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

66 2.1.3 Overview of ultrafast solid-state lasers [Ref. p. 134
2.1.3.1.2 Mode-locked rare-earth-doped solid-state lasers
The rare-earth-doped (e.g. Nd 3+ ,Tm 3+ ,Ho 3+ ,Er 3+ ,Yb 3+ ) solid-state lasers have favorable properties
for diode-pumped high-power operation, but cannot be used in the high-power regime for
femtosecond pulse generation because of their relatively small amplification bandwidth. These
lasers have 4f-electrons responsible for the laser transition, which are shielded from the crystal
host. Thus the gain bandwidth is normally not very large and pulse durations are limited to a
few 100 fs (Table 2.1.1 and Table 2.1.2). Shorter pulses can only be obtained in inhomogeneously
broadened rare-earth-doped lasers in glass hosts for example but at the expense of lower power
due to the limited thermal conductivity of glasses (Table 2.1.1 and Table 2.1.2).
Typical examples are Nd 3+ :YAG and Nd 3+ :YVO 4 . With high-power laser diodes, one or several
conventional end-pumped or side-pumped laser rods and a SESAM for mode-locking, up to 27 W
of average power in 19-ps pulses has been achieved with Nd 3+ :YAG [00Spu], or 23.5 W in 22-ps
pulses with Nd 3+ :YVO 4 [01Che1]. Significantly shorter pulse durations have been achieved with
Nd:YAG at lower output powers, down to 1.7 ps with 25 mW [90Goo] using the technique of
Additive Pulse Mode-locking (APM). For all these Nd 3+ -doped crystals, the relatively large laser
cross sections usually make it relatively easy to achieve stable mode-locked operation without
Q-switching instabilities, if the laser mode area in the gain medium is not made too large.
Phosphate or silicate glasses doped with rare-earth ions such as Nd 3+ or Yb 3+ have been
used for pulse durations down to ≈ 60 fs [97Aus, 98Hoe] and output powers of a few hundred
milliwatts. The relatively poor thermal properties make high-power operation challenging. Up to
1.4 W of average power in 275-fs pulses [00Pas1], or 1 W in 175-fs pulses [97Aus], have been obtained
from Nd 3+ :glass by using a specially adapted elliptical mode pumping geometry [00Pas2] initially
developed for diode-pumped Cr:LiSAF lasers [97Kop1, 95Kop3]. Here, a strongly elliptical pump
beam and laser mode allow the use of a fairly thin gain medium which can be efficiently cooled
from both flat sides. The resulting nearly one-dimensional heat flow reduces the thermal lensing
compared to cylindrical rod geometries, if the aspect ratio is large enough. A totally different route
toward high peak powers is to use a cavity-dumped laser; with such a device, based on Yb 3+ :glass,
400-nJ pulses with more than 1 MW peak power have been generated [04Kil].
Yb 3+ :YAG has similar thermal properties as Nd 3+ :YAG and at the same time a much larger
amplification bandwidth. Another favorable property is the small quantum defect. However, challenges
arise from the quasi-three-level nature of this medium and from the small laser cross sections,
which favor Q-switching instabilities. High pump intensities help in both respects. An end-pumped
laser based on a Yb 3+ :YAG rod has generated 340-fs pulses with 170 mW [95Hoe]. As much as
8.1 W in 2.2-ps pulses was obtained from an elliptical-mode Yb 3+ :YAG laser. In 2000, the first
Yb 3+ :YAG thin-disk laser has been passively mode-locked, generating 700-fs pulses with 16.2 W
average power [99Aus]. The concept of the passively mode-locked thin-disk laser has been demonstrated
to be power scalable, which so far lead up to 80 W in 0.7-ps pulses [06Inn].
In recent years, a few Yb 3+ -doped crystalline gain materials have been developed which combine
a relatively broad amplification bandwidth (sufficient for pulse durations of a few hundred
femtoseconds) with thermal properties which are better than those of other broad-band materials,
although not as good as e.g. those of YAG or sapphire. Examples are shown in Table 2.1.1 and
Table 2.1.2. With an end-pumped Yb 3+ :KGW rod, 1.1 W of average power have been achieved
in 176-fs pulses [00Bru]. A Kerr-lens mode-locked Yb 3+ :KYW laser produced pulses as short as
71 fs [01Liu], while a SESAM mode-locked Yb 3+ :KYW produced pulses as short as 107 fs [04Pau].
Around 70-fs pulses have been obtain with SESAM mode-locked Yb 3+ :NGW laser with 23 mW
average power [06Riv], Yb 3+ :BOYS laser with 80 mW [02Dru1], Yb 3+ :SYS laser with 156 mW
[04Dru]. However, so far the shortest pulses are still obtained with Yb:glass lasers [98Hoe]. Note that
some of these media exhibit rather low emission cross sections and therefore make stable passive
mode-locking difficult, while they might be very useful e.g. in regenerative amplifiers. Tungstate
crystals (Yb 3+ :KGW, Yb 3+ :KYW) have been rather useful for passive mode-locking since they
have relatively high cross sections. Yb 3+ :KYW has been applied in a thin-disk laser, generating
Landolt-Börnstein
New Series VIII/1B1