Miniaturised multi-plane light converters via laser-written geometric phase holograms

Multi-plane light converters (MPLCs) are an emerging 3D beam shaping technology capable of deterministically mapping a basis of input spatial light modes to a new basis of output modes. The ability to perform such spatial reformatting operations has many future applications in both classical and quantum photonics, spanning from optical communications to photonic computing and imaging. MPLCs are intricate optical systems consisting of a cascade of inverse-designed diffractive optical elements, typically separated by free-space. In this work we investigate the fabrication of miniaturised fully-encapsulated transmissive MPLCs within a glass chip using single-step direct laser writing. Our approach relies on the formation of femto-second laser induced birefringent nanogratings with a spatially controllable fast axis orientation. The glass chip is internally patterned with layers of these nanogratings to create multiple geometric phase holograms which imprint controllable phase patterns onto circularly polarised light propagating through them. We experimentally demonstrate two proof-of-concept glass-embedded 700x700x2000 micrometer cubed MPLCs: a 3-mode and a 10-mode Hermite-Gaussian mode sorter. We discuss the fabrication challenges and future improvements of these devices. Our work plots a path towards the rapid prototyping of robust monolithic MPLC technology.