# Demonstration of an optical-coherence converter

## TL;DR

This paper demonstrates experimentally that coherence can be reversibly exchanged between polarization and spatial degrees of freedom of an optical field, enabling control over coherence properties without energy loss.

## Contribution

It introduces a method to convert and exchange coherence between different optical degrees of freedom, verified through experimental demonstration.

## Key findings

- Coherence can be reversibly exchanged between polarization and spatial DoFs.
- A spatially incoherent polarized field can be transformed into a coherent unpolarized field.
- The entropy transfer makes the field resistant to polarization scrambling effects.

## Abstract

Studying the coherence of an optical field is typically compartmentalized with respect to its different optical degrees of freedom (DoFs) -- spatial, temporal, and polarization. Although this traditional approach succeeds when the DoFs are uncoupled, it fails at capturing key features of the field's coherence if the DOFs are indeed correlated -- a situation that arises often. By viewing coherence as a `resource' that can be shared among the DoFs, it becomes possible to convert the entropy associated with the fluctuations in one DoF to another DoF that is initially fluctuation-free. Here, we verify experimentally that coherence can indeed be reversibly exchanged -- without loss of energy -- between polarization and the spatial DoF of a partially coherent field. Starting from a linearly polarized spatially incoherent field -- one that produces no spatial interference fringes -- we obtain a spatially coherent field that is unpolarized. By reallocating the entropy to polarization, the field becomes invariant with regards to the action of a polarization scrambler, thus suggesting a strategy for avoiding the deleterious effects of a randomizing system on a DoF of the optical field.