| Abstract [eng] |
Diffractive optical elements are typically made by photolithography or other surface structuring techniques. However, volume direct laser writing technology attracts more interest, which allows to record phase diffractive optical elements or phase masks inside the volume of a transparent material. This technology is based on recording refractive index change modifications. Modifications are stretched along the beam propagation direction because of mismatch between refractive indices of two different materials. Because these modifications are described by their axial length, recorded element layers can be positioned in such way that the layers overlay by the same distance. This guarantee that the element will be homogeneous, but in to make the element more homogenous it is necessary to optimize the refractive index through the whole element. This is done by measuring the refractive index change with different parameters and using these results to record elements. The aim of this work was to record refractive index change modifications inside fused silica with femtosecond laser, to find suitable parameters to obtain the highest homogeneous refractive index change and use these optimized parameters for recording phase masks. For modification axial length measurement different parameters were used: pulse energy, scanning speed, radiation focus position and polarization. Afterwards, modulation of refractive index was measured. For this reason, rectangular parallelepiped structures made from I type modifications with different overlay values were recorded. For the measurements Michelson interferometer was used. Results were used for writing a linear pulse energy reduction formula to optimize refractive index through the whole thickness of the element. When the homogeneous refractive index was obtained, calculations were done to find phase shift of 2π. Calculations for volume Fresnel lens and fraxicon were made to figure out the technological capabilities of this recording technology. Volume Fresnel lens focus position is 15 % smaller compared to the focus position written in program recipe and the highest base angle of the volume fraxicon can be only 2,1 °. Results were used to record these two elements inside the fused silica sample. Measured volume Fresnel lens position differed from the written in program recipe and diffraction efficiency reached only 38 %, while fraxicon base angle was like the written in the program and diffraction efficiency reached 58 %. |
