New! Presentation of the results (in Romanian)

Results 2024

The main results consist of the characterization and testing of the following optical components: spiral phase plates with large diameter for high power lasers, spiral axicons with large diameter for generating long propagation Bessel beam for free space optical communications and free form optical components for sorting OAM beams based on log – pol transformation.

i) Optical vortices with orbital angular momentum (OAM) generated by an incident Gaussian beam with a large diameter diffracted by the entire surface of a spiral phase plate (SPP) optical element. Two types of experiments were performed. The first experiment was carried out using a Gaussian beam with a wavelength of λ=635 nm diffracted by an SPP optical element designed so that the optical path difference between the lowest level and the highest level corresponds to a phase difference of 2π. In this case, the configuration of the optical field is that of an optical vortex with OAM 1. The second type of experiment considered the situation when the Gaussian beam presents the wavelength λ=635 nm and is diffracted by an SPP optical element designed for the wavelength λ= 810 nm so that the optical path difference between the lowest level and the highest level corresponds to a phase difference of 9π/4. In this case, the obtained configuration is that of a fractional vortex.

(ii) Bessel beams with long propagation distances generated by the diffraction of a Gaussian beam with a diameter of 50 mm with a spiral axicon optical element of the transmission phase. The diffracted beam was characterized both in the immediate proximity of the optical element and at a distance of 10 meters, where the beginning of the formation of a Bessel beam showing OAM 1 was observed. In addition, the following experiment was performed: A zero-order Bessel beam was generated by diffraction an incident Gaussian field by an axicon configuration, the resulting beam being in turn diffracted by the manufactured optical element. Finally, Bessel beams with OAM=1 were generated, the difference being that they are formed at a smaller distance from the diffraction plane.

(iii) Characterization of the optical vortex beams diffracted by an aspherical optical element with a phase distribution described by a logarithmic-polar transformation combined with a spherical lens type. In this case, the structure was optimized. Considering that the optical element manufactured and characterized in stage 2023 [2] has a large focal distance, its use in classical and quantum optical vortex sorting applications represents a challenge, the reduction of the focal distance implies for optical elements with 32 discrete levels the realization of details with dimensions of the order of micrometers, a fact that considerably complicates the technological process. In order, to circumvent these difficulties, the Fourier formalism of the phase function of the optical element was employed. More specifically, the Fourier terms corresponding to an optical element with four discrete levels were considered. Thus, with the help of a spatial light modulator, comparisons were made between the shape of the optical fields generated by the diffraction of optical vortices with OAM=1, respectively, OAM=8 by logarithmic-polar transformed optical elements with continuous phase distribution respectively discretized phase distribution in four levels. The configuration of the intensities of the diffracted fields represented both in the linear  and logarithmic scale indicates a remarkable similarity between them, a fact that suggests the possibility of using optical elements in four levels for optical vortex sorting applications.
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Results 2023

The main results consists of the physical realization of the following optical components: spiral phase plates with large diameter for high power lasers, axicons and spiral axicons with large diameter for generating long propagation Bessel beam for free space optical communications and free form optical components for sorting OAM beams based on log – pol transformation.

Technological experiments were carried out for establishing the process parameters necessary for fabrication of the structures that respect both the functional characteristics and the optical quality requirements. Thus, it was investigated (1) the wet etching solutions in order to configure the optical components with 2n discrete levels in a controlled manner that provide a low roughness of the processed surface respectively a high optical quality, (2) the technological characteristics of the type of photoresist used in the process photolithography, (3) The alignment of the photolithographic masks in order to obtain structures with the required configuration, (4) optimization of the entire technological process. The fabricated components were morphologically analyzed by AFM, SEM and optical microscopy.
An optical experimental set-up was realized for the functional characterization of the central parts of the optical components consisting of the diffraction of a laser beam with a variable waist of 1-10 mm by the fabricated components. The functional investigation on the central area of the components provides useful information on the quality of the manufactured optical components. The experimental results are in good agreement with the numerical ones.

 

Results 2022

Numerical analysis and design of large diameter phase spiral optical elements for high power lasers.
A formalism was developed that calculates both theoretically and numerically the shape of the wavefront diffracted by a multilevel optical element. This formalism is based on the diffraction integral and the Fourier analysis of the phase function of the optical element. The investigated optical component has a phase function exp(𝑖𝛷) where
Φ=mφ. Where φ is the azimuthal coordinate and m is an integer called the topological charge. The resulting wavefront is an optical vortex, an optical configuration that possesses orbital angular momentum. The developed formalism follows the steps of the technological flow of photolithography and etching, and can also take into account the influence of possible technological deviations on the shape of the diffracted field. The results of this activity are the technical specifications for large-diameter phase spiral optical elements for high-power lasers, specifically, the configuration of photolithographic masks.

Numerical analysis and design of axicon and spiral axicon optical elements with large diameters for generating Bessel beams with long propagation distance for applications in free space optical communications.

Higher-order Bessel-type beams generated by a spiral axicon type optical element were numerically calculated by the diffraction integral method. The optical elements present a phase function exp(𝑖𝛷) of the form Φ=mφ-kαr where m is the order of the higher-order Bessel function or the topological charge of the resulting vortex, φ the azimuthal coordinate, k the wave vector, and α is the angle that characterizes the lens conic or axicon. The results of this activity are the technical specifications for axicon and spiral axicon optical elements for generating Bessel beams with a long propagation distance for applications in optical communications through free space, specifically the configuration of the optical elements and the formulas that generate the configuration of the photolithographic masks.

Numerical analysis and design of free-form optical elements for sorting beams with a helical wavefront as a function of the orbital kinetic moment.

A class of optical components whose configuration is given by the log-pol transformation was investigated. This class of components converts an optical vortex of topological charge m into a linear segment whose transverse wave vector depends on the topological charge of the vortex. The formalism of the diffraction integral and the stationary phase approximation was used in order to determine the shape of the wavefront of an optical vortex diffracted by the optical component that presents the log-pol configuration. The results of this activity are the determination of the geometric parameters that determine the configuration of the log-pole optical component.

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Estimated results

1) Optical reflecting spiral phase plates with high quality surface and large diameters generating helical wavefronts with various OAM TRL4

2) Spiral axicons with large diameters operating in reflection or transmission generating Bessel beams with large propagation length TRL4

3) Optical components for sorting beams with helical wavefronts as a function of their OAM TRL4