Fabrication Of Diffractive Optical Elements Research Articles

Diffractive optical elements with radial four-level microrelief fabricated by two-photon polymerization

The two-photon polymerization technique is applied for the fabrication of diffractive optical elements (DOE) with a four-level microrelief. These DOEs form longitudinal intensity distribution (axial light segment) with dimensions required for ophthalmological applications.

Fabrication of Diffractive Optical Elements in Polymers by 400-nm Femtosecond Laser Pulses

This report describes fabrication of volume gratings inside various matrix polymers using femtosecond laser irradiation at a wavelength of 400 nm. We investigated the diffraction efficiencies of volume gratings with a period of 10 μm by focusing 400-nm laser pulses with a 0.1-numerical aperture objective lens. We compared the polymers’ diffraction efficiencies at 400 nm and 800 nm. The largest refractive index change was induced in polymers with low density. The large refractive index change was attributed to its volume contraction based on its low density.

Cross-diffractive optical elements for wide angle geometric camera calibration

Diffractive optical elements (DOEs) can generate multiple two-dimensional (2D) diffraction grids that can be used to calibrate cameras for photogrammetry. However, several factors limit the accuracy and the functionality of this technique. One of the most important is the DOE fabrication itself. A large DOE with wide 2D fan-out grids is very difficult and costly to develop. Consequently, the calibration is limited to small aperture cameras and/or limited angles. To overcome these problems, we present a low cost solution. We propose to use two large, commercially available, crossed phase DOEs that generate 15×15 equally spaced dots. As the DOEs are not perfect, the unwanted secondary diffractive orders are used as calibration targets to expand the calibration field of view. We show that the use of the primary and secondary diffractive orders provides a valuable calibration tool for wide angle aerial cameras.

Designing and fabricating diffractive optical elements with a complex profile by interference

We demonstrate a novel (to our knowledge) method for the design and the fabrication of diffractive optical elements (DOEs) with an arbitrary complex phase profile based on interference. The DOEs are designed to modulate the complex light wave by the analytical formulas, and an asymmetric holographic DOE with cubic phase modulation is fabricated by a two-step exposure technique. The desired Airy beams are produced experimentally, which demonstrates the validity of this method. It is a simple approach with a low cost for the design and the fabrication of DOEs with a large area and arbitrary phase distribution.

Material aspects for the diamond machining of submicron optical structures for UV-application

The paper discusses material aspects governing the diamond machining process for submicron structures which serve as diffractive optical elements (DOE). Therefore, first the cutting process for fabrication of DOEs is introduced. The microstructured surface is generated by diamond turning using a nano Fast Tool Servo (nFTS), which enables a variation of the depth of cut up to 350 nm at a maximum frequency of 10 kHz. Such DOEs are typically used with a wavelength of 632 nm. A new application for UV-radiation with a wavelength below 350 nm requires alternative materials with sufficient reflectivity. Cutting experiments and UV-reflectivity tests were performed in order to identify suitable materials. High machining accuracy and UV-reflectivity were achieved with ultrafine grained aluminium as workpiece material.

Lithographic fabrication of diffractive optical elements in hybrid sol-gel glass on 3-D curved surfaces

We demonstrate the lithographic fabrication of diffractive optical elements (DOEs) in hybrid SiO2/TiO2 sol-gel glass on 3-D curved surfaces. The concentric circular gratings with periods of 20 μm, 10 μm and 5 μm have been fabricated in sol-gel glass on concave lens by laser direct writing successfully. Continuous 3-Dimensional surface relief with a height of 435 nm, 110 nm and 50 nm has been obtained for the period of 20 μm, 10 μm and 5 μm respectively. The optical test results of the fabricated DOE shows only a little bit deviation from the theoretical calculated results which can be explained by 3-D curved surface effect. We believe this technology can be an effective method to fabricate DOEs with even more complex surface profile on 3-D curved surfaces in terms of its simplicity and cost-effectiveness.

Fabrication of diffractive optical elements on 3-D curved surfaces by capillary force lithography

We demonstrate the fabrication of diffractive optical elements (DOEs) on 3-Dimensional curved surfaces by capillary force lithography (CFL). Curved gratings with a period of 20mum and 820nm have been successfully fabricated in polymer on concave surfaces by CFL. The experiment results indicate that the capillary force lithography is an effective method to replicate DOEs on curved surfaces with a very high fidelity and a relatively fast speed. In addition, we found that the growth rate of the polymer in the sub-microfabrication is much faster and the step height is much closer to the master than that in the microfabrication for CFL, which makes CFL more attractive in the fabrication of DOEs with a sub-microscale or even nanoscale feature size than a microscale feature size.

