Page 378 - High Power Laser Handbook

346 So l i d - S t at e La s e r s Ultrafast Lasers in Thin-Disk Geometry 347

2.0 Yb: YAG 2.0 ∆T: 0 K 30 K 60 K Gain
Gain cross section (10 −21 cm 2 ) 1.5 Yb: LuScO 3 Gain (a.u.) 1.5
Yb: Lu O
2 3
Field enhancement
β = 0.15
1.0
1.0
0.5
0.5
0.0 0.0
1020 1040 1060 1080 940 960 980 1000
Wavelength (nm) Wavelength (nm)
(a) (b)

Figure 13.8 (a) Gain spectra of Yb:YAG, Yb:Lu O , and Yb:LuScO for an inversion
3
2
3
level β of 0.15. (b) Field enhancement and resulting gain spectra for a typical
VECSEL structure at different temperatures. DT represents the temperature
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difference to the designated operation temperature.
used for Fig. 13.8 uses seven QW layers in successive maxima of the
standing wave pattern ). However, the generation of transform-lim-
91
ited pulses in the femtosecond regime, which exploit a significant
fraction of the bandwidth, is challenging. Most SESAM mode-locked
VECSELs operate at few-picosecond pulse durations, with an optical
bandwidth below 1 nm in the slow saturable absorber regime.
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The first subpicosecond pulses from a VECSEL were obtained
108
with a special SESAM, utilizing the AC Stark effect. In this device,
the strong electromagnetic field during the pulse leads to a blue shift
of the absorption. Hence, for wavelengths longer than the peak
absorption wavelength, the absorption decreases. Because no carriers
are involved in this process, the recovery time is comparable to the
pulse duration and is much faster than in conventional SESAMs. To
further decrease the SESAM’s recovery time, the single QW was
placed near the surface to enable fast recombination for carriers by
tunneling into surface states. By applying such an AC Stark SESAM,
pulses as short as 477 fs with 100-mW average output power at a
repetition rate of 1.21 GHz were realized as early as 2002. Six years
70
later, an improved AC Stark SESAM and a carefully tailored gain
spectrum of the VECSEL resulted in 260-fs pulses with 25 mW of
69
average output power.
Even shorter pulses of only 190 fs could be obtained by optimiz-
ing the spectral position of the SESAM’s absorption maximum in
relation to the VECSEL’s gain maximum by varying the temperature
of both devices. However, in this sensitive operation regime, band-
width-limited pulses could be observed only in a temperature range
of about 10°C, which limited the applicable pump power. Therefore,
the average output power did not exceed 5 mW for the shortest
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observed pulses. A recent breakthrough was the demonstration of
a mode-locked VECSEL with only 60-fs pulses; however, the output