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.
In high power laser system, it is of great interest to combine two or more diffractive structures, in particular, the beam-sampling gratings (BSG) and the color separation gratings (CSG), onto one element. However, the combined element with diffractive structure on both surfaces, may cause serious laser induced damage to the element itself. So, this paper use Fourier modal method to analyze the near field characteristic of CSG and BSG combined element. Through theoretically analysis and numerical calculation, amplitude and phase distribution of electric field are present both inside and outside the diffractive structural region, and the maximum peak-to-average modulation in near field is also given. Based on this study, the most possibility of optical damage induced by beam modulation of CSG and BSG combined element appears in the neighborhood of the interface.
Diffractive gratings, such as 1 grating and beam sampling grating (BSG), are used in the inertial confinement fusion (ICF) driver because of their high diffractive efficiency. Under high power laser condition, it demands that near fields of the diffractive gratings, mainly affected by input laser energy and beam modulation, must be less than their damage threshold, otherwise the diffractive gratings will be damaged. In this paper, Fourier modal method based on the rigorous electromagnetic theory is introduced to rapidly and accurately analyze the distribution of near fields of the diffractive gratings. Its physical concept is clear and concise, and computation cost is small. Through numerical simulation, it indicates that the results calculated by Fourier modal method are accurate and effective, compared with those calculated by other method. The near fields of 1 grating used in final optical system of ICF driver are obtained. In addition, fabrication errors effects on the near field modulation are simulated. It shows that the sidewall slope errors are the main cause of optical field modulation. With theoretical analysis and numerical simulation, it is useful to understand mechanism of damage and help how to control fabrication process errors of the optical elements used in the optical system of ICF.
Grating lenses are diffractive optical elements with gradually variant space and line, and are widely applied to various optical systems, such as harmonic wave separation and diagnosis of high power laser system, optical communication and spectral analysis. Because of its small feature size (just about several times of the illumination wavelength) and gradually variant space and line, the simulation results are not accurate when using the scalar diffractive theory. In this paper, the grating lenses are subdivided into smaller areas, and every sub-area is regarded as periodic microstructure because variance of adjacent period is very small. Then Fourier modal method is adopted to analyze their diffractive properties in sequence, and finally total diffractive efficiency of grating lenses can be easily obtained. Its physical concept is clear and concise, and computation cost is small. Through numerical simulation, diffractive efficiency of grating lens for beam sampling and harmonic separation in high power laser system is calculated. It indicates that the method presented in this paper is accurate and valid. In addition, fabrication errors effects on diffractive efficiency are also simulated in order to obtain the relationship between process errors and diffractive efficiency of grating lenses. It suggests that grating lenses not only can be easily realized in fabrication process, but also can meet practical demand in high power laser system. In experiment, a beam-sampling grating with diameter 100 mm was fabricated, and its experimental diffractive efficiency is consistent with result calculated by the method in this paper.
In the final optics assembly of ICF driver, Diffractive Optical Elements (DOEs) are applied to achieve some important functions. Due to the advantage of easy integration, DOEs that achieve different functions can be combined into one element, and thus the system structure and performance of ICF driver can be optimized. In this paper, we present a new means to fabricate color separation grating (CSG) and beam sampling grating (BSG) on two surfaces of one fused-silica substrate with two-surface exposure method. The mask for CSG can be fabricated with common optical method since the period of CSG is large; whereas the mask for BSG cannot be made with normal micro-fabrication system whose resolution cannot meet the requirement of fabrication precision of slowly changed BSG fringes (from 2μm to 4μm). Therefore, we use e-beam direct writing method to fabricate mask for BSG. The alignment of CSG and BSG is designed in accordance with the requirement of actual system. Thus the functions of harmonic wave separation and beam sampling are realized through one silica plate. The satisfying experimental results are illustrated.
Aiming at correcting chromatic aberrations in a far-infrared band, the fabrication of a hybrid microlens array with one-step lithography is proposed, by using a coding grey-level mask. The designed hybrid microlens consists of a refractive microlens and a diffractive microlens in physics. Its structure parameters, in order to achieve the best correction of chromatic aberrations, are evaluated and optimized with the software OSLO to design the layout the grey-level mask. Based on the theory of partial coherent light, the photoresist exposure model and development model, the profile of hybrid microlens in the photoresist have been simulated, the nonlinear errors in the lithography process can be pre-compensated by correcting the mask design. A hybrid microlens array is fabricated through use of the designed mask.
