image: Figure | Working principle of the programmable optical processor. (a) Conceptual schematic of the photonic platform. Three liquid-crystal spatial light modulators (LC-SLMs), cell-wise driven by a modulated electrical field, implement the target operator U by imprinting a programmable phase pattern (256 discrete levels) onto the input optical field. Optical modes, featuring transverse phase modulation, are imaged on a sensor in the focal plane of a lens (f), where each region corresponds to a different spatial mode. (b) Representative experimental results. Each run of the experiment, controlled via software, realizes the evolution corresponding to a given number of steps of a quantum walk on discrete lattices. The image shows the normalized spatial probability distributions for a quantum walk on a two-dimensional grid at different time steps (t). Experimental results are in good agreement with theoretical predictions.
Credit: Francesco Di Colandrea et al.
Programmable photonic platforms are used to manipulate different degrees of freedom of light, enabling applications in communication, information processing, and simulation. Integrated circuits are the most common ones, exploiting waveguide arrays to couple spatial modes. Free-space optical processors are now emerging as a compelling alternative to integrated solutions, providing flexible access to many co-propagating structured modes. These consist of multilayer architectures, where the platform depth scales with the number of modes, making low-loss, accurate, and programmable implementations a central challenge. To mitigate this, recent studies have focused on resource-efficient few-layer platforms.
In a new paper published in Light: Science & Applications, a team of scientists, led by Prof. Filippo Cardano from Università degli Studi di Napoli Federico II and Prof. Ebrahim Karimi from the University of Ottawa, has demonstrated that an efficient compression scheme, originally developed by members of the same collaboration, can also be implemented with commercially available programmable liquid-crystal spatial light modulators. The scheme implements large-scale translation-invariant unitaries in one- and two-dimensional spatial configurations using only three patterned layers, enabling flexible free-space photonic circuits with unprecedented reconfigurability. Similar functionalities can be achieved using other technologies, such as dielectric metasurfaces; however, these devices are typically static and lack reconfigurability.
Liquid-crystal (LC) spatial light modulators (SLMs) are pixelated arrays of LC-filled cells, each of which can be controlled independently via software. The circuit performance has been validated both with a classical laser source and single photons, implementing more than 300 different processes, coupling single input modes to up to 7000 output ones. Beyond the realization of free-space photonic circuits, which is the focus of the present work, the platform also enables programmable, arbitrary space-dependent polarization transformations.
“We have demonstrated a reconfigurable photonic circuit implementing a wide class of complex unitary transformations via optical manipulation at three layers only” – they explain. “The setup consists of three SLMs arranged in a relay imaging system to cancel free-space propagation between consecutive reflections. At each run of the experiment, analytical solutions for the patterned layers are uploaded and displayed on the SLMs.”
“Although SLMs are usually employed as phase-only elements on scalar fields,” – they continue– “our platform provides full control over both the spatial and vectorial degrees of freedom of light. This work represents the first implementation of such an architecture as a photonic circuit on spin-orbit modes, within the class of unitary matrices featuring discrete translation invariance. This establishes a novel paradigm for optical information processing and quantum simulation with structured light in free space.”
Suitability for quantum optical experiments has also been demonstrated:
“We also tested the platform in the single-photon regime, validating its suitability for quantum experiments. Our proof-of-concept demonstration lays the basis for future experiments, which will benefit from a platform that is compact, programmable, and is now ready-to-use.”
Journal
Light Science & Applications
Article Title
Compact and programmable large-scale optical processor in free space