As applied to a maskless laser lithography writing system Heidelberg DWL 66+ and the photoresist-chromium technology, we have found the tolerances on fabricating the binary-phase Dammann gratings having 7x7 useful diffraction orders and a structure separable in X and Y components and illuminated by a beam with a wavelength of 633 nm. It has been found that the obtained tolerances guarantee the production of Dammann gratings with a total diffraction efficiency close to the theoretical one and a root-mean-square error of departure of the intensity of useful diffraction orders of less than 10%.
The paper describes methods for manufacturing of diffractive optical elements by means of only "dry" processes starting from direct laser writing on titanium-containing films. According to first approach, direct laser writing onto thin Ti film forms surface oxide mask. Reactive ion etching removes non-oxidized Ti film and develops "latent" oxidized image. Subsequent thermal annealing of the oxidized Ti structure in air makes the mask more stable for following reactive ion etching of fused silica substrate to ensure proper phase depth of the binary diffractive structure. This makes it possible to avoid liquid etching, which reduces the yield and accuracy. The phase structure of the diffractive elements manufactured using the described method consists of the grooves etched in the fused silica substrate between ridges covered by TiO2 between them. We found out also that covering the Ti film by very thin Si layer helps to increase laser energy absorption at direct writing and creates quite resistant masking layer TiSi2 for the reactive ion etching. Preliminary estimates show that dual layer Si/Ti films can be used to create amplitude reflective DOEs. Possible application area for the developed methods is manufacturing of the diffractive optical elements used for precision generation of reference wavefronts in interferometric measurements of spherical and aspherical surfaces.
Bifocal diffractive-refractive intraocular lenses (BDRIOL) are used in ophthalmology to replace natural lenses that are clouded by cataracts. In Russia, the REPER-NN enterprise developed a BDRIOL manufacture technology on the base of a liquid photopolymer frontal polymerization placed between two transparent fused silica moulds. One of them has a spherical shape, the other has a flat surface with a diffractive optical element (DOE). In our BDRIOL design the diffractive structure depth changes so that the energy distribution over the foci weakly depends on the pupil diameter. Zeroth and first orders efficiencies should almost the same. Measurement of this distribution is complicated due to the diffractive profile depth corresponds to the operation of a polymer lens in the eye and is significantly greater than required for working in air. We have proposed to use temporal planarization of the diffractive mould to measure energy distribution over foci. The diffractive structure is filled with a material that provides approximately the same module of refractive index difference with fused silica as between the polymer and vitreous body of the eye. This difference is adjusted by the temperature and time selection at the photoresist baking. The sign inversion of the refractive index difference in comparison with the operating conditions in human eye changes the DOE focus to positive. The paper discussed specific photoresist treatment, refractive index measurement and preliminary results of the developed method.
A completely “dry” one-step method for the formation of reflective phase diffractive optical elements (DOE) using direct laser writing on thin Zr film without subsequent liquid or plasma treatment has been demonstrated. The method is based on effect of nanostructure formation during the thermochemical oxidation of the zirconium film deposited on a fused silica substrate. Direct laser writing of diffractive structures is performed by scanning a laser beam with a wavelength of 405- 532 nm, focused into a spot with a diameter of 400-700 nm. When line-by-line scanning with a step of 200-500 nm, the formation of nanogratings from tracks with a width of 70-100 nm and with a period equal to the scanning step was observed. It was experimentally established that lines in the form of cracks or deformations arise along the contour of the spatial distribution of temperature induced by the laser beam heating. Cracks or deformations occur under the influence of thermomechanical stresses at the boundary between the metal film and the oxide line, the thickness of which increases sharply during the oxidation of the metal. The formed nanogratings significantly change the optical properties of the film surface. Measurements by a white light interferometer show the presence of a relief with a depth of -200 to +500 nm, although when oxidizing a 100-nm thick Zr film, the increase in thickness cannot exceed 50-60 nm. Binary diffractive structures with a period of 0.9 - 10 μm, the diffraction efficiency of which exceeded 30%, were made by the new method. The one-step method can be useful for the manufacture of computer-generated holograms used in testing of aspherical surfaces.
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.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.