Use Of Diffractive Optical Elements Research Articles

Diffractive optical elements for calibration of LIDAR systems: materials and fabrication

The promise of fully autonomous vehicles to replace the judgment of human drivers with real-time algorithmic decision-making based on optoelectronic systems relies fundamentally on the quality of the available data. Limitations imposed by sensor-resolution, available optical power, and achievable signal-to-noise ratios have been well studied in the light detection and ranging (LIDAR) application space. Additionally, the problem of integrating multiple sources of image data as well as the need to establish and maintain the system calibration over life are critically important to system reliability and safety. These latter concerns will receive even greater attention as self-driving vehicles begin to transition toward fully autonomous operation. Because of the importance of calibration to system performance and safety, the process of validating and recalibrating the system will ideally be integrated into the LIDAR system itself with calibration occurring automatically “anywhere and at any time,” without dedicated external infrastructure. Mass-market adoption is also being driven by the systems’ size and weight, as well as reliable manufacturability and resilience to environmental stresses. Due to their extreme stability, manufacturability, and small size, diffractive optical elements (DOEs) are well suited for use as optical calibration references. Current three-dimensional (3D) mapping systems based on structured light illumination already rely on DOEs as precision pattern generators to provide 3D depth sensing in a wide array mobile devices. We examine the potential use of DOEs as calibration elements in multicamera or LIDAR systems, including appropriate choices of materials, designs, and fabrication methods to ensure reliable long-term performance under automotive use conditions. We present simulations of the impact of DOE material properties on the accuracy of the generated dot patterns and consequently on the depth accuracy and lateral distortion of the 3D image. Additionally, we present requirements for DOE manufacture using conventional semiconductor fabrication technologies optimized for creating engineered surface nanostructures capable of transforming the output of a laser or other narrow-band source into a precise reference pattern.

Holography with high-power CW coherent terahertz source: optical components, imaging, and applications

This paper presents the results of 15 years of studies in the field of terahertz holography at the Novosibirsk free electron laser. They cover two areas: research on obtaining holographic images in the terahertz range and the use of diffractive optical elements to form high-power terahertz radiation fields with specified characteristics (intensity, phase, and polarization), using well-studied and widely applied in the optical range methods of optical (analog), digital, and computer-generated holography. All experiments were performed with the application of high-power coherent monochromatic frequency-tunable radiation from the Novosibirsk free electron laser. The features of hologram registration in the terahertz range are described. Methods, technologies, and optical materials for terahertz holographic elements are discussed. A wide range of promising applications of high-power terahertz fields with a given spatial structure is considered. The results of the study of terahertz holograms recorded as digital holograms, as well as radiation-resistive optical elements realized as computer-synthesized holograms, are presented.

Perifoveal retinal augmented reality on contact lenses

We present an innovative augmented reality display to project information on the perifovea. Our system uses a contact lens embedding various diffractive optical elements (DOE) and switchable light sources. The DOEs are located at the iris periphery, keeping the visual axis free of any disturbance while allowing AR content projection onto the perifovea. The use of DOEs limits the quantity of displayable information for instance to some warning symbols but allows the design of more easily manufacturable elements. A proof of concept using a mock up eye (scale 2:1) to assess the impact of laser injection and mydriasis on image reconstruction quality is presented together with a video showing how the system operates dynamically.

Modelling of temperature fields in DP1000 steel during laser treatment using diffractive optical elements

Diffractive optical elements (DOEs) with their unique properties allow to form a predetermined beam intensity profile in the focal plane. The use of DOEs in laser material processing technologies reveals new possibilities for controlling the properties and operational characteristics of processed parts. The beam intensity profile formed by DOE is presented in the form of an analytical expression that was used to set a surface heat source for modelling thermal processes in DP 1000 steel. Experimental studies of samples under laser heating were performed. Simulation output results correlate well with the experimental data. The proposed simulation model, based on a precise heat input definition, is an intermediate step to the final goal, which is the prediction of structural changes in the zone of laser beam irradiation.

