2.1 Ultrafast solid-state lasers - ETH - the Keller Group

Ref. p. 134] 2.1 Ultrafast solid-state lasers 39
and recover faster than the gain, while the recovery time of the saturable absorber can be much
longer than the pulse duration. Dynamic gain saturation means that the gain experiences a fast,
pulse-induced saturation that then recovers again between consecutive pulses (Fig. 2.1.5a).
For solid-state lasers we cannot apply slow saturable absorber mode-locking as shown in
Fig. 2.1.5a, because no significant dynamic gain saturation is taking place due to the small gain
cross-section of the laser. The upper state lifetime of solid-state lasers is typically in the μs toms
regime, much longer than the pulse repetition period, which is typically in the nanosecond regime.
In addition, the gain cross-section is 1000 or even more times smaller than for dye lasers. We
therefore do not observe any significant dynamic gain saturation and the gain is only saturated to
a constant level by the average intracavity intensity. This is not the case for dye, semiconductor
and color-center lasers for which Fig. 2.1.5a describes most mode-locking processes. Therefore it
was assumed that without other pulse-forming mechanisms (such as soliton pulse shaping) a fast
saturable absorber is required for solid-state lasers. Kerr-lens mode-locking is nearly an ideal example
for fast saturable absorber mode-locking. However, SESAM mode-locking results revealed
that even a slow saturable absorber can support significantly shorter pulses even though a net gain
window remains open after the short pulse (Fig. 2.1.5c). At first this seems surprising, because
on the trailing edge of the pulse there is no shaping action of the absorber and even worse one
would expect that the net gain after the pulse would destabilize the pulse. However, we have shown
that in the picosecond regime without soliton formation, a more strongly saturated slow saturable
absorber can stabilize a shorter pulse because the pulse is constantly delayed by the absorber and
therefore swallows any noise growing behind himself [01Pas1]. This means that even with solid-state
lasers we can work with relatively slow saturable absorbers that have approximately a recovery
time in the range of 10 to 30 times the absorber recovery time. In the femtosecond regime soliton
formation is actually the dominant pulse-forming mechanism and the slow saturable absorber
needs only to be fast enough to stabilize this soliton – which is referred to as soliton mode-locking
[95Kae1, 95Jun2, 96Kae].
2.1.3 Overview of ultrafast solid-state lasers
2.1.3.1 Overview for different solid-state laser materials
Table 2.1.1 to Table 2.1.3 give a full overview of the different results that have been achieved using
different solid-state lasers. The long list as shown in Table 2.1.2 demonstrates how active the field
of cw mode-locked lasers has been. The shortest pulses in the two optical cycle regime are being
generated by the Ti:sapphire laser using Kerr-Lens Mode-locking (KLM). Otherwise, more recent
results clearly demonstrate that the emphasis has shifted towards SESAM mode-locking because
more stable and self-starting mode-locking can be achieved and the saturable absorber can be
optimized independently from the cavity design. This allowed us to push the frontier in terms of
pulse repetition rate and pulse energy by several orders of magnitude. Today, we can obtain a
pulse repetition rate of about 160 GHz as compared to around 1 GHz in 1990. In addition, we have
increased the pulse energy from the nJ-regime to more than 10 μJ from a passively mode-locked
diode-pumped solid-state laser at 10–50 MHz pulse repetition rates during the last decade which is
an increase of more than four orders of magnitude. More results are summarized in Table 2.1.2 and
discussed below in Sect. 2.1.3.1.1 to Sect. 2.1.3.1.3. Q-switching results are restricted to microchip
lasers because the pulse duration scales with the photon cavity lifetime. Thus, the shorter the laser
cavity, the shorter the pulses that can be generated as discussed in Sect. 2.1.3.1.4.
In the past few years, a novel type of laser has bridged the gap between semiconductor lasers
and solid-state lasers. The Vertical-External-Cavity Surface-Emitting Laser (VECSEL) [99Kuz]
Landolt-Börnstein
New Series VIII/1B1