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.
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.
Light with complex structures in polarization, phase and amplitude, has attracted a lot of attention in a broad range of applications and fundamental studies in classical and quantum optics. Along with the increased interest in structured light comes a need for efficient modulation platforms operating simultaneously for many modes. Multi plane light conversions (MPLC), i.e., multiple consecutive phase modulations in combination with free space propagation, have enabled such unitary transformations, which are usually built by bulky optical components, limited to scalar modulation, or rely on advanced nanofabrication techniques. Here, we demonstrate an efficient, monolithic MPLC architecture through direct laser writing in standard fused silica glass, resulting in a device with a compact form factor of only a few cubic millimeters. Our scheme is based on volumetric engineering of the glass's birefringence through laser-written nanogratings, which enables spatial control over full vectorial light structures. To showcase the approach's potential for integrated multimode-multipath optical networks, we demonstrate multi-mode unitary transformations, mode conversions, and complex beam-splitting for scalar light. We further extend the MPLC operation to vectorial light and implement various polarization-controlled spatial mode operations as well as the transformation of the topology of an optical Skyrmion. Finally, we highlight our scheme's promise for optical communications and implement a miniaturized multiplexer for spatial modes and polarization operating at telecom wavelength.
We present a compact and efficient realization of Multi-Plane Light Conversion (MPLC) for full vectorial light modulation, enabling control over polarization, phase, and amplitude. Conventional MPLC implementations based on phase-only modulation suffer from losses and limited polarization control, while metasurface-based solutions require complex nanofabrication processes. Our approach employs laser-written birefringent waveplates in silica slides, each with fixed retardance and a spatially varying optical axis orientation. The structures are fabricated using direct laser writing of nanogratings in silica, achieving ~90% transmission and ~1–2 µm spatial resolution. We demonstrate high-dimensional quantum gates and a polarization-controlled single-photon CNOT gate using spatial and polarization degrees of freedom. This cleanroom-free platform shows strong potential for high-dimensional quantum processing, optical communication, and beam shaping, with future prospects for extending modulation into the frequency domain.
Spatial transformations of light are ubiquitous in optics, with examples ranging from simple imaging with a lens to quantum and classical information processing in waveguide meshes. Multi-plane light converter (MPLC) systems have emerged as a platform that promises completely general spatial transformations, i.e., a universal unitary. However until now, MPLC systems have demonstrated transformations that are far from general, e.g., converting from a Gaussian to Laguerre-Gauss mode. Here, we demonstrate the promise of an MLPC, the ability to impose an arbitrary unitary transformation that can be reconfigured dynamically. Specifically, we consider transformations on superpositions of parallel free-space beams arranged in an array, which is a common information encoding in photonics. We experimentally test the full gamut of unitary transformations for a system of two parallel beams and make a map of their fidelity. We obtain an average transformation fidelity of 0.85±0.03. This high-fidelity suggests MPLCs are a useful tool implementing the unitary transformations that comprise quantum and classical information processing.
Abstract The ability to manipulate the spatial structure of light is fundamental for a range of applications, from classical communication to quantum information processing. Multi-plane light conversion (MPLC) addresses the limitations of single-plane modulation by enabling full control over multiple spatial modes of light through a series of phase masks separated by free-space propagation. In this tutorial, we present a step-by-step guide for building and aligning a 10-plane programmable light converter using a single spatial light modulator and standard optical components. Our method allows precise alignment, achieving single-pixel accuracy with a relatively simple setup. We hope that this guide will help other researchers to quickly adopt and adapt MPLC technology for their own experiments.
Programmable linear optical interferometers are a core primitive in optical signal processing, quantum information processing, and photonic computing. Existing photonic-integrated implementations realize arbitrary M-mode unitaries using Mach--Zehnder-interferometer meshes whose footprint and accumulated loss scale with O(M2) optical components. Here we analyze and experimentally demonstrate a programmable architecture for implementing linear optical transformations directly on spatially tiled free-space {\em pixel modes} using multi-plane light conversion (MPLC). In this architecture, M spatial modes arranged on a transverse lattice undergo a unitary transformation and are mapped to M output modes of identical geometry through a sequence of programmable phase masks separated by free-space propagation segments. Numerical simulations show that arbitrary M-mode unitaries can be compiled to a desired high fidelity using a number of phase planes that scales approximately linearly with M. Using a spatial-light-modulator-based MPLC, we experimentally demonstrate programmable interferometers acting on up to 16 spatial pixel modes, including tunable beamsplitters, Hadamard unitaries, spatial permutations, and partial unitaries on select subsets of modes. These results establish MPLC-based pixel-mode interferometers as a promising architecture for programmable linear optics with applications in classical and quantum optical interconnects, photonic switching, and quantum information processing.
Spatial transformations of light are ubiquitous in optics, with examples ranging from simple imaging with a lens to quantum and classical information processing in waveguide meshes. Multi-plane light converter (MPLC) systems have emerged as a platform that promises completely general spatial transformations, i.e., a universal unitary. However, until now, MPLC systems have demonstrated transformations that are far from general, e.g., converting from a Gaussian to Laguerre-Gauss mode. Here, we demonstrate the promise of an MLPC, the ability to impose an arbitrary unitary transformation that can be reconfigured dynamically. Specifically, we consider transformations on superpositions of parallel free-space beams arranged in an array, which is a common information encoding in photonics. We experimentally test the full gamut of unitary transformations for a system of two parallel beams and make a map of their fidelity. We obtain an average transformation fidelity of 0.85 ± 0.03. This high-fidelity suggests that MPLCs are a useful tool for implementing the unitary transformations that comprise quantum and classical information processing.