Phase mask for spatial and temporal control of ultrashort light pulses focused by lenses

The field amplitude associated with ultrashort light pulses was analyzed by using the phase-space formalism of the Wigner distribution function (WDF). The diffraction integral was properly modified to take into account the dispersion effects (up to second order). A two-dimensional WDF associated with a reduced pupil function was derived, from which the on-axis irradiance was obtained for varying times. A two-dimensional and rotationally symmetric quartic-phase mask to control the temporal stretching of femtosecond light pulses passing through optical systems was proposed and analyzed. A Gaussian spatial and temporal pulse passing through a single lens with and without the phase mask was investigated.     

Absorption of ultrashort optical pulses in water

We investigate the linear propagation of 800 and 1530 nm ultrashort optical pulses in water. For all pulse repetition rates studied, we observe pure exponential decay down to a transmission of 2.5 x 10(-5). We further demonstrate that previous observations of nonmonoexponential decay and pulse splitup in broadband pulses are consistent with Beer's law in the purely linear regime.     

An electroacoustic method for measuring the intensity of high-powermicrowave pulses

A microwave electric field intensity meter for single high-power radiowave pulses is described. A surface acoustic wave (SAW) dielectric acoustic line is used which has nonlinear electroacoustic properties and is comprised of two transducers. The first (nonlinear) transducer is affected by a microwave electric field and provides the SAW excitation due to nonlinear effects. A delayed SAW signal, carrying information about the electric field strength, is received by a linear interdigital transducer     

On the second order characterization of ultrashort pulses

Abstract

The spectral properties of almost-Gaussian functions are considered and applied to the characterization of the second-order approximation in the expansion of the coefficients of almost-perfect optical pulses. Specifically, adding small amounts of odd-order Hermite-Gaussians to a Gaussian induces a second-order increase in the time-bandwidth product, while the increase in the time-bandwidth product from adding even-order Hermite-Gaussians is higher-order and hence smaller. We indicate the class of small perturbations of Gaussian functions which change neither the temporal profile of the intensity nor the intensity of the spectral profile.     

Frequency doubling of ultrashort laser pulses in biological tissues

Theoretical and experimental studies of second-harmonic generation (SHG) in biological tissues was performed by use of ultrashort laser pulses (<1 ps). A simplified one-dimensional model for the generation and the propagation of frequency-doubled light inside tissue was developed. This model was tested in vitro against measurements of pig and chicken tissue and human tooth. The experimental results indicate that the intensity of SHG varies significantly among tissue types and between test sites in individual tissue. Possibilities of using this nonlinear tissue property in imaging and diagnostics are discussed.     

Propagation of ultrashort RF pulses in a polluted atmosphere

The effect of absorption and dispersion on the spectral characteristics and the shape of ultrashort RF pulses propagating in a polluted atmosphere is studied theoretically. The frequency band occupied by the pulses essentially covers the rotational spectrum of gas molecules encountered in a real atmosphere.     

Time-delay compensated monochromator in the off-plane mount for extreme-ultraviolet ultrashort pulses

The design of ultrafast monochromators using grazing-incidence gratings in the off-plane mount for the spectral selection of extreme-ultraviolet femtosecond pulses in a broad spectral region is presented. Their application in the selection of high-order laser harmonics is analyzed in detail. The main advantage of the off-plane mount is a much higher efficiency than that of the classical mount. It is shown that two-grating configurations preserve the length of the optical paths of different diffracted rays, maintaining the extremely short time duration of the pulse. Configurations with plane or toroidal gratings are discussed. As a test case, the design of a monochromator for the 17-61 nm region with a time compensation better than 1 fs is presented.     

Fiber-based device for the detection of low-intensity fluctuations of ultrashort pulses

We describe a fiber-based device that can significantly enhance the low-intensity fluctuations of an ultrashort pulse train to detect them more easily than with usual direct detection systems. Taking advantage of the Raman intrapulse effect that progressively shifts the central frequency of a femtosecond pulse propagating in an anomalous dispersion fiber, a subsequent spectral filtering can efficiently increase the level of fluctuations by more than 1 order of magnitude. We show that attention has to be paid to maintain the shape of the statistical distribution unaffected by the nonlinear process.     

Spatial and temporal properties of ultrashort pulses propagating in free space

Abstract

By means of the Rayleigh diffraction integral and analytic signal complex representation, the propagation equation of ultrashort pulses in free space is derived, which permits us to study spatial and temporal properties of ultrashort pulses during propagation in free space. It is shown that the pulse deformation and broadening, spectrum redshifting, narrowing and distortion can take place with increasing diffraction angle, and the pulse form changes with propagation distance in the near field, but is preserved in the far field. A comparison with previous work is also made.     

Absorption of ultrashort electromagnetic pulses by metal nanospheres in a dielectric medium

The absorption of ultrashort electromagnetic pulses on silver nanosphere embedded into glass in vicinity of plasmon resonance is studied theoretically in the frame of perturbation theory. The calculations are made for corrected Gaussian shape of incident pulse which enables us to consider both the short duration and the long duration regimes. Analysis based on numerical calculations reveals the specific features of considered process so as the change of absorption spectra for different pulse length and nonlinear dependence of absorbed energy upon pulse duration.     

