Alexander G. Poleshchuk

Processing parameter optimization for thermochemical writing of DOEs on chromium films

Computer-generated holograms are limited by conventional lithographic fabrication capabilities which rely on accurate deposition, exposure, and developing of photosensitive chemicals. We present an alternate fabrication technology that uses a focused laser beam to write patterns by inducing a thermochemical change in a bare metal film. The patterns are developed using a single etching step that dissolves the non- exposed metal. The thermochemical writing method allows holograms to be directly written onto large-diameter, thick, and non-flat substrates, requiring no intermediate steps that compromise the ultimate accuracy. Circular patterns for optical testing were written using a polar-coordinate laser writer. The laser power and control requirements are shown to be modest and the etching is shown to be tolerant of temperature and concentration variations. The technology is demonstrated with the fabrication of CGHs up to 136 mm in diameter used for optical testing.

Fabrication of gray-scale masks and diffractive optical elements with LDW glass

In the last years the application of gray-scale masks (GSM) for diffractive optics manufacturing attracts attention because of cost-effective possibility to produce a lot of diffractive elements on hard and heat-resistant thermally stable substrates. Direct laser writing of GSMs and fabrication of diffractive optical elements are effectively realized with application of LDW-glass (material for Laser Direct Write from CANYON MATERIALS, Inc). An important advantage of this material is the real-time change of transmittance in a single-step process without liquid development. It is shown that optimal transmittance range in which track width is not more than 1 micrometers is from 5-10% (transmittance of unexposed area) to 60-65% for LDW-glass type I having thinner colored layer. Power modulation and surroundings dependent peculiarities of direct laser writing on LDW-glass are discussed. Results of fabrication of diffractive optical elements using LDW-glass masks are presented. Among several types of LDW glasses studied the advantages of new GS-11 glass are elaborated. Application of GS-11 glass for GSMs allowed to fabricate blazed diffractive structures with backward slope width of 0.8 micrometers .

Polar coordinate laser writing systems: Error analysis of fabricated DOEs

Diffractive optics is a field where the progress is defined by fabrication technology. Diffractive optical elements (DOEs) are generally planar structures, typically fabricated using X-Y image generators designed for semiconductor industry. However there are some kinds of DOEs for which the polar scanning geometry, where the optic rotates under a writing beam, is more preferable. In some cases polar coordinate machines provide the only practical method of fabricating DOEs with the required accuracy. It is necessary to take into account the DOE specification when choosing the fabrication method. The present paper considers peculiarities of polar coordinate laser systems for large size and high precision DOEs fabrication. The specific error sources for these systems are described and compared with those of X-Y systems. An optimal writing strategy is discussed. The wavefront aberrations of rotationally symmetric DOEs caused by fabrication errors were measured interferometrically. Different types of aberrations were identified and can be referred to certain writing errors. Interferometric measurements of the wavefront errors for binary zone plates with a 64 mm diameter and 0.45 numerical aperture have shown that the wavefront root-mean-square error does not exceed 0.009 (lambda) wavelength.

Efficient testing of segmented aspherical mirrors by use of a reference plate and computer-generated holograms. II. Case study, error analysis, and experimental validation

Segmented mirrors present unique challenges to fabrication and testing that are absent for monolithic optics. Since traditional asphere tests do not address segmented optics adequately, we validate a previously developed method to test large quantities of segments accurately, quickly, and economically. In this test, the aspheric shape of each segment is controlled to high accuracy by use of computer-generated holograms, and the radius of curvature is tightly controlled by use of the reference plate. In an adjoining paper [Appl Opt 43, 5303 (2004)] we developed the theory for this test, and now we present a complete system design and optimization for measuring the 1.4-m segments from a 30-m F/1 primary. A complete tolerance analysis predicts a test accuracy of 4.8-nm rms surface and excellent accuracy for controlling the geometry of the segment. In addition, a laboratory demonstration using 30-cm optics is presented that demonstrated 3.9-nm rms surface accuracy.

