The formation of two grating types in SMF-28 fiber by focusing 125 fs, 0.5–2 mJ pulses through a phase mask onto a fiber sample is studied. The first type, specified as type I-IR, occurs below the damage threshold of the medium. The scaling behavior of the type I-IR gratings with field intensity and annealing properties suggests that their formation is related to nonlinear absorption processes, possibly resulting in color center formation. The second type, denoted as type II-IR, occurs coincidentally with white light generation within the fiber. These type II-IR gratings are stable at temperatures in excess of 1000 °C and are most likely a consequence of damage to the medium following ionization.
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