**1.2. Gain narrowing during amplification**

The making of the output short pulse shorter and keeping the same energy paved another way to the next milestone of the ultra-high peak power. Nevertheless, pulse duration of CPA lasers is strongly limited by gain narrowing of the pulse spectra in the gain medium of the multipass amplifiers (to around 30 fs of 100 TW–1 PW lasers based on Ti:Sa). Different approaches were entertained to overcome this limitation. Among them, there is a gain narrowing control by introduction of the thin film etalon into the front-end regenerative amplifier cavity. Impressive results were reached with this technique (pulse duration—16 fs with spectral width—72 nm) [19], but this method still suffers some restrictions, such as losses of the energy, a finite possibility of the broadening of the spectra due to the fluorescence spectrum limitation of the amplifier crystal, further gain narrowing in the buster and final amplifiers, as well as the limitation of the spectral transmission of the grating compressor.

The application of concept of optical-parametrical chirped pulse amplification (OPCPA) for broadband pulse allowed to reach a pulse duration of around 10 fs [20], but extremely severe requirements on the parameters of the pump laser which discussed above restrict output energy only to a few mJs. This way, an adequate method for shortening high-energy pulses has not yet been found. Two promising methods of the saving and restoring spectral bandwidth will be discussed also in this chapter.
