Diffractive Optical Elements Research Articles

Holographic Display System to Suppress Speckle Noise Based on Beam Shaping

In this paper, a holographic system to suppress the speckle noise is proposed. Two spatial light modulators (SLMs) are used in the system, one of which is used for beam shaping, and the other is used for reproducing the image. By calculating the effective viewing angle of the reconstructed image, the effective hologram and the effective region of the SLM are calculated accordingly. Then, the size of the diffractive optical element (DOE) is calculated accordingly. The dynamic DOEs and effective hologram are loaded on the effective regions of the two SLMs, respectively, while the wasted areas of the two SLMs are performed with zero-padded operations. When the laser passes through the first SLM, the light can be modulated by the effective DOEs. When the modulated beam illuminates the second SLM which is loaded with the effective hologram, the image is reconstructed with better quality and lower speckle noise. Moreover, the calculation time of the hologram is reduced. Experiments indicate the validity of the proposed system.

High-harmonic diffractive lens color compensation.

Large diameter, high-harmonic diffractive lenses could find applications in future space telescopes. Residual chromatic aberrations from these lenses can cause significant blurring. Solutions to reduce chromatic dispersion and other aberrations to diffraction-limited performance are discussed. A design example based on a 240-mm-diameter, 1-m focal length multi-order diffractive engineered lens operating over the astronomical R-Band (589-727 nm) is presented. The design example uses a relay subsystem with four times smaller diameter than the primary. This color corrector includes both refractive and diffractive optical elements and reduces the longitudinal chromatic aberrations by more than a factor of 30 compared to the primary lens alone, while maintaining the effective focal length and numerical aperture of the system.

Laser lift-off of polyimide thin-film from glass carrier using DPSS laser pulses of top-hat square profiles

Laser lift-off (LLO) process using a diode pumped solid state (DPSS) laser with wavelength of 355 nm and pulse width of 37 ns was investigated experimentally and theoretically for non-contact delamination of the polyimide (PI) substrate from the glass carrier. Each pulse emitted from the DPSS laser was shaped into a top-hat square beam by an optical system including a diffractive optical element lens and a spatial filter. The typical FWHM size of the top-hat beam was 315μm×328μm. The train of pulses was irradiated on the specimen in the form of a quasi-line beam through a scanner system for two representative pulse repetition rates (PRR) of 1 and 150 kHz. Experimental observations and theoretical simulations using the photothermal ablation model clarified that the intensive laser pulse induced extremely fast ablation inside the PI substrate, which caused the PI to delaminate internally along the PI/glass interface, leaving a residual PI layer of about 100 nm in thickness on the glass carrier. The successful non-overlap LLO by each pulse was confirmed optically by the Fizeau fringes at the threshold fluence of 240 mJ/cm2. The overlapping irradiation of laser pulses, which minimized overlay or stitch errors, was able to lower the threshold fluence of each pulse by about 50% compared to the non-overlapping irradiation by a single pulse. With increasing PRR for the same number of irradiation overlaps, the threshold fluence was reduced considerably due to the thermal accumulation effect, but the thickness of the residual PI layer remaining on the glass carrier increased slightly.

3D printable diffractive optical elements by liquid immersion

Diffractive optical elements (DOEs) are used to shape the wavefront of incident light. This can be used to generate practically any pattern of interest, albeit with varying efficiency. A fundamental challenge associated with DOEs comes from the nanoscale-precision requirements for their fabrication. Here we demonstrate a method to controllably scale up the relevant feature dimensions of a device from tens-of-nanometers to tens-of-microns by immersing the DOEs in a near-index-matched solution. This makes it possible to utilize modern 3D-printing technologies for fabrication, thereby significantly simplifying the production of DOEs and decreasing costs by orders of magnitude, without hindering performance. We demonstrate the tunability of our design for varying experimental conditions, and the suitability of this approach to ultrasensitive applications by localizing the 3D positions of single molecules in cells using our microscale fabricated optical element to modify the point-spread-function (PSF) of a microscope.

