Page 347 - High Power Laser Handbook

316 So l i d - S t at e La s e r s Ultrafast Solid-State Lasers 317

Figure 12.11 Schematic of optical parametric chirped pulse amplification
(OPCPA). The pump laser is usually a 10 to 100-ps source at ~1 µm.

This process may seem simple and free of thermal issues, because
there is no storage medium and thus no quantum defect. For this
process to be efficient, however, the pump pulse must be square in
time due to the high gain in the system. Single-pass gains can be
greater than 1000; therefore, if we want to amplify a chirped pulse,
the gain (which is now related to the pump pulse shape) must be flat;
otherwise, gain narrowing can be quite severe. In addition, if the
pump pulse is gaussian, we can only amplify in the narrow central
region of the gaussian intensity profile, which leaves the temporal
wings of the pump laser unconverted, reducing the efficiency (see
Fig. 12.12).
A spatially flattop or super-gaussian mode profile is also desired
to avoid massive mode reshaping of the amplified beam. An OPCPA
system’s gain bandwidth can be very large, in some cases support-
ing less than 10-fs pulses. This gain bandwidth is a direct result of
phase matching in the crystal used. In the case of OPCPA, cryocool-
ing is not necessary; however, single-mode, high-beam-quality
picosecond pump lasers must be used. The major advantage of this
technology is the wavelength tunability for the entire system. The
same architecture may be used for many different wavelengths,
from the ultraviolet into the midinfrared. Figure 12.13 shows a
34
scaled version of a recently demonstrated 3.0-µm OPCPA system.
This system uses a fiber oscillator (Er:Fiber), which is split and

Usable gain region

Gaussian pump pulse Square pump pulse

Figure 12.12 Pump pulse temporal profile for efficient OPCPA. The super-
gaussian, or “square,” pulse leaves less energy behind, greatly improving
efficiency.