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In this contribution, we discuss the features of design and polarimetric inspection of terahertz achromatic waveplates. The design of the crystalline quartz half-wave plate was performed taking into account the ellipticity and the introduced phase difference between the orthogonal components of the output radiation polarization vector. The designed waveplate are relatively thin, work equally efficiently in the frequency range from 0.4 to 1.4 THz, and, most importantly, are cheap to manufacture. The modification of the experimental terahertz time-domain spectroscopy polarimetric setup involving direct waveform detection is proposed. The proposed polarimetry THz time-domain spectrometer with electro-optic detection shows parasitical signals absence and easier measurement procedure.

In this contribution, we investigate the features of the two-beam interference of a set of wave trains, in which one of the beams contains an orbital angular momentum caused by the presence of inline topologically charged vortices in each spectral component. The infrared pump beam that generates terahertz radiation is first doubled in Mach-Zehnder interferometer with a specific time delay value to form the wave train with quasidiscrete temporal spectrum. Then, the two delayed terahertz pulses are split into two arms. One of the arms contains a delay line and geometric phase shaping elements for broadband uniformly topologically charged beam formation. The resulting structures are combined with a beamsplitter and detected with a terahertz holographic system upon their propagation. We analyze the features of the resulting spatio-frequency structures and discuss the possibility to implement the information encoding without spectral decomposition.

Terahertz pulse time-domain holography (THz PTDH) is an ultimate technique both for the measurement of object properties in the THz range and broadband wavefront sensing. In this proceeding, we reveal the key principles of the technique, including the layout solutions for recording a collimated THz wavefront in the form of spatio-temporal profiles. The possibilities to investigate ultrashort THz field propagation dynamics based on the data measured in one transverse plane is discussed. The evolution for both transverse and longitudinal components of the electromagnetic field thus can be estimated. We illustrate these possibilities on the example of Bessel-Gaussian pulsed THz beam propagation formed by an on axicon lens.

Complex wavefront manipulation is a promising technique for many applications of optics and photonics. In this report we will present our results on development of DMD wavefront correction experimental setup and discussion of its performance for various parameters of binary fringe pattern and 1st diffraction order filtration aperture. It was shown that trade-off between spatial resolution and discretisation of the desired amplitude and phase distribution should be achieved. Decrease of the binarized interference fringes width results in higher spatial resolution of the modulated complex wave but increase discretisation of amplitude and phase distributions as well. The correction of wavefront aberrations using digital micromirror device was performed. We observed significant reduction in wavefront phase error by conduction of Zernike polynomials decomposition.

We investigate an approach to short and medium-range wireless communications based on the use of terahertz beams possessing an orbital angular momentum (OAM) that allows for noise-resistant broadband carrier. A the- oretical model of the proposed beams generation is developed and numerical predictions are given for propagation and visualization of complex-structured THz beams, including ones carrying a unit topological charge on a large number of spectral components of broadband terahertz radiation. The assessment method which in our case is terahertz pulse time-domain holography allows for analyzing spatio-temporal and spatio-spectral evolution of arbitrary shaped THz wave trains during their propagation in free space and interaction with obstacles.

Terahertz pulse time-domain holography (THz PTDH) is an ultimate technique both for the measurement of object optical properties and broadband wavefront sensing. However, THz PTDH has valuable restriction connected with low signal-to-noise ratio which becomes a serious issue in coherent measurements. This noise problem could be solved by filtering with use of modern block-matching algorithms based on nonlocal similarity of small patches of images existing in investigated objects. Here we present the study on the use of denoising algorithms applied for hyperspectral THz data in the spatio-temporal and spatial-spectral domain. We provide a numerical simulation of denoising in case of broadband uniform topologically charged (BUTCH) beam of pulsed THz radiation. publishedVersion

The results of experimental studies aimed at improving the efficiency of the “telescope-deformable mirror” scheme for the purposes of long-range propagation of intense light channels in the air are presented.

Three methods of multimode fiber interferometer signal averaging are investigated theoretically and experimentally: the ensemble averaging, the averaging over a long realization, and the averaging over transverse coordinates of an output speckle pattern. The analysis is performed for two wavelengths and two launch conditions of the graded-index multimode fiber: few mode and multimode. Applicability of methods for practical applications is discussed.

In this work we demonstrate wavefront complex modulation of semiconductor light sources via digital micromirror device (DMD). Proposed holographic configuration allows to correct the aberrations caused by the imperfections of the DMD and the optical elements. We consider the blazing effect of the DMD surface and configured the optimal condition of the optical setup for certain DMD and the wavelength of the radiation source. The technique was approved with the experiment of obtaining different kinds of wavefront distributions from the semiconductor laser with high M2 factor.

