Type Of Diffractive Optical Element Research Articles

Tuneable optical diffractive structures from liquid crystalline materials incorporated into periodic polymeric scaffolds

ABSTRACTDiffractive optical elements (DOEs) are micro-structured components used in various photonic devices to manipulate propagation direction and often also polarisation state of optical beams. The simplest type of DOEs are one-dimensional diffraction gratings. We perform theoretical investigation of electrical and magnetic tunability of such gratings composed as a periodic assembly of polymer and ferromagnetic nematic liquid crystal (LC) layers. We analyse how various grating parameters and boundary conditions influence the LC configuration in the assembly. For static response to external electric and/or magnetic fields, the distortion profile of the director field is determined numerically and analytically by minimisation of the Landau-de Gennes free energy, while for the dynamic response only numerical solutions are provided. The resulting optical diffraction properties are determined numerically via solving the Maxwell’s equations by using the rigorous coupled-wave analysis. We demonstrate that the described methodology provides an efficient tool for designing various new types of polarisation-sensitive LC-based DOEs, such as three-state polarisation switches for application in spectroscopic, optical communication, and remote sensing systems.

Investigation of a spectral lens for the formation of a normalized difference vegetation index NDVI0.705

Subject of study. A new type of diffractive optical element separating radiation at the wavelengths of 705 and 750 nm into the ±1 diffraction orders is investigated as a basis for a sensor for the detection of the normalized difference vegetation index NDVI0.705. Method. The study was performed as a natural experiment using a white light source with a continuous spectrum. Main results. A diffractive optical element separating radiation at the wavelengths of 705 and 750 nm into the ±1 diffraction orders was investigated. Using this optical element allows index images to be obtained directly instead of through the complex acquisition of a full hyperspectral image for the detection of NDVI0.705 in the region of a near-infrared slope. The diffractive optical element was fabricated using direct laser writing in a photoresist. The height of the formed microrelief was 4 µm, which implies that a combination of diffractive and refractive properties of this element contribute to the image formation process. The suggested term for these diffractive optical elements is a spectral diffractive lens. A laboratory setup was assembled based on the fabricated spectral diffractive lens to confirm the possibility of operating this lens in the imaging mode. In first experiments, a high-power halogen lamp behind an opaque screen with holes with diameters of 0.4–0.5 µm (individual and in linearly positioned groups) was used to simulate small-scale radiating objects. Two diffraction orders were recorded on a light-sensitive matrix, namely, −1 for radiation with a wavelength of 705 nm and +1 for radiation with a wavelength of 750 nm. The similarity of the geometrical parameters of the images of the group of sources in the +1 and −1 orders confirmed the possibility of using this spectral diffractive lens in the imaging mode. Therefore, a slit aperture was mounted instead of the linearly positioned sources in the next experiment. Scanning of a section of a color table with subsequent assembly of an index image was performed through this aperture. Practical significance. The spectral diffractive lens proposed in this work can be used to create simple compact sensors for the real-time monitoring of vegetation cover and for specialized agricultural equipment.

Spectral splitting and concentration of broadband light using neural networks

Compact photonic elements that control both the diffraction and interference of light offer superior performance at ultra-compact dimensions. Unlike conventional optical structures, these diffractive optical elements can provide simultaneous control of spectral and spatial profiles of light. However, the inverse design of such a diffractive optical element is time-consuming with current algorithms, and the designs generally lack experimental validation. Here, we develop a neural network model to experimentally design and validate SpliCons; a special type of diffractive optical element that can achieve spectral splitting and simultaneous concentration of broadband light. We use neural networks to exploit nonlinear operations that result from wavefront reconstruction through a phase plate. Our results show that the neural network model yields enhanced spectral splitting performance for phase plates with quantitative assessment compared to phase plates that are optimized via the local search optimization algorithm. The capabilities of the phase plates optimized via the neural network are experimentally validated by comparing the intensity distribution at the output plane. Once the neural networks are trained, we manage to design SpliCons with 96.6% ± 2.3% accuracy within 2 s, which is orders of magnitude faster than iterative search algorithms. We openly share the fast and efficient framework that we develop in order to contribute to the design and implementation of diffractive optical elements that can lead to transformative effects in microscopy, spectroscopy, and solar energy applications.

Polarization holographic recording of vortex diffractive optical elements on azopolymer thin films and 3D analysis via phase-shifting digital holographic microscopy.

