Research of refractive index mismatch induced aberration effect on focused laser radiation

Distortion of intensity distribution in the near- field focus region due to the index-mismatch of sample and surrounding medium is commonly faced issue in various laser micro-machining tasks, such as: fiber Bragg grating and waveguide fabrication. Aberrations resulting from the material index-mismatch greatly reduce the quality of focal spot and in order to maintain precision of fabricated structures, compensation of aberrations is necessary. In this work a method of aberration compensation and results of experimental testing are presented. By the use of simple geometrical optics rules, compensational phase distribution is numerically calculated and experimentally tested using spatial light modulator. In order to increase effectiveness of proposed method due to the discretizational nature of spatial light modulator, additional axial shift of the focusing objective is iteratively computed with the aim of minimizing defocus and spherical aberrations. Key results of carried study are as follows: 1.Applying compensational phase distribution, by the use of spatial light modulator, and while a small focus depth inside material is maintained, partial compensation for the position of generated defects in axial direction was observed. In the case when depth of focus is more than 150 μm, error in the axial position of nominal and measured focus exceeds 25 %. Further increasing focus depth beyond 200 μm leads to no observed compensational effects. 2.By applying certain axial displacement of the focusing lens, the influence of first-order optical aberrations was successfully reduced. Due to the decreesed values of compensationl phase distribution, increase in aberration compensational effectiveness can be achieved. 3.By applying axial displacement of the focusing lens and compensational phase distribution, by the use of Spatial Light Modulator, compensation of focus axial displacement and decrease of intensity distribution distortion in the focus near-field region was absorved in a wide range of focus depths. Additional axial objective displcement allows use of presented phase correction method in processes of laser material proccessing, which require formation of precise, spatially defined defects in material volume