Micromachining bulk glass by use of femtosecond laser pulses with nanojoule energy

Using tightly focused femtosecond laser pulses of just 5 nJ, we produce opt ical breakdown and structural change in bulk transparent materials and demo nstrate micromachining of transparent materials by use of unamplified laser s. We present measurements of the threshold for structural change in Cornin g 0211 glass as well as a study of the morphology of the structures produce d by single and multiple laser pulses. At a high repetition rate, multiple pulses produce a structural change dominated by cumulative heating of the m aterial by successive laser pulses. Using this cumulative heating effect, w e write single-mode optical waveguides inside bulk glass, using only a lase r oscillator. (C) 2001 Optical Society of America.