Fused silica transmission grating plays an important role in the ultra-short laser pulse compression system. Fused silica transmission grating have the advantages of high diffraction efficiency, high damage threshold, long life and no shelter. The design and fabrication of pulse compression grating are investigated theoretically and experimentally in this paper. Rigorous coupled wave theory is used for design transmission grating with trapezoidal structure. The trapezoidal structure has better diffraction efficiency than that of the rectangular structure. The deep-etched fused silica transmission grating (1250lp/mm) is fabricated by holographic recording and ion beam etching. The aperture of transmission grating is Φ65mm, and its thickness is 1mm.The absolute -1st diffraction efficiency is about to 98%(@808nm). Experimental results are coincident with the theoretical analysis.
The metal grating has been widely used in the tunable laser to select frequency, the diffraction efficiency and damage threshold of the gratings are very important to the performance of high power tunable laser. However, because of the light absorption of metal material, the damage threshold of the metal grating is usually not high especially at short wavelength, it is hard to meet the requirements of high power laser. This paper presents the laser frequency selection with multilayer dielectric grating, in this method, the high diffraction efficiency is ensured, and the damage threshold of the grating is improved. Based on the rigorous coupled wave (RCWA) theory, the model of multi-layer dielectric film grating is established, and the theoretical design for the 473nm laser is given. After a series of optimal design, the following results are obtained. HfO2 and SiO2 are selected as multi-layer material, and the multi-layer structure is S(HL)^12HTA .The groove density is 3875 lines/mm. The profile of grating grooves is rectangular .The duty cycle of surface relief structure is between 0.31-0.35, the groove depth is between 270-310nm, the sum of residual thickness and groove depth is between 310-320nm.The -1st diffraction efficiency of the grating is over 98% (TE polarization) at the Littrow angle (66.4 degrees).The diffraction efficiency is higher than that of ordinary metal grating. At the same time, the electric field distribution of the grating is optimized, the peak electric field is avoid located at surface relief structure, and the laser induced damage threshold can be improved.
Based on holographic recording technique, a study of the precise control of grating masks has been carried out. The incident light can be heavily absorbed even that photoresist thickness is under the submicron scale. The absorption rate of the photoresist will change greatly along the exposure processing, and it can serious influence the exposure dose of the photosensitive resin in the photoresist. In the old photographic models, it can’t get practical exposure dose distribution in the photoresist. In this paper, a new nonlinear photographic model is proposed, which is based on the absorption rate changing with the exposure dose. The new model can get more accurate exposure dose than that of the old models. The experiment of the developing speed changing with the exposure dose is carried out, and the developing speed variation curve is obtained. The experiment of the absorption rate changing with the exposure dose is also carried out, and the absorption rate variation curve is obtained. The experiment of fabricate holographic grating is also carried out. The dynamic analysis model of the grating mask structure agrees with the experimental results. The new nonlinear photographic model is of great significance to improve the control precision of holographic grating mask.
The stylus profiler is a conventional instrument in surface topography measurement. The interferometric stylus profiler which uses cylindrical phase grating as standard obtains the surface topography information via measuring the changes of phase. In this way, it is apt to realize wider range, higher accuracy and higher resolution simultaneously in topography measurement. Currently, cylindrical phase grating is ordinarily fabricated by means of holographic record. Subject to the present method and technics, the microstructure parameters of the grating, such as spatial frequency, groove depth and duty cycle can hardly been uniform in every area of grating. Concerning the working principle of interferometric stylus profiler with cylindrical phase grating as sensor, the quality of cylindrical hologram phase grating (80mm in radius of curvature, 1200lp/mm in center spatial frequency) is analyzed comprehensively in this paper. Effects of the inconsistency distribution of microstructure parameter of grating over cylinder surface on the phase difference between ±1st order diffraction wave-fronts in different incidence angle are discussed in detail. The theoretical analysis and numerical calculation results show that: the holographic recording parameters determine the distribution of spatial frequency of cylindrical hologram phase grating; the inconsistency of spatial frequency on the cylinder surface is the primary cause affecting measurement accuracy; the inconsistency of duty cycle of grating will have influence on measurement accuracy when the incidence angle is not equal to zero (i.e.in a small incidence angle). Therefore, the process tolerances of cylindrical phase grating are presented and a new fabrication method of high precision cylindrical phase grating is proposed.
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