Tip optics for illumination NSOM: extended-zone approach

We examine the coupling of light from a metal-coated fiber through a tapered conic section to a sub-wavelength aperture at the taperʹs apex. Using geometric optics and introducing a repeated-zone scheme, we develop expressions for throughput and gain of this linear taper. The expressions allow for consideration of arbitrary angular distribution of light at the large end of the taper, and thus permit analysis in terms of distributions which represent light in propagating modes of a fiber. Solutions for throughput and gain of a taper pulled from a fiber are presented. Extension to geometric optics analysis of non-linear tapers is outlined.