaper that is used in femtosecond laser pulse compression. First, the theory behind compression of femtosecond lasers is covered and initial pulse compression efforts using static elements are discussed. Next, all aspects of the SLM pulse shaper construction are covered, including part selection, alignment, and calibration curve determination. This technical report also discusses the theory, construction, and evaluation of 2 separate algorithms, a modified genetic algorithm and the multiphoton intrapulse interference phase scan (MIIPS) algorithm, used to optimally compress the femtosecond laser pulses using the pulse shaper. The efficacy of these 2 algorithms when used for pulse compression was evaluated, and it was found that the MIIPS algorithm was superior to the genetic algorithm for pulse compression.

We describe an interferometric method for the reshaping and amplification of a mode-locked train of short laser pulses, based on a ring cavity containing a semiconductor laser amplifier. The amplifying medium provides both the gain and the phase nonlinearity needed for pulse reshaping. Numerical simulations have shown that the best reshaping occurs with the use of a low finesse cavity operated at laser threshold. Upon these conditions, calculations have shown that both pulse compression and amplification are obtained over a broad range of values of the linear phase shift between the cavity and pulse train. Pulse amplification and compression factors exceeding 50 and 100, respectively, resulting in peak power amplification factors up to 1000, are predicted for a 25 dB amplifier. These factors can be increased by decreasing the ratio of the input pulse energy over the saturation energy of the amplifier.

Lasers that provide a focal energy encompassed by a volume of a few cubic wavelengths (λ3) can produce relativistic intensity with maximal gradients, using minimal energy. With particle-in-cell simulations we found, that single 200 attosecond pulses could be efficiently generated in a λ3 laser pulse reflection, via deflection and compression from the relativistic plasma mirror driven by the pulse itself. An analytical model of coherent radiation from a charged layer confirms the pulse compression and is in good agreement with simulations. This novel technique is efficient (~10%) and can produce single attosecond pulses from the millijoule to the joule level.