Near-field measurement of modal interference in optical nanofibers for sub-Angstrom radius sensitivity

TL;DR

This paper presents a nondestructive, high-resolution method using a microfiber to measure and map the radius of optical nanofibers by analyzing modal interference patterns, achieving sub-Angstrom sensitivity.

Contribution

The authors introduce a novel microfiber-based technique for real-time, nondestructive measurement of optical nanofiber radius with 40 picometer resolution, surpassing previous methods.

Findings

Achieved 40 pm spatial resolution in radius measurement.

Validated measurements against SEM with high accuracy.

Demonstrated immunity to polarization and vibration effects.

Abstract

Optical nanofibers (ONF) of subwavelength 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 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 radius, 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 micrometer window) along the 10 mm ONF waist with 40 picometer resolution and high signal-to-noise ratio. The measurements agree with scanning electron microscopy (SEM) to within SEM instrument resolution. This fast method is immune to polarization, intrinsic birefringence, mechanical vibrations, scattered light, and provides a set of constraints to protect from systematic errors in the measurements