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314 So l i d - S t at e La s e r s Ultrafast Solid-State Lasers 315

100,000

Thermal lens focal length (cm) 1000 100 K (−173°C)
10,000
50 K (−223°C)
77 K (−196°C)
100

10
233 K (−40°C)
1 300 K (27°C)
1 20 40 60 80 100
Pump power for 500 µm pump spot (W)

Figure 12.10 Thermal lens focal length as a function of deposited power in
a Ti:sapphire rod from 300 to 50 K.

for a regenerative amplifier, because they act as a strong spatial filter.
However, more loss leads to higher overall gain to reach the desired
output and, therefore, to more phase distortion and gain narrowing.
For high-power applications with greater than 20 W of pump, cryo-
cooling is preferable.

12.4.1 Optical Parametric Chirped Pulse Amplification
Classically near- and midinfrared ultrafast pulses have been gener-
ated using Ti:sapphire-amplified laser systems in conjunction with
an optical parametric amplifier (OPA). This system can generate
very short (< 50 fs) pulses in the OPA idler around 2 µm. However,
in this scheme, the Ti:sapphire laser (though a rugged technology)
can have a large footprint and require laboratory-like conditions.
These systems also tend to be quite expensive (~$300,000). In
addition, in terms of reliability, Ti:sapphire systems require bulky,
frequency-doubled Nd laser systems. Although fiber-based green
pump lasers are now available and have been used to pump high-
power Ti:sapphire oscillators, they are a very new technology with
energy scalability issues.
Optical parametric chirped pulse amplification (OPCPA) pro-
33
vides an alternative to laser amplification. It uses the nonlinear pro-
cess of parametric generation (Fig. 12.11), which splits a pump photon
into two parts: the signal (the high-energy photon) and the idler (the
low-energy photon). It also has the advantage of being able to use
standard stretching and compression techniques, such as CPA and
DPA.