“Perfect” Terahertz Vortex Beams Formed Using Diffractive Axicons and Prospects for Excitation of Vortex Surface Plasmon Polaritons

Transformation of a Bessel beam by a lens results in the formation of a “perfect” vortex beam (PVB) in the focal plane of the lens. The PVB has a single-ring cross-section and carries an orbital angular momentum (OAM) equal to the OAM of the “parent” beam. PVBs have numerous applications based on the assumption of their ideal ring-type structure. For instance, we proposed using terahertz PVBs to excite vortex surface plasmon polaritons propagating along cylindrical conductors and the creation of plasmon multiplex communication lines in the future (Comput. Opt. 2019, 43, 992). Recently, we demonstrated the formation of PVBs in the terahertz range using a Bessel beam produced using a spiral binary silicon axicon (Phys. Rev. A 2017, 96, 023846). It was shown that, in that case, the PVB was not annular, but was split into nested spiral segments, which was obviously a consequence of the method of Bessel beam generation. The search for methods of producing perfect beams with characteristics approaching theoretically possible ones is a topical task. Since for the terahertz range, there are no devices like spatial modulators of light in the visible range, the main method for controlling the mode composition of beams is the use of diffractive optical elements. In this work, we investigated the characteristics of perfect beams, the parent beams being quasi-Bessel beams created by three types of diffractive phase axicons made of high-resistivity silicon: binary, kinoform, and “holographic”. The amplitude-phase distributions of the field in real perfect beams were calculated numerically in the approximation of the scalar diffraction theory. An analytical expression was obtained for the case of the binary axicon. It was shown that a distribution closest to an ideal vortex was obtained using a holographic axicon. The resulting distributions were compared with experimental and theoretical distributions of the evanescent field of a plasmon near the gold–zinc sulfide–air surface at different thicknesses of the dielectric layer, and recommendations for experiments were given.

Dispersion-engineered nanocomposites enable achromatic diffractive optical elements

The use of diffractive optical elements (DOEs) in broadband optical systems can often significantly reduce their size or enhance their performance but is mostly prevented by stray light in unwanted diffraction orders. This is because dispersion causes the diffraction efficiency to decrease as a function of the wavelength. Here we introduce nanocomposites as a material platform that allows for the design of dispersion-engineered materials. We show that these materials enable the design of almost dispersion-free, i.e., achromatic, echelette-type gratings with diffraction efficiencies of close to 100% in the entire visible spectral range. Using numerical simulations, we demonstrate that such high efficiencies are maintained across the range of incidence angles and grating periods required for most optical systems. This concept of dispersion-engineered nanocomposites can also be applied to other applications, and nanocomposite-enabled DOEs have the potential to be an enabling technology for a new generation of better and more compact optical systems.

New development of laser illumination technology for high-speed photography

In this paper, laser speckle and interference fringe were effectively restrained by the use of diffractive optical element for smoothing beam, multimode fiber and the lens coupling expander technology. The vignette phenomenon on the imaging plane was eliminated while taking the picture of objective illuminated by wide-view parallel-light. Based on this research, the 300 mm wide-view laser uniform illumination and the clear imaging of objective were realized, and it was successfully used in precise physical experiments of cylindrical implosion compression, space junk and so on. The successful combination of high-speed photography technology and laser illumination will effectively restrain the excessive exposure caused by intensive self-illumination in precise physical experiments, and improve the quality of imaging. Consequently, the image boundary of physical phenomenon will be clearer and easier to interprete and recognize.

Research & Technology: Optik & Photonik 2/2018

AbstractThe use of diffractive optical elements (DOE) leads to new applications in all optical fields. The accurate measurement of the micro structures is an important part of production. Mahr offers manufacturers of these optics convenient and highly accurate solutions for the measurement and characterization of DOEs.

Optically read displacement detection using phase-modulated diffraction gratings with reduced zeroth-order reflections

Displacement detection using optical interferometric techniques allows for low minimum detectable displacements which are unmatched by other displacement measurement methods as device sizes are scaled down. The use of diffractive optical elements as beam splitters has proven an effective way to realize miniature and robust optical interferometers. Diffraction gratings commonly used in such applications, however, can generate a zeroth-order reflected beam, which results in reduced sensor performance, packaging limitations, and laser instability. A diffraction grating concept has been designed, fabricated, and tested, which has the effect of reducing the zeroth-order component by imparting a half-wavelength phase shift to a portion of the reflected light. The design criteria for zeroth-order beam elimination are illustrated using a simple model based on phasor arithmetic. The microfabrication process used to prototype gratings is presented, and experimental measurements collected from the prototype are reported. The minimum detectable displacement achievable in sensor applications is found to be 3.6 fm/√Hz, which is comparable to sensors built using more conventional gratings. Finally, comparisons are made between the test results and the simple model predictions.