Two-photon excitation of atoms by ultrashort electromagnetic pulses in a discrete spectrum

Abstract

The paper is devoted to the theoretical investigation of two-photon excitation of atom in a discrete energy spectrum by ultrashort electromagnetic pulses of femto- and subfemtosecond ranges of durations. An analytical expression for the total probability of the process is derived. Numerical simulations are made for hydrogen and sodium atoms. It is shown that the total probability of the process is nonlinear function of pulse duration and character of this function depends strongly on the frequency detuning of pulse carrier frequency from two-photon resonance.     

Free-space propagation of ultrashort pulses: Space-time couplings in Gaussian pulse beams

Abstract

The importance of space-time couplings and the validity of the paraxial approximation are discussed for the case of ultrashort freely-propagating Gaussian pulses whose spectral components all have the same frequency-independent Rayleigh length. Simple closed-form expressions describing the electric field of such pulses are derived for model spectral distributions. Space-time couplings are found to be significant only for subcycle pulses or far from the focus.     

Focusing of ultrashort laser pulses by the combination of diffractive and refractive elements

Hybrid elements, containing optical power with both diffractive (holographic) and refractive components, are shown to be able to eliminate the effect of propagation time difference. The consideration is provided through a paraxial approximation of diffraction theory.     

Application of generalized differential quadrature method to structural problems

This paper presents the first endeavour to exploit a generalized differential quadrature method as an accurate, efficient and simple numerical technique for structural analysis. Firstly, drawbacks existing in the method of differential quadrature (DQ) are evaluated and discussed. Then, an improved and simpler generalized differential quadrature method (GDQ) is introduced to overcome the existing drawback and to simplify the procedure for determining the weighting coefficients. Subsequently, the generalized differential quadrature is systematically employed to solve problems in structural analysis. Numerical examples have shown the superb accuracy, efficiency, convenience and the great potential of this method.     

Dynamics of ultrashort light pulses in a semiconductor superlattice in the presence of a magnetic field

A system of equations that describes the propagation of ultrashort light pulses (optical solitons) in a semiconductor superlattice in the presence of a magnetic field is obtained using coupled Maxwell equations for the electromagnetic field and the Boltzmann equation written in the relaxation time approximation for the one-electron distribution function. It is shown that an initial linearly polarized light pulse induces a field with the orthogonal polarization in the sample. The dynamics of joint propagation of the initial and induced pulses in the sample is studied.     

Analytical method to evaluate the dependability of manufacturing systems

The association between Stochastic Petri nets and Markov's chains constitutes a powerful tool for analysis. However, the Markovian's models obtained for complex manufacturing systems are so large that their storage and analysis is very expensive and very time consuming. The method based on decomposition and iterative analysis is very efficient for the number of states combinatory explosion. In this paper, we present a method for modular modelisation, an algorithm for iterative analysis and some numerical results.     

Effect of material dispersion on the focusing of isodiffracting ultrashort pulses by a lens

Abstract

Based on the Fourier transform, the focusing of isodiffracting ultrashort pulses by a lens is studied, where the material dispersion of first, second and higher order is taken into account, respectively. Numerical calculation results for spatial and temporal intensity distributions, photon flux and energy density of focused isodiffracting ultrashort pulses are given and illustrated. It is shown, compared to the dispersion-free case, that the first-order dispersion leads to a broadening of the pulse form, photon flux and energy density, and a decrease of their peak values. The second-order dispersion results in a further broadening of the pulse form and photon flux, and a further decrease of their peak values, whereas the higher-order dispersion plays a relatively minor role.     

Modified toughness used to evaluate the effect of polymer modified asphalt on SMA

Abstract

Toughness, as defined in ASTM D5801, is the work used to stretch a specimen until fracture, and is used to evaluate the ability of polymer modified asphalt (PMA) to resist deformation. Fracture elongations in PMA are usually longer than 10 cm. However, it is almost impossible for asphalt concrete, with or without PMA, to endure such large deformation before fracture. It is presumed that an effective elongation exists for more effective determination of toughness. Principle component analysis (PCA) and single regression analysis were used in this study to evaluate the correlation between physical tests of PMA, including toughness and performance tests of Stonic Mastic Asphalt (SMA). Meanwhile, performance tests, including resilient modulus tests, creep tests and indirect tensile tests, were conducted on SMA samples. According to the results form Principle Components Analysis (PCA), it was observed that only a common factor affects the performance tests. Regression analyses were used to find common factors from physical tests of PMA. Correlation coefficients between toughness and performance tests were found to be better than other physical tests. When toughness was calculated with effective elongation (6.5 cm), R ² was 0.90. In our opinion, the desirable PMA should provide SMA enough work to resist the deformation while the deformation is still small. This result was also confirmed by observation of SEM and Rheological analysis. Modified toughness (calculated with effective elongation) considered as the common factor, is a simple method to evaluate the microstructure of PMA. Overall, modified toughness seems promising for use in evaluation of the effect of PMA on SMA.