Direct fabrication of complex diffractive optical elements (DOEs) on photosensitive thin films is of critical importance for the development of advanced optical instruments. In this paper, we design and investigate DOEs capable of generating optical vortices. Analog and digital approaches for one-step polarization holographic recording of vortex DOEs on new carbazole-based azopolymer thin films are described. First configuration involves analog polarization holographic recording using a vortex phase retarder and has as a result the DOE producing a diffraction pattern with phase singularities aligned in a single line. Similar diffraction picture is achieved by the single-beam digital holographic recording setup with an integrated spatial light modulator. In the third system, the implemented double-beam digital polarization holographic recording setup yields simultaneously a spatial multiplexed vortex pattern. Diffraction efficiency evolution of these three types of DOEs are monitored and compared. The phase-shifting digital holographic microscope with an electrically controlled liquid crystal variable retarder is applied to investigate the phase and surface topography of the inscribed diffractive optical elements. The comparison between the digital and analog micro-patterning techniques contributes new evidence to limited data on the influence of the analog and digital generation of the spiral wavefront on the performance of vortex DOEs.

Magnetically Tunable Liquid Crystal-Based Optical Diffraction Gratings

We present a theoretical analysis of optical diffractive properties of magnetically tunable optical transmission gratings composed of periodically assembled layers of a polymer and a ferromagnetic liquid crystal (LC). The orientational structure of the LC layers as a function of an applied magnetic field is calculated by minimization of the Landau-de Gennes free energy for ferromagnetic LCs, which is performed numerically and also analytically by using the one-constant approximation and the approximations of the high and the low magnetic fields. Optical diffractive properties of the associated diffraction structure are calculated numerically in the framework of rigorous coupled-wave analysis (RCWA). The presented methodology provides a basis for designing new types of diffractive optical element based on ferromagnetic LCs and simulating their operation governed by the in-plane magnetic field.

Bessel beams formation by hybrid axicons

We present a new type of diffractive optical elements (DOE) – hybrid axicons. The process of formation of identical intensity distributions in focus by different hybrid axicons on the example of Bessel beams is shown. The beams order is determined by the sum of the topological charges of the phase plate (structure charge) and the spiral axicon (zone charge). The results of mathematical modeling of light diffraction on hybrid axicons are presented.

회절광학소자 제작을 위한 레이저 직접 노광기의 공정실험

A direct laser lithography system is widely used to fabricate various types of DOEs (Diffractive Optical Elements) including lenses made as CGH (Computer Generated Hologram). However, a parametric study that uniformly and precisely fabricates the diffractive patterns on a large area (up to 200 mm X 200 mm) has not yet been reported. In this paper, four parameters (Focal Position Error, Intensity Variation of the Lithographic Beam, Patterning Speed, and Etching Time) were considered for stabilization of the direct laser lithography system, and the experimental results were presented.

Generation of controllable rotating petal-like modes using composited Dammann vortex gratings

A new type of diffractive optical element, called a composited Dammann vortex grating (CDVG), is proposed for generation of multiple equal-energy controllable rotating petal-like modes extra cavity. As an example, it is shown that a petal-like mode is well generated for each nonzero diffraction order by a binary pure-phase 1×7 CDVG. Mode decomposition is digitally implemented by a programmable spatial light modulator (SLM), and the experimental results show that those generated petal-like patterns are in high mode purity (∼90%) for all six different nonzero orders. Also, controllable rotating petal-like modes are demonstrated when the CDVG is digitally implemented by the programmable SLM, which provides the possibility to quantitatively control the rotation rate of this type of optical tweezers. Furthermore, tunable petal-like modes are also demonstrated experimentally by introducing a vortex incident field with different topological charges.

Focusing properties of a modified Fibonacci photon sieve

We propose a type of diffractive optical element, a modified Fibonacci photon sieve (MFiPS), designed by using the Fibonacci sequence with two different initial seed elements. Focusing properties of MFiPS show that it not only has less secondary foci, but also presents two equal intensity foci by optimizing the diameters of pinholes of MFiPS whose ratio of the two focal distances approaches the golden mean. Higher transverse resolution on the two focal spots is improved by using a super-Gaussian amplitude modulation technology.

