Transmissive Elements Research Articles

Optimization of a large aperture wedge prism support system based on the closed-loop structural-thermal-optical performance method.

In the instrument suite of the Thirty Meter Telescope (TMT), the linear atmospheric dispersion corrector (LADC) is positioned at the forefront of the spectrometer to mitigate atmospheric dispersion. The LADC comprises two large aperture wedge prisms, each approximately 1.5 meters in diameter. These prisms, which are transmissive optical elements, are supported radially along their outer circumferences. However, due to the rotational asymmetry of the prisms relative to the optical axis, the support forces do not align with the plane of the center of gravity, leading to a bending moment caused by gravitational and reaction forces. This misalignment results in significant deformation of the prism surfaces. The structural-thermal-optical performance integrated model method is typically employed to evaluate the impact of the support system on prism surface deformation. This process often involves extensive manual iterations and operates in an open-loop manner, making it inefficient and heavily reliant on the analyst's expertise. To address these limitations, this paper proposes a closed-loop integrated model analysis optimization method aimed at enhancing optimization efficiency. By integrating interface programs between analysis tools and applying advanced optimization algorithms, the traditional open-loop integrated model analysis process is transformed into a closed-loop system, enabling more efficient and reliable optimization. The closed-loop integrated model method incorporates Optimal Latin Hypercube Design (Opt LHD) and Particle Swarm Optimization (PSO) algorithms to optimize the prism support structure, reducing the analysis time from one week to just two days. Compared to the original support structure, the root mean square (RMS) value of optical surface deformation decreased by 49.3%, from 155.0 nm to 78.6 nm, under gravity and 2°C temperature coupled conditions; and by 62.5%, from 289. 1 nm to 108. 4 nm, under gravity and 42°C temperature coupled conditions. These results demonstrate that the closed-loop integrated model optimization method not only improves efficiency but also achieves significantly better outcomes.

The influence of space environmental factors on the laser-induced damage thresholds in optical components

Abstract This paper systematically investigated the impact mechanisms of proton irradiation, atomic oxygen irradiation and space debris collision, both individually and in combination, on the laser damage threshold and damage evolution characteristics of HfO2/SiO2 triple-band high-reflection films and fused silica substrates using a simulated near-Earth space radiation experimental system. For the high-reflection film samples, the damage thresholds decreased by 15.38%, 13.12% and 46.80% after proton, atomic oxygen and simulated space debris (penetration) irradiation, respectively. The coupling irradiation of the first two factors resulted in a decrease of 26.93%, while the combined effect of all the three factors led to a reduction of 63.19%. Similarly, the fused silica substrates exhibited the same pattern of laser damage performance degradation. Notably, the study employed high-precision fixed-point in situ measurement techniques to track in detail the microstructural changes, surface roughness and optical-thermal absorption intensity before and after proton and atomic oxygen irradiation at the same location, thus providing a more accurate and comprehensive analysis of the damage mechanisms. In addition, simulations were conducted to quantitatively analyze the transmission trajectories and concentration distribution lines of protons and atomic oxygen incident at specific angles into the target material. The research findings contribute to elucidating the laser damage performance degradation mechanism of transmissive elements in near-Earth space environments and provide technical support for the development of high-damage-threshold optical components resistant to space radiation.

2D-smoothing of laser beam fluctuations in optical compressor

We propose a modification of the four diffraction gratings Treacy compressor (TC)—a double-smoothing grating compressor (DSGC)—which enables smoothing of spatial fluctuations of a laser beam in two directions. Smoothing along the groves is due to oblique incidence on the gratings or tilted grooves. Smoothing in the direction normal to the grooves is achieved due to the use of nonidentical pairs of gratings. It is shown that the far-field fluence and the focal beam intensity after the DSGC are like those after the TC. Smoothing is a consequence of spatial harmonics lagging behind or overtaking the main pulse in proportion to the transverse wave vector. Analytical expressions are obtained for the spectrum of fluence fluctuations and fluence rms at the DSGC output. The efficiency of suppressing small-scale self-focusing in transmissive optical elements after the DSGC, for example, in the case of post-pulse compression is assessed.

