Femtosecond laser direct writing of fiber Bragg gratings for multiparameter sensing in extreme environments

Fiber Bragg gratings (FBGs) are the most widely used sensing elements for structural health monitoring (SHM). However, conventional UV-induced FBGs cannot survive at temperatures above 300 °C and are unable to achieve SHM in extreme environments. A femtosecond (fs) laser, featuring ultra-short pulses and ultrahigh peak intensity, has been used for inscribing high-temperature-resistant FBGs in almost all types of fibers, even without photosensitivity. In the past decade, the fs laser direct writing technology has been developing rapidly due to its extremely high flexibility in inscribing FBGs and serial- or parallel-integrated FBG arrays. Here, we present a comprehensive overview of the historic developments and recent advances in fs laser direct writing of FBGs. The grating formation mechanisms, including the index change mechanism and coupled mode theory, are introduced. Four direct writing methods, including point-by-point (PbP), line-by-line (LbL), plane-by-plane (Pl-b-Pl), and helical inscription, together with beam shaping methods, are introduced for fabricating high-quality FBGs and complex-profile FBGs. Moreover, we present the fabrication of large-scale serial or parallel ultra-weak FBG arrays and their applications for high-performance distributed temperature sensing, distributed strain sensing, distributed acoustic sensing, and 3D shape sensing. We also discuss the fabrications and applications of distributed Bragg reflector fiber lasers for highly sensitive vibration or ultrasonic sensors and sapphire FBGs for ultrahigh temperature sensing at 1900 °C and strain sensing at 1600 °C. Consequently, the fs laser directly written FBGs are promising for multiparameter sensing and SHM in extreme environments, such as hypersonic vehicles, aircraft engines, nuclear reactors, power plants, and metallurgical furnaces.