At the 29th International Conference on Optical Fiber Sensors (OFS 2025), researchers from CSEM, LioniX International, and PHIX presented their latest work on the characterization of heterogeneously integrated periodically poled lithium niobate (PPLN) using optical frequency-domain reflectometry (OFDR).
This research, conducted as part of the Horizon Europe LOLIPOP Project, focuses on developing hybrid photonic integration platforms that bring together silicon nitride and thin-film lithium niobate, a powerful combination that supports nonlinear optical functions essential for both classical and quantum applications.
Highlights of the Work:
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5 mm-long PPLN waveguides designed for efficient second-harmonic generation (SHG), enabling wavelength conversion from 1560 nm to 780 nm.
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The device is fabricated using heterogeneous integration, combining a silicon nitride interposer with a thin-film lithium niobate chip, and assembled by PHIX.
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A high-resolution OFDR system (1.5 mm spatial resolution in air) was used to analyze reflections caused by refractive index mismatches at coupling facets.
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Three strong reflections were identified, precisely corresponding to the optical interfaces of the module: the fiber array, silicon nitride spot-size converter, and the PPLN waveguide.
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These reflections were measured at -27.8 dB, -31.7 dB, and -21.4 dB, respectively, demonstrating the effectiveness of OFDR for detailed analysis of photonic packaging quality.
Why It Matters
Reflections at coupling points are a major challenge in hybrid integration, especially for applications like quantum communication where signal integrity is critical. This study not only demonstrates the feasibility of assembling such complex devices but also introduces a powerful in-situ diagnostic tool for optimizing their design and integration.
The team will continue this work with higher-resolution reflectometry to further explore the micro-structured periodic domain inversion inside the PPLN — a key step for future scalable, high-performance photonic integrated circuits.
📄 Find the full article and poster presentation at the OFS 2025 proceedings: https://horizon-de-lolipop.eu/publications/