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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80 <strong>2.1</strong>.4 Loss modulation [Ref. p. 134<br />
Table <strong>2.1</strong>.5. Saturable absorber quantities, <strong>the</strong>ir defining equations and units.<br />
Quantity Symbol Defining equation or measurement Unit<br />
Saturation fluence F sat,A measurement R (F p,A) orT (F p,A), (Fig. <strong>2.1</strong>.9) J/cm 2<br />
Recovery time τ A measurement R (t) orT (t), (Fig. <strong>2.1</strong>.8) s<br />
Incident beam area A A measurement cm 2<br />
Saturation energy E sat,A E sat,A = A AF sat,A J<br />
Saturation intensity I sat,A I sat,A = F sat,A/τ A W/cm 2<br />
Modulation depth ΔR or ΔT measurement R (F p,A) orT (F p,A), (Fig. <strong>2.1</strong>.9)<br />
Nonsaturable loss ΔR ns or ΔT ns measurement R (F p,A) orT (F p,A), (Fig. <strong>2.1</strong>.9)<br />
Incident pulse energy E p measurement J<br />
Incident pulse fluence F p,A F p,A = E p/A A J/cm 2<br />
Incident intensity I A (t) F p,A = ∫ I A (t)dt W/cm 2<br />
Reflec tivity pump− probe signal<br />
100 fs excitation pulse<br />
4 ps excitation pulse<br />
-10 -5 0 5 10 15 20 25 30<br />
Pump−probe delay [ps]<br />
Fig. <strong>2.1</strong>.8. Typical measured impulse response of a<br />
SESAM measured with standard degenerate pumpprobe<br />
measurements using two different excitation pulse<br />
durations. The saturable was grown at low temperature,<br />
which reduced <strong>the</strong> recovery time to about 20 ps. The<br />
short intraband <strong>the</strong>rmalization recovery time results in<br />
negligible modulation depth with a 4 ps excitation pulse.<br />
Thus, only <strong>the</strong> slower recovery time due to carrier trapping<br />
is important in <strong>the</strong> picosecond regime.<br />
response DR (t) was measured for two different excitation pulse durations using a semiconductor<br />
saturable absorber. For excitation with a picosecond pulse <strong>the</strong> pump-probe trace clearly shows<br />
no significant modulation depth with a fast time constant. In <strong>the</strong> femtosecond pulse regime we<br />
normally have to consider more than one absorber recovery time. In this case <strong>the</strong> slow component<br />
normally helps to start <strong>the</strong> initial pulse formation process. The modulation depth of <strong>the</strong> fast component<br />
<strong>the</strong>n determines <strong>the</strong> pulse duration at steady <strong>state</strong>. Fur<strong>the</strong>r improvements of <strong>the</strong> saturable<br />
absorber normally require some better understanding of <strong>the</strong> underlying physics of <strong>the</strong> nonlinearities<br />
which can be very interesting and ra<strong>the</strong>r complex. A more detailed review about <strong>the</strong> microscopic<br />
properties of ultrafast semiconductor nonlinearities for saturable absorber applications is given in<br />
a recent book chapter [00Sie]. <strong>Ultrafast</strong> semiconductor dynamics in general are discussed in much<br />
more detail by [99Sha]. However, <strong>the</strong> basic knowledge of <strong>the</strong> macroscopic properties of <strong>the</strong> absorber<br />
and how to measure [04Hai] and adjust <strong>the</strong>m to a certain value is normally sufficient for stable<br />
pulse generation.<br />
The saturation fluence F sat,A is determined and defined by <strong>the</strong> measurement of <strong>the</strong> nonlinear<br />
change in reflectivity R (F p,A ) as a function of increased incident pulse fluence (Fig. <strong>2.1</strong>.9). The<br />
traveling-wave rate equations [89Agr] in <strong>the</strong> slow absorber approximation give normally a very<br />
Landolt-Börnstein<br />
New Series VIII/1B1