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We show that the output mode of a single-mode optical fiber can be directlyfocused to a sub-wavelength waist with a finite working distance by taperingthe fiber to a diameter of the order of the wavelength and terminating it witha spherically/hemispherically shaped tip. Numerical simulations show that abeam waist with a width of as small as 0.62\lambda can be formed. We fabricatemicro-lensed fibers and construct a probe-scanning confocal reflectionmicroscope. Measurements on gold nano-particles show a spatial profile with awidth of 0.29\lambda for \lambda = 850 nm, which is in good agreement with thenumerical simulations. Due to their monolithic structures, these micro-lensedfibers will be flexible substitutes for conventional compound lenses in variousexperimental conditions such as cryogenic temperature and ultra-high vacuum.

Micro-lensed single-mode optical fiber with high numerical aperture