6: Passive Modelocking

As we have seen in chapter 5 the pulse width in an actively modelocked laser is inverse proportional to the fourth root of the curvature in the loss modulation. In active modelocking one is limited to the speed of electronic signal generators. Therefore, this curvature can never be very strong. However, if the pulse can modulate the absorption on its own, the curvature of the ab- sorption modulationcan become large, or in other words the net gain window generated by the pulse can be as short as the pulse itself. In this case, the net gain window shortens with the pulse. Therefore, passively modelocked lasers can generate much shorter pulses than actively modelocked lasers.

However, a suitable saturable absorber is required for passive modelocking. Depending on the ratio between saturable absorber recovery time and final pulse width, one may distinguish between the regimes of operation shown in Figure 6.1, which depicts the final steady state pulse formation process. In a solid state laser with intracavity pulse energies much lower than the saturation energy of the gain medium, gain saturation can be neglected. Then a fast saturable absorber must be present that opens and closes the net gain window generated by the pulse immediately before and after the pulse. This modelocking principle is called fast saturable absorber modelocking, see Figure 6.1 a).

Image removed due to copyright restrictions. Please see: Kartner, F. X., and U. Keller. "Stabilization of soliton-like pulses with a slow saturable absorber." Optics Letters 20 (1990): 16-19.

Figure 6.1: Pulse-shaping and stabilization mechanisms owing to gain and loss dynamics in passively mode-locked lasers: (a) using only a fast saturable absorber; (b) using a combination of gain and loss saturation; (c) using a saturable absorber with a finite relaxation time and soliton formation.

In semiconductor and dye lasers usually the intracavity pulse energy ex- ceeds the saturation energy of the gain medium and so the the gain medium undergoes saturation. A short net gain window can still be created, almost independent of the recovery time of the gain, if a similar but unpumped medium is introduced into the cavity acting as an absorber with a somewhat lower saturation energy then the gain medium. For example, this can be arranged for by stronger focusing in the absorber medium than in the gain medium. Then the absorber bleaches first and opens a net gain window, that is closed by the pulse itself by bleaching the gain somewhat later, see Figure 6.1 b). This principle of modelocking is called slow-saturable absorber modelocking.

When modelocking of picosecond and femtosecond lasers with semiconductor saturable absorbers has been developed it became obvious that even with rather slow absorbers, showing recovery times of a few picoseconds, one was able to generate sub-picosecond pulses resulting in a significant net gain window after the pulse, see Figure 6.1 c). From our investigation of active modelocking in the presence of soliton formation, we can expect that such a situation may still be stable up to a certain limit in the presence of strong soliton formation. This is the case and this modelocking regime is called soliton modelocking, since solitary pulse formation due to SPM and GDD shapes the pulse to a stable sech-shape despite the open net gain window following the pulse.