Polarization-Maintaining Diffractive Elements for Off-Axis Terahertz Digital Holography

Off-axis digital holography in the terahertz waves allows three-dimensional visualizations of optically opaque volumetric objects in a single exposure and with angular separation of the zero-order component. Forming the tilted reference beam usually requires a set of mirrors, which can disturb the polarization, causing the loss of contrast in fringes at the detector. Here we present a mirror-less approach with 3D-printed transmissive diffractive optical elements, providing unaltered polarization and successful holographic measurements at 0.1 THz without any pre- or postprocessing of interference fringes.

A shared-aperture transmissive/reflective bi-functional metasurface for both transmitarray and reflectarray

In this paper, a shared-aperture transmissive/reflective bi-functional metasurface is proposed. On metasurfaces, antisymmetric dipole pairs as transmissive elements and symmetric dipole pairs as reflective elements are interleaved in orthogonal polarization. By utilizing Huygens’ resonance, the metasurface can achieve near full phase coverage for orthogonally polarized transmitted and reflected wave. Then the bi-functional metasurface is used for the design of transmitarray (TA)/reflectarray (RA) bi-functional antenna. The measured results show that, for TA, the maximum gain is 30.26 dBi with antenna efficiency of 45.6 % and the 3-dB gain bandwidth is 26.3–29.3 GHz (10.79 %), and for RA, the maximum gain is 30.29 dBi with antenna efficiency of 45.7 % and the 3-dB gain bandwidth entirely covers the frequency region of 26–30 GHz. The bi-functional metasurface has important applications in the field of antenna engineering.

Increasing the stability margins using multi-pattern metasails and multi-modal laser beams

Laser-driven metasails can enable reaching velocities far beyond the chemically propelled spacecrafts, which accounts for precise engineering of the acceleration and the stability degree of the lightsail across the Doppler-broadened band. All-dielectric metasurfaces have shown great promise toward the realization of low-weight photonic platforms suitable for integrating multiple functionalities. The most paramount factor in the stability analysis of lightsail is the coupling between displacement and rotation, which mainly determines the durability of the nanocraft against displacement and rotation offsets. In this work, the marginal stability conditions of laser-propelled lightsails have been extended by replacing the reflective elements near the edges portions of the sail with broad-band transmissive elements and applying a multi-objective genetic algorithm (GA) optimization to the proposed configuration. The presented design not only remarkably suppresses the amplitude of the oscillatory motion but also can decrease the center of the mass requirement of the lightsail while maintaining an acceptable acceleration time. Next, a configuration where the payload is at the non-illuminating side of the dual-portion sail is proposed to protect the payload from the intense laser beam. In this case, a spherical phase profile is imprinted across the reflective elements while it is being propelled by a multi-modal beam.

Lensless Wavefront Parallel Processing of Vector Beams by Self‐Images of a Self‐Organized Q‐Plates Microarray

Wavefront shaping of structured light beams is attracting considerable attention in optics and related technologies. Due to the potential benefits, it is crucial to develop versatile methods for controlling the wavefront in a compact space with the capacity for high‐throughput parallel processing. Herein, a unique concept is introduced for converting the wavefront of multiple structured light beams, which are periodically packed in a microscopic area. An experimental demonstration is provided by using transmissive planar optical elements consisting of a microarray of geometric phase based on nematic liquid crystals. The periodic microstructure is fabricated by the directed self‐organization of topological defects present in a nanoscale relief obtained by area‐selective surface modification. The conversion of multiple wavefronts is realized by using the Talbot self‐imaging effect of the microarray. The integrated geometric phase microarray has potential applications in vortex beam emitters to harness structured light, such as for optical manipulation and lithography.

Freeform optics for variable extended depth of field imaging.

Imaging depth of field is shallow in applications with high magnification and high numerical aperture, such as microscopy, resulting in images with in- and out-of-focus regions. Therefore, methods to extend depth of field are of particular interest. Researchers have previously shown the advantages of using freeform components to extend depth of field, with each optical system requiring a specially designed phase plate. In this paper we present a method to enable extended depth-of-field imaging for a range of numerical apertures using freeform phase plates to create variable cubic wavefronts. The concept is similar to an Alvarez lens which creates variable spherical wavefronts through the relative translation of two transmissive elements with XY polynomial surfaces. We discuss design and optimization methods to enable extended depth of field for lenses with different numerical aperture values by considering through-focus variation of the point spread function and compare on- and off-axis performance through multiple metrics.

