# Complete polarization control for a nanofiber waveguide using directional coupling

## TL;DR

This paper introduces a novel nanofiber directional coupler that enables complete polarization control with high fidelity, overcoming the inherent polarization maintenance limitations of optical nanofibers.

## Contribution

The work presents a new crossed nanofiber coupler design that allows for precise polarization state preparation and control along a nanofiber waveguide.

## Key findings

- Achieved polarization state preparation with >99% fidelity.
- Demonstrated complete polarization control method.
- Validated directional coupling based on asymmetric dipolar emission.

## Abstract

Optical nanofiber waveguides are widely used for near-field delivery and measurement of light. Despite their versatility and efficiency, nanofibers have a critical drawback - their inability to maintain light's polarization state on propagation. Here, we design a directional coupler consisting of two crossed nanofibers to probe the polarization state at the waist region. Directionality of coupling occurs due to asymmetric dipolar emission or spin-locking when the evanescent field pattern breaks the mirror symmetry of the crossed-nanofiber system. We demonstrate that, by monitoring the outputs from the directional coupler, two non-orthogonal polarization states can be prepared at the nanofiber waist with a fidelity higher than 99%. Based on these states, we devise a simple and reliable method for complete control of the polarization along a nanofiber waveguide.