Diffractive Microrelief Research Articles

Design of Cascaded DOEs for Focusing Different Wavelengths to Different Points

We propose a task-specific method for calculating cascaded phase diffractive optical elements (DOEs) for focusing Q incident beams with different wavelengths to Q given points. Due to the utilization of a special optimization criterion, the proposed method makes it possible to calculate the elements of the cascaded DOE in a sequential way. In addition, the calculation of the diffractive microrelief of each DOE in the cascade is reduced to solving a set of independent and computationally simple “pointwise” optimization problems. Using the proposed method, cascades of two DOEs were designed to focus radiation of 11 different wavelengths to 11 different points. The presented numerical simulation results demonstrate good performance of the method. The chosen operating wavelengths correspond to a set of widely used vegetation indices enabling monitoring the vegetation status conditions and tracking the environmental stresses. This makes the obtained results promising for the application in remote sensing and smart agriculture.

Design of Diffractive Neural Networks for Solving Different Classification Problems at Different Wavelengths

We consider the problem of designing a diffractive neural network (DNN) consisting of a set of sequentially placed phase diffractive optical elements (DOEs) and intended for the optical solution of several given classification problems at different operating wavelengths, so that each classification problem is solved at the corresponding wavelength. The problem of calculating the DNN is formulated as the problem of minimizing a functional that depends on the functions of the diffractive microrelief height of the DOEs constituting the DNN and represents the error in solving the given classification problems at the operating wavelengths. We obtain explicit and compact expressions for the derivatives of this functional, and using them, we formulate a gradient method for the DNN calculation. Using this method, we design DNNs for solving the following three classification problems at three different wavelengths: the problem of classifying handwritten digits from the MNIST database, the problem of classifying fashion products from the Fashion MNIST database, and the problem of classifying ten handwritten letters from the EMNIST database. The presented simulation results of the designed DNNs demonstrate the high performance of the proposed method.

Calculation of quantum characteristics based on the classical solution of the diffraction problem in a resonator with a dielectric plate

The work is devoted to the development of the quantum theory of diffractive optical elements. Aspects of quantum optics are considered by the example of light diffraction from a dielectric plate in a resonator. The paper shows the connection between the classical and quantum solution of the problem of diffraction by a dielectric plate. Expressions are obtained for the eigenmodes of such a resonator, as well as for the operators of the vector magnetic potential and the electric field strength. The method proposed in this paper can be easily extended to dielectric plates with a diffractive microrelief, that is, to diffractive optical elements.

Silicon subwavelength axicons for terahertz beam polarization transformation

The diffractive optical element (DOE) for transforming of linearly polarized THz radiation beam into a cylindrically polarized beam is investigated. Optimal diffractive microrelief height is determined by numerical simulation.

Spectral Diffraction Lenses for Forming a Light Source Emitting Several Specified Wavelengths

Spectral diffraction lenses that have the same focusing distance for several specified wavelengths are proposed as a basis for a point light source with several specified wavelengths. A spectral diffraction lens designed to focus three wavelengths, 462, 608, and 952 nm (the wavelengths used to excite tubulin $$\beta$$ fluorescence), is considered. For the production of spectral lenses, the method of direct laser recording using a photoresist with iterative correction of the recording parameters was used, which provided an error in the formation of the height of the diffraction microrelief of less than 30 nm. An experiment is presented to determine the wavelengths focused by the resulting lens, which is based on the use of a point diaphragm located in the focus of the spectral lens.

Experiment with a diffractive lens with a fixed focus position at several given wavelengths

The paper presents results of the experimental investigation of “spectral” diffractive lenses the same focus position for several given wavelengths. Two spectral diffractive lenses designed to focus radiation of three and five specified wavelengths in the visible spectrum were investigated. Using a method of direct laser writing in photoresist with iterative correction of writing parameters, we fabricated a diffractive microrelief of the spectral lenses with the height deviation from the designed relief of less than 30 nm. Using a pinhole located at the focus of the fabricated lenses, we estimated the operation wavelengths. The point spread functions of the spectral lenses at the designed wavelengths were measured with the use of a tunable laser. The imaging properties of the spectral lenses were illustrated by the images of a reference color table.