Laser welding of dissimilar metallic materials with use of diffractive optical elements

A use of laser technology it is progressive in the welding of dissimilar materials. Pulsed laser welding of an aluminum alloy AK4 and a titanium alloy VT5-1 was performed. Processing conditions were determined, the realization of which during melting of materials in the heat-affected zone makes it possible to obtain a homogeneous structure without voids and shells, potentially producing sufficiently sound welded joints. To create the required power density distribution across the laser beam, it was found expedient to use diffractive optical elements. Aluminum and copper were welded by continuous laser beam. It is determined that laser action with its well-defined and precise localized heat input makes it possible to significantly reduce the growth of intermetallic compound layers.

Traceability of high focal length cameras with diffractive optical elements

This paper describes the use of diffractive optical elements (DOEs) for metrological traceable geometrical testing of high focal length cameras applied in the observation of large- scale structures. DOEs and related mathematical models are briefly explained. Laboratorial activities and results are described for the case of a high focal length camera used for longdistance displacement measurement of a long-span (2278 m) suspension bridge.

The diffractive achromat full spectrum computational imaging with diffractive optics

Diffractive optical elements (DOEs) have recently drawn great attention in computational imaging because they can drastically reduce the size and weight of imaging devices compared to their refractive counterparts. However, the inherent strong dispersion is a tremendous obstacle that limits the use of DOEs in full spectrum imaging, causing unacceptable loss of color fidelity in the images. In particular, metamerism introduces a data dependency in the image blur, which has been neglected in computational imaging methods so far. We introduce both a diffractive achromat based on computational optimization, as well as a corresponding algorithm for correction of residual aberrations. Using this approach, we demonstrate high fidelity color diffractive-only imaging over the full visible spectrum. In the optical design, the height profile of a diffractive lens is optimized to balance the focusing contributions of different wavelengths for a specific focal length. The spectral point spread functions (PSFs) become nearly identical to each other, creating approximately spectrally invariant blur kernels. This property guarantees good color preservation in the captured image and facilitates the correction of residual aberrations in our fast two-step deconvolution without additional color priors. We demonstrate our design of diffractive achromat on a 0.5mm ultrathin substrate by photolithography techniques. Experimental results show that our achromatic diffractive lens produces high color fidelity and better image quality in the full visible spectrum.

Single-pulse Conduction Limited Laser Welding Using A Diffractive Optical Element

Conduction limited laser welding is commonly used in electronic and battery applications, where a high width-to-depth ratio weld is desirable. A laser beam with Gaussian or top-hat distributions is often used to produce conduction limited spot welds. Both these energy distributions result in a higher proportion of the laser beam energy being introduced towards the centre of the welded spot and consequently, a reduced penetration weld towards the circumference of the beam spot. The use of diffractive optical elements to tailor the energy distribution of the laser beam has been evaluated. An incident laser beam with an energy distribution in the shape of a ring or C-shape was projected onto the material, which results in heat propagating towards the centre, producing a shallow weld with a consistent depth of penetration across the entire overlapped joint. The results confirmed a corresponding thermal model which predicted an even distribution of heat at the joint interface.

Computational imaging using lightweight diffractive-refractive optics.

Diffractive optical elements (DOE) show great promise for imaging optics that are thinner and more lightweight than conventional refractive lenses while preserving their light efficiency. Unfortunately, severe spectral dispersion currently limits the use of DOEs in consumer-level lens design. In this article, we jointly design lightweight diffractive-refractive optics and post-processing algorithms to enable imaging under white light illumination. Using the Fresnel lens as a general platform, we show three phase-plate designs, including a super-thin stacked plate design, a diffractive-refractive-hybrid lens, and a phase coded-aperture lens. Combined with cross-channel deconvolution algorithm, both spherical and chromatic aberrations are corrected. Experimental results indicate that using our computational imaging approach, diffractive-refractive optics is an alternative candidate to build light efficient and thin optics for white light imaging.

