Miroslav Miler

Recording of holographic diffraction gratings in photopolymers: Theoretical modelling and real-time monitoring of grating growth

Abstract Recording of holographic volume diffraction gratings in Du Ponts photopolymer HRS-150 is studied theoretically and experimentally. Particular attention is paid to the dynamics of the recording process. The temporal evolution of holographic gratings is monitored for a range of illumination intensities in two ways: (i) by means of a multiple-exposure approach when intensity profiles of recording beams are uniform, many holographic exposures are necessary and each exposure is carried out for a particular value of the total illumination intensity; (ii) by means of a single-exposure approach when intensity profiles of recording beams are strongly Gaussian and a volume grating with spatially distributed diffraction efficiency arises, depending on a local (average) value of the total illumination intensity. The second approach proves to be a useful tool providing us very quickly with qualitative information about the dynamics of the recording process while the first one, which is much more time-consumi...

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.

Holographic diffractive collimators based on recording with homocentric diverging beams for diode lasers

A single collimating transmission off-axis blazed surface-relief holographic diffractive element that corrects the ellipticity and the astigmatism of a diode-laser beam was designed. The procedure was applied to a hypothetical laser operating at a wavelength of 633 nm that possesses extremely large astigmatism and relatively small ellipticity together with small diverging angles of the laser beam. The element was recorded with only homocentric diverging beams that had pure spherical wave fronts. Because of the off-axis nature of the element, the aberration of asymmetry had to be corrected. The element was tested in a reciprocal way when the incident wave was a collimated He-Ne laser beam. A diffraction-limited beam was obtained that was as large as approximately half of the designed aperture.

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.

Near-field characteristics of periodic surface relief diffraction structures from far-field measurements

Abstract Near-field characteristics of periodic surface structures, i.e. their surface relief, may be determined by measuring the diffraction efficiency, i.e. from far-field measurements, if the relief is described by a known periodic function with a few unknown geometric parameters. This method is formulated for one- and two-dimensional periodic structures and demonstrated for some holographic grating structures.

Holographic optical element converting the Gaussian laser beam into a more uniform one

A surface relief holographic grating is designed for the transformation of a Gaussian laser beam into a beam with a flattened plateau in its central part. The depth of grating grooves varies over the surface in order to achieve a distribution of diffraction efficiency complementary to the intensity profile of the laser beam to be transformed. The appropriate efficiency distribution over the grating can be obtained by a technological process during the exposure and development. Experiments have proved that produced grating samples can sufficiently flatten the Gaussian beam, and that the degree of the beam flattening can be easily controlled.