Recent advances in fiber-tip integration based on femtosecond laser 3D printing

Integrating functional materials onto optical fiber tips offers significant potential for advanced sensing, communication, and imaging. However, conventional fabrication techniques are hindered by procedural complexity, insufficient resolution, and a lack of genuine three-dimensional (3D) structuring capability. Femtosecond laser 3D printing circumvents these limitations through spatially confined nonlinear light-matter interactions, which can induce multiphoton polymerization, precursor conversion, or photochemical bonding depending on the material system. This review presents the physical principles, material systems, and processing strategies tailored for both high precision and high efficiency, which underpin this technology. The principles of nonlinear two-photon absorption and the mechanisms of two-photon polymerization for the employed materials are explained. The printable materials are classified into three major categories: organic, organic-inorganic hybrids, and purely inorganic, and their respective progress in fiber-tip device fabrication is reviewed. For processing, precision-enhancing approaches include spatial confinement of optical fields, voxel control regulated by material kinetics and coupled confinement of light and matter, while efficiency-enhancing techniques include multifocus parallel writing, planar projection exposure, and volumetric projection exposure. Regarding applications, this review comprehensively covers optical surface imaging, optical coherence tomography and beam shaping, as well as microelectromechanical systems sensing for force, gas, temperature, and magnetic field. The core challenges include limitations in functional materials, constraints in printing technologies, and difficulties in fiber-tip integration. Correspondingly, future directions include constructing new fiber-integrated material systems, developing new fiber-integrated printing methods, and expanding new fiber-integrated device applications. This review aims to provide practical guidance for the rational design and engineering application of functional fiber-tip devices by summarizing key principles, processing strategies, and future perspectives to facilitate their practical adoption.