Alexander G. Kalintsev

Noncollinear parametric generation in LiIO 3 and β-barium borate by frequency-doubled femtosecond Ti:sapphire laser pulses

In LiIO(3) and BBO crystals the wave-matching conditions for femtosecond noncollinear parametric light generation at lambda = 390 nm pumping wavelength are investigated. In the LiIO(3) crystal simultaneous phase- and group-velocity-matching angles are determined. Parametric generation occurred at 0.45-2.9-mu;m wavelengths by pumping with the second harmonic of 150-fs Ti:sapphire laser pulses and is in qualitative agreement with calculated directions in both crystals.

Efficient noncollinear parametric amplification of weak femtosecond pulses in the visible and near-infrared spectral range.

We report measurement of efficient amplification of weak femtosecond supercontinuum seed pulses by use of a noncollinear optical parametric process in BBO crystal pumped with 150-fs pulses from a frequency-doubled regenerative-amplified Ti:sapphire laser at 390nm . The highest amplification factor, 10(8) , was achieved for 3x10(-16)J energy seed pulses at wavelength of 560nm.

Application of the photon average trajectories method to real-time reconstruction of tissue inhomogeneities in diffuse optical tomography of strongly scattering media

The possibility of application of the photon average trajectories (PAT) method to real-time reconstruction of tissue inhomogeneities in diffuse optical tomography of strongly scattering media has been substantiated. By this method, the inverse problem is reduced to solution of the integral equation with integration along a conditional PAT. Such an approach allows the standard fast algebraic algorithms commonly used in projection computed tomography to be applied to diffuse optical image reconstruction. To demonstrate the capabilities of the PAT method, a numerical experiment on cross-sectional reconstruction of cylindrical strongly scattering objects with absorbing inhomogeneities has been done. Relative shadows caused by inhomogeneities are simulated via numerical solution of the non-stationary diffusion equation. To solve the inverse problem, the QR-factorization least-squares algorithm and the multiplicative algebraic reconstruction technique are used. The results are compared with those obtained by a well-known software package for temporal optical absorption and scattering tomography based on multiple solution of the diffusion equation. It is shown that the PAT method allows reconstruction of the optical structure of objects with comparable accuracy while saving reconstruction time considerably.

The possibility of increasing the spatial resolution in diffusion optical tomography

This paper discusses the possibility of increasing the resolution of diffusion optical tomography, using the method of average photon trajectories. It is shown that this method makes it possible to use fast reconstruction algorithms based on both iterative algebraic procedures and integral transformations with filtering of projections. The latter case not only suppresses the discretization noise but also substantially compensates the diffusion smearing of the reconstructed images.

Nonlinear optical limiters of laser radiation based on reverse saturable absorption and stimulated reflection

We presented wide range of laser-radiation optical limiters that are based on different physical principles, operating in 400-12000 nm spectral range with rate 100-1 ns and providing wide dynamic range of limiting required for eye and sensor protection. We propose and investigate a method for reduction of the optical limiting threshold due to the Bragg reflection in a system with distributed feedback. The limiting threshold with the Borman effect, accompanying by diffractional absorption suppress and stimulated reflection from the LC Bragg grating, is about (mu) J/cm2 for nano seconds pulse. We investigate nonlinear optical limiters of laser radiation based on fullerene-contained materials, operating on the basis of reverse saturable absorption (RSA) as well as stimulated nonlinear reflection of laser radiation. Fullerenes C60, C70 and C84 were studied with the second harmonic of Nd:YAG laser, 532 nm, single-mode radiation pulses of 30 ns duration and maximum energy 0.35 J. Definite thresholds of RSA limiting vary from 0.05 to 0.1 J/M2) depending on fullerene density and limiting scheme. Dynamic range of limiting sufficiently increases for multi-pass trains, the fluence decreases up to 10000 times. We elaborate 3000-12000 nm laser-radiation attenuators based on multi-layer interference containing vanadium-dioxide film. It is shown that these attenuators can operate due to both amplitude and diffraction effect. Attenuation is up to 1x103- 1x105 times for 1 MW/cm2 beams, operating time being 30-100 ns. For sensors in cooled optical systems, radiation attenuators can be used with vanadium oxides having the phase transition at cryogen temperatures.

Superresolution in diffuse optical tomography

The possibility of applying image reconstruction methods used in optics to improve the resolution of the photon average trajectories method of diffuse optical tomography is demonstrated. The limit resolution determined by Nyquist’s consequence of the Kotel’nikov-Shannon theorem is achieved.

Photodynamics of nonlinear fullerene-containing media

The results of theoretical and experimental studies on photodynamics and mechanism of nonlinear optical processes, responsible for optical limiting of power radiation in the wavelength range from 0.3 to 1.3 microns, are presented. Peculiarities in the mechanisms of optical limiting for different fullerene-containing matrices, including solutions, solid-state and polymer systems, are shown.

Application of photon average trajectories method for separate mapping of absorbing and scattering macroinhomogeneities using time-domain measurements technique

Theoretical analysis and numerical experiments show a significant difference in a temporal dynamics of shadows caused by absorbing and scattering macroinhomogeneities. This difference is especially noticeable at the leading front of the pulse passed through the scattering medium. This makes it possible to image absorbing and scattering inhomogeneities separately using shadows obtained at subsequent time moments.

Influence of fast reconstruction algorithms on spatial resolution of optical diffuse tomography by photon average trajectories method

The influence of three reconstruction algorithms on spatial resolution of optical diffuse tomography by Photon Average Trajecotries (PAT) method was investigated. The resolution was estimated using the model of spatially invariant linear filter by the conventient procedure usually used in CT. The resolution of absorbing inhomogeneities of model objects is shown to go for a theoretical limit of PAT method and the algorithms considered appears to reconstruct optical diffuse images without complementary limitations on spatial resolution.

Optical diffuse tomography reconstruction using photon average trajectory

A simulation for a tomography reconstruction of an internal structure of the strongly scattering cylindrical body was carried out. For this purpose Photon Average Trajectories method was applied with several conventional CT algorithms such as the Max-Entropy algorithm MART and the least square algorithms NNLS and LSQR. The comparison of the reconstruction results obtained using these algorithms with ones obtained using MMTOAST10 program package based on diffuse equation Finite Element Method solver was carried out. It was shown that a satisfactory quality of the reconstruction was attainable already after few seconds of calculations at PC Pentium III for trajectory algorithms. The same quality of reconstruction with MMTOAST10 was achieved after about 1000 seconds.