Spectral and angular dependence of the efficiency of a two-layer and single-relief sawtooth microstructure

Spectral and angular dependences of the diffraction efficiency of two-layer single-relief sawtooth microstructures are analyzed. The microstructures are assembled from optical plastics and new brands of special optical glass. This makes it easy to replicate elements with diffraction microrelief by precision casting or stamping. Wide possibilities of weakening the dependence of the diffraction efficiency of such microstructures on the wavelength and the angle of incidence of radiation on the element are demonstrated.

The maker of harmonic lens microreliefs

In this paper I briefly relate the life and research endeavors of the late Oleg Yurievich Moiseev (January 6, 1959 - June 29, 2016), Candidate of Engineering and a senior researcher at the Image Processing Systems Institute of the RAS - who had unique skills in techniques for fabricating computer-aided diffractive optical elements and recording micro- and nano-optical reliefs. I describe the methods for diffractive microrelief synthesis he authored, outlining the practical uses of the optical elements and devices created by him.

Precision laser recording of microstructures on molybdenum films for generating a diffractive microrelief

We discuss a problem of reducing the thickness of lines of the contact pattern masks produced by laser ablation of thin films of refractory metals and used when synthesizing the micro-relief of diffractive optical elements (DOEs). For a contact mask for a DOE on molybdenum, patterns with features in the range 0.25-0.3 µm were recorder by laser ablation on 40-nm thick films. This is approximately 3 times smaller than the characteristic dimensions obtained by thermochemical recording chromium films of the same thickness in the standard process. A microrelief of height of up to 300 nm was formed in a quartz substrate by reactive ion etching in an inductively coupled plasma through the mask. We show that thin molybdenum films can have promising applications as metallic masks when synthesizing a DOE microrelief.

MONITORING A SPATIAL INTENSITY DISTRIBUTION OF TRANSMITTED AND REFLECTED LIGHT IN A DIFFRACTIVE STRUCTURE

We propose a method and a design of a device for an on-line monitoring of a spatial intensity distribution of transmitted and reflected diffraction orders found within a ±80° solid angle and a 360° azimuthal angle. The proposed device is intended to serve as a shape and depth profiler of the diffractive element microrelief in the course of manufacturing.

Phase modulation of Bloch surface waves with the use of a diffraction microrelief at the boundary of a one-dimensional photonic crystal

Phase modulation methods for surface electromagnetic waves propagating at the interface between a homogeneous medium and a one-dimensional photonic crystal have been analyzed numerically and theoretically. Modulation is performed by changing the geometrical parameters of the microrelief on the surface of the photonic crystal. The phase modulation methods under consideration can be used to create optical elements for surface waves, in particular, lenses, prisms, and diffraction gratings. A Bragg grating has been calculated as an example. According to the simulation results, the average coefficient of reflection of a surface wave in the band gap is 0.95.

Precision laser recording on a molybdenum films for diffractive microrelief formation

The problem of reducing the thickness of the lines of contact pattern masks used in the formation of micro-relief of diffractive optical elements (DOEs) and produced by laser ablation of thin films of refractory metals. For contact mask of DOEs on molybdenum films with thickness of 40 nm using a laser ablation patterns recorded with elements of the picture width 0.25–0.3 µm. This is approximately 3 times smaller than the characteristic dimensions, obtained by thermochemical recording chromium films of the same thickness in the standard process. Reactive ion etching in an inductively coupled plasma through a mask was formed micro-relief height up to 300 nm in a quartz substrate. We have shown promising applications of thin films of molybdenum as a metallic mask in the formation of microrelief of DOEs.

