Libmonster ID: BY-1572
Author(s) of the publication: Alexander POLESHCHUK

by Alexander POLESHCHUK, Dr. Sc. (Eng.), Head of the Laboratory of Diffractive Optics, Institute of Automation and Electrometry, RAS Siberian Branch (Novosibirsk)

Optics in its classical representation is based on lenses, prisms and mirrors, i.e. elements, which have achieved the height of perfection long ago. The further advance in this field of science and technology is connected with a wide application of diffractive optical elements (DOE) in the form of thin glass plates, one side of which has surface microrelief features and a depth of a part of micron size, to be more precise, up to a half of light wavelength (0.4-0.7 micron). They can replace customary lens systems, transform laser radiation and form images of virtual objects, designed by computer, while their application opens up a prospect of creating cheap, light, compact but, at the same time, complex optical devices. The range of their use is very wide, from artificial human crystalline lenses to optics of space telescopes.

DIFFRACTION AND REFRACTION

The diffraction* phenomenon was first revealed and described by Italian physicist and astronomer Francesco Grimaldi and British naturalist Robert Hooke in the

* Diffraction is any deviation of wave propagation near obstacles, therefore due to which waves can get to a geometric shadow area, round obstacles, penetrate through small holes in screens, etc.-Ed.

second half of the 17th century, but it was used to manufacture optical devices long after. Modern diffraction optics is an outcome of the age of information technologies, and it could not appear earlier due to lack of such tools as laser and computer.

The manufacturing methods of DOE were first developed at the Institute of Automation and Electrometry in

стр. 4

The simplest transformations of light beams by traditional (A) and diffractive (B) elements.

the 1970s under the guidance of Voldemar Koronkevich, Dr. Sc. (Eng.). The main efforts were directed at creation of laser systems for diffraction structure recording and development of thermochemical technology of chromium masks manufacturing and later to new manufacturing methods of phase DOE with complex surface relief.

It will be discussed later, but first there are some explanations. Lens is a basic element not only in classical optics but also in diffractive optics. It is designed for light focusing and object imaging, i.e. for geometric and wave transformation of light beams. For example, it transforms a parallel beam (plane wave) at the input to a convergent beam (spherical wave) at the output. But a diffraction lens differs from a classical refraction lens*.

To design a discrete (diffraction) structure based on a regular plane-convex lens, we shall make use of the geometric method. Therefore, we must divide conditionally the lens into thin spherical layers of equal thickness. The layer radius will be equal to the curvature radius of the lens spherical surface, and its thickness (h) will be equal to the value Nλ(n-1), where λ-light wavelength, N-integer, n-refraction factor of the lens material. So, if the lens is 5 mm thick, then at λ=0.5 urn, n=1.5 (glass), the number of spherical layers of 1 μm thick will be equal to 5,000. Besides, on a flat surface of the lens different layers can be incorporated into a discrete stepped

* Refraction lens is characterized by the refraction (deflection) phenomenon. Essentially, this is a change of direction of electromagnetic radiation propagation, occurring at the interface of two media transparent for these waves or in the depth of a medium with ever varying properties.-Ed.

structure by two lines parallel to the optical axis. The configuration thus obtained is called a zone plate or a diffraction lens, and its effect on a light wave is similar to that produced by a refraction lens. But the latter has a fixed length of optical path from the object point to the object image for all beams crossing the lens aperture*. It operates at the expense of refraction and deflection of a light beam on its surface (lens can be imagined conditionally as a totality of prisms with different angles increasing from the center to the periphery, therefore refraction angles of light beams getting to each of the prisms will be also different).

In diffraction lens the length of optical path at interfaces of zones undergoes jumps equal to λ (at N=1) or Nλ, if in the second case lens has a "deep" profile (N>1). Besides, it operates at the expense of diffraction phenomena on a circular thin array, whose pitch decreases to the lens periphery and can reach values of the order of light wavelength for large apertures. The diffraction lens has another important and useful feature, i.e. it can be thousands of times thinner if compared to its analog equal in optical power.

METHODS OF PRODUCTION

The potentials of DOEs are mostly determined by a technology of their production, which shall provide reconstruction of a wavefront form with the designed accuracy reaching 1/1000 of light wavelength for some applications.

* Aperture is a characteristic of an optical device, describing its ability to collect light and withstand diffractive blurring of image details.-Ed.

стр. 5

Transformation of a plane-convex lens into a diffractive lens (a) and its 3-D model (b).

Until recently the same equipment was used for micropatterning of optical surfaces as for production of microelectronic chips. However, a topological structure of DOE surface has an arbitrary shape, while that of microcircuit is, as a rule, of the form of a line-rectangle pattern. In customary methods a diffractive structure was represented as a set of elementary images such as trapezia and rectangles oriented along two fixed orthogonal axes. DOEs were produced by scanning of a light-sensitive layer by a focused electron or laser beam in the Cartesian coordinate system, which resulted in distortion of a formed image. But it is inadmissible, for example, for DOE designed for testing of aspherical mirrors* of modern telescopes. Such DOE shall have an error in production of a circular diffractive structure not more than 0.1 μm if the overall dimensions of the item make up several hundreds of millimeters. Elements of such type can be produced only by means of equipment, which uses the polar coordinate system for recording. Moreover, it is preferable also for a major part of the focusing optics. What are its features?

By using such method of recording, the substrate with light-sensitive layer rotates at a constant angular velocity, while a focused beam moves along a straight line, which crosses the center of rotation. Besides, at spiral scanning realized by slow continuous movement of a recording spot, DOEs are produced as integral with no stops and flybacks. It allows significant cutting of recording time and avoiding of any stitchings typical of equipment operating in the Cartesian coordinate system. Circular scanning which is optimal for DOE with axial symmetry is realized by discrete displacement of a recording spot. This method is optimal also for synthesis of DOE with arbitrary pattern.

* Lenses and mirrors are called aspherical, if one or two of their surfaces are not spherical.-Ed.

This principle was used at the Institute of Automation and Electrometry for development of a precision circular laser recording system designed for production of DOE in the polar coordinate system. It is well-known that laser radiation can be focused on a spot sized less than a light wavelength (~0.5 μrn) and to obtain high-density power there. As a result, a substance located near the focus heats instantaneously up to a high temperature. Execution of control of a laser beam displacement and radiation power by computer makes it possible to provide the required properties to a light-sensitive material surface. We use, in particular, chromium films in this capacity as they form a latent "thermochemical" image under the action of heating caused by laser. It allows direct recording of high-quality computer generated holograms, linear and angular scales, encoders, grids and different masks.

It is noteworthy that diffractive optics uses elements of both a rectangular and sawtooth surface relief. Creation of a relief with a rectangular pattern starts from coating of a chromium film of about 50-80 nm to a glass optical substrate surface. The required structure is recorded then on the chromium film by a focused beam of a powerful laser.

It was determined at our laboratory that amorphous chromium films change their internal structure as affected by radiation, and a thin layer of chromium oxides is formed on their surfaces. The control of a laser spot displacement and radiation power by computer, makes it possible to form in the chromium film a latent image, which is then developed in a selective developer whereby metal alone dissolves promptly, and the exposed areas remain. Thus, a microstructure is formed on the substrate surface. It is used as a mask for obtaining of a relief of the required depth already in glass proper by means of reactive ion etching. Then chromium is etched away, and a diffractive element with a binary structure is finished.

стр. 6

DOE recording in the Cartesian (A) and polar (B) coordinate systems. The arrows indicate a moving direction of the recording spot and address grids.

In the end, structures with minimal dimensions of parts of micron are obtained.

The laser technology is also used when it is required to obtain DOE with a sawtooth surface pattern. First, spin coating is used to get resist film onto the substrate. This film can change a rate of dissolution in the developer depending on the exposure level, i.e. by changing of a dissolution rate we can change a film thickness. To form a relief the resist film surface is exposed by a moving focused laser beam with changing power. A relief of the given shape is formed in the film after its developing. Reactive ion etching is carried out at the final stage, and a relief is transferred from the resist film to the substrate material.

