Quality Of Compressed Pulses Research Articles

Characterizing post-compression of mJ-level ultrafast pulses via loose focusing in a gas cell

The ability to generate high-intensity ultrashort laser pulses is a key driver for advancing the strong-field physics and its applications. Post-compression methods aim to increase the peak intensity of amplified laser pulses via spectral broadening through self-phase modulation (SPM), followed by temporal pulse compression. However, other unavoidable nonlinear self-action effects, which typically occur parallel to SPM, can lead to phase distortions and beam quality degradation. Here we study the ability to compress high-energy pulses by loose focusing in a noble gas to induce nonlinear spectral broadening, while limiting unwanted nonlinear effects such as self-focusing. We introduce ptychographic wavefront sensor and FROG measurements to identify the regimes that optimize pulse compression while maintaining high beam quality. Using a 700 mbar argon-filled double-pass-based scheme, we successfully compress 2 mJ, 170 fs, 1030 nm laser pulses to ∼35 fs, achieving 90% overall flux efficiency and excellent stability. This work provides guidelines for optimizing the compressed pulse quality and further energy scaling of double-pass-based post-compression concepts.

Sub-50 fs pulses generation from an all-fiber monolithic gain-managed nonlinear amplifier with CFBG-based pre-compressor

We demonstrate an all-fiber monolithic laser source from a gain-managed nonlinear (GMN) fiber amplifier with a CFBG-based pre-compressor that generates high-contrast pulses with a repetition rate of 80 MHz, average power of 1.6 W, and pulse duration of sub-50 fs. The compressed pulse quality can be optimized by tuning the parameters of the seed pulses injected into the GMN amplifier by controlling the pump power of the fiber pre-amplifier. This compact and cost-effective laser system is a simplified design of the GMN amplification-based femtosecond fiber laser systems. With alignment-free characteristics in the amplifiers and clean compressed pulses, it is easy to assemble and use in applications such as multi-photon microscopy.

Dispersion engineering in nonlinear multipass cells for high-quality pulse compression

A dispersion-engineered multipass cell operating in the enhanced frequency regime is presented. Through the use of dispersive cavity mirrors, the nonlinear interaction is reshaped resulting in a smoother broadened spectrum, which yields a significant improvement in compressed pulse quality. The 70 W average power output of an Yb:fiber laser at 50 kHz repetition rate is compressed from 205 fs to 32 fs with more than 96% of the energy contained in the temporal main feature of the pulse. This first, to the best of our knowledge, experimental demonstration of a pulse quality improvement through enhanced frequency chirping in a multipass cell displays the opportunities for dispersion-tailored pulse compression.

27nJ, 114fs Pulses From an Environmentally Stable All-Normal All-PM Yb-Doped Fiber Laser Mode-Locked With a Nonlinear Amplifying Loop Mirror

We report on a self-starting mode-locked all-polarization-maintaining fiber laser using a nonlinear amplifying loop mirror. It delivers a 7.81 MHz train of pulses with an average power of 213 mW, corresponding to pulse energy higher than 27 nJ. The output chirped pulses can be recompressed down to ∼114fs with a grating pair compressor. The nonlinear phase shifts accumulated in the long fiber cavity can be used to compensate for the mismatched third order dispersion and increase the compressed pulse quality. The laser shows excellent stability and environmental robustness during the test.

Single-stage few-cycle nonlinear compression of milliJoule energy Ti:Sa femtosecond pulses in a multipass cell.

We report on the nonlinear temporal compression of mJ energy pulses from a Ti:Sa chirped pulse amplifier system in a multipass cell filled with argon. The pulses are compressed from 30 fs down to 5.3 fs, corresponding to two optical cycles. The post-compressed beam exhibits excellent spatial quality and homogeneity. These results provide guidelines for optimizing the compressed pulse quality and further scaling of multipass-cell-based post-compression down to the single-cycle regime.

Pre-Chirp-Managed Adiabatic Soliton Compression in Pressure-Gradient Hollow-Core Fibers

Post-pulse-compression is demanded to produce energetic few-cycle pulses. We propose pre-chirp-managed adiabatic soliton compression (ASC) in gas-filled pressure-gradient hollow-core fibers to suppress the detrimental pedestals and therefore significantly improve the compressed pulse quality. We show that two-stage ASC can compress 125 μJ, 130 fs pulses at 2 μm to a nearly two-cycle pulse 15 fs in duration. Our analytical analysis suggests that ASC is in favor of compressing pulses centered at a longer wavelength. As an example, a 280 μJ, 220 fs Gaussian pulse at 4 μm is compressed to 60 fs with minimal pedestals. We expect that the resulting high-quality, energetic few-cycle pulses will find important applications in high-field science.