Ablation of microstructures applying diffractive elements and UV femtosecond laser pulses

A new method for simple and economic fabrication of diffractive optical elements (DOEs) with three and four phase levels, by UV nanosecond (ns) laser ablation is presented. The technique is based on the combination of two sequentially generated complementary 2-level phase elements. During the fabrication, complete ablative removal of a highly absorbing silicon suboxide layer by pixelated backside illumination ensures the necessary high precision and optical quality. Full functionality of the new DOEs is demonstrated by fabricating micro-structures using UV femtosecond pulses.

Resist evaluation for fabrication of diffractive optical elements (DOEs) with sub-micron resolution in a MEMS production line

Diffractive optical elements (DOEs) represent small, lightweight and potentially low-cost alternatives to conventional optical components. We have evaluated photoresists and processes for fabrication of silicon micro-machined DOEs with a sub-micron pattern using an MA150 (Suss) proximity aligner. The resists HiPR 6512 (Fuji film), AZ ECI 3007 (AZ Electronics Materials), IX335 H (JSR Micro) and UVIII (Rohm and Haas) were all able to resolve the desired 0.8 µm pattern, but the wall angle obtained with IX335H was a superior 86°. Double development of the resists proved possible in a KOH-based developer but unfeasible in a TMAH-based developer. The final DOE device was successfully realized based on the optimized photolithography process.

Spatial-phase-modulation-based study of polyvinyl-alcohol/acrylamide photopolymers in the low spatial frequency range

Photopolymers are appealing materials for the fabrication of diffractive optical elements (DOEs). We evaluate the possibilities of polyvinyl-alcohol/acrylamide-based photopolymers to store diffractive elements with low spatial frequencies. We record gratings with different spatial frequencies in the material and analyze the material behavior measuring the transmitted and the reflected orders as a function of exposition. We study two different compositions for the photopolymer, with and without a cross-linker. The values of diffraction efficiency achieved for both compositions make the material suitable to record DOEs with long spatial periods. Assuming a Fermi-Dirac-function-based profile, we fitted the diffracted intensities (up to the eighth order) to obtain the phase profile of the recorded gratings. This analysis shows that it is possible to achieve a phase shift larger than 2pi rad with steep edges in the periodic phase profile. In the case of the measurements in reflection, we have obtained information dealing with the surface profile, which show that it has a smooth shape with an extremely large phase-modulation depth.

Application of an interferometric phase contrast method to fabricate arbitrary diffractive optical elements

A novel approach for the fabrication of diffractive optical elements is described. This approach is based on an interferometric phase contrast method that transforms a complex object wavefront into an intensity pattern. The resulting intensity pattern is used to expose a photoresist layer on a substrate. After development, a diffractive phase object with an on-axis diffraction pattern is achieved. We show that the interferometric phase contrast method allows a precise control of the resulting intensity pattern. An array of blazed Fresnel lenses is realized in photoresist by using kinoform or detour-phase computer holograms for the interferometric phase contrast setup.

Surface relief diffraction gratings fabricated in ZnSe by frequency doubled Nd:YAG laser micromachining

A novel and rapid laser ablation method for the fabrication of diffractive optical elements (DOE) in ZnSe that takes advantage of the relatively low intensity damage threshold of the material is presented. The structures were characterized in terms of their shape and diffraction efficiency at normal incidence under 10.6μm radiation for TE and TM polarizations. Sample surface polishing as well as the possible effect of the melted zone and structural modification of the material around the ablated region on the power transmission capability of the grating are also discussed.

Fabrication of high aspect ratio nano gratings using SR lithography

In this paper, the fabrication technique for high-aspect-ratio diffractive optical element (DOE) is introduced. The 500 nm-width and 1,000 nm-width line-and-space pattern has been successfully fabricated. It was fabricated by synchrotron radiation (SR) lithography for the application of nano gratings, and poly-methylmethacrylate (PMMA) was used as X-ray resist. The nanoscale grating with the aspect ratio of 4.4 and 2.2 was achieved. So far, there are a number of reported techniques for fabrication of DOEs yet the height of those gratings is not sufficient. Therefore, we have attempted to investigate the fabrication of high-aspect-ratio nano gratings by a high-resolution X-ray lithography using SR source at Ritsumeikan University, Japan. Nevertheless, the evaluation of various factors influencing the high-aspect-ratio structure fabricated by our recommended technique is discussed. So far the fabricating process, such as, proximity gap of exposure, the exposure dosage, and the development time have been optimized to fabricate the gratings.

