In situ observation of photoinduced refractive-index changes in filaments formed in glasses by femtosecond laser pulses

We investigated the relationship between the formation of filaments and loc al refractive-index changes induced by femtosecond laser pulses in silica g lass. In situ observation revealed that the location of a filament coincide d with that of the refractive-index change. Observation also showed that th e region of refractive-index change was elongated toward the upstream direc tion of the laser pulses with increasing exposure time. The region of refra ctive-index change was several hundred micrometers long, and its diameter w as smaller than 2 mum. The refractive-index change was confirmed by two of three different methods to be as large as 0.8 x (C) 2001 Optical Society of America OCIS codes: 130.2790, 190.7110, 190.3270, 230.7370, 320.7110, 160. 2750.