Performance Enhancements of Femtosecond Fiber Amplifier by Pump Wavelength Optimization

We demonstrate an efficient scheme to accelerate the self-similar pulse evolution and reduce the intensity noise of a free-running femtosecond fiber amplifier based on the pump wavelength optimization. Experiments and simulations indicate the enhanced tolerances of the pulse self-similar amplification to the seed signal power and pump wavelength fluctuations, with the optimum 915 nm pump wavelength. ∼20% increase in the compressed pulse quality and ∼31% reduction in the amplifier root-mean-square (RMS) relative intensity noise (RIN) (1.5 kHz to 5 MHz) have been observed, even with a more than 4 times higher pump laser diode (LD) RIN than the case of 976 nm. ∼50 fs transform-limited pulses are generated with the ∼0.03% amplifier RMS RIN. The proposed scheme can lower the requirements of low-noise self-similar femtosecond fiber amplifiers on the power stability of the seed oscillator and the thermal control of the pump LD, thus denoting potentials for the various satellite-based high-precision applications of femtosecond laser in space.

Spectrally shaped chirped pulse amplification of an all-normal dispersion Yb-doped fibre laser

We show experimentally and theoretically that the spectral profile of the seed pulse of an Yb-doped fibre-based chirped pulse amplifier (CPA) plays a dominant role for achieving an ultrashort compressed pulse. The picosecond seed pulse from an all-normal dispersion Yb-doped fibre laser can be compressed to 350 fs with a peak power of 25 MW. Better compressed pulse quality and a shorter pulse was achieved by the spectrally shaped CPA system as opposed to the system without spectral shaping.

Application of nonlinear pulse shaping of femtosecond pulse generation in a fiber amplifier at 500 MHz repetition rate

We numerically and experimentally demonstrate that a nonlinear pulse shaping technique based on pre-chirping management in a short gain fiber can be exploited to improve the quality of a compressed pulse. With prior tuning of the pulse chirp, the amplified pulse express different nonlinear propagating processes. A spectrum with s flat top and more smooth wings, showing a similariton feature, generates with the optimal initial pulse chirp, and the shortest pulses with minimal pulse pedestals are obtained. Experimental results show the ability of nonlinear pulse shaping to enhance the quality of compressed pulses, as theoretically expected.

Spectral and temporal breathing self-similar evolution in a fiber amplifier for low-noise transform-limited pulse generation

We demonstrate a simple scheme for high-power low-noise high-contrast ultrashort pulse generation. It is enabled by the spectral and temporal breathing self-similar pulse evolution with an optimized negative pre-chirp. Experiments and simulations indicate the enhanced tolerances of this scheme to the gain-shaping distortions and pump fluctuations. It can lead to ∼16% increase in the compressed pulse quality with more than a two times wider spectrum and ∼31% reduction in the root-mean-square (rms) relative intensity noise (RIN). Transform-limited pulses as short as 36 fs are generated with the rms RIN of 0.029% (1 kHz-5 MHz) from a 2 m Yb-fiber amplifier.

Femtosecond fiber laser for high-stability and high-quality pulse compression

We demonstrate a femtosecond fiber master oscillator power amplifier system of high-stability and high-quality pulse compression, which is applied to corneal refractive surgery. The nonlinear-polarization-evolution mode-locking in a hybrid-cavity Yb-fiber oscillator consisting of both polarization-maintaining (PM) and non-PM fibers is demonstrated. In this paper, the PM-fibers introduced into the mode-locked fiber oscillator partially replace non-PM fibers. This alternative approach is conducive to reducing the adverse effect of uncontrolled fiber birefringence, which originates from the on-PM fiber suffering environmental temperature fluctuation and mechanical vibration. Once the length of non-PM fiber is comparable to the fiber beat length, the uncontrolled fiber birefringence caused by environment starts to abate the laser robustness, repeatability, and reliability of mode-locking. The stability becomes notoriously worse for long-cavity Yb-fiber oscillators. The PM-fibers adopted in the mode-locked fiber oscillator could improve the mode-locked stability of nonlinear-polarization-evolution self-started with long-cavity Yb-fiber oscillators. We study the dependence of the compressed pulse quality on the parameters of input pulse pre-chirp injected into the Yb-doped fiber amplifier. Due to the nonlinear-chirp and third order dispersion, the mode-locked pulse shape in time-domain will produce distortion during power being amplified in the Yb-doped fiber amplifier. A diffraction grating to adjust the pre-chirp of the input pulse from positive value to negative value launched into the Yb-doped fiber amplifier is placed between mode-locked Yb-fiber oscillator and Yb-doped fiber amplifier. We vary the pre-chirp by changing the distance between the diffraction grating and triangular prism and then adjust the second grating pair to compress the amplified pulses into its shortest pulse duration of full-width at half maximum measured by an autocorrelator. The experimental results show that the best compression quality of mode-locked pulse occurs at the negative pre-chirp with a measured pulse width of 183 fs. Deviation from this optimum pre-chirp degrades the compressed-pulse quality and features an increased temporal pedestal. The fiber laser produces self-started mode-locking at a repetition rate of 19.4 MHz, an average power of 1.2 W, a pulse width of 183 fs. The homemade femtosecond fiber laser is used to perform surgery on ex vivo pig corneas. The surgey shows that the light burst of femtosecond laser in corneal tissues could cut pig corneas, which proves that the femtosecond fiber laser satisfies the surgical operation requirements of animal corneas.