One of the problems of interferometric methods is the difficulty of measuring surface shape with sharp boundaries due to the wavelength-limited dynamic range of the measurement. To circumvent this limitation multiwavelength methods or techniques based on hologram capturing at the different tilt of the illumination beam are applied. In this work we examine the performance of the digital holographic interferometry with multi-inclination illumi- nation in the numerical and real experiments. Lensless implementation of the technique implies the wavefront propagation by numerical algorithms. In this regard the speckle scattering in the Fresnel diffraction area caused by surface roughness and the impact of distance from the object to the registration plane are analyzed. Since shape measurement is based on the calculation of phase difference for the wavefronts recorded with tilt of the object illuminating beam, the requirements to preciseness of measurements of the angle of incidence of this beam are considered. The algorithm of the inclination angle determination are developed. The performance of noise suppression techniques, namely sine-cosine and BM3D methods are considered for high noisy conditions, when the phase distributions are formed by reflecting object with a great roughness and height differences.

In this proceeding, we discuss the method that allows for field of view and reconstruction quality enhancement of pulsed THz holograms recorded by matrix detectors that do not exceed the the object transverse dimensions, at distances, that are comparable with the object size. The method comprises the use of random phase mask situated between the object and the hologram, at the hologram registration process. The introduced phase variation levels out the input from closer and further (to the hologram pixel) points of the object, and thus improves overall reconstruction quality. Here, we study numerically this approach and demonstrate 4 times increase of the properly reconstructed object area, if compared to the undisturbed hologram recording, and consecutive increase of the correlation between the reconstructed and actual object from 0.34 to 0.82.

In this proceeding, we present the description of the numerical approach for resolution enhancement, field of view widening and noise reduction in pulse time-domain holography. The approach comprises iterative procedure of the recorded hologram self-extrapolation into wider spatial area, and consecutive ‘self-healing’ of an object. The concept has been proofed on a synthetically generated pulse time-domain holograms. The proposed method is sought after, especially in THz range, where the distance between the object and the hologram lies in the order of several tens of wavelengths, and the detector sizes are usually limited, and with minor modifications can be applied for other THz holographic approaches.

The transverse structure formation processes of a laser beam after multiple filamentation were investigated. It is shown that the spectra of post-filamentation light channels (PFC), rings and beam differ substantially. The spectrum of PFC has a significant and symmetrical spectral broadening and covers a range of 600-1100 nm. The broadening of the spectrum of the rings is asymmetric and is directed mainly to the short-wavelength region of the spectrum. The broadening data are stable and do not change when moving away from the multiple filamentation zone. The annular structure of radiation in the cross section of the beam is formed around individual filaments within the region of multiple filamentation, and at a distance of tens of meters from it begins to form a common ring structure surrounding postfilamentation channels.

The stabilization system implemented has allowed one to achieve phase residual instability ~0.17 radian (rms) for the 30 fs-pulse, which is sufficient for nonlinear interaction radiation with optical medium in forthcoming lightwave electronics experiments.

We propose a new algorithm for absolute phase retrieval from multiwavelength noisy phase coded diffraction patterns in the task of surface contouring. A lensless optical setup is considered with a set of successive single wavelength experiments. The phase masks are applied for modulation of the multiwavelength object wavefronts. The algorithm uses the forward and backward propagation for coherent light beams and sparsely encoding wavefronts which leads to the complex-domain block-matching 3D filtering. The key-element of the algorithm is an original aggregation of the multiwavelength object wavefronts for high-dynamic-range profile measurement. Numerical experiments demonstrate that the developed approach leads to the effective solutions explicitly using the sparsity for noise suppression and high-accuracy object profile reconstruction. acceptedVersion

The method of amplification of hologram was applied to the so-called Rozhdestvenskiy hooks, that were obtained in the Rozhdestvenskiy interferometer (Michelson interferometer, combined with a grating spectrograph). In such a device the absorption lines reveal themselves as specific “hooks”, whose curvature provides the information about the atomic oscillator force. The holographic amplification “smoothes” the hooks and thus makes their analysis much simpler.

Measurements of average phase shifts introduced by living HeLa cells to probe wave front were carried out. Variations of this value were monitored in the course of morphological changes caused by photodynamic treatment at various irradiation doses. Observations of changes in living cells were also performed by means of far field optical microscopy and confocal fluorescent microscopy. Quantitative analysis of the data obtained shows that average phase shift introduced by the cells may either increase or decrease depending upon major parameters of the treatment.

