Abstract

Recently much emphasis has been placed on mode locking the Ti:sapphire laser owing to the unique characteristics of this new solid state laser medium. Novel passive amplitude modulation techniques allowed self-starting and/or self-sustaining passive mode locking of continuous wave Ti:sapphire lasers.^1,2 However, the comparatively inefficient all-optical modulators have not been capable of generating femtosecond pulses in the presence of net positive dispersion in the cavity. A breaktrough was achieved by exploiting solitonlike shaping mechanisms.^2,3 In a Ti:sapphire laser the high intracavity power and long optical path within the amplifier medium (compared with dye lasers) lead to strong self-phase modulation. With the introduction of negative group-velocity dispersion this nonlinearity can result in efficient pulse shortening down to the femtosecond regime. Femtosecond Ti:sapphire lasers are different from CPM dye lasers in three respects: i) steady-state pulse duration is determined primarily by phase-modulation rather than amplitude modulation, ii) steady-state pulse parameters may be significantly different at different positions in the cavity iii) the broad gain bandwidth does not limit pulse duration down to 10 fsec. This implies that special attention has to be paid to phase affecting cavity elements in the design of these lasers.

© 1992 The Author(s)