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
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
Ref. p. 134] <strong>2.1</strong> <strong>Ultrafast</strong> <strong>solid</strong>-<strong>state</strong> <strong>lasers</strong> 35<br />
Shorter pulses are only obtained with external pulse compression. Sub-4-femtosecond pulses<br />
have been demonstrated with external pulse compression [03Sch] for <strong>the</strong> first time using cascaded<br />
hollow fiber pulse compression. External pulse compression into <strong>the</strong> few optical cycle regime [99Ste]<br />
is ei<strong>the</strong>r based on optical parametric amplification [99Shi], compression of cavity-dumped pulses in<br />
a silica fiber [97Bal], hollow fiber pulse compression [97Nis] or more recently through filamentation<br />
[04Hau1, 05Hau]. Especially <strong>the</strong> latter two allow for pulse energies of more than 100 μJ withonly<br />
a few optical cycles which fulfill a central task in <strong>the</strong> generation of attosecond eXtreme UltraViolet<br />
(XUV) pulses [01Dre]. For such applications using intense few-cycle pulses in <strong>the</strong> near infrared<br />
driving extreme nonlinear processes <strong>the</strong> electric field amplitude ra<strong>the</strong>r than <strong>the</strong> intensity envelope<br />
becomes <strong>the</strong> important factor.<br />
Femtosecond pulses in <strong>the</strong> near infrared reached a bandwidth large enough to only support one<br />
to two optical cycles underneath <strong>the</strong> pulse envelope. Therefore, <strong>the</strong> position of <strong>the</strong> electric field<br />
underneath <strong>the</strong> pulse envelope becomes important. This Carrier-Envelope Offset (CEO) [99Tel]<br />
can be stabilized in laser oscillators with attosecond accuracy [03Hel]. At <strong>the</strong> beginning this was a<br />
challenging task as it was not obvious how to obtain this because normally only <strong>the</strong> pulse intensity<br />
and not <strong>the</strong> electric field is detected. Also mode-locking <strong>the</strong>ory fully confirms that <strong>the</strong> position<br />
of <strong>the</strong> peak electric field underneath <strong>the</strong> pulse envelope can have very large fluctuations by much<br />
more than one optical cycle. The stabilization is done in <strong>the</strong> frequency domain using <strong>the</strong> extremely<br />
stable and broadband frequency comb generated with femtosecond <strong>solid</strong>-<strong>state</strong> <strong>lasers</strong> [99Tel]. The<br />
simplest approach is based on a f-to-2f-interferometer [99Tel] which was first realized with a<br />
spectrally broadened Ti:sapphire laser pulse using photonic crystal fibers [00Jon, 00Apo].<br />
The emphasis of this chapter is to give an updated review of <strong>the</strong> progress in ultrafast <strong>solid</strong>-<strong>state</strong><br />
<strong>lasers</strong> since 1990 when mode-locking became a hot topic again and <strong>the</strong> era of ultrafast dye <strong>lasers</strong><br />
has come to its end. The topics mentioned above will be discussed in more details. The goal is to<br />
give also to <strong>the</strong> non-expert an efficient starting position to enter into this field without providing<br />
all <strong>the</strong> detailed derivations. Relevant and useful references for fur<strong>the</strong>r information are provided<br />
and a brief historic perspective is given throughout this chapter. A basic knowledge in <strong>lasers</strong> is<br />
required. The emphasis is on <strong>solid</strong>-<strong>state</strong> <strong>lasers</strong> because <strong>the</strong>y will dominate <strong>the</strong> field in <strong>the</strong> future.<br />
More extended reviews and books have summarized <strong>the</strong> dye laser era [88Sha, 90Die]. Here, no<br />
emphasis is put on fiber and semiconductor <strong>lasers</strong>, but some useful references to recent review<br />
articles and book chapters will be provided.<br />
<strong>2.1</strong>.2 Definition of Q-switching and mode-locking<br />
<strong>2.1</strong>.<strong>2.1</strong> Q-switching<br />
The history of Q-switching goes back to 1961, when Hellwarth [61Hel] predicted that a laser<br />
could emit short pulses if <strong>the</strong> loss of an optical resonator was rapidly switched from a high to a<br />
low value. The experimental proof was produced a year later [62McC, 62Col]. The technique of<br />
Q-switching allows <strong>the</strong> generation of laser pulses of short duration (from <strong>the</strong> nanosecond to <strong>the</strong><br />
picosecond range) and high peak power. The principle of <strong>the</strong> technique is as follows: Suppose a<br />
shutter is introduced into <strong>the</strong> laser cavity. If <strong>the</strong> shutter is closed, laser action cannot occur and<br />
<strong>the</strong> population inversion can reach a value far in excess of <strong>the</strong> threshold population that would<br />
have occurred if <strong>the</strong> shutter were not present. If <strong>the</strong> shutter is now opened suddenly, <strong>the</strong> laser will<br />
have a gain that greatly exceeds <strong>the</strong> losses, and <strong>the</strong> stored energy will be released in <strong>the</strong> form of<br />
a short and intense light pulse. Since this technique involves switching <strong>the</strong> cavity Q-factor from a<br />
low to a high value, it is known as Q-switching. Ideally Q-switched <strong>lasers</strong> operate with only one<br />
axial mode because strong intensity noise is observed in a multi-mode Q-switched laser.<br />
Landolt-Börnstein<br />
New Series VIII/1B1