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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Ref. p. 134] <strong>2.1</strong> <strong>Ultrafast</strong> <strong>solid</strong>-<strong>state</strong> <strong>lasers</strong> 81<br />
100<br />
Reflectivity R [%]<br />
R ns<br />
95<br />
F sat,A<br />
= 18 J/cm 2<br />
Rns= 3.7%<br />
R<br />
= 4.9%<br />
R 0<br />
R ns<br />
90<br />
0 50 100 150 200 250 300<br />
Incident pulse fluence F p,A [ J/cm 2 ]<br />
Fig. <strong>2.1</strong>.9. Measured nonlinear reflectivity as a<br />
function of incident pulse fluence on a typical<br />
SESAM. Theoretical fit determines <strong>the</strong> macroscopic<br />
saturable absorber parameters: saturation fluence<br />
F sat,A, modulation depth ΔR and nonsaturable loss<br />
ΔR ns.<br />
good fit and determine <strong>the</strong> saturation fluence F sat,A , modulation depth ΔR and nonsaturable<br />
losses ΔR ns of <strong>the</strong> absorber [95Bro1, 04Hai].<br />
The modulation depth is typically small (i.e. a few percent to a fraction of a percent!) to prevent<br />
Q-switching instabilities in passively mode-locked <strong>solid</strong>-<strong>state</strong> <strong>lasers</strong> [99Hoe1]. Thus it is reasonable<br />
to make <strong>the</strong> following approximation:<br />
ΔR =1− e −2q0 ≈ 2q 0 , q 0 ≪ 1 , (<strong>2.1</strong>.6)<br />
where q 0 is <strong>the</strong> unsaturated amplitude loss coefficient.<br />
The saturation of an absorber can be described with <strong>the</strong> following differential equation [89Agr]:<br />
dq (t)<br />
dt<br />
= − q (t) − q 0<br />
τ A<br />
−<br />
q (t) P (t)<br />
E sat,A<br />
, (<strong>2.1</strong>.7)<br />
where q (t) is <strong>the</strong> saturable amplitude loss coefficient that does not include any nonsaturable losses<br />
and P (t) is <strong>the</strong> time-dependent power incident on <strong>the</strong> absorber. Note that (<strong>2.1</strong>.7) may not be<br />
precise for high excitations where inverse saturable absorption can start to become important: For<br />
example, high excitation many times above <strong>the</strong> saturation fluence can result in additional effects<br />
such as two-photon absorptions, free carrier absorption, <strong>the</strong>rmal and even various damage effects<br />
[99Tho], [05Gra2]. Two-photon absorption only starts to become significant in <strong>the</strong> femtosecond<br />
pulse width regime and results in an earlier roll-off of <strong>the</strong> nonlinear reflectivity at high incident<br />
pulse fluences. This is a well-known effect and has been used in power-limiting devices before<br />
[86Wal]. In this regime, however, <strong>the</strong> absorber is operated more closely to <strong>the</strong> damage threshold<br />
which needs to be evaluated separately. Our experience is that damage in semiconductor saturable<br />
absorbers typically occurs at around 100 times <strong>the</strong> saturation fluence of <strong>the</strong> absorber with longterm<br />
degradation observed close to below this damage threshold. Therefore, we normally operate<br />
<strong>the</strong> device a least an order of magnitude below this damage threshold, ideally at an incident pulse<br />
fluence of 3 to 5 times <strong>the</strong> saturation fluence of <strong>the</strong> absorber. We <strong>the</strong>refore neglect <strong>the</strong>se very<br />
high-fluence effects in <strong>the</strong> following discussion. They, however, will become important again for<br />
suppressing Q-switching instabilities and will be discussed in more detail in Sect. <strong>2.1</strong>.6.8.<br />
At any time t <strong>the</strong> reflected (or transmitted) intensity I out (t) from <strong>the</strong> saturable absorber is<br />
given by<br />
I out (t) =R (t) I in (t) =e −2q(t) I in (t) (<strong>2.1</strong>.8)<br />
for a given input pulse I in (t). Then <strong>the</strong> total net reflectivity is given by<br />
Landolt-Börnstein<br />
New Series VIII/1B1