Method for removing longitudinal chromatism in full color holographic projection system

Hologram, as a type of diffractive optical element, is sensitive to wavelength in the process of optoelectronic reconstruction. Due to the different wavelengths of three prime colors used in full color holographic projections, there are chromatisms which badly spoil the reconstructed image. The chromatisms are composed of transverse and longitudinal chromatisms. For a computer generated hologram, transverse chromatism can be compensated by resampling the object information. However, it becomes more complex for longitudinal chromatism. This paper analyzes how the image is reconstructed from the phase-type hologram in a holographic projection system and the causations of longitudinal chromatism. To remove it, this paper proposes loading a specially designed phase distribution on a phase-type hologram. The advantage of this method is that it can be achieved by computer calculation and without adding any hardware such as achromatic optical element. A time-sharing system for a full-color hologram projection is developed in this paper. Comparisons have been made between the reconstructed images with and without chromatism. The experimental result shows that the method is effective in removing longitudinal chromatism and the quality of the reconstructed image is improved.

Focusing light with a reflection photon sieve

An advanced type of diffractive optical element is presented that combines the concept of the photon sieve with an off-axis, off-normal incidence reflection geometry. Compared to transmission optical elements, the signal-to-background ratio is significantly increased by separating the first from other diffraction orders without drastically reducing the size of the smallest diffractive element. The reflection photon sieve produces sharp foci at maximum contrast and offers the advantages of effective heat dissipation and a large working space above the focal plane. Experimental results for a device working at a photon energy of 100 eV are presented and compared to theory.

Analyses of pseudo-nondiffracting beams at oblique incidence

We design three types of diffractive optical elements (DOEs) which generate pseudo-nondiffracting beams (PNDBs) in different regions by using the conjugate gradient method. In order to analyze the influence of the tilt angles of the DOEs, the coordinate-transformation method is used to simplify the calculation. The uniformity of the out-coupling illuminance along the optical axis is calculated. The analyzed results show that when the tilt angle increases, the uniformity in the region [140 mm, 200 mm] has more violent fluctuations than those in the regions [280 mm, 340 mm] and [380 mm, 520 mm]. The analyses also reveal that the uniformity of DOEs with a longer focal length is less sensitive to the tilt angle, and some points within the PNDB region have small variations of axial illuminance while the tilt angle increases.

Mode-matched phase diffractive optical element for detecting laser modes with spiral phases

A new type of diffractive optical element for detecting and measuring the power distribution of transverse modes emanating from radially symmetric laser resonators is presented. It is based on a relatively simple straightforward design of a phase-only diffractive optical element that serves as a matched filter, which correlates between specific prerecorded transverse modes with a certain azimuthal mode order and those in the incident laser light. Computer simulations supported by experimental results demonstrate how such elements can accurately detect modes with spiral phases and provide quantitative results on the modal power distribution.

Performance of diffractive optical elements for homogenizing partially coherent light

The effectiveness of different types of diffractive optical element (DOE) for homogenizing partially coherent beams is analyzed, both analytically and numerically. The effectiveness is described by the homogenizing parameter, defined as the inverse of the normalized variance of the dose distribution. For an important class of DOEs designed with common discrete-Fourier-transform methods, it is found that the homogenizing parameter is only of the order of the number of coherence cells in the illuminating beam. However, for a different type of DOE that produces distinct beams under coherent illumination, the homogenizing parameter can be an order of magnitude higher. The inherent dehomogenizing effect caused by the limited temporal duration of the beam, a phenomenon sometimes referred to as dynamic speckle, is also considered.

A Diffractive Optical Element Designed for Arbitrary Pattern Beam Shaping by Use of Iterative Optimization Algorithm

Techniques of diffractive optical elements (DOEs) based on computer-generated holograms are surveyed. This type of DOE is numerically designed and optimized by using an iterative diffraction algorithm or a nonlinear search algorithm. In this report, a beamshaper designed by employing the former type of algorithm is presented. The beamshaper is capable of transforming an incident Gaussian beam into a specific beamshape, which has a complicated pattern modeled on a printed circuit with a spatial resolution of tens of micron. The actual procedure for designing the beamshaper and the criteria used to determine the design parameters are reported. Furthermore, an evaluation of the designed and fabricated beamshaper and several ideas for improvingit are discussed.

