Lithium niobate on insulator devices for photonic integrated circuits

Passive photonic components play a central role in photonic integrated circuits (PICs), where optical power distribution, power combining, and wavelength-selective routing must be achieved within compact, low-loss, and fabrication-aware platforms. This thesis addresses the design, analysis, and experimental assessment of passive integrated Multimode Interference (MMI) devices based on thin-film lithium niobate (TFLN). Moreover, hybrid silicon nitride–thin-film lithium niobate (SiN–TFLN) add drop filters are designed and analyzed. A major part of this work is devoted to multimode interference (MMI) couplers as broadband passive splitters and combiners. A systematic design methodology is developed for 1×N and N×1 configurations with 2 ≤ N ≤ 8, starting from the self-imaging principle and extending to a detailed numerical investigation of geometrical design parameters and performance metrics. The analysis includes insertion loss, output power uniformity, broadband behavior, and combined performance evaluation, together with the role of output-port spacing and taper geometry. The study is further extended from splitter operation to combiner operation, where inter-port phase relations, wavelength stability of the phase, ideal combining performance, and sensitivity to amplitude and phase errors are examined. In addition, fabrication tolerance is assessed through thickness-related variations, providing a broader view of the practical design space of TFLN-based MMI devices. The numerical design study is complemented by the fabrication and experimental characterization of selected MMI structures. The fabricated devices are analyzed within the technological constraints of the adopted TFLN platform, including actual fabrication conditions and thickness-induced geometrical variations. Experimental measurements are performed using grating-coupled configurations in the C-band, outside the original O-band design range. The measured responses are interpreted together with the corresponding numerical predictions in order to clarify the effects of off-design operation, coupling and normalization uncertainties, and fabrication-induced deviations, thereby establishing a direct link between idealized design and realistic device performance. The thesis also investigates wavelength-selective routing through a grating-assisted add–drop filter implemented on a hybrid SiN–TFLN platform. Starting from a single-channel reference configuration, the study develops dual-channel and multi-channel structures based on superimposed gratings, enabling multiple wavelength-selective drop operations within a single device footprint. The analysis addresses spectral response, field distribution, effect of grating-period separation, inter-drop crosstalk, wavelength positioning, bandwidth stability, and the optimization of the number of grating periods. Fabrication-oriented considerations, including duty-cycle sensitivity, tolerance analysis, and minimum feature-size constraints, are also incorporated. Overall, this thesis establishes a fabrication-aware design perspective for passive photonic devices in lithium-niobate-based platforms, combining systematic numerical design, experimental characterization, and multi-channel wavelength-routing concepts to provide robust building blocks for integrated photonic systems. This research was carried out within the framework of the RESTART project (Spoke 1), funded by the Italian National Recovery and Resilience Plan (PNRR).