Accuracy potential of circular laser writing of DOEs

The circular laser writing system for writing of arbitrary patterns of diffractive elements in chromium films and photoresists has been developed as an alternative method for laser or e-beam writing in x-y system of coordinates. This system is able to generate binary amplitude and continuous- tone patterns with features of less than 1 micrometers and accuracy of 0.1 micrometers on substrates up to 300 mm diameter. Construction of the circular laser writing system (CLWS) designed at the IA&E and writing strategies such as laser beam addressing in polar coordinate system are described. This article presents the overview and analysis of writing errors. The characteristics of zone plates and linear gratings fabrication and methods of errors measurement are given. The measurements of wavefront errors of binary f/1.1 zone plates and linear polar system gratings with period of less than 5 micrometers fabricated by CLWS demonstrate the high quality of writing strategy.

Fabrication of diffractive optical elements by direct laser-writing with circular scanning

A new circular laser writing system for fabrication of computer generated holograms is described. The results of diffractive element synthesis without photoresists usage are presented.

Application of gray-scale LDW-glass masks for fabrication of high-efficiency DOEs

The perspective opportunity to fabricate gray-scale masks was given LDW-glasses (LDW--Laser Direct Writing) from Canyon Materials, Inc. LDW-glass blanks contain a large number density of coloring specks of silver in a surface glass layer. A focused laser beam is used to heat erase these coloring specks. Experiments on the influence of laser radiation on LDW-glasses was carried out using a circular laser writing system. The transmittance value from the blank of 0.1 - 5% up to 70 - 80% depending on laser beam power is obtained with the current write scheme. Results of research of LDW-glasses behavior in a wide range of laser beam scanning speeds are described. Laser pattern generation in LDW-glass including the spatial resolution is discussed. The technology of fabrication of continuous-phase diffractive elements was tested by making exemplary Fresnel lenses. Total 80% efficiency for quartz Fresnel lenses with minimal zone width of 8 micrometers was readily achieved in the preliminary experiments.

Methods for certification of CGH fabrication

A method for certifying the direct writing process for computer generated holograms is offered and experimentally validated. This method effectively reveals both random errors and slow drift during writing process. The techniques are demonstrated with a 64 mm diameter, f/1 binary zone plate.

Requirements and approaches to adapting laser writers for fabrication of gray-scale masks

The photolithography using gray-scale masks (GSM) with multilevel transmittance is now one of promising ways for manufacturing of high efficiency diffractive optical elements and microoptics. Such masks can be most effectively fabricated by laser or electron-beam writers on materials with a transmittance changing under influence of high-energy beams. The basic requirements for adaptation of existing and developed scanning laser writers are formulated. These systems create an image by continuous movement of a writing beam along one coordinate and overlapping of adjacent written tracks along another coordinate. Several problems must be solved at the GSM manufacturing: the calibration of the influence of the laser beam on a recording material without transferring the gray-scale structure into photoresist; the transmittance at the current exposed pixel depends on surrounding structures generated before recording of the current track and a character of the laser beam power modulation; essential increasing of the computed data in comparison with binary elements. The offered solutions are based on the results of investigations of the materials with variable transmittance (LDW-glass, a-Si film) and takes into account the specificity of diffractive blazed microstructures. The reduction of data amount for fabrication of multi-level DOEs is effectively performed using offered vector-gradient data format, which is based on piecewise-linear approximation of phase profile. The presented approaches to adaptation of laser writers are realized in software and hardware, and they allow to solve the basic problems of manufacturing GSMs.

New application for x-ray lithography: fabrication of blazed diffractive optical elements with a deep-phase profile

The use of the X-ray lithography to produce blazed diffractive optical elements (DOEs) is described. The proposed method allows one to make highly efficient blazed DOE with a deep phase profile (ten wavelengths and more) using a single X-ray mask with a binary transmission pattern. Unlike the well-known multilevel DOEs, blazed ones do not involve fabrication and aligning of a set of masks. DOEs with a profile depth of 10 micrometers and more and zone sizes of down to 1 micrometers can be obtained due to the short wavelength and high penetrability of X- rays. The first experimental samples of blazed DOEs with a 10 micrometers -height profile (lenses and gratings) were fabricated by X-ray lithography with synchrotron radiation using the X-ray masks, prepared in accordance with the pulse-width modulation algorithm. Diffraction efficiency for lenses was measured for white light. It is higher than 80 percent for the central part of the lenses (inside a 10 mm diameter) and about 60 percent for an area of 20 mm diameter.