2.1 Ultrafast solid-state lasers - ETH - the Keller Group
2.1 Ultrafast solid-state lasers - ETH - the Keller Group
2.1 Ultrafast solid-state lasers - ETH - the Keller Group
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76 <strong>2.1</strong>.3 Overview of ultrafast <strong>solid</strong>-<strong>state</strong> <strong>lasers</strong> [Ref. p. 134<br />
<strong>2.1</strong>.3.3 Laser cavity designs<br />
<strong>2.1</strong>.3.3.1 Typical picosecond <strong>lasers</strong><br />
The setups of picosecond <strong>lasers</strong> typically do not differ very much from those of <strong>lasers</strong> for continuouswave<br />
operation. Some mode-locker is installed, which might be ei<strong>the</strong>r an Acousto-Optic Modulator<br />
(AOM) for active mode-locking (Sect. <strong>2.1</strong>.4.1) or e.g. a SESAM (Sect. <strong>2.1</strong>.4.2 and Sect. <strong>2.1</strong>.4.3)<br />
for passive mode-locking. Also, <strong>the</strong> cavity design needs to fulfill a few additional demands. As an<br />
example, we refer to Fig. <strong>2.1</strong>.7a, which shows <strong>the</strong> setup of a high-power Nd 3+ :YAG laser [00Spu],<br />
passively mode-locked with a SESAM. The cavity design must provide appropriate beam radii<br />
both in <strong>the</strong> laser head (where <strong>the</strong> fundamental Gaussian mode should just fill <strong>the</strong> usable area)<br />
and on <strong>the</strong> SESAM to obtain an appropriate degree of saturation. This depends on a number of<br />
factors: <strong>the</strong> output power, <strong>the</strong> output coupler transmission, <strong>the</strong> cavity length, and <strong>the</strong> saturation<br />
fluence of <strong>the</strong> SESAM. Obviously <strong>the</strong> cavity length must be chosen to obtain <strong>the</strong> desired repetition<br />
rate. The equations given in Sect. <strong>2.1</strong>.6.8 can be used to ensure that <strong>the</strong> chosen design will not<br />
suffer from Q-switching instabilities. The laser head is side-pumped in <strong>the</strong> mentioned example, but<br />
end-pumped laser heads can also be used, where <strong>the</strong> pump light is typically injected through a<br />
folding mirror which is highly reflective for <strong>the</strong> laser light.<br />
The SESAM should typically be used as an end mirror, not as a folding mirror. O<strong>the</strong>rwise a<br />
tendency for <strong>the</strong> generation of multiple pulses, which would meet on <strong>the</strong> SESAM, might be induced.<br />
Similar setups can be used for actively mode-locked <strong>lasers</strong>, where <strong>the</strong> SESAM is replaced by an<br />
AOM. The AOM should be placed close to an end mirror, for similar reasons as discussed above<br />
for <strong>the</strong> SESAM.<br />
Brewster<br />
plate<br />
Dichroic<br />
mirror<br />
SESAM<br />
DCP<br />
laser head<br />
Output<br />
coupler<br />
Pump<br />
Nd:YVO 4<br />
SESAM<br />
Output<br />
Output coupler coating<br />
a<br />
b<br />
SESAM<br />
M3<br />
OC<br />
M1 Gain<br />
medium<br />
M2 Pump<br />
optics<br />
Laser<br />
diode<br />
c<br />
Fig. <strong>2.1</strong>.7. (a) Setup of a passively mode-locked high-power Nd 3+ :YAG laser, containing a Direct-Coupled<br />
Pump (DCP) laser head, two curved mirrors, a SESAM, and an Output Coupler mirror (OC) with 7 %<br />
transmission. (b) Quasi-monolithic setup as used for passively mode-locked Nd 3+ :YVO 4 <strong>lasers</strong> with repetition<br />
rates above 20 GHz. (c) Typical setup of a femtosecond laser. The gain medium is pumped with a<br />
laser diode. A prism pair is used for dispersion compensation, and a SESAM as mode-locker.<br />
Landolt-Börnstein<br />
New Series VIII/1B1