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The generation of ultrashort light pulses in a colliding pulse mode-locked dye laser is studied by carrying out a numerical simulation of the pulse propagation in the laser cavity. The phenomena considered in this simulation were saturable amplification, saturable absorption, bandwidth limiting element, self-phase modulation, group velocity dispersion, and linear loss.

We report on the dynamics of nonequilibrium distributions of dye molecules, such as, Nile Blue, Cresyl Violet, sulforhodamine 640, and rhodamine B, excited by pump pulses of 65-fs duration and probed by pulses compressed to durations of 10 fs. We examine a spectral region centered at the pump wavelength, 618 nm, and ranging from 570 to 670 nm. Differential absorption spectra are taken at intervals of 25 fs following the initial excitation of the dye. We observe optically induced changes which reach equilibrium within a few hundred femtoseconds following excitation. Of special interest is hole burning at the pump wave-length as well as bleaching of replica holes at energies shifted from the pump energy by a vibrational quantum energy. We also observe transient excited state absorption in near-infrared dyes due to pump-induced nonequilibrium population of the vibrational levels of the upper electronic state and transient-induced transmission in dyes such as rhodamine B where our 618-nm pump pulse lies below the absorption edge of the dye. We discuss the relationship of these optically induced dynamics with ultrashort pulse formation in lasers.

A principal problem in obtaining optical pulses with bandwidths approximating that of the visible spectrum and durations approaching the transform limit has been the uncompensated cubic phase distortion of pulses compressed by grating pairs. We describe the use of combined prism and grating sequences to compensate not only the quadratic, but also the cubic, phase distortion of optical pulses frequency broadened in an optical fiber. We obtain pulses of 6-fs duration which can be used on a routine basis for time-resolved spectroscopy. We have used these pulses to study the dynamics of spectral hole burning in organic dye molecules and in semiconductor materials. Examples are given of recent work on molecules, such as cresyl violet and Nile blue, and on semi-conductor materials, such as GaAs. We have also used these 6-fs pulses to examine the optical Stark effect in organic dye molecules such as rhodamine B and rhodamine 6G. We obtain, in particular, direct observations of the dynamic shift of the rhodamine B absorption spectrum on a femtosecond time scale. We also use the intensity dependence of these shifts as an alternate means of measuring the molecular dipole moment.

Cross-phase modulation (XPM) is important whenever two optical fields copropagate and at least one of them is intense. In this work we investigate the role of walk-off between two optical fields copropagating in an optical fiber. We consider a cw signal in the anomalous dispersion regime and an intense picosecond pulse in the normal dispersion regime. The continuous wave is modulated through XPM and can evolve into a short pulse whose duration and amplitude depend on the degree of walk-off. The signal pulses are longer and less intense than in the no walk-off case, and a longer fiber is necessary to form it. When the walk-off length is shorter than the necessary length for short pulse formation the signal wave becomes amplitude modulated, with a modulation depth of a few percent. In this case a frequency chirp exists in the signal wave and can be used to generate short pulses on propagation through an appropriate dispersive element.