Characterization of profilograms of piecewise-continuous diffraction microrelief

Questions of the profilometric analysis of the piecewise-continuous microrelief of high-efficiency diffraction elements have been virtually ignored in the literature, even though the market for diffraction optics has substantially increased in recent years, and high-efficiency diffraction elements have gone from the realm of the exotic to ordinary practice of optical instrumentation. In this paper, the authors consider several techniques for evaluating microrelief that make it possible to compute the “approximated depth,” almost regardless of the type of profilometer being used. The proposed techniques are based on a polynomial approximation of the diffraction-zone profile with minimization of the influence of the unreliable data close to its boundaries.

Thin copper film for plasma etching of quartz

We discuss a method for generating a diffractive optical microrelief by plasmochemical etching with the use of a masking copper layer.

Studying fabrication errors of the diffraction grating on the end face of a silver-halide fiber

A technology for fabricating a diffractive microrelief on an end face of a polycrystalline IR fiber (PIR-fiber) is studied. We discuss how fabrication imperfections of a beam-splitter implemented on the fiber’s output end face could affect the specified intensity distribution of diffraction orders. Recommendations for improving the characteristics of the microrelief to manufacture are worked out.

Design of on-fiber diffractive microrelief for efficient graded-index fiber mode excitation

The problem of energy-efficient excitation of high-order fiber modes is under discussion. Some examples of mode excitation using diffractive optical elements (DOEs) and on-fiber microreliefs are given. For the graded-index fiber modes an approach is proposed and a numerical procedure is described that allows calculating a microrelief height function that provides a phase incursion necessary for the energy-efficient excitation of the high-order modes. The results of numerical and optical experiments are presented that prove the appropriateness of the approach.

Formation of diffractive microrelief on diamond film surface

The fabrication of diamond-based optical elements for high-power CO 2 lasers is of particular interest because of low optical absorption, high thermal conductivity and weak temperature dependence of the refractive index of diamond. In earlier publications, we presented the results of the implementation of ion-chemical etching technology for generating a diffraction grating on the surface of a diamond film. The results on generating diamond diffractive optical elements and a sub-wavelength antireflection structure by plasma etching were also presented. In this paper, we study the generation of the grating on the diamond surface by ion-chemical etching. The choice of (Ar+O 2) mixture as a working gas and use of niobium as a masking layer are substantiated. The results of studying a diffraction microrelief generated on the diamond-film surface through ion-chemical etching are discussed.

A method for the diffractive microrelief formation using the layered photoresist growth

A technology of diffraction relief fabrication without substrate etching is proposed. The step-wise relief is produced by the multiple applying of photoresist layers (layer over the layer). The layers are previously exposed to light through the appropriate photomasks and subject to all conventional stages of resist processing.

A method for estimating the DOE's energy efficiency

A method has been developed for estimating the variations of the energy efficiency of diffractive optical elements (DOEs) after the introduction of a technological error in the diffraction microrelief formation. For DOEs with radial symmetry the equations describing the energy efficiency variations due to a piecewise linear approximation of microrelief zones are obtained.

The morphology of moth and butterfly wing scales which exhibit reflective diffraction phenomena

The butterfly and moth families illustrate how nature has used diffractive micro-relief structures to achieve unique optical effects. Whereas the majority of insects use pigments (absorption) or occasionally thin film multi-layer structures (interference) to create colour, the wings of many families of butterfly and moth have complex 2-D or 3-D arrangements of submicron grating structures which produce zero and higher order diffraction conditions.The special properties of a diffraction grating can be understood by light incident perpendicularly on a transmission grating. The light is diffracted into a number of grating orders at angles θn given by the grating equation sin θn = n λ/d where λ is the wavelength of the light, n = 0, ± 1, --- and d is the grating period. The same conditions are valid for reflective diffraction structures. If the grating period is finer than the wavelength ie. d < λ, no first order diffraction exists for normal illumination.