Fast profile measurement of micrometer-sized tapered fibers with better than 50-nm accuracy

The forward scattering of light illuminating a transparent dielectric cylinder, such as a tapered fiber, from the side can be understood as interference of the diffracted, reflected, and transmitted light. Additionally, light can be resonantly coupled into the fiber if a multiple of the wavelength matches the circumference. Using a suitable laser setup with a novel evaluation algorithm allows us to quickly extract the fiber radius from the complex diffraction pattern, obtaining an accuracy of better than 50 nm. We demonstrate experimentally our method, which is noncontact and allows one to simultaneously measure the profile of a several-centimeter-long fiber waist with a diameter near the diffraction limit.