4.7: Summary

Starting from a simple two level laser and absorber model, we characterized the dynamics of solid-state lasers mode-locked and Q-switched by a saturable absorber. The unique properties of solid-state laser materials, i.e. their long upper-state life time and their small cross sections for stimulated emission, allow for a separation of the laser dynamics on at least two time scales. One process is the energy build-up and decay, which occurs typically on a time scale of the upper state lifetime or cavity decay time of the laser. The other process is the pulse shaping, which occurs within several roundtrips in the cavity. Separating these processes, we can distinguish between the different laser dynamics called cw-Q-switching, Q-switched mode locking and cw-mode locking. We found the stability boundaries of the different regimes, which give us guidelines for the design of absorbers for a given solid state laser to favour one of these regimes. Semiconductor absorbers are a good choice for saturable absorbers to modelock lasers, since the carrier lifetime can be engineered by low temperature growth [20]. When the pulses become short enough, the laser pulse saturates the absorber much more efficiently, which stabilizes the laser against undesired Q-switched mode locking. It has been demonstrated experimentally, that this technique can control the laser dynamics of a large variety of solid-state lasers, such as \(\ce{Nd: YAG}\), \(\ce{Nd:YLF}\), \(\ce{Nd:YV}0_4\), [18] in the picosecond regime.

With semiconductor devices and soliton formation due to negative GVD and SPM, we can use similar semiconductor absorbers to modelock the lasers in the femtosecond regime [35]. The stability criteria derived here can be ap- plied to both picosecond and femtosecond lasers. However, the characteristics of the absorber dynamics may change drastically when going from picosecond to femtosecond pulses [36]. Especially, the saturation energy may depend not only on excitation wavelength, but also on the pulsewidth. In addition there may be additional loss mechanismes for the pulse, for example due to soliton formation there are additional filter losses of the pulse which couple to the energy of the pulse via the area theorem. This has to be taken into account, before applying the theory to fs-laser systems, which will be discussed in more detail later.

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