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arXiv:2604.18885

Volumetric Processing of Structured Light Integrated in Glass

Oussama Korichi, Markus Hiekkamaki, Robert Fickler

physics.opticsquant-phSubmitted Apr 20, 2026• Announced Apr 22, 2026
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Abstract

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.

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Optics Express•Jan 15, 2025•5 citations
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.
Multi-plane light converters (MPLCs) are an emerging beam shaping technology capable of deterministically mapping a basis of input spatial light modes to a different basis of output modes. The ability to perform such multi-modal spatial reformatting operations has many future applications in both classical and quantum photonics, spanning from optical communications to photonic computing and advanced imaging. In this work we fabricate miniaturised transmissive MPLCs fully-encapsulated within a fused silica glass chip using single-step 3D direct laser writing. Our approach relies on the formation of femto-second laser induced birefringent nanogratings with a spatially controllable slow-axis orientation. Multiple layers of these nanogratings are laser-written throughout the volume of the glass to create a sequence of axially separated geometric phase holograms which imprint controllable phase patterns onto circularly polarised read-out light propagating through them. We construct and test a range of proof-of-concept laser-written MPLCs operating in the visible (lambda = 633nm). These miniature beam multiplexers are formed from up to 5 separate phase masks of width ~260um, cascaded along a total length of ~2.7mm, thus occupying a compact volume of ~0.15 mm3. We first demonstrate Hermite-Gaussian (HG) mode sorters capable of diverting the energy carried by up to 28 overlapping HG modes into spatially separated output channels. We next create a 7-mode orthogonal speckle sorter, highlighting the universal nature of the spatial transformations it is possible to encode. Finally, we show analogue optical matrix multiplications achieved by passively scattering light through these 3D structured glass elements. Our work begins to merge the concepts of free-space optics with 3D integrated photonics in glass and plots a path towards the rapid prototyping of robust monolithic MPLC technology.
This thesis explores the development and application of multi-plane light conversion (MPLC) technol-ogy, focusing on its integration into experimental setups that push the boundaries of quantum opticsand spatial state tomography. Central to this study is a custom-built, research-grade MPLC devicedesigned around a programmable spatial light modulator (SLM), which facilitates two groundbreak-ing experiments: the manipulation of high-dimensional qudits for quantum gate operations, and thedevelopment of a high-dimensional Stokes-space spatial beam analyser for single-shot spatial statetomography.The first experiment involved implementing quantum gates — specifically the X-gate, Z-gate, andDFT-gate — for a 17-dimensional qudit, represented through the first 17 azimuthal Laguerre-Gaussianspatial modes on a MPLC device. This experiment not only demonstrated MPLC’s capability tohandle complex, high-dimensional quantum states but also underscored its potential to enhance thescalability and efficiency of quantum computing architectures. By leveraging the precise control overspatial modes afforded by MPLC and the programmability of SLM-based MPLC, the experimentdemonstrated high-dimensional gate operations, paving the way for more complex quantum computingoperations for qudits.The second project focused on using MPLC to develop a device capable of performing high-dimensional Stokes-space spatial beam analysis. This innovative instrument marks a significantbreakthrough in optical beam characterisation, providing a reference-free, single-shot technique forconducting spatial state tomography on beams characterisable by a six-mode or six-dimensional system.The device’s capacity to swiftly and precisely characterise the wavefront and coherence of opticalfields renders it an invaluable asset for advancing optical communication technologies and enhancingoptical sensing methodologies.Throughout this thesis, the theoretical foundations of MPLC technology are presented, with adetailed account of the MPLC device design and operational principles. Experimental results arepresented to validate the effectiveness of the MPLC device in both quantum gate operations andspatial state tomography. Comparisons with existing technologies highlight the MPLC’s enhancedperformance and versatility.This work not only expands the understanding and application of MPLC in quantum and opticalsciences but also sets the stage for future innovations in these fields. The continuous refinement andadaptation of MPLC technology promise to unlock new possibilities in both fundamental research andpractical applications.
Programmable linear optical interferometers are a core primitive in optical signal processing, quantum information processing, and photonic computing. Existing photonic-integrated implementations realize arbitrary MMM-mode unitaries using Mach--Zehnder-interferometer meshes whose footprint and accumulated loss scale with O(M2)O(M^2)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, MMM spatial modes arranged on a transverse lattice undergo a unitary transformation and are mapped to MMM output modes of identical geometry through a sequence of programmable phase masks separated by free-space propagation segments. Numerical simulations show that arbitrary MMM-mode unitaries can be compiled to a desired high fidelity using a number of phase planes that scales approximately linearly with MMM. Using a spatial-light-modulator-based MPLC, we experimentally demonstrate programmable interferometers acting on up to 161616 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.
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.
Structured light beams are essential in optical communications, quantum information processing, and light-matter interactions. The generation and conversion of high-purity structured light beams are crucial for these applications. In this study, we propose a bidirectional high-purity structured light beam transformation approach based on the multi-plane light conversion (MPLC) method. This approach enables mutual conversion of arbitrary orthogonal modes and precise control of system loss and mode purity. Using the proposed method, we demonstrate the conversion of 7-modes Gaussian beams array (GBA) and perfect vortex beams (PVB), achieving a bidirectional output modes signal-to-noise ratio (SNR) exceeding 30 dB and a loss of 5.23 dB. For generation of 13-modes, an SNR above 25 dB and an insertion loss below 5.45 dB are obtained. Furthermore, the validity of methods is confirmed by generating multi-wavelength and structured light beams with more complex field distribution. The work paves the way toward structured-light-beam-based optical communications, high-dimensional quantum communications, and advancements in MPLC.