ASPHERICAL WAVEFRONTS

Spherical and plane wavefronts are formed in optics naturally by means of lenses, prisms and spherical mirrors. A point-light source (for instance, end of the optical fiber) forms an almost ideal spherical wavefront. The shape of such wavefront can be tested to a very high accuracy by comparing with a primary standard (plane or sphere) through wavelength of a laser interferometer. Primary standards of plane and spherical surfaces with error less than 1 nm are developed today at the All-Russia Research Institute of Metrological Service. For example, mercury (or oil) mirror is used as a standard of plane surfaces.

However, until the present days there were no metro-logical standards of aspherical wavefronts (AW), and problem of their development was not discussed. This was connected with the fact that it is impossible to check directly and expressly the shape of AW, if it differs (more than several wavelengths) from plane or spherical wave-fronts. Besides, the formation of such wavefront by methods of classical optics (lenses, mirrors) has significant limitations.

A major problem of creating AW standards with error less than 1 nm is necessary for development of a number of optics areas such as X-ray optics (telescopes and microscopes), astronomic (including space-based) optics, synchrotron radiation, deep ultraviolet nanolithography*, etc.

Both lenses (or mirrors) and DOE are widely used now for AW formation. All these elements transform an initial plane or spherical wavefront from a point source to an aspherical wavefront, i.e. they essentially are wavefront correctors.

Refraction correctors consisting of a set of ordinary spherical lenses were widely used until recently for testing of aspherical mirrors though they have significant residual aberrations (up to 0.05 wavelength), but lens sizes are rather high. For instance, the lens corrector for test of the aspherical mirror of 6.5 m diameter of the Magellan telescope (USA) is about 1.5 m long at the lens diameter up to 200 mm, whereby the admissible error of lens alignment is only several microns. The similar corrector developed at the Institute of Automation and Electro-metry consists of the only one DOE and has residual aberrations less than 10-5 of wavelength, i.e. it is ideally suited for such kind of wavefront transformations. Lately, after new methods of microrelief formation were developed, DOE are widely used for interferometric testing of aspherical optics and correction of laser wavefronts.

* Nanolithography is a production process of microcircuit chips with element dimensions less than 100 nm by ultraviolet or X-ray resist exposure.-Ed.

стр. 7

The manufacturing methods of a DOE microrelief with rectangular (a) and sawtooth (b) profiles.

However, for such correctors to become metrological standards, it is necessary to provide certification of AW. The Hubble Space Telescope (USA) illustrates exceptional importance of this requirement. Right after its space launching in 1990, it has been found that the 2.4 m aspherical mirror had a manufacturing defect and spherical aberration did not allow achieving of even a tenth of the target resolution. This error took place at assembly of a lens corrector, when one of the lenses was installed with a slight displacement from the planned position, and AW characteristics were not checked. The error in the formed AW led to 2 μm error of mirror shape against 0.015 μm standard value. The unscheduled mission of Space Shuttle (USA) was required in 1993 to install a correction optical system into the telescope, and the repair cost made up US$ 700 mln for NASA.

Afterwards all new telescopes developed in the USA were tested by the DOE (or computer generated holograms). It is noteworthy that such method was suggested in the USSR almost 20 years earlier.

In recent years our institute is engaged in development of AW certification methods providing nanometer accuracy for testing of shapes of telescope aspherical mirrors. Using AW imitators in combination with interferometer is one way of solving the problem. This is how it works. Spherical wavefront from interferometer output passes through DOE corrector and transforms into AW. DOE imitator is installed successively with DOE corrector for certification and testing of AW. Wavefront is reflected and diffracts on DOE imitator, passes again through DOE corrector and returns to interferometer. If both DOE are precision-made, interferometer will not register distortions, and it can be expected that AW corresponds to ideal. If there are residual aberrations, it is a signal that something is wrong. Analysis of distortions allows identifying their source in some cases.

Another way of solving the problem of AW certification is using combined diffractive optical elements (CD). They are designed and manufactured in such a way that several independent wavefronts are formed as a result. In case when one of them is aspherical and the other is spherical, a standard spherical mirror installed into a spherical wavefront will allow to test beam path in the optical system. That is, in this case a spherical mirror serves as DOE imitator.

