Yixiao Zhang

Refractive micro lens array made of dichromate gelatin with gray-tone photolithography

A novel method is proposed to fabricate refractive micro lens array using dichromate gelatin (DCG) and an enzyme. DCG has optical properties similar to glass but is sensitive to UV light. The enzyme solution is used to develop the optically exposed DCG instead of a conventional water-isopropanol developer, which greatly enhances the surface relief depth. The continuous surface relief structure is realized by a coded grey-tone mask. A computer program has been developed to code an arbitrary 3D surface relief shape into a grey-tone mask design. The DCG properties and the lithography process are described. The grey-tone coding principle is presented. Unlike the current processes where patterned photoresist relief structure has to be transferred to a substrate material, the fabricated DCG micro lens array can be directly used as an optical element. The new process opens a new route of micro lens fabrication, which is simple and low cost

Coding gray-tone mask for refractive microlens fabrication

A new method has been developed to design gray-tone masks for microlens fabrication. Unlike previous methods which modulate the light intensity by changing the cell size or cell pitch only, the method relays on adjusting both the shape and position of a cell which gives an extra freedom to control the design accuracy. Based on the theory of partial coherent light and the resist development model, the intensity distribution through the gray-tone mask and exposure of photoresist have been simulated. Nonlinear effects in aerial image and resist development have been take into account to correct the mask design. A grey tone mask for microlens array has been fabricated and printed by a g-line stepper. Hemispherical photoresist structures have been achieved.

Decomposition storage of information based on computer-generated hologram interference and its application in optical image encryption.

An information-encryption method based on computer-generated hologram (CGH) interference is presented. In this method the original information is decomposed into two parts, and then each part is encoded on a separate CGH. When these two encoded CGHs are aligned and illuminated, a combined interference pattern is formed. The original information is obtained from this pattern. It is impossible to decrypt the original information from one CGH alone; two matched CGHs must be put together to make it available.

Design of hybrid micro optical elements with coded gray-tone mask

A new method to fabricate hybrid micro-optical elements in one-step lithography is proposed in this paper. A gray-tone coding method has been developed to achieve one step photolithography of achromatic hybrid micro-optical elements. Design and simulation of hybrid lens with grey-tone mask are presented. The aberrations of optical elements are pre-corrected in the lithography process. The operating range of the hybrid optical element is in infrared band (8.0∼12.0 μm) with the central wavelength at 10.6 μm.

Electron-beam lithography to improve quality of computer-generated hologram

Abstract The quantized errors in computer generated holograms (CGH) have been analysed theoretically. In order to evaluate quantitatively the influence of these errors on the quality of reconstructed image, a signal-to-noise ratio (SNR) is introduced. It is found that the SNR of a reconstructed image is very sensitive to the accuracy of amplitude and phase. Improving the accuracy of amplitude and phase is much more effective for improvement of CGH image quality than simply increasing the sampling cells. The accuracy of amplitude and phase of a sampling cell is represented by the fidelity of sampling cell geometry. CGH plates with the same encoded image have been fabricated by both conventional photoreduction method and e-beam direct write method. The reconstructed images from both plates have been compared. The experimental result has confirmed the theoretical analysis: e-beam direct write can produce CGHs with quality superior to conventional photoreduction technique

Enhancement of photolithography resolution by fractional Fourier domain filtering

A new spatial filtering technique is proposed for improving image contrast and depth of focus in projection photolithography. The technique is based on fractional Fourier domain filtering. Unlike the current pupil filtering method, the fractional filter can be placed at any location along the projection optical path other than the pupil plane. The theory of partial coherent diffraction combined with fractional Fourier domain filtering is presented. Phase filters for contact hole and line-space patterns have been designed. Computer simulation of complete imaging process including fractional Fourier domain filtering have been carried out. The simulation has demonstrated that the new filtering technique can significantly improve image fidelity, reduce the optical proximity effect and increase the depth of focus. Because of the flexibility in filter location, it is predicted to be easier to implement in a practical optical lithography system.

Multiple fractional fourier transform hologram by use of holographic lens

Abstract A new method is presented to record multiple fractional Fourier transform holograms by use of a holographic lens. With holographic lenses, several fractional Fourier transform holograms of different objects can be recorded in a simple way, and images of these recorded objects can be reconstructed in different positions and directions in three-dimensional space. In this paper, the theory and characteristics of the hologram recorded using the holographic lens are analysed. Using this method, a multiple fractional Fourier transform hologram was fabricated, with satisfactory results.

Simple geometry to record fractional Fourier transform hologram with holographic lens

FRTH is a new kind of hologram, which is different form common Fresnel holograms and Fourier transform holograms. It can be applied for fractional Fourier transform filtering and anti-counterfeiting, etc. Due to the flexibility of holographic lens, we present a method that uses the -1 diffraction wave of holographic lens as the object wave and the 0 diffraction wave as the reference wave to record FRTH. It provides a new simple way to record FRTH. In this paper, the theory of achieving FRT and recording FRTH with holographic lens has been discussed, and the experimental results are also presented.

Study of focusing combination of large-caliber coherent laser beams

A focusing combination scheme based on four incident plane waves with a caliber of 290mm is investigated to enhance the peak power of the shooting laser in the ICF fast ignition. Calculated and simulated results indicate that about 67% energy is focused in the center spot using an optimized paraboloidal mirror, and the light peak intensity is enhanced about 13.95x1010 times. Besides, the tolerances of the light spot position and the initial phases are analyzed in detail for experiment.

Error diffusion method applied to design combined CSG-BSG element used in ICF driver

In the final optics assembly of Inertial Confinement Fusion (ICF) driver, Diffractive Optical Elements (DOEs) are applied to achieve some important functions, such as harmonic wave separation, beam sampling, beam smoothing and pulse compression etc. However, in order to optimize the system structure, decrease the energy loss and avoid the damage of laser induction or self-focusing effect, the number of elements used in the ICF system, especially in the final optics assembly, should be minimized. The multiple exposure method has been proposed, for this purpose, to fabricate BSG and CSG on one surface of a silica plate. But the multiple etch processes utilized in this method is complex and will introduce large alignment error. Error diffusion method that based on pulse-density modulation has been widely used in signal processing and computer generated hologram (CGH). In this paper, according to error diffusion method in CGH and partial coherent imaging theory, we present a new method to design coding mask of combine CSG-BSG element with error diffusion method. With the designed mask, only one exposure process is needed in fabricating combined element, which will greatly reduce the fabrication difficulty and avoid the alignment error introduced by multiple etch processes. We illustrate the designed coding mask for CSG-BSG element with this method and compare the intensity distribution of the spatial image in partial coherent imaging system with desired relief.