Andrey G. Sedukhin

Beam-preshaping axicon focusing

An approach to efficient axially symmetric focusing of a light beam to an extended line segment is studied wherein the on-axis intensity and the scale of transverse intensity distribution can be controlled precisely and simultaneously at any point of the segment. Schematic realization is based on employing two phase-only optical components in tandem, the first of which, with a disk form, performs prior central light beam shaping and marginal correction to smooth the edge slopes, while the second one, with an annular form, is used for the appropriate axicon-type focusing of a light beam to the segment. An instance is given of converting a collimated Gaussian laser beam into an oscillation-free nondiffracting zero-order Bessel beam reproduced on a finite interval.

Marginal phase correction of truncated Bessel beams

Approximate analytic expressions are obtained for evaluating the axial intensity and the central-lobe diameter of J0 Bessel beams transmitted through a finite-aperture phase filter. A reasonable quality factor governing the axial-intensity behavior of a phase-undistorted truncated Bessel beam is found to be the inverse square root of the Fresnel number defined, for a given aperture, from the axial point of geometrical shadow. Additional drastic reduction of axial-intensity oscillations is accomplished by using marginal phase correction of the beam instead of the well-known amplitude apodization. A procedure for analytically calculating an optimal monotonic slowly varying correction phase function is described.

Discontinuity-free edge-diffraction model for characterization of focused wave fields

A model of discontinuity-free edge diffraction is proposed that is valid in the framework of the scalar Debye approximation and describes the formation process and approximate structure of the stationary diffracted field of a monochromatic converging spherical wave of limited angular opening throughout the whole space about the focus. The field is represented semianalytically in terms of the sum of a direct quasi-spherical wave and two edge quasi-conical waves of the zeroth and first order. The angular spectrum amplitudes of all these waves have smooth continuous variations of the real and imaginary parts in polar angle and radius, the separable nonanalytic functions defining the polar-angle variations of the amplitudes being found by optimization techniques.

Diffraction technique for testing the resolution and sensitivity of Hartmann and Shack-Hartmann sensors.

A diffraction technique for testing the resolution and sensitivity of Hartmann and Shack-Hartmann sensors is proposed, substantiated theoretically, and studied experimentally. It is based on the generation and measuring of the wavefronts of monochromatic plane waves with a weak, one-dimensional, damped chirp modulation and without this modulation. The modulated and nonmodulated wavefronts are produced by the insertion of a half-plane straight-edge screen into the field of a plane wave and by withdrawal of the screen out of the field. Also, the respective intensity distributions of these waves are measured. Testing the resolution and sensitivity of the sensors is performed by analyzing the difference patterns of the phase and intensity distributions recorded with the induced wavefield modulation and without it.

Refinement of a discontinuity-free edge-diffraction model describing focused wave fields

Two equivalent forms of a refined discontinuity-free edge-diffraction model describing the structure of a stationary focused wave field are presented that are valid in the framework of the scalar Debye integral representation for a diffracted rotationally symmetric converging spherical wave of a limited yet not-too-low angular opening. The first form describes the field as the sum of a direct quasi-spherical wave and a plurality of edge quasi-conical waves of different orders, the optimum discontinuity-free angular spectrum functions of all the waves being dependent on the polar angle only. According to the second form, the focused field is fully characterized by only three components--the same quasi-spherical wave and two edge quasi-conical waves of the zero and first order, of which the optimum discontinuity-free angular spectrum functions are dependent on both the polar angle and the polar radius counted from the geometrical focus.

Direct laser writing of gray-scale microimages with a large dynamic range in chromium films

A method for direct laser thermochemical writing of gray-scale microimages in thin chromium films is developed and studied. The method includes exposing a chromium film with a focused laser beam with variable power intensity and developing it in a selective etchant. The range of variation of the transmission by more than 100 times is obtained. The nonlinearity of the dependence of the chromium film transmission on the power intensity of the exposing beam is eliminated by software correction. The samples of the raster (with a size of 64×64 and a step of 176 μm) of apertures (36 μm) with the gray-scale Gaussian transmission function are manufactured experimentally. This raster is used in the modified Shack — Hartmann sensor.

Application of diffractive optical elements in laser metrology

Laser metrology often requires the precision forming of a probe laser beam with necessary wavefront shape or given intensity distribution. This problem is solved optimally by application of diffractive optical elements (DOE). In the IA&E SB RAS a circular laser writing system (CLWS) was created for high precision DOE fabrication. This system allows to fabricate arbitrary DOEs with minimum feature sizes below 0.6 ?m and the radial position accuracy of about 0.1 ?m over 300 mm substrates. The investigations carried out have confirmed promising prospects of application of high precision DOEs fabricated with the help of CLWS for solving various metrological problems.

Application of the Shack–Hartmann Wavefront Sensor for Monitoring the Parameters of a Supersonic Gas Jet

Results of an experimental study of the density distribution in a small-size (1–2 mm in diameter) supersonic gas jet in vacuum are reported. The measurements are performed by the developed Shack–Hartmann wavefront sensor, which consists of a microlens array with 100 × 100 elements and a video camera with a resolution of 2048 × 2048 pixels. The reliability of measurements in terms of the space and time resolution, as well as in terms of the minimum levels of phase changes induced by the tested object, is analyzed.

Comparison of the energy characteristics of tightly focused needle beams with longitudinal polarization

A comparative analysis is made of spatial variations in the energy characteristics of tightly focused, longitudinally polarized needle laser beams generated by an optical system with selective thin-film linear-to-radial polarization conversion, followed by their spatial filtering and tightannular focusing to subwavelength sizes. For the important special case of ideal radial polarization of a focused beam, longitudinal cross-sections of spatial distributions of the electric energy density and the Poynting vector modulus in the vicinity of the focus were compared by numerical simulation. It is shown that the degree of their difference increases substantially with decreasing angular zone of annular focusing and with the introduction of spatial-frequency filtering. It is established that the dimensions of the axial zone of beam focusing determined for their central lobes in the first approximation do not depend on the choice of energy characteristics used for their measurements.

Effect of multiple equidistant imaging: analyzing the techniques of its monochromatic reproduction and the forms of manifestation

Monochromatic imaging systems with spatial-frequency filters in the form of Fabry-Perot interferometers, concentric ring masks, and diffractive multifocal lenses are shown to realize the same effect of multiple equidistant imaging. However, the forms of manifestation of this effect are not identical due to the difference in spectral content of generated wave fields. Self-imaging fields with a discrete angular spectrum inherent in the systems with masks and interferometers are found to comprise a subclass of periodically focused fields with a continuous angular spectrum peculiar to the systems with diffractive multifocal lenses. The advantages of the latter systems are the extremely high total light efficiency and the sharply defined longitudinal localization of generated wave fields, which enhance the brightness of the reproduced images and decreases their parasitic diffraction dispersion, background noise, and blurring.