The algorithm for estimating the linewidth for metal oxide grating embedded in the grooves of metal grating by analyzing diffractometric measurements in transmitted and reflected light are considered. To calculate the diffraction efficiency under of rigorous electromagnetic theory, the developed algorithm uses the functions of the GD-Calc toolkit. The estimation of the linewidth from the diffraction efficiency analysis for 1μm period of the grating formed on a Ti film has demonstrated discrepancy near 5% with result obtained from analysis microimages.
Diffractive optics are promising for use in interferometry as a Transmission Sphere (TS) due to the fact that it is simpler and cheaper, since it consists of one component. However, in the manufacture and use of Diffractive TS (DTS) with an aperture up to f/1, many problems arise that are not specific for classical TS. One of these problems is the effects of polarization, which can affect the interferogram. These negative effects can occur when diffractive TS has a period close to the interferometer wavelength. This work is devoted to computer simulation of such effects and development of technique for their partial compensation. To avoid the harmful effects of laser beam depolarization and to decrease the two-pass transmittance of a typical DTS at its margins, we decompose the local linear polarization into the radial and azimuthal components and evaluate separately the forward and backward transmittance of each polarization component for the case with a spatially variable prescribed local duty cycle.
The accuracy of the asphere testing using computer-generated holograms (CGH) is determined by the precision of the diffractive pattern fabrication. In this paper, two methods for quality control of CGHs are presented. These methods are based on writing small marks, which have microgratings with 2-5 µm period. These marks consist of two parts, one of which is formed before the fabrication of CGH. The second one is embedded during the writing process of the main diffractive pattern. The shift between the first and second parts of the mark makes it possible to determine the drift of the positioning system for both coordinates.
We present the results of high-ordered periodic surface structure formation by femtosecond laser pulses (λ = 1026 nm, τ = 232 fs) using an astigmatic Gaussian beam with a diameter of ≈100 μm on the surface of metal films (thickness of 15 nm and 150 nm) of Hf sputtered on the glass substrate. Continuous and uniform structures with a period of ≈700 nm are formed at a scanning speed up to 2000 μm/s on the film with a thickness of 15 nm. In addition, the formation of ablative periodic substructures with a period of about 160 nm oriented perpendicularly to the main ones was observed on the oxide protrusions. In the case of 150 nm thick film, defects, cracks and phase shifts accompanied a uniform structure with a period of ≈940 nm formed at a speed up to 500 μm/s.
Computer-generated holograms are widely used in interferometry for aspherical mirror testing. However, CGH has relatively low numerical aperture, which is limited both by the spatial resolution of the manufacturing process and by the need to do the design in a frame of the rigorous diffraction theory. As a result, the typical minimum period for phase CGH is limited to about 2 μm, if a wavefront error does not exceed λ/20. It limits now f/number at value of f/1.5 for 633 nm wavelength.
In this paper we discuss the technology of direct laser writing on metal films of the titanium group and chromium. It allows one to fabricate microstructures with a period of up to 1 μm at 0.7 μm spot size. Thermochemical effects of laser radiation on the films of various metals have been studied for a long time. The spatial resolution of thermochemical writing on metal films can be significantly improved by through oxidation, which dramatically increases negative feedback at light absorption. The study of laser writing on Zr and Ti films demonstrated their future promise for the technology of creating computer-generated holograms. However, the technology of laser through-oxidation is limited by relatively low scanning speeds of up to 500 mm/s. It limits CGH size when using circular scanning of the laser beam. Reactive ion etching of Ti films through the laser-induced oxide mask can be used to fabricate binary phase structures. The technology is studied for producing diffractive transmission spheres (TS) with an aperture of up to f/0.75.
The important effects, techniques, and factors are considered that aim to increase the spatial resolution of a scanning direct laser writing of diffractive structures on thin films of transition metals from titanium group (Ti, Zr, and Hf). The writing process is based on metal oxidation under the thermal action of a tightly focused laser beam. Scanning speed of the laser beam and film thickness were varied to get a regime of through oxidation (TO) of the metal film under laser heating. It results in strong increase of the film transmission in exposed area. TO ensures a strong threshold due to feedback connected with decreasing of laser power absorption near center of focused gaussian laser spot. To the best of our knowledge, the direct laser writing of amplitude diffractive structures on Zr and Hf films were performed for the first time. The new regime of direct laser writing on thin Zr films was revealed. It allows forming tracks with width of 100 nm and less at laser spot diameter of 700 nm and laser wavelength of 532 nm. In this work, the spectral dependence of the refractive index and extinction coefficient of hafnium films was first experimentally determined in the wavelength range of 250–1100 nm.
The development of specialized non-destructive methods for monitoring of microstructured optical elements is necessary for introduction of diffractive, micro-optical and conformal optical elements into production. However, a wide variety of such elements, as well as their production technologies, set many tasks in the implementation of process control both at the final and at the intermediate stages of the formation of a multilevel and binary phase element. All this requires the use of various expensive equipment, each of which individually does not allow solving the whole range of tasks. Multichannel scanning measuring system that implementing spectroscopic and diffraction testing methods was developed at IAE SB RAS. The device includes four measuring channels allowing to measure the following parameters and characteristics: transmission function of half-tone masks in zeroth order or as sum of all diffraction orders; thickness of transparent films covered on substrate (by the spectroscopic method in reflection); measurement of the zero-order intensity of diffraction structures, both in transmission and in reflection in range of wavelengths from 200 nm to 1100 nm; measurement of the diffraction efficiency of the first and zero orders of diffraction in reflected light (including for structures made in optically transparent materials) for monitoring the parameters of the formed relief of multilevel elements.
One of the promising methods for manufacturing high-aperture and sub-wavelength diffractive optical elements (DOE) is direct laser writing on thin metal films in regime of through oxidation. The amplitude diffractive metal/oxide structure is formed directly by localized laser heating without post-processing using liquid or dry etching. Optical diffractometry based on an analysis of the diffraction pattern obtained from a locally illuminated region of the grating is quite suitable indirect method for inspecting the metal/oxide gratings with a submicron period. Measurement of the angular distribution of diffraction orders allows determining local period of the DOE structure, and measuring the intensity distribution over all diffraction orders allows determining the duty cycle of the grating. In this paper, the configurations of instrumentation schemes of metal/oxide gratings monitoring and algorithms for processing of diffraction patterns formed at illuminating the metal/oxide gratings by probe laser beam are considered.
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