стр. 8

For manufacture of etalon DOE for testing of aspherical optics at the Institute of Automation and Electrometry, a soft-hardware complex for direct laser recording of micro-structures in the polar coordinate system based on the laser recording system CLWS-300IAE was developed and constructed for the first time in the world. The new adaptive correction methods of external actions (temperature, vibration, deformation, atmospheric pressure, etc.) provided a recording error less than 10 nm during 10 hours and more of continuous operation of the complex.

Etalon DOE for testing shapes of aspherical mirror surface of a number of unique Russian and foreign telescopes were manufactured at our institute in recent years. By means of our etalon DOE, the following mirrors were manufactured at the Lytkarino Optical Glass Factory (Moscow Region): 1.7 m mirror for the Spektr-UF promising Russian space telescope; 4.1 m mirror for the VISTA wide-angle telescope, which is already operating at the Southern European Observatory (ESO) on the Paranal Mountain in Chile; 3.7 m mirror for the ARIES telescope (India) and others.

Etalon DOEs for testing of optical systems of SALT, E-ELT, Magellan, LBT and JWST telescopes were also developed and manufactured under international contracts.

The large SALT telescope with an 11-m mirror was officially opened in South Africa in 2005, but it proved to be inoperative due to problems with adjusting of its optical system. Its adjustment became possible by means of a large etalon DOE manufactured at the Institute of Automation and Electrometry. Now the specialists of the South African Astronomical Observatory have completed the repair works and already obtained images of stars of excellent quality. The first results and expression of gratitude to our institute were published in 2010.

The extremely large European E-ELT telescope with a 39-m mirror will be biggest in the world (it is planned to put it into operation in 2018). An optical system based on

Checking of AW shape by DOE-imitator (a) and CD-corrector with an etalon spherical mirror (b).

стр. 9

a precision amplitude-phase DOE for checking of prototypes of telescope segments is developed and designed at the Institute of Automation and Electrometry. Inaccuracy of the formed aspherical wavefront with the budget of all manufacturing errors made up 5.3 nm.

Besides, our institute developed and manufactured for the Steward Observatory of the University of Arizona (USA) etalon DOEs for testing of aspherical mirrors of Magellan telescopes (6.4-m mirror), the Large Binocular Telescope (2 mirrors of 8.4-m diameter each) and a new infra-red space telescope for the James Webb observatory (JWST) with a 6.6 m mirror, whose launching to Lagrange L2 point* is planned in 2018.

In conclusion let's point out several key moments, which are important for development of diffractive optics.

* Lagrange points are points in a system of two massive bodies, in which the third body with negligible mass, on which no other forces act except gravitational forces on the part of the first two bodies, can remain immovable relative to these bodies. They are named after the French mathematician, astronomer and mechanic Joseph Louis Lagrange, who was the first to discover this phenomenon in 1772. In point L2, (about 1.5 mln km from the Earth) the orbital period of an object becomes equal to the orbital period of our planet. It is an ideal place for deployment of space observatories and telescopes as it exists within the shadow of the Earth-Ed.

The development of a circular laser recording system stimulated research works directed at working out of new technologies of diffractive structure synthesis. Manufacture of DOE in the polar coordinate system proved to be very productive. Accuracy in the formation of a wave-front shape by diffractive elements reached λ/100, which corresponds to the highest standards of classical optics.

The development of laser thermochemical technology of DOE manufacture in thin chromium films was a significant stimulator for research works. The technology suggested by the Laboratory of Diffractive Optics is characterized by exceptional simplicity and reproduction in combination with obtained high-quality micro-structures.

The methods of design and manufacture of precision DOE for testing of aspherical optics with guaranteed accuracy are modified.


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Alexander POLESHCHUK, OPTICS IN THE AGE OF INFORMATION TECHNOLOGIES // Minsk: Belarusian Electronic Library (BIBLIOTEKA.BY). Updated: 02.11.2021. URL: https://biblioteka.by/m/articles/view/OPTICS-IN-THE-AGE-OF-INFORMATION-TECHNOLOGIES (date of access: 04.12.2021).

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