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Modal interference in optical nanofibers for sub-Angstrom radius sensitivity

TitleModal interference in optical nanofibers for sub-Angstrom radius sensitivity
Publication TypeJournal Article
Year of Publication2017
AuthorsF. K. Fatemi, J. E. Hoffman, P. Solano, E. F. Fenton, G. Beadie, S. L. Rolston, and L. A. Orozco
Date PublishedJan
KeywordsFiber characterization, Fiber optics, Nanophotonics and photonic crystals

Optical nanofibers (ONFs) of sub-wavelength dimensions confine light in modes with a strong evanescent field that can trap, probe, and manipulate nearby quantum systems. To measure the evanescent field and propagating modes and to optimize ONF performance, a surface probe is desirable during fabrication. We demonstrate a nondestructive near-field measurement of light propagation in ONFs by sampling the local evanescent field with a microfiber. This approach reveals the behavior of all propagating modes, and because the modal beat lengths in cylindrical waveguides depend strongly on the radius, it simultaneously provides exquisite sensitivity to the ONF radius. We show that our measured spatial frequencies provide a map of the average ONF radius (over a 600&\#x00A0;&\#x03BC;m window) along the 10&\#x00A0;mm ONF waist with a 40&\#x00A0;pm resolution and a high signal-to-noise ratio. The measurements agree with scanning electron microscopy (SEM) to within SEM instrument resolutions. This fast method is immune to polarization, intrinsic birefringence, mechanical vibrations, and scattered light and provides a set of constraints to protect from systematic errors in the measurements.