The effect of laser beam intensity distribution on weld characteristics in laser welded aluminum alloy (AA5052)

The distribution of laser beam intensity can play a key role in laser welding, since it is deeply related to the temperature gradient and cooling rate of the melt pool. In this study, aluminum alloy butt welding was conducted using a normal Gaussian beam and modulated beams with various intensity profiles, produced with a diffractive optical element (DOE). Their effect on the weld characteristics was investigated. Welding with the modulated beams generally required larger heat input for full penetration compared with the Gaussian beam welding (GBW) since laser energy was spread by the core and ring beam combination. This laser energy spreading reduced weld defects and produced a funnel-shaped weld cross-section in the modulated beam welding (MBW). Microstructure analyses found that the GBW and MBW had different grain growth behaviors based on the shape of the melt zone boundary. A straight melt zone boundary formed along the vertical direction induced columnar grain growth along the horizontal direction in the GBW. For the MBW, it grew along the diagonal direction, because the melt zone boundary had a slanted shape. The highest tensile strength (208.2 MPa, which corresponded to ~ 91% of the tensile strength of the base material) was obtained with the MBW, owing to reduced average grain size. It was found that the re-crystallization process induced by laser welding significantly increased the grain size of the original material.

APPLICATION OF DIFFRACTIVE OPTICAL ELEMENTS FOR OPTICAL-ELECTRONIC DIMENSIONAL INSPECTION SYSTEMS

The work is devoted to the application of diffractive optical elements in systems using the structured illumination method to the geometric parameters inspection of industrial articles. The objects of inspection in these systems are: weapon barrels, fuel pellets, fuel elements, spacer grids, ceramic ring insulators. The used diffractive optical elements are computer-synthesized holograms that focus laser radiation into geometric shapes, the configuration of which is optimally combined with the inspected objects shape.

Centimeter-scale low-damage micromachining on single-crystal 4H–SiC substrates using a femtosecond laser with square-shaped Flat-Top focus spots

Femtosecond (fs) lasers have been proved to be reliable tools for high-precision and high-quality micromachining of ceramic materials. Nevertheless, fs laser processing using a single-mode beam with a Gaussian intensity distribution is difficult to obtain large-area flat and uniform processed surfaces. In this study, we utilize a customized diffractive optical element (DOE) to redistribute the laser pulse energy from Gaussian to square-shaped Flat-Top profile to realize centimeter-scale low-damage micromachining on single-crystal 4H–SiC substrates. We systematically investigated the effects of processing parameters on the changes in surface morphology and composition, and an optimal processing strategy was provided. Mechanisms of the formation of surface nanoparticles and the removal of surface micro-burrs were discussed. We also examined the distribution of subsurface defects caused by fs laser processing by removing a thin surface layer with a certain depth through chemical mechanical polishing (CMP). Our results show that laser-induced periodic surface structures (LIPSSs) covered by fine SiO2 nanoparticles form on the fs laser-processed areas. Under optimal parameters, the redeposition of SiO2 nanoparticles can be minimized, and the surface roughness Sa of processed areas reaches 120 ± 8 nm after the removal of a 10 μm thick surface layer. After the laser processing, micro-burrs on original surfaces are effectively removed, and thus the average profile roughness Rz of 2 mm long surface profiles decreases from 920 ± 120 nm to 286 ± 90 nm. No visible micro-pits can be found after removing ~1 μm thick surface layer from the laser-processed substrates.

Kerr lens effect induced by a super-Gaussian laser beam

Recently, a new type of laser cavity has been proposed. It includes two Diffractive Optical Elements set inside the resonator against the cavity mirrors in order to achieve the structuring of the fundamental mode. The latter is a perfect Gaussian beam (M2 = 1) from the output mirror side, and super-Gaussian in the amplifying medium. Such a special cavity allows for improving the energy extraction, and it should be interesting to mode-lock such a laser cavity by using the KLM technique. For that, we aim to model the Kerr lensing effect, induced by a super-Gaussian beam. An analytical expression for the Kerr focal length has been deduced from a Zernike polynomial development. The obtained results are in a good agreement with a diffraction analysis.