A new method is presented to encode and decode images with computer-generated hologram of fractional Fourier transform. The fractional Fourier transform computer-generated holograms (FRT-CGH) of several objects with different orders are respectively recorded on one holographic plate. In order to reconstruct images of the objects, several fractional Fourier transform systems of certain orders are needed. This method is agile and convenient on design and fabrication. It can be encoded and decoded either by numerical method or by optical method. Because of its particularity of image reconstruction, FRT-CGH is regarded as a new optical security system and may be widely used in the future.
Digital implementation of fractional Fourier transform hologram (FRTH) is studied. From the definition of fractional Fourier transform (FRT), its discrete form is obtained and then a fast FRT algorithm is generated based on fast Fourier transform (FFT). Therefore, with the algorithm whose numerical efficiency is equal to the FFT, FRT can be quickly calculated and moreover FRTCGH can be efficiently encoded and reconstructed. The digital implementation for FRTH can lead its advantages, such as flexibility of fabrication and variation with recording system parameters, to expanding its application fields in FRT and holography.
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.
In this paper, we present a new method to improve the image quality and resolution of photolithography by filtering in fractional Fourier domain. Introducing a filter into fractional Fourier domain can not only increase the flexibility of the filtering operation, but also enhance the image quality and the depth of focus in photolithography. The corresponding simulation results are illustrated.
In this paper, we introduce the recording and reconstruction theories of the multiple fractional Fourier transform hologram (M-FRTH). We fabricated a multiple fractional Fourier transform hologram, and obtained satisfying reconstruction results. The experimental result shows that the M-FRTH has a high anti-counterfeiting capacity and can be used in the fabrication of the trademark, ID, and the notes.
A new method has been developed to fabricate refractive microlens by etching ammonium dichromate gelatin (ADG) with enzyme solution. Unlike previous methods which are used to fabricate refractive microlens with photoresist, the process of fabricating microlens by etching ADG with enzyme solution doesn't require the use of expensive equipment, and it isn't sophisticated and time consuming. The light exposes ADG through a high contrast binary mask, then the exposed parts of ADS generate cross- linking reaction. Usually, the relief achieved by water developing is very shallow (<1um) when nonpre-harden gelatin is used, so we compound a certain concentration enzyme solution, and because of surface tension, ADG turns to spherical structure after developing. The optimum technique parameters of this process are presented. Results are presented for experiments and evaluated by profile meter and interference microscope.
Based on the energy conservation law, a modified proximity function is proposed to describe the absorbed energy distribution in photoresist during laser writing. The measured data for photoresist absorbing energy are fitted well to the modified proximity function. We analyze the proximity effect in laser writing by using the new model, it is helpful to further develop the precompensation and multi- exposure methods to correct proximity effects in laser direct writing.
An anti-counterfeiting method using CGH phase coding is presented in this paper. The phase of a wave-front is coded by means of detour phase coding and then interfere with a reference wave, which is also generated by CGH. The object recorded in the hologram can be read out from the interference patterns. The object can be easily generated by computer, caught by CCD camera or scanner, both black-white and grayscale object can be used in our method. The features of this method lies in that it is flexible in designing, easy to observation and reconstructed by common expanding white light source. The reference wave can be a plane wave or another phase coding wave, then it's difficult to duplicate an identical CGH based on the decoding information, this made it enjoy a high anti-counterfeiting ability.
The fabrication of refractive microlens by introducing the melting process with coding gray-tone mask is reported. The applied mask is obtained by the coding method, nonlinear effects in intensity distribution through the gray-tone mask have been taken into account to correct the mask design. A continuous relief is formed in photoresist after exposing, and then the excellent surface shape microlens can be gained by melting. The technical parameters of this process are also presented. Results are presented for experiments and evaluated by profile meter and scanning electronic microscope. The fabrication technology of refractive microlens by this process is simple and it indicates the enormous potential to extend the fabrication range ofrefractive microlens.
In this paper, a new method has been proposed to realize fine OPC with gray-tone coding mask instead of gray-tone mask. The relationships of the gray-tone coding mask and gray-tone mask have been discussed, and OPC simulation results are given with the gray-tone coding mask.
This paper aims at the request of dividing harmonic waves in the high power laser system used to perform Inertial Confinement Fusion. Dividing harmonic waves is realized by introducing binary optical element. Based on scalar diffraction theory, the distribution of its diffraction field was calculated and the fabrication parameters were also optimized. The element is fabricated with RIE. We also measured the relief structure and diffraction efficiency of each harmonic wave and analyze the errors.