Yongjun Xie

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.

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.

Polar-coordinate laser writer: analysis of exposure dose distribution

In the single point laser direct writing (LDW) process, there are differences between the exposure dose distribution and the light intensity distribution, and the differences will bring the line profile errors. In this paper, the equations to calculate the exposure dose distribution for the polar coordinate laser direct writing system are presented. The differences between the exposure dose distribution and the light intensity distribution are discussed. The line profile in the photoresist after development is predicted. The experimental results agree well with the theoretical forecast.

Application of harmonic diffraction in infrared

Harmonic diffractive surface element is successfully introduced to the system of infrared dual band in this paper. It has been simultaneously accomplished that the rectification of the Transverse ray, Lateral color and Longitudinal aberration in both band, wave front aberration less ¼ wave length and Modulation Transfer Function of dual band approaching or attaining the diffraction limit. The properties of action spectrum of harmonic diffractive are between refractive element and diffractive, which debased the demand for technical level. The practical design not only shows that the system is compact, few elements and high rates of transmission but also has better weaken-ray aberrations character and 100% cool diaphragm efficiency. The harmonic diffractive element offers a new component for optics designs.

Hybrid diffractive-refractive head-mounted display with reflective relay system using micro-liquid-crystal on silicon

As the micro-display applied to head-mounted display, the optical system not only suits for the small size of the micro-display, but also provides sufficient eye relief and exit pupil, and it becomes perplex. For settling this problem, a head-mounted display, which combines a hybrid diffractive-refractive eyepiece with a reflective relay system using a liquid crystal on silicon (LCOS) with the diagonal size 18mm, was designed. Basing on a Zeiss (60°) eyepiece, and replacing the doublet of it of a diffractive-refractive doublet, a hybrid eyepiece with 20mm eye relief and 10mm exit pupil was designed. The weight greatly reduced and the optical performance improved of the eyepiece compared with the Zeiss one. Considering the space for illuminating source of LCOS, a reflective relay system was used, which includes a flat half mirror and a concave mirror. The magnifying power of the relay system also makes the optical system suitable for the small size of LCOS. The system is with high performance, sufficient exit pupil and eye relief, and reasonable size and weight in the specific application of head-mounted display.