극자외선 영역에서 곡면 DOEs에 관한 연구

본 연구에서는 여러 형태의 회절광학요소의 field performance를 조사하였다. Zemax 프로그램을 이용하여 다섯 개의 회절광학요소, 즉 투과형 평면 DOE, 투과형 곡면 DOE, 반사형 포물면 DOE, 반사형 평면 DOE, 그리고 반사형 곡면 DOE를 설계하였다. 그리고 이들 회절광학요소에 극자외선 파장인 13 nm와 가시광선 파장인 632.8 nm를 적용시켰다. 이들 DOEs에 사입사 조명시의 회절 한계 하에서의 시야각의 크기 및 파장에 따른 특성, 그리고 주된 수차의 형태를 상호분석 비교하였다. 회절한계 하에서 투과 및 반사 형태 모두에서 곡면 DOEs의 시야각이 평면 DOEs의 시야각보다 훨씬 크다는 것을 알 수 있었다. 또한 사입사 경우에 평면 DOEs와 포물면경의 주된 수차는 코마이며, 곡면 DOEs의 주된 수차는 비점수차와 상면만곡의 혼합된 형태로 나타남을 알 수 있었다. 측정을 통하여 얻은 시야각의 크기와 수차의 종류가 이론적인 결과와 잘 일치함을 알 수 있었다. 또한 극자외선영역에서 평면형 DOEs에 비해 곡면형 DOEs의 field angle의 증가율이 가시광선에서보다 더 효과적임을 알 수 있었다. Field performance of several different types of diffractive optical elements(DOEs) has been carried out. Using Zemax model, we have designed five different types of DOEs, such as transmissive flat-DOE, transmissive curved-DOE, reflective flat-DOE, reflective curved-DOE and parabolic mirror, We have applied two different wavelengths, i.e., 13 m(EUV) and 632.8 nm(visible) to above DOEs. Off_axis dominate aberrations and the diffraction limiting (Rayleigh limit) field angles have been investigated and compared at both wavelengths for each DOE. At diffraction limit, field angle of curved-DOEs was much greater than that of flat-DOEs for both transmission and reflective types. We also showed that dominated off_axis aberration of flat-DOEs was coma, but that of curved-DOEs was mixture of astigmatism and curvature of field. The measured field angle and expected OPD aberrations were well coincided with theoretical ones. Increasing the ratio of field angle with wavelength was more effective in curved-DOEs than flat-DOEs.

Computer-generated stratified diffractive optical elements

We present what is to our knowledge a new type of diffractive optical element (DOE), the computer-generated stratified diffractive optical element (SDOE), a hybridization of thin computer-generated DOEs and volume holograms. A model and several algorithms for calculating computer-generated SDOEs are given. Simulations and experimental results are presented that exhibit the properties of computer-generated SDOEs: the strong angular and wavelength selectivity of SDOEs makes it possible to store multiple pages in a computer-generated SDOE, which can be read out separately (multiplexing). The reconstruction of an optimized SDOE has a higher quality than the reconstruction of optimized one-layer DOEs. SDOEs can be calculated to have only one diffraction order.

Laser-triggered high-voltage plasma switching with diffractive optics

High-power lasers can be used to induce ionization of gases and thereby enable rapid triggering of electrical discharge devices, potentially faster than any devices based on mechanical or solid-state switching. With diffractive optical elements (DOEs) the laser light can conveniently be directed to positions within the gas so that an electrical discharge between two high-voltage electrodes is triggered reliably and rapidly. Here we report on two different types of DOE used for creating an electrical discharge in pure argon for potential high-voltage applications. One is the diffractive equivalent of a conventional axicon that yields an extended, and continuous, high-intensity focal region between the electrodes. The other is a multiple-focal-distance kinoform--a DOE that is designed to produce a linear array of 20 discrete foci, with high peak intensities, between the electrodes. We show that DOEs enable efficient, rapid switching and may provide increased flexibility in the design of novel electrode configurations.

Stratified volume diffractive optical elements as high-efficiency gratings

We propose stratified volume diffractive optical elements as a new type of diffractive optical element that is capable of functioning as a high-efficiency grating in applications with requirements not suited to traditional holographic or diffractive optical techniques. In this approach, diffractive optical fabrication methods are used to construct an optical structure that emulates volume grating behavior. We discuss the diffraction properties of stratified volume diffractive optical elements and compare them with those used previously in both volume holographic optical elements and stratified volume holographic optical elements. A systematic design process is then presented for deriving structure parameters. We illustrate this process by designing a prototype stratified volume diffractive optical element to meet the operational specifications for a beam-scanning element in a spaceborne coherent wind lidar. We use numerical simulation to assess the performance of the prototype element, including sensitivity to fabrication errors.