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

7
as the precision machining of explosives without detonation and the
cutting of the corneal flap for refractive corrective surgery, just to
8
name a few.
Ti:sapphire, however, has its limitations. For example, until
recently it was not directly diode pumpable. Although it can be
9
pumped with 4XXnm diode lasers in an oscillator, output power
is limited because these pumps are low power, and the nonlinear
absorption effects are quite severe. New directly diode-pumped
materials have become more widespread. Ytterbium-, chromium-,
and erbium-doped materials can have broad emission bands and
low quantum defect (reduced thermal problems) and are directly
pumped by high-power laser diodes. The first years of the 21st
century have seen continued rapid progress in the development of
higher average-power ultrafast lasers, with the introduction of
widespread thermoelectric and cryogenic cooling technology for
ultrafast laser amplifiers to mitigate large thermal effects. These
effects plague laser systems across the board and are not unique to
femtosecond lasers; however, they can have a dramatic effect on
the generation of short pulses.
This chapter describes ultrafast sources, amplification methods,
thermal mitigation, and ways to measure the fastest events ever
recorded in human history.

12.2 Ultrafast Laser Sources and Oscillators

Modern ultrafast sources are predominantly solid state and pas-
sively mode locked. Two specific types of mode locking are used
today—Kerr lens mode locking and mode locking from saturable
absorbers, specifically semiconductor saturable absorber mirrors
(SESAMs). 10

12.2.1 Kerr Effect
In 1990, the modern solid-state ultrafast laser was developed by Wilson
11
Sibbett at the University of St. Andrews. This laser used a new,
passive mode-locking mechanism and a third-order effect, known as
the Kerr effect, that was given by a change in the index of refraction
in Ti:sapphire:
3
3 χ () 3 χ ()
3
n() = w n( w 0 ) + I(); n ( w 0 ) = (12.1)
w
2
n w )
8 n(w 0 ) 8( 0 0
(3)
where n(w) is the index of refraction, χ is the third-order susceptibility
tensor component magnitude, and I(w) is the intensity of light. The
nonlinear index n (w ) gives rise to a lensing effect at the very peak
0
2
of the intensity profile, with a value of ~2 × 10 –16 cm /W for
2
Ti:sapphire. If an optical cavity is designed with the lens shown in
Eq. (12.2) in mind, passive mode locking can occur.