Pulse Shape Influence on the Accuracy of Z-scan Measurements

Although the laser pulses with durations ranging from nanoseconds to femtoseconds and various pulse shapes are utilized for the Z-scan measurements, the influence of the temporal pulse shape on the measurement results is often neglected. In this paper, we tried to differentiate the influence of the temporal pulse shape on the common Z-scan technique with a small on-axis aperture in two cases: when the pulse peak intensity at the beam waist is known (for relatively long pulses), or when the total pulse energy and full width at half maximum (FWHM) of the correlation functions or FWHM of pulse durations are known (for short pulses)

Some Aspects of the Inverse Problem of Determination of Cr^(4+) :YAG Absorption Cross Sections Using Experimental Data of Transmission

Mathematical modelling of nonlinear transmission of Cr4+:YAG crystals excited by Nd:YAG laser pulses of different duration are reported. Numerical simulation of transmission using a five-level scheme which included finite excitation lifetimes in Cr4+:YAG, focusing and diffraction of transversely nonhomogeneous pump radiation allowed to specify more precisely the limits of applicability of the four-level model. Critical analysis of typical simplifications commonly used for solving an inverse problem of determination of absorption cross sections is undertaken. It is shown that some of these assumptions that seem plausible enough might lead to considerable deviations of the determined absorption cross sections from their “true” values. A technique for determination of confidence intervals in case of the nonlinear regression is emphasized

Numerical Analysis of Short Pulse Optical Parametric Amplification Using Type I Phase Matching

The possibilities of efficient amplification and additional shortening of faster moving short fundamental harmonic pulses by the more slowly moving longer second harmonic pulses for type I phase matching are numerically analyzed for initially collimated axially symmetric beams, taking into account diffraction, group velocity mismatch and dispersion of the pulses

Capture of particles of dust by convective flow

Interaction of particles of dust with vortex convective flows is under theoretical consideration. It is assumed that the volume fraction of solid phase is small, variations of density due to nonuniform distribution of particles and those caused by temperature nonisothermality of medium are comparable. Equations for the description of thermal buoyancy convection of a dusty medium are developed in the framework of the generalized Boussinesq approximation taking into account finite velocity of particle sedimentation. The capture of a cloud of dust particles by a vortex convective flow is considered, general criterion for the formation of such a cloud is obtained. The peculiarities of a steady state in the form of a dust cloud and backward influence of the solid phase on the carrier flow are studied in detail for a vertical layer heated from the sidewalls. It is shown that in the case, when this backward influence is essential, a hysteresis behavior is possible. The stability analysis of the steady state is performed. It turns out that there is a narrow range of governing parameters, in which such a steady state is stable.Comment: 14 pages, 10 figures, published in Physics of Fluid

2-YEAR EXPERIENCE OF CORNEAL COLLAGEN CROSSLINKING IN KERATOCONUS TREATMENT

Aim. To assess reliability, efficacy, and stability of corneal collagen crosslinking (CXL) results for stage I‑II keratoconus.Patients and methods. 2‑year (2012-2014) results of CXL were analyzed. CXL was performed on 20 eyes of 15 patients (13 men and 2 women, mean age 31 years). Stage 1 keratoconus was diagnosed in 13 eyes, stage 2 keratoconus was diagnosed in 2 eyes. Primary stage 1‑2 keratoconus was diagnosed in 17 eyes, keratoconus after corneal refractive surgery (LASIK, radial keratotomy) was diagnosed in 3 eyes. Preoperative and postoperative examination included uncorrected (UCVA) and best-corrected (BCVA) visual acuity measurement, pachymetry, corneal topography (total astigmatism measurement), and biomicroscopy. Follow-up period varied from 1 to 24 months.Results. All patients reported on vision improvement. Preoperatively, mean UCVA and BCVA were 0.4 and 0.64, respectively. Postoperatively, mean UCVA and BCVA were 0.49 and 0.66, respectively. Preoperatively and postoperatively, total astigmatism was 2.1 D and 2.0 D, respectively, while mean central corneal thickness was 454 μm and 447 μm, respectively.Conclusions. Corneal collagen crosslinking for stage 1‑2 keratoconus provides mild UCVA and BCVA improvement, decrease in total astigmatism and central corneal thickness. CXL provides stable results of treatment of primary stage 1‑2 keratoconus and keratoconus after corneal refractive surgery