Combined diffractive optical elements with adjustable optical properties controlled by a relative rotation: tutorial.

A pair of adjacent transmissive diffractive optical elements (DOEs) forms a combined DOE with tunable optical properties, as, for example, a diffractive lens with an adjustable focal length. The optical properties are controlled by a relative movement of the two DOEs, such as a translation or a rotation around the optical axis. Here we discuss various implementations of this principle, such as tunable diffractive lenses, axicons, vortex plates, and aberration correction devices. We discuss the limits of the tuning range and of diffraction efficiency. Furthermore, it is demonstrated how chromatic aberrations can be suppressed by using multi-order DOEs.

Step-by-step guide to 3D print motorized rotation mounts for optical applications.

Motorized rotation mounts and stages are versatile instruments that introduce computer control to optical systems, enabling automation and scanning actions. They can be used for intensity control, position adjustments, etc. However, these rotation mounts come with a hefty price tag, and this limits their use. This work shows how to build two different types of motorized rotation mounts for $1^{\prime \prime}$ optics, using a 3D printer and off-the-shelf components. The first is intended for reflective elements, such as mirrors and gratings, and the second for transmissive elements, such as polarizers and retarders. We evaluate and compare their performance to commercial systems based on velocity, resolution, precision, backlash, and axis wobble. Also, we investigate the angular stability using Allan variance analysis. The results show that our mounts perform similarly to systems costing as much as $\$ 2500\,\rm USD $, while also being quick to build and costing less than $\$ 220\,\rm USD$. As a proof of concept, we show how to control lasers used in an optical tweezers and Raman spectroscopy setup. When used for this, the 3D printed motorized rotational mounts provide intensity control with a resolution of 0.03 percentage points or better.

High-Peak-Power Long-Wave Infrared Lasers with CO2 Amplifiers

Long-wave infrared (LWIR) picosecond pulses with multi-terawatt peak power have recently become available for advanced high-energy physics and material research. Multi-joule pulse energy is achieved in an LWIR laser system via amplification of a microjoule seed pulse with high-pressure, mixed-isotope CO2 amplifiers. A chirped-pulse amplification (CPA) scheme is employed in such a laser to reduce the nonlinear interaction between the optical field and the transmissive elements of the system. Presently, a research and development effort is underway towards an even higher LWIR peak power that is required, for instance, for promising particle acceleration schemes. The required boost of the peak power can be achieved by reducing the pulse duration to fractions of a picosecond. For this purpose, the possibility of reducing the gain narrowing in the laser amplifiers and post-compression techniques are being studied. Another direction in research is aimed at the increased throughput (i.e., repetition rate), efficiency, and reliability of LWIR laser systems. The transition from a traditional electric-discharge pumping to an optical pumping scheme for CO2 amplifiers is expected to improve the robustness of high-peak-power LWIR lasers, making them suitable for broad implementation in scientific laboratory, industrial, and clinical environments.

Characterization of Absorption Losses and Transient Thermo-Optic Effects in a High-Power Laser System

(1) Background: The modeling, characterization, and mitigation of transient lasers, thermal stress, and thermo-optic effects (TOEs) occurring inside high energy lasers have become hot research topics in laser physics over the past few decades. The physical sources of TOEs are the un-avoidable residual absorption and scattering in the volume and on the surface of passive and active laser elements. Therefore, it is necessary to characterize and mitigate these effects in real laser systems under high-power operations. (2) Methods: The laboratory setup comprised a 10-kW continuous wave laser source with a changeable beam diameter, and dynamic registration of the transient temperature profiles was applied using an infrared camera. Modeling using COMSOL Multiphysics enabled matching of the surface and volume absorption coefficients to the experimental data of the temperature profiles. The beam quality was estimated from the known optical path differences (OPDs) occurring within the examined sample. (3) Results: The absorption loss coefficients of dielectric coatings were determined for the evaluation of several coating technologies. Additionally, OPDs for typical transmissive and reflective elements were determined. (4) Conclusions: The idea of dynamic self-compensation of transient TOEs using a tailored design of the considered transmissive and reflecting elements is proposed.

