Fengyou Li

Lithographic fabrication of large diffractive optical elements on a concave lens surface.

We demonstrate experimentally the lithography technique to fabricate a large computer-generated diffractive optical element (DOE) pattern on a concave lens surface with precise alignment by using a laser direct writer. We obtained photoresist film with uniform thickness on the large concave substrate by selecting proper spin-coating parameters, which mainly involve spin rate, spin acceleration, and viscosity of the photoresist. We obtained a square line profile on the concave lens surface. We can write lines that range in width from 0.7 to 10 microm using a single pass of the laser beam. We have designed and fabricated a grating on the concave lens surface using the laser direct writing lithography technique. It is believed that this technique can also transfer large DOE patterns, with a continuous surface relief, onto a convex or concave lens (mirror) surface.

Fabrication of large diffractive optical elements in thick film on a concave lens surface

We demonstrate experimentally the technique of fabricating large diffractive optical elements (DOEs) in thick film on a concave lens surface (mirrors) with precise alignment by using the strategy of double exposure. We adopt the method of double exposure to overcome the difficulty of processing thick photoresist on a large curved substrate. A uniform thick film with arbitrary thickness on a concave lens can be obtained with this technique. We fabricate a large concentric circular grating with a 10-ìm period on a concave lens surface in film with a thickness of 2.0 ìm after development. It is believed that this technique can also be used to fabricate larger DOEs in thicker film on the concave or convex lens surface with precise alignment. There are other potential applications of this technique, such as fabrication of micro-optoelectromechanical systems (MOEMS) or microelectromechanical systems (MEMS) and fabrication of microlens arrays on a large concave lens surface or convex lens surface with precise alignment.

Lithographic fabrication of large curved hologram by laser writer.

We fabricated a large curved computer-generated hologram pattern on the concave substrate with diameter of 110mm and radius of curvature of 504mm. The line width of the hologram varied form 39um to 810um. We adopt the single pass and the screw-lines to fabricate this curved hologram precisely and efficiently. 80% of the fabrication time is saved by this method. This work will be useful to the measurement of large convex secondary mirrors that is also hardness at present.

Using curved hologram to test large-aperture convex surface

A valid optical system with a computer-generated hologram fabricated on a concave lens surface, for measuring large-aperture convex surface, is demonstrated experimentally. The CGH employed in this system has been constructed using a new technology that combines laser direct writing and lithography. This technology allows precise alignment, superior linear profile and high resolution of the gratings that compose the CGH. The particular characteristics of this new type of CGH could derive higher accuracy, efficiency and lower cost for testing aspherics in comparison to other CGH employed previously by other authors. We have designed and fabricated one system and measured a 110 mm-diameter convex surface of errors 300.6 nm P-V after compensating the alignment errors. It is believed that this kind of system can be used to measure even large aperture convex surface.

Method for correcting the joint error of a laser writer.

We present what we believe to be a new method for correcting the joint error of a laser writer without reduction of throughput. By digitization, we optimize the intensity data of the incident beam at the vicinity of the start point and the end point. The joint error will not be sensitive to lengthening of the start or end point by optimization. The advantage of this method over multipass writing and error scattering is that it requires only a single pass, and thus 80% of the fabrication time will be saved. Experimentation shows the method to be effective.

Electromagnetic diffraction analysis of columned grid gratings

Binary optics processing methods may be applied to a glass or silicon substrate to generate an array of small columns in order to enhance transmission. We employ the rigorous vector coupled-wave theory to analyse the electromagnetic diffraction nature of two-dimensional columned grid gratings. By computer simulation and calculation, the relation between the reflectivity and the structure parameters of such gratings is presented. These results show that a glass grating can achieve antireflection over almost the whole visible light waveband for unpolarized light, while a silicon grating with a columned grid substrate can achieve low reflectance over as broad a field of view as a tapered silicon grating structure, but it can be fabricated more simply.

Design of a novel hologram for full measurement of large and deep convex aspheric surfaces

We proposed a valid method with a novel computer-generated hologram (CGH) to test large-aperture convex aspheric. The CGH consisted of two zones with different amounts of power: the central zone has a larger amount of power than the marginal zone. Compared with other CGHs used for convex aspheric testing [SPIE.2576.258 (1995)], it could overcome the difficulty of measuring the central region of the convex surface under test, while relaxing the requirement for the illumination optics and CGH of the test system. We have designed an optical test system with the novel CGH to test a 150 mm-diameter convex surface with full aperture by using optical design software Zemax. The simulated result verified the efficiency of the novel CGH. It is believed that this kind of CGHs can be used to measure any large and deep convex surface with full aperture.

Diffractive axicons fabricated by laser direct writer on curved surface

We report a computer-generated element: diffractive axicon, which is fabricated by laser beam lithography on a concave lens surface, for obtaining more uniform axial line focus over a long interval, compared with traditional methods. The on-axis intensity distribution of this kind of diffractive axicon fabricated by our laser direct writer was tested and the results agree well with the theoretical analysis.

Redistribution of output weighting coefficients for complex multiplexed phase-diffractive elements

The formation of multiplexed phase-only holograms with more weighted phase functions creates spurious cross terms and nonlinear scaling. We extend previously reported work [Appl. Opt. 25, 3767 (1986)] by proposing a normal method to analyze multiplexed holograms mathematically. We show that the output of holograms with any number weighted phase function can be written as a new linear combination for the original phase function with new weights. The relationship between the original weights and the new weights is developed for real-time optimization of hologram performance. We focus on the analysis of two and three multiplexed holograms to demonstrate the effectiveness of this approach.

The analysis of line profile in laser direct writing process

In the analysis of line profile in single point Laser Direct Writing process, the line profile errors will increase if we replace the exposure dose distribution by the intensity distribution of focus plane because there are differences between them. The exposure dose distribution in the photoresist is analyzed as well as the line profile after developed. The experimental results agree well with the theoretical forecast.