Propagation Of Femtosecond Laser Pulses Research Articles

Features of focused propagation of intense femtosecond laser pulses in air under low pressure

The results of theoretical and experimental investigations of the influence of air pressure on the dimensional and power parameters of preliminary focused GW femtosecond laser radiation are presented. A nonlinear region of the focus of a laser beam is examined. It is established that in rarefied air the achievement of higher values of laser radiation intensity in the focal waist is possible due to a decrease in the plasma blocking action.

飞秒激光在晶体中传输的级联非线性过程

飞秒激光在晶体中传输的级联非线性过程

Broadband Asymmetric Conical Emission via Cascaded Second-Order Nonlinear Polarization during the Propagation of Femtosecond Laser Pulses in a BBO Crystal

We investigate the propagation of femtosecond laser pulses in a 5-mm-thick BBO crystal along the direction of type-I phase-matched second-harmonic generation. An intensity-asymmetric broadband conical emission (500–2000 nm) is demonstrated when a suitable chirp is introduced. It is generated by optical parametric amplification pumped by the second-harmonic light and seeded by the fundamental light which is broadened by cascaded nonlinear processes during second-harmonic generation.

Fabrication of hollow optical waveguides in fused silica by three-dimensional femtosecond laser micromachining

We report on the fabrication of hollow optical waveguides in fused silica using femtosecond laser micromachining. We show that in such hollow waveguides, high-intensity femtosecond laser beams can be guided with low optical loss. Our technique, which was established earlier for fabrication of optofluidic structures in glass, can ensure a high smoothness at the inner surfaces of the hollow waveguides and provide the unique capability of fabrication of hollow waveguides with complex geometries and configurations. A transmission of ∼90% at 633 nm wavelength is obtained for a 62-mm-long hollow waveguide with an inner diameter of ∼250 μm. In addition, nonlinear propagation of femtosecond laser pulses in the hollow waveguide is demonstrated, showing that the spectral bandwidth of the femtosecond pulses can be broadened from ∼27.2 to ∼55.7 nm.

Geometrical study of a hexagonal lattice photonic crystal fiber for efficient femtosecond laser grating inscription

We have studied transverse propagation of femtosecond pulse duration laser light through the microstructure of hexagonal lattice photonic crystal fibers. Our results provide insight in the role of the microstructure on the amount of optical power that reaches the core of the PCF, which is of particular importance for grating inscription applications. We developed a dedicated approach based on commercial FDTD software and defined a figure of merit, the transverse coupling efficiency, to evaluate the coupling process. We analyzed the propagation of femtosecond laser pulses to the core of a wide range of PCFs and studied the influence of the PCF orientation angle, the air hole pitch and air hole radius on the energy reaching the core. We have found that the transverse coupling efficiency can benefit from a dedicated design of the microstructured cladding and an accurate fiber orientation. We designed a dedicated PCF microstructure that enhances transverse coupling to the core at a wavelength of 800 nm.

Characteristics of filaments during high-power femtosecond laser radiation propagation in air and water: I. Qualitative analysis

The problem of propagation of high-power femtosecond laser pulses in a self-focusing regime and formation of filaments is considered. Comparative analysis of key parameters of the filaments formed in a laser beam in two physically distinct media (atmospheric air and water) is carried out. It is established that the cardinal distinctions in optical parameters of water and air, and, first of all, in values of the Kerr coefficient, chromatic dispersion, and photoionization probability of molecules lead to changes in key characteristics of light and plasma filaments generated in the channel.

Filamentation of femtosecond Gaussian pulses with close-to-linear or -circular elliptical polarisation

A numerical investigation was made of the formation and development of filaments in the propagation of high-power femtosecond Gaussian laser pulses in argon, whose polarisation is close to the linear or circular one. Filaments produced by close-to-circularly polarised pulses were found to be more uniform, greater in diameter, and higher in intensity than the filaments produced by close-to-linearly polarised pulses. For incident pulses with a close-to-linear (circular) polarisation, the degree of ellipticity of the radiation on the axis of the resultant filament becomes equal to zero (unity) at the instant of the peak of the local intensity.