Generation of low-order Laguerre-Gaussian beams using hybrid-machined reflective spiral phase plates for intense laser-plasma interactions

In laser-plasma interactions (LPI), the laser beam mode is a critical parameter when trying to explore new physical phenomena. Of the various spatial beam modes, the Laguerre-Gaussian (LG) mode with vortex phase has attracted considerable attention due to its unique features, including the ability to carry an orbital angular momentum. Due to this, it has been actively applied to LPI, which mainly utilize ultrashort intense laser pulses. However, existing transmissive phase-manipulating optical elements have several limitations when applied in LPI due to critical issues such as pulse broadening, attenuation, and beam shape–all of which have an influence on the beam quality, as well as, geometry, size, simplicity, and cost–all of which are related to processing technologies. In this paper, we present a series of procedures to obtain high-quality low-order (l = 1 and 2) LG vortex beams from large-sized off-axis reflective spiral phase plates (ORSPPs). The geometric designs for various surface structures, electromagnetic wave simulations in the extra-large domain, hybrid-mechanical processing technique attempted newly, and experimental demonstrations are involved. Experimental observations of LG intensity distributions and interference fringes were verified with the simulation results of Poynting vector, phase, and angular momentum densities. The beam quality of LG intensity distributions was analyzed quantitatively through the investigation of an annular zone formed from the uniformity of the stepped and continuous surface structures of ORSPPs. Furthermore, we numerically investigated the physical phenomena on the high-intensity angular momentum transfer from light to matter, considering ORSPP–driven low-order LG vortex laser pulses, by performing 3D particle-in-cell simulations.

Features of the Development of the Small-Scale Self-Focusing in Superpower Femtosecond Lasers

The physical reason for the difference between the small-scale self-focusing of nanosecond and femtosecond pulses is that the typical intensity of the latter is three orders of magnitude higher, i.e., TW/cm2 versus GW/cm2. This causes a significant shift of the growth-rate maximum of the Bespalov–Talanov instability to the region of high spatial frequencies. During free propagation, a decrease in the spectral density of noise and the self-filtering of the beam lead to the noise-density decrease in the region of the maximum growth rate and, therefore, slowing of the self-focusing development. This is shown to shift the restriction on using the transmissive optical elements in the superpower lasers towards high powers.

Full-coloration based on metallic nanostructures through phase discontinuity in Fabry-Perot resonators.

We demonstrate a flexible full-color plate using Fabry-Perot (FP) resonators with two different types of silver nanostructures, a uniform nanofilm and a layer of nanoislands, for transmissive color elements. Two different nanostructures with deep-subwavelength features are selectively generated according to the layer thickness during vacuum deposition with no patterning process. In the nanofilm case, the primary optical mode accountable for generating the color shifts to blue from the original FP resonance while in the nanoislands case, it shifts to red so that a wide spectrum in the visible range is available through the phase discontinuity in the FP resonators. The peaks in the FP resonance shifted toward the opposite directions are attributed to the opposite signs of the phase retardations by a nanofilm and nanoislands. This approach paves a new way of constructing full-color elements for a variety of display devices and image storage systems.

Liquid-Crystal-Mediated Geometric Phase: From Transmissive to Broadband Reflective Planar Optics.

Planar optical elements that can manipulate the multidimensional physical parameters of light efficiently and compactly are highly sought after in modern optics and nanophotonics. In recent years, the geometric phase, induced by the photonic spin-orbit interaction, has attracted extensive attention for planar optics due to its powerful beam shaping capability. The geometric phase can usually be generated via inhomogeneous anisotropic materials, among which liquid crystals (LCs) have been a focus. Their pronounced optical properties and controllable and stimuli-responsive self-assembly behavior introduce new possibilities for LCs beyond traditional panel displays. Recent advances in LC-mediated geometric phase planar optics are briefly reviewed. First, several recently developed photopatterning techniques are presented, enabling the accurate fabrication of complicated LC microstructures. Subsequently, nematic LC-based transmissive planar optical elements and chiral LC-based broadband reflective elements are reviewed systematically. Versatile functionalities are revealed, from conventional beam steering and focusing, to advanced structuring. Combining the geometric phase with structured LC materials offers a satisfactory platform for planar optics with desired functionalities and drastically extends exceptional applications of ordered soft matter. Some prospects on this rapidly advancing field are also provided.

