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Multimode devices and components have attracted considerable attention in the last years, and different research topics and themes have emerged very recently. The multimodality can be seen as an additional degree of freedom in designing devices, thus allowing for the development of more complex and sophisticated components. The propagation of different modes can be used to increase the fiber optic capacity, but also to introduce novel intermodal interactions, as well as allowing for complex manipulation of optical modes for a variety of applications. In this roadmap we would like to give to the readers a comprehensive overview of the most recent developments in the field, presenting contributions coming from different research topics, including optical fiber technologies, integrated optics, basic physics and telecommunications.

We investigate frequency doubling of focused Bessel-like higher order fiber modes in a one-dimensionally quasi-phase matched structured KTP crystal. A single higher-order fiber mode, LP0,7, was generated at 971 nm using a fiber optical parametric amplifier pumped by sub-nanosecond pulses at 1064 nm. Frequency doubling to 485.5 nm was achieved with a maximum conversion efficiency of 48%, which produced a highly structured blue beam without any brightness improvement with respect to the fundamental beam. By optimizing the phase mismatch, we also experimentally demonstrate clean on-axis conversion-resulting from cascaded chi((2)) : chi((2)) processes, which is in good agreement with our numerical simulations, that give rise to brightness enhancement., QC 20211122

We demonstrate an energetic dual-wavelength ultrashort pulsed source by exploiting the inherent features of the newly discovered process of soliton self-mode conversion (SSMC) in a multimode fiber. The generated pulses are at wavelengths of 1205 nm and 1273 nm, respectively, and the pulse energies are approximately 30 nJ. The natural group-velocity-locking feature of SSMC ensures minimal relative timing jitter, hence highlighting the utility of exploiting the new degrees of freedom afforded by field of multimode nonlinear fiber optics. The relative timing jitter is evaluated by measuring the power fluctuations of generated sum-frequency signals. When compared to a conventional fiber based dual-wavelength source based on traditional frequency-shifted solitons, the relative timing jitter is found to be reduced by greater than 11 dB. Since this process is wavelength-agnostic within the transparency window of optical fibers, our source provides an attractive means of achieving integrated multi-color ultrashort pulse sources for a variety of applications.

Quantum networks are the ultimate target in quantum communication, where many connected users can share information carried by quantum systems. The keystones of such structures are the reliable generation, transmission, and manipulation of quantum states. Two-dimensional quantum states, qubits, are steadily adopted as information units. However, high-dimensional quantum states, qudits, constitute a richer resource for future quantum networks, exceeding the limitations imposed by the ubiquitous qubits. The generation and manipulation of such D-level systems have been improved over the last ten years, but their reliable transmission between remote locations remains the main challenge. Here, we show how a recent air-core fiber supporting orbital angular momentum (OAM) modes can be exploited to faithfully transmit D-dimensional states. Four OAM quantum states and their superpositions are created, propagated in a 1.2-km-long fiber and detected with high fidelities. In addition, three quantum-key-distribution protocols are implemented as concrete applications to assert the practicality of our results. This experiment enhances the distribution of high-dimensional quantum states, attesting the orbital angular momentum as a vessel for the future quantum network