Ultrashort Laser Pulse Phenomena, 2nd Edition

426 Femtosecond Pulse Amplification

fact that the gain is essentially unsaturated until the pumping process stops. The

latter is responsible for the shaping of the trailing edge of the signal pulse. With

the onset of saturation (Region II) the pulse duration increases, a tendency which

becomes more pronounced with significant GVD.

7.6. OPTICAL PARAMETRIC CHIRPED PULSE

AMPLIFICATION (OPCPA)

The idea of optical parametric chirped pulse amplification (OPCPA) is to

replace the laser gain media of a CPA system [21], (cf. Section 7.4), by a non-

linear crystal. Amplification by stimulated emission is substituted by parametric

amplification of the signal pulse in the presence of a pump pulse. The appara-

tus is sketched in Figure 7.20. Since the introduction of the concept of chirped

parametric amplification by Dubietis et al. [51] in 1992 and an analysis of the

prospects for high-power amplification by Ross et al. [52] several amplifier sys-

tems have been developed producing sub-ps pulses in the TW range. A summary

of various concepts and recent progress can be found in Ross et al. [52] and

Butkus et al. [53].

The OPCPA concept relies on the fact that a chirped pulse can be parametri-

cally amplified without significant distortion of the phase if the OPA bandwidth

is large enough. Note that the bandwidth is determined by material parame-

ters, the pump and signal wavelength, and the geometry favoring noncollinear

Signal

Stretcher Compressor

Nonlinear

crystal

Pump

Figure 7.20 Schematic diagram of an OPCPA device. The pulse to be amplified is stretched and

chirped and combined with a pump pulse in a nonlinear optical crystal. The parametrically amplified

chirped output is subsequently compressed.