Multi-Plane Light Conversion for Scalar and Vectorial Spatial Mode Transformations

Abstract

Light can be structured across its degrees of freedom — spatial amplitude, phase, polarization, spectral, and temporal domains — to produce optical fields with tailored structures. Controlling these structures has become a central field in modern photonics, enabling advances in optical communications, quantum information processing, and imaging. Many of these applications require devices that can transform sets of spatial modes in an arbitrary and lossless manner. Multi-plane light conversion (MPLC) — the cascading of several phase-modulation planes with free-space propagation — has emerged as a powerful framework for this task. This thesis develops MPLC as a tool to interface with, transform, and process structured light across different photonic platforms, and introduces a new route for implementing it in a compact form.

I first used a spatial light modulator (SLM) to build reconfigurable MPLC interfaces between platforms whose modes do not naturally match: a fully automated interface that couples the complex output of a multimode fiber into a single-mode fiber in real time, and a passive interface that converts free-space modes into the guided modes of an integrated silicon waveguide across the telecom band. I then address the bulk, alignment sensitivity, and scalar-only operation of conventional SLM-based MPLC by introducing a monolithic, volumetric MPLC inscribed directly into fused silica glass by femtosecond laser direct writing. Birefringent nanogratings form structured half-wave plates that act on both circular-polarization components via the geometric Pancharatnam–Berry phase, thereby providing full control over the scalar and polarization degrees of freedom within an intrinsically aligned chip a few cubic millimeters in size. A wide range of scalar and vectorial transformations are demonstrated, including high-dimensional unitary quantum gates, mode conversion and beam splitting, polarization-controlled spatial operations, optical-skyrmion topology conversion, and high-dimensional mode sorting at telecom wavelength. Together, these results establish laser-written volumetric MPLC as a compact and versatile platform for integrated structured-light processing in both classical and quantum applications.