Simulation of DOE fabrication using DMD-based gray-tone lithography

Digital-multi-micromirror device (DMD)-based photolithography technique is a promising tool for the fabrication of microstructure. In this paper an imaging model was developed for describing its complicated imaging process, and then the fabrication processes of some typical diffractive optical elements (DOEs) were simulated. The simulated results demonstrate that the DMD-based lithography technique allows to realize a wide variety of microoptical elements, e.g., Dammann grating, Fresnel lens, beam shaping element or refractive microstructures.

Design and fabrication of diffractive optical elements by use of gray-scale photolithography

Diffractive optical elements (DOEs) are key components in the miniaturization of optical systems because of their planarity and extreme thinness. We demonstrate the fabrication of DOEs by use of gray-scale photolithography with a high-energy-beam sensitive glass photomask. We obtained DOE lenses with continuous phase profiles as small as 800 microm in diameter and 5.9 microm in the outermost grating pitch by selecting a suitable optical density for each height level and optimizing the process variables. Microlenses patterned with eight levels and replicated by UV embossing with the polymer master mold showed a diffraction efficiency of 81.5%, which was sufficiently high for the devices to be used as optical pickups. The effects of deviations in diffraction efficiency between the DOE height and profile design were analyzed.

Holographic patterning of acrylamide-based photopolymer surface

The patterning of an acrylamide-based photopolymer surface by holographic recording is studied. The patterns are induced by light alone and no post-processing is required. Periodic surface modulation is observed in addition to a volume phase grating. An investigation has been carried out using white light interferometry into the dependence of the amplitude of the photo induced surface relief modulation on the spatial frequency, intensity of recording and sample thickness. The observed dependencies indicate that the diffusion of material during the holographic recording plays a major role in surface relief formation. The possibility for inscription of surface relief patterns opens the door to at least two new applications for this photopolymer: fabrication of diffractive optical elements and biosensors.

Characterization of the gray-scale photolithography with high-resolution gray steps for the precise fabrication of diffractive optics

To fabricate a diffractive optical element (DOE) with optimum surface profile, we characterize the relation between the developed height profile on the photoresist and the optical density (OD) value of a high-energy-beam-sensitive (HEBS) gray-scale photomask with high-resolution gray steps. The lithography characteristics of a gray step less than 10 µm are inspected in detail. The gray patterns with various gray-step widths are fabricated for the investigation of the relation between the OD values and the height profiles on the specific photoresist. We demonstrate the dependence of the photoresist height profiles on the width of the gray step, and this dependence agrees well with the simulation results from our model. We provide a proper method to design the OD value of a gray-scale photomask for the fabrication of DOEs with precise surface profiles.

Design of diffractive optical elements for high-power laser applications

Diffractive optical technology is rapidly becoming an indispensable tool for the creation of high-efficiency optical beam-shaping devices within a wide range of industrial and research activities. The high damage thresholds of typical materials used in diffractive optical element fabrication make them highly suited for deployment in high-power laser systems. We present two high-power applications of diffractive optical elements designed using an ultrahigh fidelity, ultrahigh efficiency design technique and demonstrate the advantages, in terms of optical alignment, that can be achieved by combining the diffractive structures with a multilevel diffractive lens.

Fabrication of Diffractive Optical Elements in Silica Glass by Femtosecond Laser

Fabrication of Diffractive Optical Elements in Silica Glass by Femtosecond Laser

Selective etching of chalcogenides and its application for fabrication of diffractive optical elements

Bulk samples of As and Ge based chalcogenides were synthesized from high purity elements, and thin layers (thickness d≈100–5000 nm) were deposited by vacuum thermal evaporation and/or spin coating. Photoinduced structural changes were studied by Raman and IR spectroscopy. The kinetics of dissolution of thin layers in aqueous and non-aqueous alkaline solvents were studied. Both, positive and negative selective etching behaviours were found and used for production of diffractive optical elements (DOEs). Exposure with a halogen lamp through a Cr mask, classical holographical exposure using an Ar+ laser, and direct writing laser lithography using a He–Cd laser were used to produce amplitude/phase DOEs with feature sizes of 0.37–4 μm. Prepared low efficiency and low lifetime amplitude/phase type DOEs were transformed into corrugated relief type phase DOEs by subsequent selective etching. Positive and/or negative copies of the pattern were obtained depending on the thin layer composition, its prehistory and the applied etching bath. The profiles of the structures produced by etching were subsequently investigated using AFM and REM techniques and basic performance parameters of formed DOE were measured and compared with theoretically calculated values.