Morphology and Thermal Stability of Fiber Bragg Gratings for Sensor Applications Written in ${\rm H}_{2}$-Free and ${\rm H}_{2}$-Loaded Fibers by Femtosecond Laser

Bragg gratings written in H
₂
-free and H
₂
-loaded SMF-28 fiber by use of infrared (IR) femtosecond laser radiation with a phase mask have been investigated in terms of their axial and cross-sectional morphology. It was found that Type I-IR and Type II-IR refractive index changes existed simultaneously in a single grating (which is termed a hybrid grating in this paper) written in either H
₂
-free or H
₂
-loaded fiber. The proportion of the two types of refractive index changes is a key factor in determining the thermal stability of the grating and spectral properties such as cladding mode coupling and polarization dependent loss (PDL). High-pressure molecular hydrogen loading (H
₂
loading) can dramatically enhance the efficiency of grating inscription while preventing the laser-induced damage, due to the enhanced color center formation and the suppressed multiphoton ionization (MPI). The H
₂
-loaded fiber Bragg gratings thus exhibit spectral properties and annealing behavior which is distinguishable from the H
₂
-free gratings. The findings of this work are of particular importance in the design of optical fiber sensors which can operate at very high temperatures for prolonged periods and thus can have important uses in industry.