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> 79<br />
typical 1-kHz rate. This greatly increases <strong>the</strong> signal-to-noise ratio in measurements [07Mar] and<br />
reduces space-charge effects that tend to hide <strong>the</strong> underlying interesting physical processes with<br />
current sources at 1 kHz.<br />
<strong>2.1</strong>.4 Loss modulation<br />
<strong>2.1</strong>.4.1 Optical modulators: acousto-optic and electro-optic modulators<br />
Many textbooks, for example [86Sie, 84Yar, 98Sve], have reviewed optical modulators for pulse generation.<br />
Today <strong>the</strong> most important optical modulators for short pulse generation are <strong>the</strong> acoustooptic<br />
and electro-optic modulators. The acousto-optic modulators have <strong>the</strong> advantage of low optical<br />
insertion loss and can readily be driven at high repetition rates. They are typically used for cw<br />
mode-locking. However, for Q-switching <strong>the</strong>ir loss modulation is limited and <strong>the</strong> switching time is<br />
ra<strong>the</strong>r slow. Therefore, acousto-optic modulators are primarily used for repetitive Q-switching of<br />
cw-pumped <strong>lasers</strong> (e.g. Nd:YAG) and electro-optic modulators are used for Q-switching in general.<br />
For mode-locking <strong>the</strong> acousto-optic modulator typically consists of an acousto-optic substrate<br />
(typically fused quartz) and a transducer that launches an acoustic wave into <strong>the</strong> substrate. An<br />
acoustic resonator is formed when opposite to <strong>the</strong> transducer <strong>the</strong> crystal substrate is air backed.<br />
Then <strong>the</strong> acoustic wave is reflected and an acoustic standing wave is formed which produces a<br />
light modulator at twice <strong>the</strong> microwave drive frequency. At higher frequencies (a few hundreds of<br />
megahertz to a few gigahertz) <strong>the</strong> loss modulation is strongly reduced by <strong>the</strong> acoustic attenuation<br />
in <strong>the</strong> substrate. Thus, at higher modulation frequencies a sapphire [90Kel1, 90Wei] or a GaP<br />
[90Wal] substrate has been used successfully.<br />
<strong>2.1</strong>.4.2 Saturable absorber: self-amplitude modulation (SAM)<br />
Saturable absorbers have been used to passively Q-switch and mode-lock many <strong>lasers</strong>. Different<br />
saturable absorbers, such as organic dyes, color filter glasses, dye-doped <strong>solid</strong>s, ion-doped crystals<br />
and semiconductors have been used. Independent of <strong>the</strong> specific saturable absorber material, we<br />
can define a few macroscopic absorber parameters that will determine <strong>the</strong> pulse generation process.<br />
The relevant macroscopic properties of a saturable absorber are <strong>the</strong> modulation depth, <strong>the</strong><br />
nonsaturable loss, <strong>the</strong> saturation fluence, <strong>the</strong> saturation intensity and <strong>the</strong> impulse response or<br />
recovery times (Table <strong>2.1</strong>.5). These parameters determine <strong>the</strong> operation of a passively mode-locked<br />
or Q-switched laser. In our notation we assume that <strong>the</strong> saturable absorber is integrated within a<br />
mirror structure thus we are interested in <strong>the</strong> nonlinear reflectivity change R (t) as a function of<br />
time and R (F p,A ) as a function of <strong>the</strong> incident pulse energy fluence on <strong>the</strong> saturable absorber. If<br />
<strong>the</strong> saturable absorber is used in transmission, we simply characterize <strong>the</strong> absorber by nonlinear<br />
transmission measurements, i.e. T (t) andT (F p,A ). Both <strong>the</strong> saturation fluence F sat,A and <strong>the</strong><br />
absorber recovery time τ A are determined experimentally without any needs to determine <strong>the</strong><br />
microscopic properties of <strong>the</strong> nonlinearities. The saturation fluence of <strong>the</strong> absorber does not only<br />
depend on material properties but also on <strong>the</strong> specific device structure in which <strong>the</strong> absorber is<br />
integrated, which gives significantly more design freedom.<br />
Standard pump-probe techniques determine <strong>the</strong> impulse response R (t) and <strong>the</strong>refore τ A .In<br />
<strong>the</strong> picosecond regime we typically only have to consider one picosecond recovery time, because<br />
much faster femtosecond nonlinearities in <strong>the</strong> saturable absorber give negligible modulation depth.<br />
This is shown with a semiconductor saturable absorber in Fig. <strong>2.1</strong>.8, where <strong>the</strong> differential impulse<br />
Landolt-Börnstein<br />
New Series VIII/1B1