The high efficiency of the perturbation theory for the calculation of electromagnetic field in the non-paraxial laser
radiation beam is revealed. The perturbation method is compared with the exact solution of Maxwell's equations for a
non-paraxial Gaussian wave. Calculations on the microscopic particles manipulation in the laser radiation field are
performed under the actual experiments conditions.
A transformation of a higher Gaussian beam in general astigmatic optical systems is described in terms of
rotations in 3D space. This way is simpler than direct Fourier integral calculation and preferable for numerical
simulations. Two examples of optical systems and corresponding transformations, connecting with mode
converter and fractional Fourier transform, are discussed.
The intensity of spiral beams remains unchanged under propagation and focusing neglecting scaling and rotation. The spiral beam with predetermined intensity in the shape of any planar curve can be generated by use of amplitude and phase elements concurrently. We introduce the new method of singular laser fields formation, close to spiral type, by means of pure phase modulation. Our algorithm is based on the well-known Gerchberg-Saxton phase retrieval algorithm and spiral beams optics. It demonstrates fast convergence and some other advantages: phase distributions obtained are stable to spatial resolution changing (it is enough 128 x 128 pixels for some patterns), theoretical energy efficiency is about 85 % with acceptable intensity homogeneity. We demonstrate theoretical results on fields formation in the shape of closed-curves (triangular, square, "snowflake") and open-ended curve (Archimedes spiral) by means of elements on dichromate gelatin. Besides, the example of experiment on micromanipulation with the use of the square-shaped field is presented.
Spiral beams while propagating and focusing, keep their intensity structure unchanged neglecting scale and rotation. One of the experimental ways to obtain spiral beams is the astigmatic transformation method. It allows producing spiral beams by means of structurally one-dimensional amplitude-phase elements and simple cylindrical optics. The paper is dedicated to modeling of spiral light fields formed with one-dimensional spatial light modulators. The effect of the ultimate resolution of the modulator and ultimate accuracy of the intensity and phase choice on the quality of the synthesized field in a far-zone of diffraction has been estimated in this work. The results can be of interest for various laser applications including the laser manipulation with micro-objects.
The range of possibilities ofthe laser manipulation with microscopic objects could be sufficiently expanded by using of the beams with predetermined spatial intensity and orbital momentum density distributions in the focusing plane. Such beams permit to realize rotation and fixed trace movement of absorbing particles. The spiral beams having intensity in the shape of triangular boundary, the line with self-intersection and Archimedes spiral were formed by composition of amplitude and phase masks produced on the base of bichromated gelatin. The spiral beams keep their intensity structure unchanged under propagation except scale and rotation. The Ar-laser and microscope MIN-8 with immersion micro objective (60x, NA=O.85) were used in experimental set-up. Particles of the cetylpiridiniumbromide and colored latex spheres were chosen as an objects for manipulation. Experimental results are presented on microobjects movement effectuated with spiral beams along different fixed trajectories. The motion direction is determined by the direction of the beams orbital momentums.
Development of methods for generating of laser beams with predetermined values of intensity and angular momentum distributions is a challenge of great interest for various laser technologies including laser manipulation by microscopic objects. The suggested method oftransformation of laser radiation to complex structure modes in build-up beam rotator interferometer has been theoretically evolved and experimentally tested. Its main advantage over the others is that the method doesn't require complex diffractive optical elements to be used. Experiments were performed using tunable (adjustable) diode laser and interferometer formed by three mirrors. The beam rotation has been achieved by Dove prism inserted into the interferometer. The evolution of the transformed beam was observed with alternation of the prism rotation angle and the injection current of the laser diode.
Possibilities of the laser manipulation with microscopic objects could be sufficiently expanded by using of the beams with predetermined spatial intensity and orbital momentum density distributions in the focusing plane. The experiments on the visualization of the wave front of the beams with nonzero orbital momentum by using controllable liquid crystal plate are described. The experimental results presented on trapping, rotation and motion along a specified trajecotry of absorbing particles by means of the beams.
The major directions of manipulations with micro objects by laser beams are considered. Using spiral beams can essentially extend the potential of manipulation. In ray optics approximation the calculation of the torque transmitted from a spiral beam to an absorbing particle is made in accordance with the wave optics. The experimental observation of torque transfer from non zero orbital moment beams to particles trapped into the laser beam focus is described.
