Jan Pala

Holographic Gaussian to flat-top beam shaping

Recording and development of Gaussian gratings are analyzed, and utilizing these gratings for beam shaping is demonstrated. It is shown that by using only holographic recordings of gratings, diffractive shapers for transformation of Gaussian beams to super-Gaussian beams of the fourth degree, and more as well, can be obtained. Holographic shapers are fabricated by interference of natural-size laser beams as elements of a matrix, which can be used for flat-top beam shaping of laser beams of natural size. If appropriate shaping of a beam is demanded, the necessary shaper can be found among a number of realizations in the matrix, or in a set of such matrices, without any calculations or preliminary measurements.

Application of holographic diffractive doublets as collimators for highly elliptical light beams edge emitted from diode lasers

Two schemes for collimation of diode laser light beams with high cross-sectional ellipticity by means of a doublet of holographic diffractive elements are proposed and designed; one of the schemes is realized and tested. In both the schemes the first element of the doublet collimates the beam in the plane of the longer axis of the ellipse, and the second element collimates it in the perpendicular plane. Each element simulates a cylindrical lens. The set-up with the focal line of the cylindrical beam oriented perpendicular to the meridional plane is realized experimentally. The elements are holographic surface-relief gratings recorded in photoresist. For recording, only homocentric diverging beams are used, which minimizes potential aberrations and optical dirt. The parameters of the elements are computed using four equations, including one equation for compensation of the aberration of the lowest order. The doublet is proposed for the He—Ne red wavelength. A collimated He—Ne laser beam is employed for quality testing of collimation in a reverse way, with this beam impinging upon the second element. Characteristics of an outgoing beam from the first element of the doublet are recorded with a charge-coupled device camera. Calculated spot diagrams are compared with cylindrical focal lines captured separately from both the elements.

Off-axis Fresnel diffraction approximation

Diffraction is analyzed for oblique propagation of light beam under a large angle. Fresnel diffraction approximation is valid provided the beam is deflected into the direction of oblique propagation, the structure of the diffraction screen is projected onto the plane perpendicular to the propagation direction, and the diffraction pattern is observed in the plane perpendicular to the propagation. The task is illustrated by the diffraction due to a circular aperture.

Compensation of beam ellipticity and astigmatism based on holographic diffractive elements recorded onto spherical substrates

Abstract Edge emitting diode lasers with their divergent, highly elliptical and astigmatic beams in the visible spectrum are widely used in all branches of photonics. Usually the beams must be transformed into circular anastigmatic beams for the majority of applications. Holographic diffractive elements on spherical substrates are devised for transformation of beams to circular collimated beams. An off-axis holographic set-up is used to record diffractive elements into a thin photoresist layer as shallow surface-relief gratings working in reflection mode with curved and chirped grooves. The elements are destined for the diode lasers emitting at a suitable wavelength and with appropriate ellipticity and astigmatism. The performance of the elements is tested on the basis of intensity patterns and the elements produced at a focal plane on their illumination with a collimated expanded beam of a HeNe laser.

Diffraction of cylindrical waves due to elliptical and rectangular apertures

Operation of some optical devices such as a surface plasmon resonance compact sensor relies on cylindrical waves. On the contrary, optical power is usually transmitted via collimated beams. A holographic grating structure may serve in coupling of a collimated laser beam and its transformation into the beam with the cylindrical wave front, and the vice versa transformation on outcoupling. The paper deals with the role of an aperture shape in focusing of the cylindrical waves. Elliptical and rectangular apertures are discussed, and the transversal as well as longitudinal diffraction in the zone of the focal line is studied. A special feature of this problem is that in the focal plane the Fraunhofer diffraction takes place only in the direction perpendicular to the focal line. The Fresnel difracton causes a modulation along the focal line. A consequence of the elliptical aperture is that the focal line is not illuminated uniformly: illumination declines from the center to the ends. For practical purposes the rectangular aperture turn out to be much more suitable.

Beam ellipticity compensation by holographic optics

Ellipticity of optical beams can be compensated by means of diffraction gratings. Holographic gratings are a simple alternative, which is noted for the possibility of recording gratings with curved and non-equidistant grooves. Using spherical wave fronts off-axis diffractive imaging elements can be recorded. They can be used for a beam transformation. The usual transformation task is to collimate beams emitted by edge-emitting diode lasers. Holographic recording and reconstruction, taking into account aberrations of the third order and assuming a recording layer to be on a spherical substrate, are described. Experiments verifying theoretical presumptions were carried out. Fraunhofer diffraction pattern with a very narrow central maximum was obtained.

Axial diffraction patterns in the vicinity of the focus of the off-axis holographic diffractive collimator

Off-axis holographic optical elements can be used for collimation of optical beams from edge emitting diode lasers. Such a single element collimator can simultaneously compensate large elliptic cross-section and considerable astigmatism of that beams and also reduce two selected lower aberrations. Beside a current transverse diffraction pattern in the focus also patterns in the vicinity of the focus should take attention. Diffraction patterns due to an elliptic aperture of such a collimator are very complicated and for their theoretical description sophisticated methods must be used. This contribution involves some preliminary results of the study. Theoretical transverse diffraction patterns in the vicinity of the focus have been mainly presented and their description analyzed.

Phase elements by means of a photolithographic system employing a spatial light modulator

The system employs a spatial light modulator (SLM), between a pair of crossed polarizers, and an electronic shutter. Transmission of the SLM with the polarizers is controlled by graphical software that defines which pixels are fully transparent and which are fully opaque. While a particular binary graphics is on the SLM the electronic shutter allows light to pass for a certain time. The graphics is imaged, by an objective, onto a photoresist plate. A mercury lamp is used as a light source. The graphics changes after each exposition and the whole sequence of images determines the resultant surface-relief modulation.

Modifications of holographic diffractive doublets for collimation of highly elliptical beams from edge-emitting diode lasers

Holographic diffractive doublets are optional systems for collimation of highly elliptical light beams, which are emitted from edge of diode-lasers. The first element converts an impinging astigmatic divergent beam into a cylindrical divergent beam, and the second element transforms the cylindrical beam into a collimated beam. In contrast to previous concept of the doublet with perpendicular input and output, another non-parallel scheme is considered, with the cylindrical wave front from the first element perpendicularly impinging upon the second element. A strongly off-axis holographic set-up is used for recording of diffractive elements in photoresists layers deposited on glass substrates.

Line curvature in plane-grating spectra

The curvature of spectral lines in a plane-grating spectrum is much smaller than that in a prismatic spectrum. The effect of curvature need not always be entirely negligible. Our contribution deals with the more exact solution of the curvature based on a propagation vector diagram of incident and diffracted rays.