Fabrication And Investigation Of Etching Process Of Broadband Pulse Compression Gratings

Large-size pulse compression gratings (PCG) are the key components in large aperture pulse compression systems. The PCGs for high-power laser systems include multilayer dielectric grating (MDG), metal multi-layer dielectric grating (MMDG) and gold-coated grating.The main purpose of this thesis is to fabrication of the broadband pulse compression grating and to improve the diffraction efficiency of the multi-layer dielectric grating. The subject matter of the thesis as the follows:(1) Fabrication of sinusoidal broadband gold-coated gratings. Compared with other gratings, gold-coated gratings have the advantages of simple structure and low cost etc. More importantly, this kind of gratings can get high diffraction efficiency within a broadband range. Based on above advantages gold-coated gratings have gained lots of attention in the fields of the short-pulse (shorter than 100fs), high-power laser pulse compression systems. In order to satisfy the needs of the high-power laser system, Broadband sinusoidal gold-coated gratings (BSGG) with line densities of 1740 lines/mm and groove depth of 210 nm have been successfully fabricated via the methods of trapezoidal grating-coating-deposition (TGCD) and holographic lithography-deposition (HLD). The average diffraction efficiency at the-1 order (Littrow mount) is above 87% and the peak value is 90% for TM polarized light spanning wavelengths from 750 to 850 nm. The experimental results show that:Gratings groove depth can be easily controlled by the methods of TGCD and HLD. After removing the coated photoresist, the substrate could be reused. Diffraction efficiency and bandwidth meet the requirements of the domestic general broadband pulse compression gold-coated gratings. Those two methods take into account the requirements of large aperture broadband gold-coated gratings groove depth uniformity, therefore this paper has a good reference value to the further fabrication of larger aperture gold-coated gratings.(2) Fabrication and testing results of sine-top, high-efficiency, broadband gold-coated gratings (BGCG) for high power laser pulse compression applications are reported. These gratings differ from conventional metal-on-photoresist pulse compression gratings (PCG) in that the gratings patterns are generated by etching the quartz substrate directly. The groove depth and duty cycle of the photoresist mask was controlled by changing photoresist thickness and adjusting exposure and development time, respectively. The duty cycle of the photoresist mask was further corrected by oxygen plasma etching. Using this method, high efficiency, sine-top, BGCG with line densities of 1740 lines/mm was achieved. The average diffraction efficiency at the -1st order was 89.2% and the peak value was 90% for TM polarized light as the wavelength increases from 750 to 850 nm.(3) Etching process of the metal multi-layer dielectric gratings. As a new type of pulse compression grating, MMDGs are still in the stage of lab research. So it is necessary to have a systematic and detailed study on the fabrication of MMDGThe etching process of the MMDGs consists two parts: ① The judgments of initial photoresist mask; ② The graphic transfers during the ion beam etching. In order to reduce the difficulty of photoresist mask processing, we use CHF3 as the work gas. A qualitative judgment was received by large number of statistical results, which mask must be at a high degree of> 260 nm and duty cycle> 0.15. The MMDG groove depth is about 300 nm, duty cycle is in the range of 0.3-0.4.(4) Improve the diffraction efficiency of the multilayer dielectric gratings. In this paper, we analyze the diffraction efficiency of PCG, when top layer is SiO2. Diffraction efficiency is the critical parameter of the pulse compression gratings, and optimization of grating shape parameters (groove depth and duty cycle) can achieve higher diffraction efficiency. Ashing and oxygen etching methods are used to reduce the photoresist mask duty cycle, and pressing method is used to increase the duty circle of photoresist mask. Best photoresist grating mask could be obtained by effectively combining the above two methods. After etching, the better duty cycle is in the range of 0.35-0.5. The best groove depth is in the range of 260-460 nm when the top layer SiO2 thickness is 431 nm. If the photoresist grating mask bottom is not clean, the side walls would not be steep and duty cycle would be too big or too small, thus multi-layer dielectric grating diffraction efficiency would decrease. Solving these problems is the key to improve the diffraction efficiency.