Generation and conversion of mode beams and their polarization states on the basis of diffractive optical elements application

We propose and analyze new polarization converters to transform the linearly polarized laser modes into axially symmetric modes, which show more promise in various applications, as well as generating various inhomogeneously polarized configurations. Designed converters are based on the coherent composition of the modal beams with different amplitude-phased distribution and polarization states. What makes the systems simple and universal is the use of diffractive optical elements (DOEs) to generate required mode patterns with specific space orientation alongside the simultaneous generation of different beams with different transverse mode content, which can be subsequently combined. The numerical modeling of the polarization mode converters has made it possible to analyze its performance and capabilities. The DOEs in question have been designed and fabricated. Natural experiments that demonstrate the generation of vector higher-order cylindrical beams have been conducted.

Generating inhomogeneously polarized higher-order laser beams by use of diffractive optical elements

We propose an improved version of the earlier developed optical arrangement for generating inhomogeneously polarized laser light modes with the aid of a diffractive optical element (DOE) with carrier frequency. By eliminating lenses from the optical arrangement, we achieve the miniaturization, reduced light losses, a smaller number of parameters being matched, and a simpler system adjustment procedure. Note that all the capabilities of the previous version, namely, the universality and simple readjustment to different polarization types, are fully retained. The numerical modeling of the polarization mode combiner has made it possible to analyze its performance and capabilities. In the experiments, the quality of the resulting beams is shown to be improved. For generating higher-order cylindrical beams, a lower-order mode at the output of the polarization mode combiner is additionally transformed with a DOE that operates in the zero diffraction order, introducing radial phase changes.

Grating-based optical scheme for the universal generation of inhomogeneously polarized laser beams

We propose and analyze a new optical system to transform linearly polarized laser modes into axially symmetric (radial or azimuthal) modes that show more promise in various applications, as well as generating various inhomogeneously polarized configurations. The system is based on the coherent composition of modal beams with phase diffraction gratings that allow the intermode phase shift to be varied without the need for auxiliary components. What makes the system simple and universal is the use of diffractive optical elements to generate required mode patterns with specific space orientation along with the simultaneous generation of different beams with different transverse mode content, all of which can subsequently be combined.

Optofluidic generation of Laguerre-Gaussian beams

Laguerre-Gaussian (LG) beams have been extensively studied due to their unique structure, characterized by a phase singularity at the center of the beam. Common methods for generating such beams include the use of diffractive optical elements and spatial light modulators, which although offering excellent versatility, suffers from several drawbacks, including in many cases a low power damage threshold as well as complexity and expense. This paper presents a simple, low cost method for the generation of high-fidelity LG beams using rapid prototyping techniques. Our approach is based on a fluidic-hologram concept, whereby the properties of the LG beam can be finely controlled by varying the refractive-index of the fluid that flows through the hologram. This simple approach, while optimized here for LG beam generation, is also expected to find applications in the production of tunable fluidic optical trains.

Focusing of high polarization order axially-symmetric polarized beams

We study the high numerical aperture focusing properties and typical applications of axially-symmetric polarized beams (ASPBs) with high polarization orders. We calculate the field distribution near focus of an aplanatic system for incident ASPBs with different polarization orders and initial azimuthal angles, and based on the simulation results, we find some unique focusing properties of the beams, such as ever on-axis energy null, strong longitudinal field, and flowerlike intensity distribution at focus. In addition, we can manipulate the three-dimensional (3D) focused field distribution flexibly by use of diffractive optical elements (DOEs), which will give rise to some interesting applications, and we also discuss possible applications and present an example of a 3D optical chain at last.

Fibre sensors based on transverse mode selection

Use of diffractive optical elements (DOEs) in fibre sensors is very promising. This work is dedicated to the investigation of dependence of the transverse mode content in step-index fibres on the microbending value by means of diffractive optical elements. We describe the experimental set-up and methods of experimental data processing that afford precise measurements of mode power. The LP mode power dependencies on the fibre microbending value are obtained. The results obtained allow the designing of fibre sensors with advanced characteristics such as the dynamic range and accuracy.