Specialty active fibers for fiber-optic sensing networks: from gain media to active sensing units

As the Internet of Everything (IoE) continues to connect physical objects, sensing units, and information systems, traditional passive optical fibers face practical limitations in some large-scale and complex sensing networks, particularly regarding signal-to-noise ratio (SNR) degradation and limited system integration. By enabling in situ high-fidelity signal excitation and online gain compensation, rare-earth-doped specialty active optical fibers provide routes to local signal generation and gain compensation, which may help mitigate these limitations in selected sensing-network architectures. This paper summarizes recent advances in specialty active fibers through three interrelated routes: material composition, waveguide design and emerging digital manufacturing. First, material and matrix engineering can partly mitigate host-glass limitations through advanced deposition processes and multi-ion designs, thereby improving gain-band coverage and luminescence stability under specific conditions. Second, at the microstructural design level, active fibers exploit engineered microstructures to provide additional degrees of freedom for optical-field manipulation. This not only helps address modal gain equalization in space-division multiplexing but also enhances evanescent field interactions, providing a structural foundation for cross-sensitivity decoupling in multiparameter sensing and the generation of spatially structured light. Finally, emerging additive manufacturing, especially 3D printing, enables spatial customization of active components and micro- to nanoscale lab-on-fiber integration, enabling more flexible design of sensing units for targeted multiphysical responses. This review also discusses key bottlenecks to deployment. These include parasitic absorption caused by residual hydroxyl and carbon-containing species, as well as scattering associated with pores and interfacial defects in additively manufactured fibers. Looking forward, large-scale fiber-optic sensing networks may benefit from integrating active gain, tailored waveguides and local sensing functions in the same fiber platform. Together, these technological routes show how active fibers are evolving from conventional gain media toward active sensing units that couple environmental transduction with local photonic functions, including signal generation, amplification and optical-field control.