In-line lensless holography is considered with a random phase modulation at the object plane. The forward wavefront propagation is modelled using the Fourier transform with the angular spectrum transfer function. The multiple intensities (holograms) recorded by the sensor are random due to the random phase modulation and noisy with Poissonian noise distribution. It is shown by computational experiments that high-accuracy reconstructions can be achieved with resolution going up to the two thirds of the wavelength. With respect to the sensor pixel size it is a super-resolution with a factor of 32. The algorithm designed for optimal superresolution phase/amplitude reconstruction from Poissonian data is based on the general methodology developed for phase retrieval with a pixel-wise resolution in V. Katkovnik, ”Phase retrieval from noisy data based on sparse approximation of object phase and amplitude”, http://www.cs.tut.fi/~lasip/DDT/index3.html.

In this paper we propose an investigation of digital optical phase conjugation (DOPC) method applicability and efficiency for the problem of three-dimensional (3D) holographic imaging. We validate the basic properties of developed DOPC-based method for different cases of imaging objects, from simple two-dimensional (2D) case to 3D figure composed of 2D polygons. We implement the method of adaptive optimization of the wavefront (AOWF) for auxiliary image formation. Since the simplicity and universality of AOWF method, this approach is useful for the fast basic 3D holographic image formation.

The results of experiments to study the spatial characteristics of the field of multiple filamentation (FMF) gigawatt pulse Ti:Sapphire laser in the glass. The dependences of the coordinates of the beginning and end of the FMF, the diameter of the laser pulse energy were received. It was found that when a certain average intensity values in the beam FMF has formed around the illuminated faces of the glass sample. It is shown that when the threshold intensity of the beam is formed by a secondary area of multiple filamentation, the spatial characteristics of which depend on the time of exposure to repetitively pulsed laser radiation. The dependence of the diameter of the secondary FMF and coordinates its start from the effects of time. The time dependence of the impact of the spatial characteristics of the primary field of multiple filamentation experiments absent.

The results of experimental studies of postfilamentation channels controlled by the track length of 150 meters for collimated beams of different diameters. It is shown that the divergence of the laser beam after the filamentation area is much greater than the divergence postfilamentation channels. It is shown that at distances from the end of the field filamentation, significantly exceeding the length of the field filamentation, postfilamentation channels contain sufficient intensity to generate multiple filamentation in the optical elements and enables us to provide a functional effect on the optical elements of the matrix.

The improved scheme of data recording process on an optical disc based on a substrate made of high-stable materials has been proposed. Information layer with depth of 115nm and width of 600nm has been obtained on sapphire substrate by ion-beam etching using improved scheme. These experimental results allow to create the sapphire optical discs for long-term data storage.

In this paper we propose a technique of the distribution of transparent particles suspended in a volume investigation. We use a method that implies division of the volume into a plurality of layers containing particles therein. An inline hologram of this volume is reconstructed in two adjacent layers, which are compared using correlation function. We have derived dependencies of correlation on particle parameters. We perform an experimental validation of this study. This technique is useful for applications that require instant assessment of the particle distribution.

The paper presents results on singlet oxygen detection in aqueous solutions of a photosensitizer based on the reconstruction of 3D temperature gradients resulting from nonradiative deactivation of excited oxygen molecules. 3D temperature distributions were reconstructed by means of the inverse Abel transformation from a single digital hologram in the case of cylindrically symmetric distribution of the temperature gradient and using holographic tomography algorithm with filtered back projection in the case of nonsymmetrical distribution. Major features of the applied techniques are discussed and results obtained by the two methods are compared.

A numerical simulation of the generation of supercontinuum (SC) in highly-nonlinear photonic crystal fibre pumped with picoseconds-long pulses has been done using the RK4IP algorithm. Simulation and experimental results are in good agreement. The supercontinuum will be used for further amplification.

Results of experiments on controlling the position and length of the filamentation zone of femtosecond laser pulses in atmospheric path length 150 m using different initial spatial focusing and defocusing. The obtained distribution of filaments along the filamentation zone, measured dependence the length of the filamentation zone of the numerical aperture of the beam, its initial radius and pulse power.

The results of experiments to study the spatial characteristics of multiple filamentation gigawatt laser pulses in the glass are presented. It is shown that with increasing pulse power multiple filamentation region increases in length and diameter, the distribution of filaments within the region has a maximum value when the power > 105 Pcr area filamentation takes the form of a hollow cone, the apex directed to the source of the laser radiation.

Results of experiments on controlling the position and length of the filamentation zone of femtosecond laser pulses in atmospheric path length 110 m using different initial spatial focusing and defocusing. The obtained distribution of filaments along the filamentation zone, measured dependence the length of the filamentation zone of the numerical aperture of the beam, its initial radius and pulse power.