Self-similar propagation of high-power parabolic pulses in optical fiber amplifiers

Self-similarity techniques are used to study pulse propagation in a normal-dispersion optical fiber amplifier with an arbitrary longitudinal gain profile. Analysis of the nonlinear Schr?dinger equation that describes such an amplifier leads to an exact solution in the high-power limit that corresponds to a linearly chirped parabolic pulse. The self-similar scaling of the propagating pulse in the amplifier is found to be determined by the functional form of the gain profile, and the solution is confirmed by numerical simulations. The implications for achieving chirp-free pulses after compression of the amplifier output are discussed.     

Dynamics of parabolic pulses in an ultrafast fiber laser

We report experimental and numerical results on the dynamics and propagation of parabolic pulses in a passively mode-locked ytterbium-doped fiber laser. Experimental data and numerical simulations are shown to match. Particular importance is attached to the pulse-shaping process in the different sections of the resonator. The paramount role of the gain fiber and saturable absorber in the laser is explicated.     

Interaction between parabolic pulses in a dispersion-decreasing fiber

The properties of the self-similar parabolic pulses interaction in a dispersion-decreasing optical fiber with normal group-velocity dispersion are firstly investigated in our paper. We find that two parabolic pulses separated by a time-delay create oscillation with a sinusoidal fit at the beginning of their overlap, and then further evolve into a train of asymptotic dark solitons. Additionally, outside the overlap regime, the evolution is equal to a single pulse’ propagating behavior. The chirp after interaction in the parabolic pulses still allows for efficient and high-quality pulse compression.     

Experimental study of parabolic bound pulses from an Er-doped fiber ring laser

We have experimentally observed various bound states of parabolic pulses in a passively mode-locked Erbium-doped fiber ring laser. Beyond the mode-locking threshold, stable single parabolic pulse with the duration of 3.29 ps has been obtained and compressed outside the cavity to 253 fs assuming a sech² profile. By increasing the pump power and rotating the polarization controllers, stable bound states of two parabolic pulses with a fixed separation of 6.5 ps has been obtained. Further increasing the pump power and carefully adjusting the intracavity PCs, multiple bound pulses have also been observed in the same cavity with the same pulse separation.     

Self-similar evolution of self-written waveguides

Numerical simulations show that channel waveguides can be self-written in photosensitive materials. As the waveguide evolves, its shape remains approximately constant, even though its depth and width change. We find an exact solution that describes this evolution, which we show to be self-similar. A wide variety of single-peaked beams form waveguides that converge to this solution.     

Effect of self-phase modulation on the evolution of picosecond pulses in a Nd:glass laser

Many authors have reported disparate characteristics of pulses from Nd:glass mode-locked lasers. From these it has become clear that the well-developed pulse has a frequency sweep or subpicosecond structure and yields a contrast ratio less than the ideal 3 in the two-photon-fluorescence measurement. On the other hand, the early pulse is well behaved, has simple temporal and spectral structure, and yields a contrast ratio of 3 in TPF. The measurements are almost all indirect. Theories explaining the early pulse have appeared, but these fail for the fully developed pulse. The authors present time-resolved spectrograms, covering pulse development from 1/50 peak intensity, where the pulse is well mode-locked, to full development, where spectral structure is complex. A numerical analysis, including self-phase modulation, non-linear absorption, amplification and dispersion, yields results that qualitatively agree very well with the experimental records, suggesting that self-phase modulation plays an important role in the evolution of mode-locked pulses in a Nd:glass laser.     

Ultrafast temperature relaxation evolution in Au film under femtosecond laser pulses irradiation

Ultrafast temperature relaxation processes in Au film including two temperature relaxation and thermal diffusion relaxation with femtosecond laser pulse excitation were investigated numerically by Finite Element Method (FEM). With the temperature dependent thermal parameters, the full 2D temperature field evolution in picosecond and nanosecond domains were obtained. It is proposed that the heat transfer depth can be alternatively localized or enhanced by the distinct temperature relaxation mechanisms. Moreover, the effect of laser parameters and Au film thickness and surface reflectivity on the two temperature relaxation time were analysed.     

Experimental investigation of spatiotemporal evolution of femtosecond laser pulses during small-scale self-focusing

Small-scale self-focusing (SF) causes rapid increase in the partial spatial intensity, breaking up the spatial profile of the beam into an intensity increasing zone (IZ) and a non-increasing zone (NIZ). We measure the evolutions of the pulse width at the IZ and NIZ with small-scale SF, and our results show that the pulse width at the IZ becomes narrower with increasing partial spatial intensity. The pulse width at the NIZ is almost unchanged with increasing laser power. We find that our experimental results are in good agreement with an approximate theoretical analysis.     

Thermodynamic evolution of materials during laser ablation under pico and femtosecond pulses

Using molecular-dynamics, we study the thermodynamic evolution of a simple two-dimensional Lennard–Jones system during laser ablation for pulse durations ranging from 200 fs to 400 ps. We briefly review results previously obtained for fs pulses where the evolution of the material was shown to be solely a function of the locally absorbed energy (provided that only thermal effects are important), i.e., is adiabatic. For longer pulses (100 and 400 ps) the situation becomes more complex, as the relaxation path also depends on the position in the target and on the timescale on which expansion occurs. We show that, in contrast to fs pulses, the material ejected following ps laser irradiation does not enter the liquid–vapor metastable region before ablation occurs, hence showing that phase explosion is not the dominant mechanism in this regime. Following on from previous work, we propose that trivial fragmentation is the main ablation mechanism. PACS 79.20.Ds; 79.20.Ap; 61.80.Az     

Self-similar solutions of laser produced blast waves

The aerodynamics of the blast wave produced by laser ablation is studied using the piston analogy. The unsteady one-dimensional gasdynamic equations governing the flow are solved under assumption of self-similarity. The solutions are utilized to obtain analytical expressions for the velocity, density, pressure and temperature distributions. The results predict all the experimentally observed features of the laser produced blast waves.     

