Monolithic 3D micro-encapsulation on optical fiber tips via femtosecond laser direct writing

Multifunctional integration of optical fiber tips has attracted tremendous attention because of the high sensitivity, compact size, and immunity to electromagnetic interference of fiber-optic devices. However, achieving stable three-dimensional (3D) micro-structuring while maintaining both miniaturization and sensing performance remains a persistent challenge. Here, we present a monolithic 3D micro-encapsulation strategy based on femtosecond laser direct writing (FsLDW) and chemical vapor polymerization (CVP), enabling the multifunctional integration of Fabry–Pérot interferometers (FPIs) on fiber tips with submicron precision and enhanced structural robustness. This approach combines an FsLDW-nanoprinted air cavity with the CVP of the Parylene C coating, reducing the overall sensor footprint from the centimeter scale to approximately 100 µm without compromising sensing performance. As a proof of concept, we demonstrate the stable operation of encapsulated optical fiber sensors under continuous fluidic and mechanical disturbances, supporting both intensity and wavelength demodulation. The encapsulated FPI cavities achieve over an order of magnitude improvement in spectral stability and a 26.4 dB increase in long-term signal contrast, with performance sustained over 10 000 repeated test cycles. This work offers a scalable route toward high-density, multifunctional fiber-optic sensors with enhanced durability and long-term reliability for next-generation photonic integrated applications.