Page 367 - High Power Laser Handbook

336 So l i d - S t at e La s e r s Ultrafast Lasers in Thin-Disk Geometry 337

structures. Typically, the pump radiation’s internal angle of incidence
is chosen in such a way that the antinodes of the pump and laser light
are brought into alignment, which makes the structure resonant for
both the pump and the laser wavelength. Initial experiments indi-
36
cate that in-well pumping bears the potential for further scaling of
the output powers of VECSELs. 36,37

13.4 SESAM Mode Locking
A SESAM acts as an intracavity loss modulator with an intensity-
dependent reflectivity. Its macroscopic nonlinear optical properties
are mainly determined by modulation depth DR, or the difference in
reflectivity between a fully saturated and an unsaturated SESAM, as
well as by the saturation fluence F , which is the pulse fluence
sat
needed to reduce the losses by 1/e of the initial value (neglecting the
nonsaturable losses R ). An example of a nonlinear reflectivity mea-
ns
surement of a SESAM is shown in Fig. 13.4a. As mentioned earlier, the
power scaling principle of disk lasers relies on increasing the mode
area on the active region. Analogous arguments apply for the SESAM,
such that a fixed set of parameters can be used in different average
power regimes. Hence, among the various techniques that can force a
laser into mode-locked operation, 45–47 passive mode locking with a
4,5
semiconductor saturable absorber mirror (SESAM) is ideally suited
for ultrafast disk lasers.
13.4.1 Pulse Formation Mechanisms
Another crucial parameter describing the dynamics of a SESAM is
the recovery time t (see Fig. 13.4b), which is defined as the expo-
1/e
nential time constant of the return to the unsaturated reflectivity after

100.0 1.0
R Fast time constant (<1 ps)
ns
99.5 ∆R ns F sat 0.8
Reflectivity (%) 99.0 ∆R ∆R (normalized) 0.6 Slow time constant
(τ
~80 ps)
1/e
0.4
98.5
R 0.2
98.0 lin
0.0
2 5 10 20 50 100 200 500 0 100 200 300 400
Pulse fluence (µJ/cm 2 ) Time delay (ps)
(a) (b)
Figure 13.4 (a) Example of a measurement of the nonlinear reflectivity of a
SESAM (crosses) as a function of the incident pulse fluence. The theoretical fit
(solid) results in F = 16.6 µJ/cm , DR = 1.95%, and DR = 0.16%. (b) Example
2
sat
ns
of the temporal response of a SESAM. The measurement was performed with
2.7-ps pulses, which were too long to resolve the fast recombination time
constant (the SESAM is described in Ref. 42).