Tiny Laser-Built Device Reshapes Light for Faster Computing and Imaging
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Researchers are developing multi-plane light converters (MPLCs), a novel 3D beam shaping technology with potential applications ranging from optical computing to advanced imaging. Unė G. Būtaitė from University of Exeter, Martynas Beresna from University of Southampton, and David B. Phillips from University of Exeter, alongside their colleagues, have demonstrated a significant advance in this field by fabricating miniaturised, fully-encapsulated MPLC devices within a single glass chip using direct laser writing. This innovative approach utilises femtosecond laser-induced nanogratings to create geometric phase holograms, enabling precise control over light propagation and offering a pathway towards robust and rapidly prototyped monolithic MPLC technology.
The team experimentally validated their design with functioning 700x700x2000 micrometer cubed devices capable of sorting Hermite-Gaussian modes, representing a crucial step towards practical implementation. These devices represent a significant advance in 3D beam shaping technology, enabling the deterministic mapping of light modes with potential applications spanning optical computing and imaging. The research demonstrates the creation of fully-encapsulated, transmissive MPLCs through a single-step direct laser writing process, circumventing the need for complex alignment procedures typically associated with these systems. This breakthrough relies on the formation of birefringent nanogratings with precisely controlled fast axis orientations, patterned internally within the glass chip. Layers of these nanogratings function as geometric phase holograms, imprinting controllable phase patterns onto circularly polarised light as it propagates through the device. By manipulating the phase of light in this manner, the MPLC can reshape and redirect optical beams with high precision.
The team successfully fabricated two proof-of-concept devices, measuring 700x700x2000 micrometers cubed, designed as 3-mode and 10-mode Hermite-Gaussian mode sorters. The fabricated MPLCs efficiently separate and redirect different transverse spatial light modes, demonstrating the potential for parallel processing of multiple input modes. This capability surpasses the limitations of single-plane devices, such as spatial light modulators, which typically address only one mode at a time. The ability to efficiently sort up to 10 Hermite-Gaussian modes within a compact volume highlights the potential of this technology for applications including optical communications, super-resolution imaging, and even linear optical computing. This work establishes a pathway towards rapid prototyping of robust, monolithic MPLC technology, offering a significant step towards realising the full potential of advanced photonic systems. The researchers are actively addressing fabrication challenges and exploring future improvements to enhance the performance and scalability of these miniaturised optical components. Fabrication of multi-plane light converters via femtosecond laser induced nanograting patterning A 72-qubit superconducting processor forms the foundation of this research into miniaturised multi-plane light converters (MPLCs). Researchers fabricated fully-encapsulated transmissive MPLC devices within a glass chip using single-step direct laser writing. This approach relies on creating femtosecond laser induced birefringent nanogratings with spatially controllable fast axis orientations. The glass chip was internally patterned with layers of these nanogratings to generate multiple geometric phase holograms. These holograms imprint controllable phase patterns onto circularly polarised light as it passes through them. A custom-built direct laser writing system was employed, utilising a femtosecond laser beam initially linearly polarised. A half wave-plate, held in a motorised rotation mount, adjusted the polarisation orientation before the beam was focused into the glass substrate by an objective lens. Three-dimensional translation stages synchronised the laser emission and half wave-plate orientation, moving the glass relative to the writing beam along Cartesian coordinates. The MPLC inverse-design algorithm yielded optimised phase profiles for each plane, subsequently mapped to the writing orientation of the nanogratings. This determined the local fast-axis of the geometric phase holograms. Experimentally, two proof-of-concept glass-embedded MPLC devices, measuring 700x700x2000 micrometers cubed, were demonstrated. These devices functioned as 3-mode and 10-mode Hermite-Gaussian mode sorters. Laser-written geometric phase holograms were created by generating birefringent nanogratings within high-purity silica glass. Controlling the fast-axis orientation of these structures via the linear polarisation state of the writing beam enabled the fabrication of geometric phase holograms acting as half wave-plates with spatially-varying fast-axis orientation. Illumination with circularly polarised light imparted a spatially-varying geometric phase, dictated by the fast-axis orientation pattern. This process imprinted pixelated phase patterns onto incident beams, resulting in fully encapsulated, miniaturised MPLC devices operating on transverse spatial modes with specified circular polarisation. Miniaturised multi-plane light converters for Hermite-Gaussian mode sorting and far-field image projection Researchers demonstrated miniaturised multi-plane light converters (MPLCs) fabricated within a glass chip using direct laser writing. These devices, measuring less than 0.8 cubic millimetres in volume, were inverse-designed to sort up to 10 Hermite-Gaussian (HG) modes. A single geometric phase hologram, one square millimetre in area and with a resolution of 3.0 micrometres, successfully projected a target image into the far-field when illuminated with a 633 nanometre wavelength Gaussian beam. Measurements revealed approximately 13 per cent of the transmitted energy was contained within the zero diffraction order when the opposite circular polarisation was filtered. The study then focused on a two-plane MPLC designed to sort three HG modes into focused spots arranged around a ring in the far-field. Nanograting layers were vertically spaced by 44 micrometres to achieve a response approximating a half wave-plate. Simulated coupling matrices for the 3-mode HG sorter exhibited a mean off-diagonal intensity value of 0.02, while experimentally measured matrices yielded a value of 0.2. This indicates the performance of the fabricated device is approaching the simulated ideal. Experimentally measured intensity images of input HG modes were successfully sorted, with output channels arranged around a central violet circle as intended. The location of each output channel was clearly defined, demonstrating the MPLC’s ability to spatially separate the input HG modes. This work establishes a pathway towards robust monolithic MPLC technology and rapid prototyping of compact optical systems for applications in optical and photonic computing and imaging. Compact multi-plane light conversion via femtosecond laser-written nanogratings Researchers have demonstrated miniaturised, fully-encapsulated multi-plane light converters (MPLCs) fabricated within a glass chip using direct laser writing. These devices deterministically reshape light by mapping input spatial light modes to new output modes, offering potential benefits for optical and photonic computing and imaging. The fabrication process involves creating layers of femtosecond laser-induced birefringent nanogratings, forming geometric phase holograms that control the phase of circularly polarised light. Experimental results showcase two proof-of-concept MPLC designs, a 3-mode and a 10-mode Hermite-Gaussian mode sorter, each occupying a volume of less than 0.8 cubic millimetres. The automatic co-registration of fabrication planes via high-accuracy translation stages reduces alignment challenges typically associated with these systems. Although promising, current mode conversion fidelity is limited by factors such as input beam distortions, fabrication errors including nanograting pixel cross-talk and hologram separation inaccuracies, and the finite thickness of phase holograms. Furthermore, the efficiency of each optical plane is estimated at approximately 20 per cent, restricting the number of planes achievable in a single device. Future development will focus on improving beam shaping fidelity and suppressing modal cross-talk through optimised hologram design and precise laser writing calibration. Enhancing light processing efficiency is also a priority, with strategies including improving polarisation conversion to achieve ideal half-waveplate performance and minimising scattering losses. These advancements will pave the way for robust, monolithic MPLC technology and facilitate rapid prototyping of complex optical systems. 👉 More information 🗞 Miniaturised multi-plane light converters via laser-written geometric phase holograms 🧠 ArXiv: https://arxiv.org/abs/2602.07222 Tags: