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

Ref. p. 134] 2.1 Ultrafast solid-state lasers 77
For high pulse repetition rate an even simpler cavity design has been used. In the 1-μm spectral
region, Nd:YVO 4 has been found to be a particularly suitable gain medium for very high repetition
rates, as already discussed in Sect. 2.1.3.1. For repetition rates of 40 GHz and above, a quasimonolithic
design [99Kra2] is useful, where the output-coupler mirror is a dielectric coating directly
fabricated on a curved and polished side of the laser crystal, while the SESAM is attached to the
other side of the crystal (see Fig. 2.1.7b). The crystal may be anti-reflection-coated on the flat
side, or just uncoated. Note that with a reflecting coating on this side, there is a cavity effect:
Depending on the exact size of the air gap between crystal and SESAM, the optical field can
more or less penetrate the SESAM structure, and accordingly the effective modulation depth and
saturation fluence of the SESAM are modified. In this case for optimum performance, one has
to manipulate the width of the air gap. A quasi-monolithic cavity design has also been used for
Q-switched microchip lasers (Fig. 2.1.6) and described in more detail in Sect. 2.1.3.1.
Counterpropagating waves overlap in the crystal, leading to the phenomenon of spatial hole
burning, which can have significant influence on the mode-locking behavior [95Bra2, 95Kae3,
01Pas2]. In particular, it allows for shorter pulses, although often with some amount of chirp,
because gain narrowing is effectively eliminated: even without a mode-locker, such lasers tend to
run with a significant emission bandwidth, and the mode-locker more or less only has to lock the
phases of the running cavity modes.
2.1.3.3.2 Typical femtosecond lasers
Most femtosecond lasers are based on an end-pumped laser setup, with a broad-band laser medium
such as Ti 3+ :sapphire, Cr 3+ :LiSAF, Nd:glass or Yb 3+ :glass (see Sect. 2.1.3.1 for an overview). In
the case of Ti 3+ :sapphire, the pump source can be either an Ar ion laser or a frequency-doubled
solid-state laser. In any case, one typically uses a few watts of pump power in a beam with
good transverse beam quality, because the mode radius in the Ti 3+ :sapphire rod is usually rather
small. Other gain media like Cr 3+ :LiSAF, Nd:glass or Yb 3+ :glass are typically pumped with highbrightness
diode lasers, delivering a few watts with beam-quality factor M 2 in the order of 10 in
one direction and < 5 in the other direction, which allows for diffraction-limited laser output with
typically a few hundred milliwatts.
The typical laser cavities (see Fig. 2.1.7c as an example) contain two curved mirrors in a
distance of a few centimeters on both sides of the gain medium. The pump power is usually injected
through one or both of these mirrors, which also focus the intracavity laser beam to an appropriate
beam waist. One of the two “arms” of the cavity ends with the output coupler mirror, while the
other one may be used for a SESAM as a passive mode-locker. One arm typically contains a
prism pair (Sect. 2.1.5.2.3), GTI (Sect. 2.1.5.2.1) or chirped mirrors (Sect. 2.1.5.2.4) for dispersion
compensation, which is necessary for femtosecond pulse generation. In most cases, femtosecond
lasers operate in the regime of negative overall intracavity dispersion so that soliton-like pulses are
formed.
Instead of a SESAM, or in addition to it, the Kerr lens in the gain medium can be used for
mode-locking (Sect. 2.1.4.4). In most cases, soft-aperture KLM is used. Here, the cavity design
is made so that the Kerr lens reduces the mode area for high intensities and thus improves the
overlap with the (strongly focused) pump beam. This is achieved by operating the laser cavity near
one of the stability limits of the cavity, which are found by varying the distance between the above
mentioned curved folding mirrors, or some other distance in the cavity.
For the shortest pulse durations around 5–6 fs, the strong action of KLM as an effective fast
saturable absorber is definitely required. Also double-chirped mirrors (Sect. 2.1.5.2.4) are required
for precise dispersion compensation over a very broad bandwidth. Typically, several dispersive
mirrors are used in the laser cavity, and additional mirrors may be used for further external
compression. A broadband SESAM allows for self-starting mode-locking.
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