Multi-scale and -contrast sensorless adaptive optics optical coherence tomography.

The roles of the retinal microvasculature and the retinal pigment epithelium (RPE) in maintaining the health and metabolic activity of the retina lend great clinical value to their high-resolution visualization. By integrating polarization diversity detection (PDD) into multi-scale and -contrast sensorless adaptive optics optical coherence tomography (MSC-SAO-OCT), we have developed a novel multi-contrast SAO OCT system for imaging pigment in the RPE as well as flow in the retinal capillaries using OCT angiography (OCTA). Aberration correction was performed based on the image quality using transmissive deformable optical elements. MSC-SAO-OCTA imaging was performed at multiple fields-of-view (FOVs) with adjustable numerical aperture (NA). Retinal flow and RPE structural images for in vivo healthy and pathological human posterior eyes were demonstrated to show clinical feasibility of the system. High-resolution imaging of retinal vasculature at both large and small FOVs, as well as characterization of RPE topology and deformation, enables more sophisticated and concise investigation of retinal pathologies for in vivo human imaging. MSC imaging may permit detection and analysis of even subtle deformations in the RPE layer using a single instrument.

Error correction associated with stray light for Faraday-effect polarimetry system on EAST

The polarimeter-interferometer system with 11 double-pass radial-view measurement chords has the ability to provide electron density and plasma current profiles, making it exceptionally useful in daily operation on the Experiment Advanced Superconducting Tokamak. However, due to limited optical access and intrinsic feedback, the stray lights arising from spurious reflections along the optical path (unwanted reflections from various optical components/mounts and transmissive optical elements such as windows, waveplates, and lenses as well as the detectors) distort the Faraday rotation measurements. Furthermore, the feedback light from the retro-reflector which is used to realize the double-pass configuration makes it even worse. A data processing approach to decrease the stray light influence is reported in this paper. Based on the theoretical model developed, the Faraday rotation angle is extracted by subtracting the deviation term which can be calculated with a simplified model. With this approach, the Faraday rotation oscillation during density ramp-up can be reduced from 2°-5° to 0.5°-1.5°, which reduces the Faraday rotation measurement errors significantly.

Liquid crystal cells with subwavelength metallic gratings for transmissive terahertz elements with electrical tunability

Abstract We report the propagation properties of polarized terahertz (THz) waves in a liquid crystal (LC) cell with subwavelength metallic gratings to produce transmissive THz elements with electrical tunability. One-dimensional metallic gratings, which can be used both as transparent electrodes and linear polarizers in the THz regime, were fabricated on the quartz glass substrates of the LC cell. The substrates with and without the metallic grating showed almost identical transmission spectra for a linearly polarized THz wave whose electric field was parallel to the grating vector. We fabricated a 90 ∘ -twisted nematic LC cell using the two substrates with the metallic grating and examined its transmission spectra in the THz regime. During these measurements, we applied a voltage to the LC via the metallic gratings. We calculated in detail the transmission spectra and its voltage dependence based on the effective medium theory for subwavelength structures, the continuum theory of twisted nematic LCs, and the optical constants of the LC in the THz regime. The observed spectra agreed well with theoretical simulations. These results demonstrate that LC cells with subwavelength metallic gratings are useful as active THz elements in applications such as attenuators and polarization rotators.

White-light hyperbolic Airy beams

Airy beams have gained attention due to their exotic properties of seemingly bending around obstacles, self-healing and being resistant to diffraction. Regular Airy beams are often generated by imposing cubic spatial phase on a Gaussian beam and Fourier transforming the resulting field with a lens. In this paper, we analyze the so-called hyperbolic Airy (HA) beams that are formed behind the cubic phase element, i.e. with no Fourier lens in the setup. We use an ultra-broadband supercontinuum laser source in combination with transmissive (refractive) and reflective cubic phase elements to create white-light HA beams. The resulting beams are sampled with a SEA TADPOLE spatial-spectral interferometer to record the hyperspectral beam profile and reconstruct a three-dimensional spatio-temporal impulse response of the cubic phase elements. We show that nondispersing beams are produced in reflective geometry, while the main lobe of the HA beam created with a refractive phase element suffered from lateral dispersion.