A beam propagation through astigmatic square-law waveguides (n(x,y) equals Kx2X2 + Ky2Y2) is investigated by theoretical and experimental means. It is shown that if certain condition between astigmatic value, length of waveguide, wave-front curvature and Gaussian parameters takes place then a Hermite-Gaussian beams transforms into Laguerre-Gaussian beams without residual astigmatism.
Structurally stable laser beams with phase singularities that are rotating under propagation (so called spiral beams) have been investigated in various aspects. Some integral invariants of general laser beams and an optical analog of the Steiner theorem in mechanics are presented. Similarity and distinction of spiral beams for different rotation behavior are shown. A usage of spiral beams shaped like a predetermined planar curve applying for construction of phase focusing element is discussed.
The real time control of the rotation rate of a microscopic particle trapped in a focused laser beam by mean of polarization ellipticity of laser radiation has been studied. The ellipticity change was carried out with the modulator using orientational S-effect in nematic liquid crystals. The dependence of the particle rotation speed on the state of polarization ellipticity was determined. The calculation of an angular momentum of rotation per a unit of energy of the light field is shown.
In this review some results on spiral beam optics are considered. Spiral beams keep their intensity structure unchanged under propagation except its scale and rotation. Some theoretically calculated spiral beams and the ways of their experimental constructing are presented. A comparison between an example of nonrotating but structurally stable beam and a corresponding spiral beam is performed.
In reference 1 it was shown that beams containing phase singularities have enough various intensity distributions, in particular, the distribution looked like an arbitrary planar curve. In this work we present a method of synthesis of these beams by means of one-dimensional phase elements. The basis of the method is the result stated in reference 1 that Fourier transform with an additional astigmatic phase converts such beams into light fields with one-dimensional structure. Thus, the synthesis of a singular beam can be reduced to formation of one-dimensional light fields with subsequent astigmatic Fourier transform of them. One-dimensional light field synthesis is carried out by means of two one-dimensional phase elements located at some distances along the beam propagation. One-dimensional phase masks were realized experimentally on dichromated gelatin layers which were made by sensibilization of standard holographic photoplates. The masks were recorded through an exposure of layers by an argon ion laser operating at the wavelength of 0.488 mkm. The laser beam was transformed into a narrow line of 10 mkm width. The recording was made by moving a layer in its own plane step-by-step with the help of an electric motor controlled by a computer. (The step size was 5 mkm.) After the exposure these layers were developed by water vapors according to a technique described in reference 2. Experimental results of synthesis of a beam whose intensity looks like a boundary of a regular triangle are presented.
It is known that one-dimensional phase problem in optics can be reduced to a search of zero positions of the analytic continuation of a light field complex amplitude. Usually this procedure is executed by means of numerical methods based on the measurement of the field intensity on several planes. In this work it is shown that the analytic continuation can be realized by optical way. Namely, two-dimensional Fourier transform with an additional astigmatic phase converts a one- dimensional object field into a singular wavefield. The field zeros are the same as that of the analytic continuation of one-dimensional Fourier transform of the initial field. Thus, it is possible to restore the object field through one measurement. Results of computer simulations are presented.
It is known that Laguerre-Gauss beams with indices n equals 0 and nonzero m have a single phase singularity of order m and the intensity shaped as a circumference. In this work a generalization of these beams is proposed, namely, for any closed curve on the plane there exists a family of singular beams depending on a pair of integer-valued parameters, any member of which is structurally stable under propagation and focusing. In particular, when the curve is a circumference Laguerre-Gauss modes and parameters n,m are obtained.
The sensor based on the unharmonic grating which operates as a set of phase compensators of a wave front curvature is investigated. Applications of the sensor for a real-time measurement semiconductor laser output beam astigmatism via a pumping current and for a lens testing are presented.
A new type of wave front sensor is proposed. The possibility of the measuring sensitivity control is shown. The results of the numerical simulation and experiments of the wave front aberrations measurements are presented.
It is regarded as a system intended for automatization of quality check-up operations of a runway surface of a bearing internal ring for the purpose of search for microdefects, deviation form defects, and as a sorter of bearings. The system contains a laser device for noncontact diagnostic and electronics tools.