Determination of geometric properties of SNOM tips by means of combined far-field and near-field evaluation

Scanning near field optical microscopes provide access to a variety of interesting material properties with a resolution in the nanometric size of scale. However, the quality of the optical fiber tip is of decisive importance. Because the production process of pulled and coated glass fiber tips is still highly empirical and full of defects, a technique would be useful to determine the tips' quality before they are shipped to the user or mounted in the microscope. This contribution shows an easy and fast full field method for the characterization of common 633 nm glass fiber SNOM tips. Size and shape as well as disturbances at the aperture can be recognized by means of evaluating the far field distribution of the emitted intensity and phase which are recorded by a CCD target. A numerical model is introduced which solves the reverse task that allows to draw conclusions from the measured intensity and phase distributions to the shape of the tip itself. Experimental investigation in a simple and robust setup and comparisons with combined near/far-field calculations show the working principle of this measurement technique for the analysis of SNOM tips.