The ability to fabricate strong and extraordinarily stable Bragg gratings in any type of fibre using femtosecond laser light has attracted much interest in the recent past. The induced index change has been attributed to glass densification brought about by nonlinear multiphoton ionisation resulting in bond breaking, local melting and rapid cooling occurring after optical breakdown by the high-energy femtosecond light.
In this paper, a preliminary comparative study between fibre Bragg grating fabrication in germanosilicate fibre using 244nm femtosecond source with a repetition rate of 250kHz and 200fs pulse duration and a 244nm CW frequency doubled argon-ion source is presented. The reflectivities achieved in both cases were close to identical for similar writing fluences, as were the isochronal annealing profiles. This indicated that despite the very high repetition rate used, the collective heat generated by the femtosecond radiation plays no role in the compaction of the glass matrix. The index change observed in this case is a result of single photon absorption into known defect centres - the laser intensity was insufficient to excite the glass band edge leading to glass compaction. No qualitative difference in index change between CW and short-pulse excitation of these defects is found.