APPLICATION OF OPTICAL DIFFRACTOMETRY FOR THE ERROR MONITORING OF MANUFACTURING OF PRECISION OPTICAL ELEMENTS

The work is devoted to an experimental study of the effectiveness of the method for measuring errors in manufacturing of computer-generated synthesized holograms using specialized marks. Each mark represents two aligned diffraction gratings with the same duty cycle and period, the first of which is formed before writing, and the second is formed in the process of forming the structure of the hologram. To determine the writing error, the method of optical diffractometry was used. The results of an experimental study of dependence of diffraction efficiency of the mark on the mutual displacement of parts of the microgratings are presented. The error manufacturing of a synthesized hologram arising during long-term writing of a diffractive optical element has been simulated experimentally.

Silicon diffractive optical element with piecewise continuous profile to focus high-power terahertz radiation into a square area

A technique of high-frequency laser ablation using a 2D beam scanner has been designed and applied to creation of a silicon diffractive optical element (DOE) with a continuous profile to focus a terahertz Gaussian beam into a square region. The microrelief of the resulting silicon DOE has been analyzed with a white-light interferometer and a scanning electron microscope. The distribution of radiation behind the DOE illuminated by a high-power beam of the Novosibirsk free-electron laser at a wavelength of 130 µm, recorded with a pyroelectric camera, has been demonstrated. The measured diffraction efficiency of the DOE is equal to 97 ± 2 % .

High-dynamic-resolution optical edge detection based on liquid crystal diffractive moiré lenses with a tunable focal length.

In this Letter, we experimentally demonstrate liquid crystal-based moiré lenses with a wide and tunable focal length by direct-writing photoalignment. Our moiré lenses, which consist of two cascaded diffractive optical elements, have a large range of refractive power between $\pm 0.85\;{{\rm m}^{- 1}}$ at 532 nm and a mutual rotation between $\pm 90^\circ$ with high diffraction efficiency ($\gt\!{75}\%$). Based on the as-designed moiré lenses, we propose high-dynamic-resolution optical edge detection without any axial shift or substitution of components. The minimum edge width is 13.2 µm and can be adjusted within 100 µm by mutual rotation of this device, which has great potential to be used in adaptive and compact optical systems.

All diffractive optical element setup for creating and characterizing optical vortices with high topological charges: Analytical models and numerical results

We introduce the concept of an all-diffractive optical element (DOE) setup for generating and characterizing optical vortices (OVs), and present analytical models and numerical results by using two-dimensional chirp z-transform (2-D CZT) algorithm. Such an all-DOE setup consists of a spiral zone plate (SZP) to generate focused OVs with topological charge (TC) up to 50, and a gradually changing-period grating (GCPG) to sensitively measure orbital angular momentum. Especially, 2-D CZT based Fresnel diffraction integral algorithm is derived for scalar wave propagation with different sampling rates on DOE and observation planes, respectively. Due to the broad applicability, flexible aperture matching can be easily realized in the all-DOE setup. The numerical comparisons show that 2-D CZT based FDI, direct integration and fast Fourier transform based methods are equivalent expressions. The computation speed of 2-D CZT based FDI is 3 orders faster than fast Fourier transform based method and 5 orders faster than direct integration method, respectively. Both the SZP and the GCPG are fabricated by standard lithography technology. The characteristic intensity distribution of the focused OVs passing through the GCPG is observed, agreeing well with the numerical simulation results. Our results provide an easy and convenient way to create all-DOE setup in the production and measurement of OVs with high TC.

Local Polarization Reversal by Ion Beam Irradiation in SBN Single Crystals Covered by Dielectric Layer.