Propagation of Intense Femtosecond Laser Pulses in Deuterated Methane and Deuterium Cluster Jets

Propagation of intense femtosecond laser pulses (60 fs, 800 nm, 120 mJ, 6 × 1017 W/cm2 in vacuum) in supersonic (CD4)N and (D2)N cluster jets at different backing pressures was studied. Pump-probe interferometry is employed to investigate the propagation of laser beams in dense cluster jets by examining the electron density distribution of plasma channels. It was found that propagation effects, including ionization-induced defocusing and laser attenuation of incident pulses, are very different in the (CD4)N and (D2)N cluster jets. Different ionization states of CD4 and D2 molecules were observed by analyzing the transverse electron density profiles of the plasma channels and should be considered as a major reason for the differences in the propagation effects. Numerical simulations of the nonlinear propagation of femtosecond laser pulses in (CD4)N and (D2)N cluster jets were performed, and the results indicated a good reproduction of the experimental data.

Generation of squeezed vacuum pulses at 810 nm using a 40-cm-long optical fiber

We experimentally demonstrate the generation of a squeezed vacuum pulse at 810 nm with a fiber polarization interferometer. During femtosecond laser pulse propagation through an optical fiber in the normal dispersion regime, only self-phase modulation within a short length contributes to pulse squeezing since the laser pulse is immediately broadened. Guided acoustic-wave Brillouin scattering (GAWBS) noise that increases in proportional to the fiber length is also lower with shorter fibers. Consequently, a maximum noise reduction of 2.1 dB (4.8 dB when corrected for losses) is obtained using a 40-cm-long single mode optical fiber.

PARAMETER CONTROL OF OPTICAL SOLITON IN ONE‐DIMENSIONAL PHOTONIC CRYSTAL

The paper deals with finding of soliton solution for Schrödinger equation with periodic linear and nonlinear properties of medium in 1D case. Such structure is named as photonic crystal. To find soliton solution the corresponding problem for finding of eigenfunctions and eigenvalues is formulated. Iterative process is proposed for solution of this problem. Using the technique of continuation on parameter we investigate a dependence of soliton location on its maximum intensity, on ratio between light frequency and frequency of structure, on ratio between dielectric permittivity of alternating linear and nonlinear layers and on position between centre of initial distribution of eigenfunction and center of considered photonic structure area. The results of this paper confirm the features of soliton self‐formation investigated early in our papers [37, 38, 39, 40, 41, 42], in which one considered a propagation of femtosecond laser pulse through nonlinear layered structure.

Investigation of wave breaking in the normal dispersion region of nano-structured photonic crystal fibers

The propagation of femtosecond laser pulses with wavelengths of 1550 nm, 1064 nm, 800 nm and 700 nm, respectively, which are in the normal dispersion region of the nano-structured photonic crystal fiber (N-PCF) with interesting broadband normal dispersion and highly nonlinear properties, is studied. For the effect of chirp variation mainly induced by group velocity dispersion (GVD) and self-phase modulation (SPM), after propagation over a short length, the wave breaking occurs. Namely, oscillatory structures are presented near pulse edges and sidelobes appear in the pulse spectrum. In the case of 800 nm, after the propagation of 20 mm, a super flat spectrum is obtained. The bandwidth of the super flat spectrum is associated with the dispersion length and the nonlinear length. By choosing N-PCF and laser pulse with appropriate parameters, a broadband super flat spectrum in a short length can be achieved.

Direct time-domain observation of laser pulse filaments in transparent media

The interplay among self-focusing, energy depletion, and plasma formation is fundamental to the understanding of laser-matter interaction. In this article, we present the first direct time-resolved observations of the propagation of intense femtosecond laser pulses experiencing conical emission, self-focusing, self-guiding, beam filamentation, plasma defocusing, and continuum generation in wide-band-gap dielectrics. We demonstrate that continuum generation involves different mechanisms as a function of deposited energy.

Slow wave plasma structures for direct electron acceleration

Two highly versatile experimental techniques are demonstrated for making preformed plasma waveguides with a periodic structure capable of supporting the propagation of ultra-intense femtosecond laser pulses up to 2×1017 W cm−2, limited by the available laser energy. These waveguides are made in hydrogen, nitrogen and argon plasmas with a length of 15 mm and a modulation period as short as 35 μm. Simulations show that these guides allow direct laser acceleration of electrons, achieving gradients of 80 MV cm−1 and 10 MV cm−1 for laser pulse powers of 1.9 TW and 30 GW, respectively. It is also shown that the periodic structure in these waveguides supresses the Raman forward instability, which could otherwise interfere with the direct acceleration scheme proposed.

