The modification of transparent materials with femtosecond lasers has a lot of interest in data processing, waveguides and diffractive optical elements (DOE) development fields. In our research, we are focused on transparent materials processing with a deeply focused Gaussian beam. As well known, the internal modifications induced in fused silica with high numerical aperture objectives are affected by spherical aberration. In most cases, this phenomenon is unwanted and needs to be compensated to get the width to height ratio close to 1:1 for high-quality waveguides writing etc. However, when the focusing is quite deep (> 1 mm), due to the energy dissipation the modification can be formed only in the central part of the laser beam. Consequently, the radial size of modifications is reduced less than the diffraction limit. This property can be successfully used to record high-density volume DOE with the diffraction efficiency > 90%.
In this work, we develop the method to record DOE with multi-level binary refractive index modification distribution in bulk fused silica. In the beginning, the transverse modification length induced by single pulse elongation to the wider area is investigated to find the conditions where only type-I modification is induced. The two-level binary phase by a single modification depth > 50 µm can be achieved. In the next step, the desired multi-level binary phase distribution is simulated according to the required intensity distribution. Then the slicing of the multi-level binary phase to the two-level binary phase images is performed. This method involves recording the phase elements slice by slice with the resolution limited to the minimal induced phase change in one separate layer.