Formation of the domain structure by ion beam irradiation was studied in thermally depolarized Ce-doped strontium barium niobate single crystals covered by a dielectric layer. Three types of irradiation regimes were used: dot exposure, stripe exposure, and line exposure. The dependences of the domain size and depth on the irradiated dose were measured. The circular shape of the isolated domains with partially switched broad domain boundary was obtained. Isotropic domain growth was attributed to the step generation at the wall by merging with the residual nanodomains that appeared after thermal depolarization. The obtained linear dose dependence of the switched area was attributed to the screening of the depolarization field by the injected charge. The shape distortion of the domains growing in the neighborhood with already created ones was attributed to the electrostatic interaction of the approaching charged domain walls. The obtained results can be applied for the creation of precise domain patterns with arbitrary orientation and shape to produce nonlinear optical devices with improved characteristics, including electrically tunable diffractive optical elements.

Geometric and physical configurations of meta‐atoms for advanced metasurface holography

AbstractMetasurfaces consisting of subwavelength structures, so‐called meta‐atoms, have steadily attracted considerable attention for advanced holography due to their advantages in terms of high‐resolution holographic images, large field of view, and compact device volume. In contrast to conventional holographic displays using bulky conventional diffractive optical elements, metasurface holography enables arbitrary complex wavefront shaping with a much smaller footprint. In this review, we classify metasurface holography according to the meta‐atom design methodologies, which can further expand hologram functionalities. We describe light‐matter interactions, particularly in metasurface systems, using the relevant the Jones matrix to rigorously explain modulations of the amplitude, phase, and polarization of light. Six different types of meta‐atoms are presented, and the corresponding achievable wavefronts that form the holographic images in the far‐field are also provided. Such a simple classification will give a straightforward approach to design and further realize advanced metasurface holographic devices.image

Inverse Designed THz Spectral Splitters

This letter reports proof-of-principle demonstration of 3-D printable, low-cost, and compact terahertz (THz) spectral splitters based on diffractive optical elements (DOEs) designed to disperse incident collimated broadband THz radiation (0.5–0.7 THz) at a prespecified distance. Via inverse design, we show that it is possible to design a diffractive optic that can split broadband incident spectrum in any desired fashion, as is evidenced from both finite-difference time domain (FDTD) simulations and measured intensity profiles using a 500–750-GHz vector network analyzer (VNA). Due to its straightforward as well as simple construction without the usage of movable parts, our approach, in principle, can have various applications such as in portable, low-cost spectroscopy as well as in wireless THz communication systems as a THz demultiplexer.

Laser beam oscillation welding of aluminum alloy using the spatially modulated beam by diffractive optical element (DOE)

In laser welding, the spatial beam intensity profile of the laser beam is an important parameter as it has a considerable effect on the temperature distribution and cooling rate of the melt pool, which determine the weld properties. In this study, a laser beam with a spatially modulated intensity profile by a diffractive optical element (DOE) was applied to the laser beam oscillation welding of an aluminum alloy (AA5052), and the weld characteristics of modulated beam welding (MBW) were analyzed in relation to those of Gaussian beam welding (GBW). According to the result of experiment verifying full penetration condition, the MBW generally required a slower maximum beam speed for full penetration than the GBW due to laser energy spreading by the core and ring beam constitution. On the other hand, this dispersed laser energy of the modulated beam was more effective in formation of a stable melt pool, which in turn reduced weld defects (bumpy bead surface, pores, and cracks) compared with the GBW. It also played a role in improving the asymmetric morphology of the weld by generating a relatively even temperature distribution of the melt pool. From an electron back scatter diffraction (EBSD) analysis, it was found that columnar grains were mostly distributed at the edge area of the weld, and equiaxed grains were largely distributed at the center area of the weld. The asymmetric feature in the microstructure was also found because of the asymmetric beam oscillation pattern and the resultant cooling rate difference. It was found that the area solidified at the lower cooling rate exhibited relatively large columnar grains. For the MBW, the asymmetric feature in the proportion of the low and high angle grain boundaries within the weld zone was diminished, which resulted from the relatively uniform temperature distribution by the dispersed laser energy. Due to the slow cooling rate, the MBW sample could have larger grains than GBW; this, however, slightly reduced the tensile strength of the joint welded by the modulated beam. The highest tensile strength obtained from the GBW and the MBW sample was equivalent to 88 % and 83.4 % of the base material’s tensile strength, respectively. It was found that the hardness value and its distribution across the weld zone was similar for the GBW and MBW samples. It was also found that the weld zone has a lower hardness value than base material owing to the increase of grain size through recrystallization.

