Page 374 - High Power Laser Handbook
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342 So l i d - S t at e La s e r s Ultrafast Lasers in Thin-Disk Geometry 343
HR@ HR@ AR
λ λ
Gain Gain Las Pump
structure structure QDs QWs
QW-SESAM QD-SESAM
(a) (b) (c)
Figure 13.7 Integration scheme, progressing from conventional VECSEL SESAM
mode locking with (a) large mode area ratios, thus limited to large cavities; (b) identical
mode areas on gain structure and SESAM, making high repetition rate and integration
possible; and (c) absorber-gain integration in a single device. The MIXSEL contains two
high reflectors (HRs), a quantum dot (QD) saturable absorber, a quantum well (QW)
gain, and an antireflection (AR) coating. The intermediate HR prevents the pump light
from bleaching the saturable absorber.
the use of QD-SESAMs, which have a lower density of states and thus
a lower total saturation energy. QD-SESAMs enabled the first demon-
stration of 1:1 mode locking in VECSELs, with 25-GHz repetition
83
rate and further scaling to 50 GHz. Recently, even the integra-
81
tion of the saturable absorber into the gain structure was realized.
This novel type of ultrafast semiconductor laser—the mode-locked
15
integrated external-cavity surface emitting laser (MIXSEL) (see
Fig. 13.7c)—is a technology that opens the way to cost-efficient mass
production for widespread applications. Furthermore, the concept is
easily scalable to even higher repetition rates. More details on this
topic can be found in Refs. 90 and 91.
The QML instabilities limit the scalability of TDLs to gigahertz
repetition rates. However, this topic has not yet been fully explored,
and the highest reported repetition rate of 81 MHz is presumably far
67
below the highest achievable repetition rate with this technology. As
an example, an Er,Yb:glass laser with a similarly low-gain cross sec-
tion already allows for a repetition rate of up to 100 GHz at 35-mW
92
average output power. However, the practical use of gigahertz TDLs
at tens of watts of average output power is limited at this time.
On the other hand, it is often desirable to operate at low repetition
rates to increase the available pulse energy. Scaling mode-locked
VECSELs to lower repetition rates is limited by the onset of harmonic
mode locking. The typical carrier lifetime in a QW-VECSEL is in the
order of nanoseconds. If the cavity roundtrip time becomes longer,
two subsequent pulses with lower energy will have a gain advantage
over a single pulse with higher energy and will therefore be favored.
93
The threshold for harmonic mode locking strongly depends on the
laser’s operation parameters. However, the lowest repetition rates
that have been demonstrated up to now are around 1 GHz, with an
average output power of 275 mW. 68