Abstract

For some time the shortest optical pulses have been produced by coupling the optical fiber pulse compressor to the colliding pulse mode-locked ring dye laser. By this method, pulses as short as 30, 16, 12, and 8 fs have been sequentially reported. Most recently, even the frequency tunable 5-ps pulses from a synchronously pumped mode-locked dye laser were compressed to 16 fs by a two-stage compressor. For all these applications an intense, ultrashort pulse was propagated through a short length of single-mode fiber used in the compressor. At relatively modest intensities, this propagation would be accurately described by the nonlinear Schrodinger equation (NLSE). However, at the high intensities (~1 TW/cm²) involved here the accuracy of the NLSE is questionable. The intense pulses induce a change in the index of refraction which is of the order of the index changes used in the fabrication of the fiber itself. Also, the intensity dependence of the group velocity becomes significant. In this report, experimental data showing the input and output pulses from a short fiber in these extreme conditions will be presented. Because these data were obtained by a cross-correlation measurement using 20-fs probing pulses, it has approximately that resolution. The corresponding spectral changes of the output pulses accompanied by measurements of their time-dependent phase are also shown. These results are compared with numerical calculations based on the NLSE and on the NLSE modified by an intensity dependent group velocity.

© 1985 Optical Society of America