Photo‐Transformable Gratings for Augmented Reality

AbstractThe seamless integration of multiple diffractive optical elements (DOEs) into a single‐level substrate has been the key for the advancements in augmented reality (AR) technologies. Presently, serial‐type lithographic manipulations such as electron‐beam lithography can successfully provide a deterministic route toward this end. The large‐scale utilization of these processes, however, is impractical owing to their high cost. The holographic inscription of photochromic materials can offer an etching‐free and wide‐scale strategy for the cost‐effective development of large‐area 1D DOEs. Given the processing time, the accessible DOE area from this holographic inscription is more than several orders of magnitude larger (108–1010 µm2 h−1) than that obtained through the aforementioned lithography (102–105 µm2 h−1). However, erasing, which is believed to be prevalent in the multiscale integration of various DOEs, deteriorates the pattern integrity and throughput. Here, it is proposed that the technical conundrum resulting from erasing and re‐inscribing the DOEs can be effectively circumvented by transforming the already formed DOEs without erasing. Centimeter‐scale mosaic DOEs can be readily developed within an hour, which in turn facilitate advanced AR demonstrations. This approach leverages the ability of holographic inscription to seamlessly integrate the wide‐scale multiple DOEs as well as their utility as a go‐to tool for AR technologies.

Aperiodic biomimetic Vogel spirals as diffractive optical elements for tailored light distribution in functional polymer layers

The biomimetic, deterministic aperiodic golden angle Vogel spiral lacks both translational and rotational symmetry and exhibits circularly symmetric scattering rings in Fourier space. We present a versatile and low cost method for maskless fabrication of a Vogel spiral relief grating by illumination of a photoresist with laser light complexly structured using a spatial light modulator and subsequent transfer of its negative into a polymer layer via replica molding. We investigate its diffractive properties and present as one application its implementation as a surface structure, leading to an efficiency enhancement of 18% in an organic solar cell.

Flat‐Top Pumped Multi‐Millijoule Mid‐Infrared Parametric Chirped‐Pulse Amplifier at 10 kHz Repetition Rate

AbstractThe advancement of mid‐infrared (MIR) parametric amplification technique serves as the main driving force of multiple strong‐field experiments in the frontier of science such as the generation of coherent soft X‐ray and isolated attosecond pulses. However, the low parametric down‐conversion efficiency especially in the MIR wavelength is a major bottleneck that hinders the further exploration of the MIR‐laser‐driven strong‐field physics. Here, a simple and robust method that doubles the efficiency of a MIR optical parametric chirped‐pulse amplifier (OPCPA) is reported, as a demonstration of the concept, through engineering a flat‐top pump profile via high‐efficiency diffractive optical elements. With the proposed flat‐top pump scheme, a MIR OPCPA system at 3 μm and 10 kHz repetition rate with both the record pulse energy of 2.7 mJ and the highest average power at 27 W among the MIR OPCPAs with kHz repetition rate is demonstrated. This work provides a promising approach for achieving high energy high power MIR few‐cycle lasers that can be readily used in emerging strong field experiments. In addition, it is believed that the developed technique in this work can become a general method of parametric efficiency enhancement, which is applicable to other nonlinear conversions and wavelength ranges.

Spatial-Light-Modulator-Based Multichannel Data Transmission by Vortex Beams of Various Orders.

We report an atmospheric multichannel data transmission system with channel separation by vortex beams of various orders, including half-integer values. For the demultiplexing of the communication channels, a multichannel diffractive optical element (DOE) is proposed, being matched with the used vortex beams. The considered approach may be realized without digital processing of the output images, but only based on the numbers of informative diffraction orders, similar to sorting. The system is implemented based on two spatial light modulators (SLMs), one of which forms a multiplexed signal on the transmitting side, and the other implements a multichannel DOE for separating the vortex beams on the receiving side. The stability of the communication channel to atmospheric interference and the crosstalk between the channels are investigated.