Separating partially coherent light

TL;DR

This paper introduces a scalable, integrated photonic method for automatically separating and measuring the coherence modes of partially coherent light, enabling advanced control in optical imaging and communication.

Contribution

The authors develop a silicon photonic interferometer architecture that finds and measures coherence modes of partially coherent light through in situ optimization, scalable with the coherence matrix rank.

Findings

Successfully separated two coherence modes from overlapping laser fields.

Demonstrated linear scalability of the method with the coherency matrix rank.

Benchmarking shows superior performance over traditional tomographic methods.

Abstract

Recent advances in optical imaging and communication increasingly involve high-dimensional, partially coherent light, creating a growing need for scalable tools to measure and manipulate coherence. Here, we demonstrate the automatic separation of spatially partially coherent light into "coherence modes" -- its orthogonal and mutually incoherent components. To make this separation possible, we exploit variational processing in layered self-configuring interferometer architectures in a silicon photonic circuit. This process formally finds and measures the eigenvectors and eigenvalues of the coherency matrix, hence measuring the partially coherent state, while leaving it intact and separated after optimization. Furthermore, we show that mutually incoherent beams, if spatially orthogonal, can be automatically separated even if they are completely overlapped, hence separating unknown laser beams based only on their mutual incoherence. Our experiment finds and separates the two strongest coherence modes starting from a nine-mode sampling of the partially or fully overlapping fields from two independent lasers. The method requires a number of physical components that scales linearly with the rank $r$ of the coherency matrix and operates through a sequence of $r$ in situ gradient-based optimizations enabled by electronic drive frequency multiplexing of interferometer phase shifters. We benchmark its performance against a mixture-based tomographic method, also implemented on chip. These results establish a scalable framework for programmable coherence analysis and control in imaging, communication, and photonic information processing.