Page 334 - High Power Laser Handbook

302 So l i d - S t at e La s e r s Ultrafast Solid-State Lasers 303

4(w L )
n
−1
f Kerr = 2 0 m P (12.2)
π w 4
Here L is the material length, w is the beam radius, and P is the
m
beam power. From this expression for a typical Ti:sapphire oscillator,
we get a focal length of ~1 m.

12.2.2 Ultrafast Oscillators
A typical ultrafast oscillator has some distinguishing characteristics.
First, it needs a pump source, whether diodes or another laser.
Second, it needs some form of dispersion compensation—either
12
prisms, chirped mirrors, or both, depending on the desired result.
Finally, some sort of starting mechanism, such as a shock (prism jog
is typical), to induce an intensity modulation to start the Kerr effect,
or a SESAM, which induces lower loss for a given intensity. Figure 12.1
shows a standard Ti:sapphire laser. Note that if the cavity is set just
right, self-mode locking can occur.
Many other femtosecond lasers have since been developed and
are widely used today. Table 12.1 gives a sampling of available femto-
second laser sources. These sources can cover a wide range of pulse
durations, from less than 10 fs to 1 ps. An advantage of some of these
ultrafast laser sources is their ability to be directly laser diode
pumped, which can reduce cost and complexity. Ti:sapphire, which
has the potential for the shortest pulses, still must be pumped by
complex intracavity-doubled Nd:YVO (neodymium-doped yttrium
orthovanadate) lasers. Although new laser diodes in the 4XXnm
regime, and potentially in the 5XXnm regime, may help this problem,
this technology has a long way to go to reach usable powers of
around 1 to 5 W at 532 nm. New optically pumped semiconductor
13

Figure 12.1 Diagram of a standard Ti:sapphire oscillator with prisms used
for phase compensation.