Investigating Diffraction Efficiency Of Grating Fabricated By Femtosecond Laser In Fused Silica Glass

It is well known that femtosecond laser direct etching has great advantages, such as a small heat effect zone, high machining accuracy and easy operation, so it has been widely used in grating fabrication on transparent materials. In the paper, a series of one-dimensional, two-dimensional and three-dimensional gratings have been fabricated in the silica glass by using a femtosecond laser. The effects of the femtosecond laser parameters and grating structures on the diffraction efficiency of grating have been carefully studied. In addition, a new approach to fabricate high efficiency diffraction grating and high density grating has been developed. The following are the main content of the thesis.1. The effect of femtosecond laser and grating etching parameters on diffraction efficiency of the one dimensional grating. (1) the effect of laser power on the diffraction efficiency of grating; (2) The effect of femtosecond laser polarization on the diffraction efficiency of grating. It has been found for those one-dimensional grating fabricated in the fused silica glass by using the Ti:sapphire femtosecond laser amplifier with a repetition rate of 1 kHz, both the laser polarization and the laser power have a great influence on their diffraction efficiency. During the process, all the gratings were fabricated with the same laser power and the same moving speed of the platform. When the gratings were fabricated with the femtosecond laser polarization perpendicular to the moving direction of the platform, the diffraction efficiency of the gratings would increase 2.83 times compared with the gratings that were fabricated with the femtosecond laser polarization parallel to the moving direction of the platform. When the laser power varying from 0.47mW to 2.06 mW, the diffraction efficiency of grating first increased and then decreased. The maximum diffraction efficiency was obtained when the fabrication laser power equals to 1.6 m W.2. The effect of grating layers on diffraction efficiency of grating. Including (1) the effect of grating layers on the diffraction efficiency of one-dimensional grating. The one-dimensional gratings were fabricated in the fused silica glass by the same laser parameters and grating constant. It was found along with the increment of the layer number, the diffraction efficiency of the grating increases as well. When the grating layer was 6, the diffraction efficiency was 6.7 times higher than the efficiency of single layer one-dimensional grating; (2) the effect of grating layers on the diffraction efficiency of two-dimensional grating. The one layer two-dimensional grating and the multi-layer two-dimensional grating were fabricated with the same laser parameters. Similar to that of one-dimensional grating, the diffraction efficiency of two-dimensional grating also increases with the increment of layers. And when the grating layer was 3, the diffraction efficiency was 5.15 times higher than the efficiency of single layer two-dimensional grating.3. High density gratings fabricated by femtosecond laser. It was found when a two-layer one dimensional grating with the identical grating constant for each layer was fabricated in the fused silica glass by the femtosecond laser, if the second layer was not fabricated right below the first layer, but offset half of a grating constant. The efficiency of the odd order diffraction of signal layer grating with the identical grating constant will decrease. Along with the decrease of the distance between layers, this phenomenon becomes more obvious. If this kind of the two-layer one dimensional grating was considered as a diffraction optical unit, and more identical units were fabricated right below the first one in the fused silica glass. It was found, along with the layer number increase, the efficiency of the odd order diffraction will decrease more. In contrast the efficiency of even order diffraction will increase. The diffraction pattern of this multilayer grating are similar to that of a one dimensional high density grating with a two times higher grating constant but with a much higher diffraction efficiency. Indicating a novel approach to fabricate gratings using femtosecond laser, which have a higher grove density and higher diffraction efficiency.