Enhanced Femtosecond Optical Pulses Compression Induced by Negative Third-Order Dispersion in Solid-Core Photonic Crystal Fibers

In this paper, the chirped- free optical pulses compression through propagating in silica - index guiding photonic crystal fibers will be studied. The results show that the Raman Effect can enhances the quality of compressed pulses and also the positive third-order dispersion (TOD) can be considered as the main reason in limiting the optical pulses compression. We will show that, the combined effects of the Raman scattering and negative TODs can causes generating much further compressed pulses in comparison with applying the only Raman effect or combination of that with the positive TODs. In this paper, We also numerically present a novel technique to solve the wave propagation equation, which is known as the concrete algorithms of split-step fourier method and fourth order Runge-Kutta (RK4) method. This method has a higher accuracy, although the computational time is longer because the number of calculation steps is chosen very large.

Dynamic parabolic pulse generation using temporal shaping of wavelength to time mapped pulses

Self-phase modulation in fiber amplifiers can significantly degrade the quality of compressed pulses in chirped pulse amplification systems. Parabolic pulses with linear frequency chirp are suitable for suppressing nonlinearities, and to achieve high peak power pulses after compression. In this paper, we present an active time domain technique to generate parabolic pulses for chirped pulse amplification applications. Pulses from a mode-locked laser are temporally stretched and launched into an amplitude modulator, where the drive voltage is designed using the spectral shape of the input pulse and the transfer function of the modulator, resulting in the generation of parabolic pulses. Experimental results of pulse shaping with a pulse train from a mode-locked laser are presented, with a residual error of less than 5%. Moreover, an extinction ratio of 27 dB is achieved, which is ideal for chirped pulse amplification applications.

Optimised one-step compression of femtosecond fibre laser soliton pulses around 1550 nm to below 30 fs in highly nonlinear fibre

Optimised compression of femtosecond fibre laser pulses in highly nonlinear fibre is demonstrated. Specifically reported are compressed pulse durations below 30 fs using a simple setup where a single sub-10 cm segment of highly nonlinear fibre is directly spliced to the output pigtail of a commercial femtosecond fibre laser. The compressed pulse quality is confirmed through frequency resolved optical gating measurements.

Nonlinear propagation analysis of few-optical-cycle pulses for subfemtosecond compression and carrier envelope phase effect

A numerical approach called Fourier direct method (FDM) is applied to nonlinear propagation of optical pulses with the central wavelength 800 nm, the width 2.67-12.00 fs, and the peak power 25-6870 kW in a fused-silica fiber. Bidirectional propagation, delayed Raman response, nonlinear dispersion (self-steepening, core dispersion), as well as correct linear dispersion are incorporated into 'bidirectional propagation equations' which are derived directly from Maxwell's equations. These equations are solved for forward and backward waves, instead of the electric-field envelope as in the nonlinear Schroedinger equation (NLSE). They are integrated as multidimensional simultaneous evolution equations evolved in space. We investigate, both theoretically and numerically, the validity and the limitation of assumptions and approximations used for deriving the NLSE. Also, the accuracy and the efficiency of the FDM are compared quantitatively with those of the finite-difference time-domain numerical approach. The time-domain size 500 fs and the number of grid points in time 2048 are chosen to investigate numerically intensity spectra, spectral phases, and temporal electric-field profiles up to the propagation distance 1.0 mm. On the intensity spectrum of a few-optical-cycle pulses, the self-steepening, core dispersion, and the delayed Raman response appear as dominant, middle, and slight effects, respectively. The delayed Raman responsemore » and the core dispersion reduce the effective nonlinearity. Correct linear dispersion is important since it affects the intensity spectrum sensitively. For the compression of femtosecond optical pulses by the complete phase compensation, the shortness and the pulse quality of compressed pulses are remarkably improved by the intense initial peak power rather than by the short initial pulse width or by the propagation distance longer than 0.1 mm. They will be compressed as short as 0.3 fs below the damage threshold of fused-silica fiber 6 MW. It is demonstrated that the carrier envelope phase (CEP) causes the difference on the temporal electric-field profile and the intensity spectrum for the initial peak power of the order of megawatts. At the propagation distance longer than the coherence length for third-order harmonics, the difference grows in the spectral components around the third-order and higher-order harmonics. The CEP can be a sensitive marker to monitor the evolution of nonlinear optical process by a few-optical-cycle electric-field wave-packet source.« less

Compression of optical pulses chirped by self-phase modulation in fibers

The use of self-phase modulation in a single-mode fiber to chirp an optical pulse, which is then compressed with a grating-pair compressor, has been shown to be a practical technique for the production of optical pulses at least as short as 30 fsec. We report the results of a theoretical analysis of this process. Numerical results are presented for the achievable compression and compressed pulse quality as functions of fiber length and input pulse intensity. These results are given in normalized units such that they can be scaled to describe a wide variety of experimental situations and can be used to determine the optimum fiber length and compressor parameters for any given input pulse. Specific numerical examples are presented that suggest that the technique will generally be useful for input pulses shorter than about 100 psec. For energies of a few nanojoules per pulse, the compressed pulse widths will typically be in the femtosecond regime.