超短脉冲激光在DBASVP分子中传播时的双光子面积演化和光限幅效应

以一维不对称π共轭分子体系（DBASVP分子）为介质,在双光子共振条件下,从双光子面积定理和严格数值求解Maxwell-Bloch方程两方面出发,分别研究超短脉冲激光在该有机分子介质中的传播过程,从而探讨双光子面积的演化规律,并分析双光子面积定理的适用性.提出了一种数值模拟分子介质光限幅特性的理论方法.分子的电子结构和电偶极矩是基于密度泛函理论利用从头计算方法得到的.研究结果表明,基于慢变幅和慢变相近似以及单模场条件下的双光子面积定理不能很好地描述超短脉冲的双光子面积在该分子介质中的演化规律.基于双光子吸收的分子光限幅特性与分子介质的厚度有关. 关键词： 双光子吸收 光限幅效应 双光子面积定理 超短脉冲激光     

Formation of parabolic pulses in inhomogeneous fiber optical amplifiers

The conditions for forming pulses with parabolic envelope asymptotics in inhomogeneous fiber optical amplifiers are investigated. The profile of normal group-velocity dispersion, which ensures formation of parabolic pulses with a reduced rate of frequency modulation, is determined. It is shown by numerical simulation that the presence of chirp in the input pulse may cause narrowing of the spectrum at a certain energy.     

Petawatt laser pulses

We have developed a hybrid Ti:sapphire-Nd:glass laser system that produces more than 1500 TW (1.5 PW) of peak power. The system produces 660 J of power in a compressed 440+/-20 fs pulse by use of 94-cm master diffraction gratings. Focusing to an irradiance of >7x10(20) W/cm (2) is achieved by use of a Cassegrainian focusing system employing a plasma mirror.     

Time evolution of synchronously pumped dye laser pulses: Picosecond pulse formation studies

The pulse-by-pulse evolution and compression of the output of a synchronously pumped dye laser as a function of the number of pump pulses was investigated with a high resolution, variably triggered streak camera. The measured pulse durations and energies for the pulse compression process are demonstrated to be in disagreement with the predictions of recent classical rate equation models. This is shown not to be due to the complete breakdown of the model but, rather, is caused by the failure to include two key loss terms in the rate equation treatment, namely, excited state absorption and thermal lensing. In addition, a highly durable dye laser compatible with the 100 MHz output of commercially available QS and ML YAG lasers is presented which is capable of 50 μJ, 20–30 ps transform limited pulses without the need of amplification. Further refinements of this laser system are discussed, based on the experimental work reported, which will lead to tunable ～ 100 fs pulses with energies in the μJ range from a single laser pump source.     

Self-similar low-noise femtosecond ytterbium-doped double-clad fiber laser

We report the generation of self-similar highly stable femtosecond pulses from an ytterbium-doped double-clad fiber laser. Positively chirped parabolic pulses with 6.4-ps duration and more than 3.2-nJ energy are obtained. These pulses are extra-cavity compressed to 140 fs. Radio-frequency noise measurements show that this regime of emission ensures low-noise operation with less than 0.05% amplitude fluctuations. PACS 42.55.Wd; 42.65.Re     

Self-focusing of intense laser pulses in a clustered gas

We report the self-focusing of intense laser pulses in gases composed of atomic clusters. This is in strong contrast to beam spreading owing to ionization-induced refraction commonly observed in nonclustered gases. The effect is explained in terms of the ensemble average transient polarizability of the heated clusters as they explode in response to the intense pulse.     

Self-similar evolution of the surface morphology of a stressed amorphous alloy foil

A time-ordered sequence of topographic images of a stressed amorphous Fe₇₀Cr₁₅B₁₅ ribbon is presented. It is shown that the surface of this material (unlike polycrystalline metal foil) has a fractal structure due to the nonequilibrium conditions of its formation. As a tensile stress of about 500 MPa is applied to the surface, the fractal dimension of the surface increases from 1.21±0.02 to 1.34±0.03, then drops to 1.12±0.03, and finally increases to 1.22±0.02. In about 1.5 hours, a complex surface morphology characterized by a roughness amplitude of several tens of nanometers evolves into a regular pattern of shear bands with amplitude of about 300 nm. Self-affine changes in surface morphology are explained by competition between several processes, including crack propagation, surface smoothing, and self-diffusion.     

Distortion of ultrashort laser pulses in a spatial filter

By means of Collins diffraction integral, the propagation expression of an ultrashort laser pulse passing through a spatial filter is derived. The effects of the magnification of spatial filter on pulse broadening and distortion of pulse-front for the ultrashort optical pulse are analytically deduced and numerically simulated. It is found that pulse broadening and propagation time difference of a laser pulse getting through spatial filter is proportional to the magnification. As a conclusion, in an ultrashort pulse laser system with a large aperture, the effect of pulse broadening and distortion of pulse-front should be considered.