Binary phase masks for easy system alignment and basic aberration sensing with spatial light modulators in STED microscopy.

The use of binary phase patterns to improve the integration and optimization of spatial light modulators (SLM) in an imaging system, especially a confocal microscope, is proposed and demonstrated. The phase masks were designed to create point spread functions (PSF), which exhibit specific sensitivity to major disturbances in the optical system. This allows direct evaluation of misalignment and fundamental aberration modes by simple visual inspection of the focal intensity distribution or by monitoring the central intensity of the PSF. The use of proposed phase masks is investigated in mathematical modelling and experiment for the use in a stimulated emission depletion (STED) microscope applying wavefront shaping by a SLM. We demonstrate the applicability of these phase masks for modal wavefront sensing of low order aberration modes up to the third order of Zernike polynomials, utilizing the point detector of a confocal microscope in a 'guide star' approach. A lateral resolution of ~25 nm is shown in STED imaging of the confocal microscope retrofitted with a SLM and a STED laser and binary phase mask based system optimization.