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.
Based on the focus principle of the zone plate and the encoding method of the computer-generated hologram, a zone plate applied in OCT to increase the focal depth is proposed. The phase distribution of this kind of zone plate is constructed by superimposing a shifted elliptical phase to the phase function of a common sine zone plate, which will confine the spot size of the focused beam within a particular range along axis and thus extend the focal depth. The numerical simulation shows that the focal segment of the zone plate meet the requirement of the commonly used OCT imaging system in the two aspects of focal depth and transverse resolution. Because the zone plate has many advantages such as flexible focal length and depth design, simple structure for fabrication, low cost and convenient micromation and integration, it is a potential long focal depth component for optical coherence tomography.
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.
An optimized coding gray-tone mask method with multi-parameters is presented in this paper. Precise calculations, which are performed on the dead area in the backward reflection of the pyramid prism, demonstrate that the fringe error caused by the coding gray-tone mask has little affect on the performance of the pyramid prisms. This not only greatly reduce the resolution requirement of the coding gray-tone mask, but also reduce the errors produced by the exposure
process. A satisfying simulation result is obtained with the designed coding gray-tone mask.
The CSG’s physical performance is decided by its structural parameters, such as width of grating stair, height of grating
stair, and the shape of stair edge. Consideration of the practical fabrication process, an error model of CSG’s structure is built to express the relationship between CSG’s performance and its structural parameters. In this model, each structural parameter of CSG is expressed as a variable, and the structural parameter error induced by fabrication process is expressed as minute disturb of relative variable. Through analysis and simulation, we obtained a general estimation standard for the structural parameter errors of CSG, which provides a theoretical direction for CSG’s fabrication and usage.
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.
The beam sampling grating is one of the important diffractive optical elements used in the field of laser sampling. It can be considered as an off-axis, binary phase, Fresnel zone plate. Base don the theory of interference, the principle of the variable period grating formation process is analyzed perfectly and the transmissive function of the BSG is obtained. This paper repots to fabricate the BSG with electron-beam direct writing. Compared with the holographic method, the new method is simpler, cheaper, and more efficient to manufacture elements on a large scale.
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 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.