We report here on the theoretical performance of blazed binary diffractive elements composed of pillars carefully arranged on a two-dimensional grid whose period is smaller than the structural cutoff. These diffractive elements operate under unpolarized light. For a given grating geometry, the structural cutoff is a period value above which the grating no longer behaves like a homogeneous thin film. Because the grid period is smaller than this value, effective-medium theories can be fully exploited for the design, and straightforward procedures are obtained. The theoretical performance of the blazed binary elements is investigated through electromagnetic theories. It is found that these elements substantially outperform standard blazed echelette diffractive elements in the resonance domain. The increase in efficiency is explained by a decrease of the shadowing effect and by an unexpected sampling effect. The theoretical analysis is confirmed by experimental evidence obtained for a 3λ-period prismlike grating operating at 633 nm and for a 20°-off-axis diffractive lens operating at 860 nm.

/pdf/design-and-fabrication-of-blazed-binary-diffractive-elements-558xsgkst7.pdf

Design and fabrication of blazed binary diffractive elements with sampling periods smaller than the structural cutoff

An extensively modified laser-lithography system specially developed for realization of micro-optical profiles on non-planar surfaces is presented. This extended system offers new possibilities of fabricating micro-optical elements without the technology related restriction of surface shape that existed so far. A diffractive lens on a convex spherical substrate is designed and fabricated as an example for hybrid achromatic refractive-diffractive elements to demonstrate the functionality of the system and the wide range of possible new applications.

Laser-lithography on non-planar surfaces.

The fabrication of continuous-relief diffractive optical elements by direct laser beam writing in photoresist is analyzed. The main limitation and tolerances are identified, and their influence on optical performance is quantified. Fabricated structures show rounded profile steps resulting from the convolution of the desired profile with the writing beam. This leads to a reduction in diffraction efficiency. Optimization techniques are presented to minimize this effect. Scaling the profile depth by a factor of μ > 1 increases the first-order diffraction efficiency for blazed elements. This method is also applied to suppress the zeroth diffraction order in computer-generated holograms. A nonlinear compensation of the exposure data for the Gaussian beam convolution results in an 18% increase of the diffraction efficiency for a blazed grating with a 10-μm period to a value of 79%.

Analysis and optimization of fabrication of continuous-relief diffractive optical elements

We describe the advantages of using diffractive (Fresnel) lenses on thin membranes over conventional optics for, among others, future space telescope projects. Fabrication methods are presented for lenses on two types of freestanding membrane up to 50 cm in size. The first is a Fresnel lens etched into a thin (380-microm) glass sheet, and the second is an approximately 50-microm-thick polymer membrane containing a Fresnel lens made by replication process from a specially made fused-silica master. We show optical performance analysis of all the lenses that are fabricated, including a diffraction-limited Airy spot from a 20-m- focal-length membrane lens in a diffractive telescope system.

Fabrication of large-aperture lightweight diffractive lenses for use in space

Polymer optics have gained increasing importance in recent years. With advancing requirements for the optical components, the fabrication process remains a challenge. In particular, the fabrication of the mold inserts for the replication process is crucial for obtaining high-quality optical components. This review focuses on fabrication technologies for optical mold inserts. Thereby, two main types of technologies can be distinguished: fabrication methods to create mold inserts with optical surface quality and methods to create optical microstructures. Since optical mold inserts usually require outstanding form accuracies and surface qualities, a focus is placed on these factors. This review aims to give an overview of available methods as well as support the selection process when a fabrication technology is needed for a defined application. Furthermore, references are given to detailed descriptions of each technology if a deeper understanding of the processes is required.

https://www.mdpi.com/2072-666X/10/4/233/pdf

Review on Fabrication Technologies for Optical Mold Inserts.

High-fidelity diffractive surfaces have been generated with single-point diamond-turning techniques. A key to the success of this technique is the ability to shape the diamond tool tip to provide the optimum phase-relief profile, given manufacturing constraints. Replication technology is used to transfer the phase-relief surface into a thin epoxy or photopolymer layer on a glass substrate. Diffraction efficiency results for a wide range of zone widths are presented to provide the reader with a baseline of expected performance for replicated visible and near-infrared diffractive optical elements. In addition, a new method for analyzing diffractive surface structures is presented. The ray-trace algorithm quickly provides accurate results of predicted diffraction efficiency for arbitrary zone profiles, which is extremely valuable in predicting manufacturing errors.

Single-point diamond turning and replication of visible and near-infrared diffractive optical elements

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Single-point diamond turning and replication of visible and near-infrared diffractive optical elements