Approaching the microjoule frontier with femtosecond laser oscillators: theory and comparison with experiment

A detailed numerical analysis of heavily chirped pulses in the positive-dispersion regime (PDR) is presented on the basis of the distributed cubic - quintic generalized complex nonlinear Ginzburg - Landau equation. It is demonstrated that there are three main types of pulse spectra: truncated parabolictop, Pi- and M-shaped profiles. The strong chirp broadens the pulse spectrum up to 100 nm for a Ti : Sa oscillator, which provides compressibility of the picosecond pulse down to sub-30 fs. Since the picosecond pulse has a peak power lower than the self-focusing power inside a Ti : Sa crystal, the microjoule energies become directly available from a femtosecond oscillator. The influence of the third- and fourth-order dispersions on the pulse spectrum and stability is analysed. It is demonstrated that the dynamic gain saturation plays an important role in pulse stabilization. The common action of dynamic gain saturation, self-amplified modulation (SAM) and saturation of the SAM provides pulse stabilization inside the limited range of the positive group-delay dispersions (GDDs). Since the stabilizing action of the SAM cannot be essentially enhanced for a pure Kerrlens mode-locking regime, a semiconductor saturable absorber is required for pulse energies of > 0.7 mu J inside an oscillator. The basic results of the numerical analysis are in an excellent agreement with experimental data obtained from oscillators with repetition rates ranging from 50 to 2 MHz.