Broad emission band of Yb^3+ in the nonlinear Nb:RbTiOPO_4 crystal: origin and applications

By means of micro-structural and optical characterization of the Yb:Nb:RbTiOPO(4) crystal, we demonstrated that the broad emission band of Yb(3+) in these crystals is due to the large splitting of the ytterbium ground state only, and not to a complex multisite occupation by the ytterbium ions in the crystals. We used this broad emission band to demonstrate wide laser tuning range and generation of femtosecond laser pulses. Passive mode-locked laser operation has been realized by using a semiconductor saturable absorber mirror, generating ultra short laser pulses of 155 fs, which were very stable in time, under Ti:sapphire laser pumping at 1053 nm.

Filamentation of high-power laser radiation in air and water: Comparative analysis

The propagation of high-power femtosecond laser pulses is considered in the self-focusing and filamentation formation regime. Similarities and differences in the character of the laser beam filamentation are analysed qualitatively and quantitatively in the atmospheric air and water. Based on the numerical results, the most important parameters (radius, intensity, free electron density) in light and plasma filaments produced by ultrashort laser pulses are shown to depend strongly on the optical parameters of the propagation medium, and, first of all, on the Kerr nonlinearity coefficients and the photoionisation rate of the medium molecules.

On the focusing limit of high-power femtosecond laser pulse propagation in air

Focused propagation of high-power femtosecond laser radiation in air is considered. Based on numerical solution of the nonlinear Schrodinger equation for complex envelope of light wave electric field, evolution of the beam effective radius is studied. The dependence of the effective (rms) size of a focal spot and the maximally achievable intensity of laser radiation at focal waist on the initial pulse power is established. It is shown that focal spot of tightly focused intensive ultrashort laser radiation can change its size during the pulse passage through the beam waist. This is the consequence of pulse intensity clamping in region of beam focusing caused by gas photoionization and plasma producing. This may prevent laser intensity from its further growth in the focal region and arrest the transversal compression of the beam in the linear focus as a whole.

SPECTRAL WIDTH VARIATION OF ULTRASHORT LASER PULSES IN MONOMODE OPTICAL FIBERS

Femtosecond laser pulse propagation in monomode optical fibers is demonstrated and investigated numerically (by simulations) and experimentally in this paper. A passively mode locked Nd:glass laser giving a pulse duration of about 200 fsec at 1053 nm wavelength and 120 mW average optical power with 100 MHz repetition rate is used in the experimental work. Numerical simulations are done by solving the nonlinear Schrödinger equation with the aid of Matlab program. The results show that self phase modulation (SPM) leads to compression of the spectral width from 5 nm to 2.1 nm after propagation of different optical powers (34, 43, 86 and 120 mW) in fibers of different length (5, 15, 35 m). The varying optical powers produced a varying phase shift. The output spectral width also changed with the fiber length at a given peak power.

Multistep third-harmonic generation of femtosecond laser pulses in periodically-poled and chirped-periodically-poled lithium niobate

Simultaneous generation of the second and third harmonics in periodically-poled and chirped-periodically-poled lithium niobate (PPLN and CPPLN) is studied numerically taking into account mismatches and dispersions of the group velocities of interacting laser pulses. It is shown that CPPLN frequency converters can be successfully used for creating multicolor short laser pulses.

Tracking propagation of ultrashort intense laser pulses in gases via probing of ionization

We use optical interferometry to study the propagation of femtosecond laser pulses in gases. We show the measurements of propagation in a nitrogen gas jet and we compare the results with propagation in He under the same irradiation conditions. We find that in the case of nitrogen, the detailed temporal structure of the laser pulse can be tracked and visualized by measuring the phase and the resulting electron-density map. A dramatically different behavior occurs in He gas jets, where no details of the temporal structure of the laser pulse are visible. These observations are explained in terms of the ionization dynamics of nitrogen compared to helium. These circumstances make N2 gas sensitive to variations in the electric field and, therefore, allow the laser-pulse temporal and spatial structures to be visualized in detail.

High-efficiency second-harmonic generation of superintense ultrashort laser pulses

The effect of instantaneous cubic nonlinearity on second-harmonic generation of femtosecond laser pulses is investigated. The possibility of improving generation efficiency and shortening pulse duration is analyzed. Energy conversion efficiency of 60% was obtained in an experiment in a 1 mm thickness KDP crystal at the peak intensity of 0.6 TW/cm(2) and the duration of 60 fs.