Spatial-mode Demultiplexing Overcomes Diffraction Limits, Enabling Superresolution Experiments Via Orthonormal Mode Projection

The fundamental limit of conventional imaging, imposed by the wave nature of light, prevents clear distinction between closely spaced objects, hindering observation of fine details. Nickolay Erin Titov from Karlsruhe Institute of Technology and colleagues address this challenge by developing a new technique called spatial-mode demultiplexing, or SPADE, which draws inspiration from methods used to estimate parameters in complex systems. This research demonstrates a practical implementation of SPADE using Multi-Plane Light Conversion, effectively projecting light onto a carefully chosen basis that overcomes the diffraction limit. The achievement opens new possibilities for super-resolution microscopy, allowing scientists to visualise structures and processes previously hidden from view and promising advancements in fields ranging from biology to materials science.

vollständig und genau angegeben zu haben, was aus Arbeiten anderer unverändert oder mit Abänderungen entnommen wurde, sowie die Satzung des KIT zur Sicherung guter wissenschaftlicher Praxis in der jeweils gültigen Fassung beachtet zu haben. I herewith declare that the present thesis is original work written by me alone, that I have indicated completely and precisely all aids used as well as all citations, whether changed or unchanged, of other theses and publications, and that I have observed the KIT Statutes for Upholding Good Scientific Practice, as amended. The research focuses on manipulating light’s spatial modes, the shapes light beams take as they propagate, to achieve greater control and efficiency. MPLC provides a powerful method for their manipulation, particularly relevant to space-division multiplexing, a technique that increases the capacity of optical fibers by transmitting multiple data streams simultaneously using different spatial modes.

The team also investigated the application of MPLC to quantum optics, exploring its use in manipulating quantum states of light for quantum information processing. A significant portion of the research focused on the theoretical underpinnings of precise measurement, statistical estimation, and the limits of precision in optical systems, exploring concepts like the Cramer-Rao bound and Fisher information. The work also details the development and application of photon number resolving detectors, crucial tools for advanced optical measurements, alongside the design and fabrication of phase masks, photonic lanterns, and 3D waveguides necessary for implementing MPLC.

Spatial Resolution Beyond the Diffraction Limit

This research presents a significant breakthrough in optical imaging, overcoming the fundamental diffraction limit that traditionally restricts the resolution of closely spaced, incoherent light sources. Scientists investigated spatial-mode demultiplexing (SPADE), a technique inspired by quantum principles, implemented using multi-plane light conversion (MPLC). Early MPLC systems sorted a limited number of modes, but a key advancement in 2017 involved the development of a “magic mapping” that dramatically reduced the number of required optical planes, enabling the construction of sorters for hundreds of modes. By 2021, this technology had progressed to a multiplexer capable of handling over a thousand modes with just fourteen optical planes. Building on this progress, the researchers designed, simulated, and experimentally validated an MPLC setup specifically for performing the SPADE measurement, aiming to overcome diffraction limits for resolving incoherent point sources. This achievement lies in the development of a high-fidelity, low-loss, and compact mode demultiplexer, engineered through careful simulation and optimization, and validated experimentally, demonstrating the potential to significantly enhance resolution in optical imaging applications and establishing a new approach to super-resolution imaging.

Super-Resolution Imaging via Spatial Demultiplexing

This research demonstrates a novel approach to optical imaging, overcoming the limitations imposed by diffraction that traditionally restrict the resolution of closely spaced, incoherent light sources. Scientists successfully designed and experimentally validated a method called spatial-mode demultiplexing, implemented through multi-plane light conversion, to achieve improved resolution in far-field imaging. The technique operates by projecting light onto a basis of orthogonal modes, effectively separating and resolving signals that would otherwise be blurred together by diffraction, offering a broadly applicable super-resolution strategy that satisfies key criteria for practical imaging. Unlike many existing super-resolution techniques, this method does not require specific experimental conditions or complex manipulations of the light source, making it suitable for diverse applications in astronomy, remote sensing, and biomedical imaging. The team acknowledges that further work is needed to optimise the system and explore its full potential, particularly in challenging imaging scenarios, with future research directions including refining the mode sorting process and investigating the technique’s performance with real-world samples and data.