Aberration control by high NA focusing in transparent media

High numerical aperture (NA) focusing of light in transparent media is important in microscopy, various techniques of optical data storage and laser microprocessing. By high NA focusing through a flat bounding surface of a medium there appears positive spherical aberration reducing light energy concentration and lowering physical resolution. The deeper is focusing and higher NA, the stronger is aberration and light scattering. By laser material processing the spherical aberration results in uncontrolled energy distribution along optical axis that makes inherently complex physical processes of laser radiation interaction with material even more complex and less predictable. To control the spherical aberration induced by internal high NA focusing it is suggested to apply specially designed aplanatic objective of patent pending optical design. The objective provides full compensation of spherical aberration for diffraction limited on-axis and off-axis high NA focusing at wide range of depths, for example from 0 to 2 mm in fused silica for NA0.8 without immersion. By setting movable components in optimum for particular depth reciprocal positions it is possible to provide not only full aberration compensation but also under- and over-compensation, thus providing control of energy distribution along optical axis at arbitrary depth and achieving special material processing effects. The paper describes design features of this optical approach as well as comparative results of spot size calculations and corresponding microphotographs of processed areas in fused silica by laser radiation focusing at various depths using NA0.8 objectives with and without compensation of aberrations.