We show that the output mode of a single-mode optical fiber can be directly focused to a sub-wavelength waist with a finite working distance by tapering the fiber to a diameter of the order of the wavelength and terminating it with a spherically/hemi
spherically shaped tip. Numerical simulations show that a beam waist with a width of as small as 0.62lambda can be formed. We fabricate micro-lensed fibers and construct a probe-scanning confocal reflection microscope. Measurements on gold nano-particles show a spatial profile with a width of 0.29lambda for lambda = 850 nm, which is in good agreement with the numerical simulations. Due to their monolithic structures, these micro-lensed fibers will be flexible substitutes for conventional compound lenses in various experimental conditions such as cryogenic temperature and ultra-high vacuum.
