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

TL;DR

This paper introduces a novel micro-lensed single-mode optical fiber with a high numerical aperture, capable of focusing light to sub-wavelength spots, and demonstrates its fabrication and application in high-resolution microscopy.

Contribution

It presents a new fiber tapering and shaping technique to create micro-lenses on optical fibers, achieving sub-wavelength focusing and practical microscopy applications.

Findings

Achieved a beam waist of 0.62λ in simulations.

Measured a spatial profile of 0.29λ on gold nanoparticles.

Demonstrated the fiber's use in a confocal reflection microscope.

Abstract

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/hemispherically shaped tip. Numerical simulations show that a beam waist with a width of as small as 0.62\lambda 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 0.29\lambda 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.