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

40 2.1.3 Overview of ultrafast solid-state lasers [Ref. p. 134
combines the best of both worlds: the semiconductor gain medium allows for flexible choice of
emission wavelength via bandgap engineering and offers a wealth of possibilities from the semiconductor
processing world. SESAM mode-locked optically pumped VECSELs have already pulled
even with solid-state lasers in the GHz pulse repetition rate regime and will be briefly reviewed in
Sect. 2.1.3.1.5. For a more detailed recent review we refer to [06Kel]. Semiconductor lasers have
the advantage that the SESAM can be integrated into the gain structure. This holds promise for
high-volume wafer-scale fabrication of compact, ultrafast lasers. Recently, this vertical integration
of ultrafast semiconductor lasers has been demonstrated for the first time and is referred to as a
Mode-locked Integrated External-cavity Surface Emitting Laser (MIXSEL) [07Bel].
Ultrafast fiber lasers also demonstrate very good performances and are being briefly reviewed
in Sect. 2.1.3.1.6.
2.1.3.1.1 Solid-state laser materials
Solid-state lasers can be grouped in two types: transition-metal-doped (Cr 2+ ,Cr 3+ ,Cr 4+ ,Ti 3+ ,
Ni 2+ , Co 2+ ) and rare-earth-doped (Nd 3+ , Tm 3+ , Ho 3+ , Er 3+ , Yb 3+ ) solid-state lasers (Table
2.1.1). Color-center lasers have also supported ultrashort pulse durations, but they require
cryogenic cooling [87Mit, 89Isl]. A similar wavelength range can be covered with Cr:YAG lasers,
for example. Table 2.1.1 to Table 2.1.3 summarize the laser parameters of these solid-state lasers
and the performance that has been demonstrated with these lasers up to date with mode-locking
(Table 2.1.2) and Q-switching (Table 2.1.3). Q-switching in Table 2.1.3 is restricted to microchip
lasers because of the very short laser cavity that can support even picosecond pulses with Q-
switching.
Several factors are important to achieve a good power efficiency: a small quantum defect, the
absence of parasitic losses, and a high gain (σ L τ L product, where σ L is the gain cross section and
τ L the upper-state lifetime of the gain medium) are desirable. The latter allows for the use of an
output coupler with relatively high transmission, which makes the laser less sensitive to intracavity
losses. For high-power operation, we prefer media with good thermal conductivity, a weak (or even
negative) temperature dependence of the refractive index (to reduce thermal lensing), and a weak
tendency for thermally induced stress fracture.
For ultrafast lasers, in addition we require a broad emission bandwidth because of large bandwidths
of ultrashort pulses. More precisely, we need a large range of wavelengths in which a
smoothly shaped gain spectrum is obtained for a fixed inversion level. The latter restrictions explain
why the achievable mode-locked bandwidth is in some cases (e.g., some Yb 3+ -doped media
[99Hoe2]) considerably smaller than the tuning range achieved with tunable cw lasers, particularly
for quasi-three-level gain media. A less obvious requirement is that the laser cross sections should
be high enough. While the requirement of a reasonably small pump threshold can be satisfied even
with low laser cross sections if the fluorescence lifetime is large enough, it can be very difficult to
overcome Q-switching instabilities (see Sect. 2.1.6.8) in a passively mode-locked laser based on a
gain material with low laser cross sections. Unfortunately, many broad-band gain media tend to
have low laser cross sections, which can significantly limit their usefulness for passive mode-locking,
particularly at high pulse repetition rates and in cases where a poor pump beam quality or poor
thermal properties necessitate a large mode area in the gain medium.
Finally, a short pump absorption length is desirable because it permits the use of a small path
length in the medium, which allows for operation with a small mode area in the gain medium and
also limits the effects of dispersion and Kerr nonlinearity. The latter is particularly important for
very short pulses. In addition, short pump absorption length is required for the thin-disk laser
concept [94Gie] which so far supports the highest pulse energies in the 10 μJ-regime directly
generated from a passively mode-locked laser [06Mar, 07Mar].
Landolt-Börnstein
New Series VIII/1B1