Victor Pavlovich Korolkov

Polar coordinate laser pattern generator for fabrication of diffractive optical elements with arbitrary structure

A precision laser pattern generator for writing arbitrary diffractive elements was developed as an alternative to Cartesian coordinate laser/electron-beam writers. This system allows for the fabrication of concentric continuous-relief and arbitrary binary patterns with minimum feature sizes of less than 0.6 microm and position accuracy of 0.1 microm over 300-mm substrates. Two resistless technologies of writing on chromium and on amorphous silicon films were developed and implemented. We investigated limit characteristics by writing special test structures. A 58-mm f/1.1 zone plate written directly is demonstrated at a lambda/50 rms wave-front error corresponding to a 0.06-microm pattern accuracy. Several examples of fabricated diffractive elements are presented.

Zone-boundary optimization for direct laser writing of continuous-relief diffractive optical elements

Enhancing the diffraction efficiency of continuous-relief diffractive optical elements fabricated by direct laser writing is discussed. A new method of zone-boundary optimization is proposed to correct exposure data only in narrow areas along the boundaries of diffractive zones. The optimization decreases the loss of diffraction efficiency related to convolution of a desired phase profile with a writing-beam intensity distribution. A simplified stepped transition function that describes optimized exposure data near zone boundaries can be made universal for a wide range of zone periods. The approach permits a similar increase in the diffraction efficiency as an individual-pixel optimization but with fewer computation efforts. Computer simulations demonstrated that the zone-boundary optimization for a 6 microm period grating increases the efficiency by 7% and 14.5% for 0.6 microm and 1.65 microm writing-spot diameters, respectively. The diffraction efficiency of as much as 65%-90% for 4-10 microm zone periods was obtained experimentally with this method.

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.

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.

Alignment of the writing beam with the diffractive structure rotation axis in synthesis of diffractive optical elements in a polar coordinate system

A method is developed to ensure precise alignment of the origin of a polar coordinate system in which the laser beam position is defined in writing diffractive optical elements with the optical workpiece rotation axis. This method is used to improve the accuracy of a circular laser writing system in writing large-scale diffractive optical elements in a polar coordinate system. Results of studying new algorithms of detection and correction of positioning errors of the circular laser writing system in the course of writing are reported.

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.

Microstructuring of optical surfaces: Technology and device for direct laser writing of diffractive structures

Results of development and testing of a scanning system of interference lithography are presented. The system is designed to form diffractive microstructures consisting of microgratings with a specified orientation, a size of 5–10 µm, and a period ranging from 0.6 to 1.5 µm. The total writing field of the system is 300 × 300 mm. The system is used to study direct laser writing of microgratings on chromium and amorphous silicon films applied by the method of magnetron sputtering onto the glass substrate surfaces. The device and technology of direct writing of microgratings can be further used to form antireflective subwave coatings of optical elements and graphical microstructured identification marks for product protection and also to manufacture diffractive attenuators of laser radiation.