Pavel A. Nosov

Accuracy of sample material parameters reconstruction using terahertz pulsed spectroscopy

New experimental and theoretical results for the material parameter reconstruction using terahertz (THz) pulsed spectroscopy (TPS) are presented. The material parameter reconstruction algorithm was realized and experimentally implemented to study the test sample. In order to both verify the algorithm and to estimate the reconstruction accuracy, test sample material parameters obtained with the TPS were compared with the results of the same sample studying by the use of the backward-wave oscillator (BWO) spectroscopy. Thus, high reconstruction accuracy was demonstrated for the spectral range, corresponding to the BWO sensitivity and located between 0.2 and 1.2 THz. The numerical simulations were applied for determining the material parameter reconstruction stability in the presence of white Gaussian noise in TPS waveforms as well as fluctuations in the femtosecond (FS) optical pulse duration. We report a strong dependence of the inverse problem solution stability on these factors. We found that the instabi...

Aberrational synthesis of optical systems intended for the conversion of laser beams

This paper presents a technique of aberrational synthesis of laser optical systems with small wave aberrations for maintaining similarity of the fields when multimode laser beams are being transformed. The minimization of the wave aberrations of laser optical systems is based on the detected connection of its design parameters with the expansion coefficients of the wave aberration. Examples are given of the automatic synthesis of laser optical systems to illustrate the effectiveness and high response rate of the proposed algorithm.

An impact of multiple wave reflections in a flat sample on material parameter reconstruction using THz pulsed spectroscopy

The present paper is dedicated to studying the accuracy of sample material parameter reconstruction using terahertz (THz) pulsed spectroscopy. The technique for characterizing the material parameters of thin flat samples allowing to take into account multiple wave reflections in a flat have been considered. While transmitting through the resonant sample, THz pulse undergoes multiple reflections, which result in satellite pulses in THz waveform. The accuracy of material parameter reconstruction strongly depends on the number of satellite pulses. We have analytically estimated an impact of satellite pulses on material parameter reconstruction by considering the local linearization of theoretical sample transfer function based on the model of quasi-Fabry-Perot-resonator.

Investigation of the spatial structure of a high-power fiber laser beam

Analysis of experimental results and theoretical studies has confirmed that the free- space propagation and the transformation of a high-power fibre laser beam by an optical system can be described by the theory of laser optics. The need to use the theory of laser optics to develop modern high-performance optical systems of laser processing heads, which form the beam of the high-power fibre laser, is discussed.

Wavelet-domain de-noising technique for THz pulsed spectroscopy

De-noising of terahertz (THz) pulsed spectroscopy (TPS) data is an essential problem, since a noise in the TPS system data prevents correct reconstruction of the sample spectral dielectric properties and to perform the sample internal structure studying. There are certain regions in TPS signal Fourier spectrum, where Fourier-domain signal-to-noise ratio is relatively small. Effective de-noising might potentially expand the range of spectrometer spectral sensitivity and reduce the time of waveform registration, which is an essential problem for biomedical applications of TPS. In this work, it is shown how the recent progress in signal processing in wavelet-domain could be used for TPS waveforms de-noising. It demonstrates the ability to perform effective de-noising of TPS data using the algorithm of the Fast Wavelet Transform (FWT). The results of the optimal wavelet basis selection and wavelet-domain thresholding technique selection are reported. Developed technique is implemented for reconstruction of in vivo healthy and deseased skin samplesspectral characteristics at THz frequency range.

A Comparison of Terahertz Pulsed Spectroscopy and Backward-Wave Oscillator Spectroscopy

This paper presents new experimental and theoretical results for the material parameter reconstruction utilizing the terahertz (THz) pulsed spectroscopy (TPS). The material parameter reconstruction algorithm was realized and experimentally implemented to study the test sample. The algorithm takes into account multiple reflections of THz pulse within the flat sample during the transmission mode measurements. Therefore the samples with small thickness or low refractive index could be studied utilizing the proposed method. In order to estimate the reconstruction accuracy, test sample material parameters, obtained with the TPS, were compared with the results of the same sample studying by the use of the backward-wave oscillator (BWO) spectroscopy. Thus, high reconstruction accuracy was demonstrated.

Analysis of Multielement Laser Cavities of an Arbitrary Configuration

A practical method is proposed and developed for calculating characteristics of the laser beam formed by stable and unstable cavities of an arbitrary configuration.

Accounting the in uence of sphericity of real cavity mirrors on laser beam parameters

On the basis of the scalar diffraction theory, the problem of calculating the characteristics of a laser beam in linear cavities of arbitrary configuration, taking into account the aberrations of the mirrors, is solved. The configurations of cavities in which the aberrations of the mirror surfaces have a significant influence on the characteristics of the formed laser radiation are determined.

Elaboration of power optics for laser processing heads

The need to use the theory of laser optics to develop modern high-performance optical systems of laser processing heads, which form the beam of the high-power fiber laser, is discussed. A technique for synthesis of high-performance optical heads of laser systems for various purposes is elaborated and software-implemented.

In vivo terahertz pulsed spectroscopy of dysplastic and non-dysplastic skin nevi

The results of the in vivo terahertz (THz) pulsed spectroscopy (TPS) of pigmentary skin nevi are reported. Observed THz dielectric permittivity of healthy skin and dysplastic and non-dysplastic skin nevi exhibits significant contrast in THz frequency range. Dysplastic skin nevus is a precursor of melanoma, which is reportedly the most dangerous cancer of the skin. Therefore, the THz dielectric spectroscopy is potentially an effective tool for non-invasive early diagnosis of melanomas of the skin.