Integrated Optical Fiber Devices: A Comprehensive Review of In-Fiber Photonic Integration

• • The review consolidates fabrication methods including direct fiber-drawing, side and end-face polishing, chemical etching, thermal splicing, diffusion, tapering and twisting, femtosecond laser micro-machining, and assembly of microdevices and metasurfaces, providing a comprehensive toolkit for in-fiber integration. Industrial impact: enables rapid prototyping and scalable manufacturing of integrated fiber devices, reducing development cycles and cost. • • Applications span three-dimensional shaping sensing, refractive index sensing, chemical and biochemical sensing, polarizers and modulators, broadband photodetectors, beaming focusing and manipulation, OCT imaging, spectral filtering, vortex beam generation, and optical microfluidic devices. Clinical impact: potential for minimally invasive biomedical diagnostics and treatments with high sensitivity and specificity. • • The review identifies future directions: miniaturization of three-dimensional optical devices for embedded systems, distributed sensing units seamlessly integrated with optical fiber systems, optoelectronic hybrid chip and fiber integration, and integrating multifunctional photonic components into a slender optical fiber for biomedical applications. Industrial impact: guides R&D investment toward next-generation fiber-optic systems with enhanced functionality and reduced form factor. • • The paper emphasizes historic background and methods of integration, structures and materials, functionalities and performances, as well as applications in sensing, communications, signal manipulation and processing, medical diagnosis and treatment. Industrial impact: provides a roadmap for cross-disciplinary innovation, bridging photonics, materials science, and biomedical engineering.