Femtosecond‐Laser‐Inscribed Cladding Fiber Bragg Gratings: Architectures, Design Trade‐Offs and Future Sensing Opportunities

Femtosecond (fs) laser inscription enables fiber Bragg gratings (FBGs) to be positioned beyond the fiber core, including within the cladding, thereby expanding control over modal interactions and directional sensing. These cladding fiber Bragg gratings (CLFBGs) extend FBG functionality through interactions with core, cladding, and evanescent fields. Their operation does not involve fundamentally different grating physics; rather, core‐mode, cladding‐mode, and coupled‐mode responses depend on power distribution, modal overlap with the laser‐inscribed refractive‐index modulation (RIM), grating position, fiber geometry, modal excitation, and interrogation configuration. This Perspective reviews advances in CLFBG fabrication, optical characteristics, sensing mechanisms, and applications. Cladding‐waveguide‐assisted, evanescent‐field‐coupled, and direct cladding‐mode‐excitation architectures are compared in terms of mechanisms, design trade‐offs, and performance, and distinguished from conventional core FBGs and tilted FBGs (TFBGs) regarding directional sensitivity, spectral complexity, environmental interaction, multiplexing, and application suitability. Capabilities in vectorial and multiparameter sensing, including curvature, bending direction, torsion, acceleration, temperature, and environmental monitoring, are discussed. Challenges including limited reflectivity, birefringence‐induced distortion, multiplexing constraints, fabrication repeatability, and long‐term stability are assessed alongside mitigation strategies involving plane‐by‐plane inscription, mode‐field‐engineered fibers, hybrid distributed interrogation, and data‐driven demodulation. This Perspective outlines pathways toward reliable, multifunctional, multiplexed, and distributed sensing in harsh environments.