Laser chains delivering ulta-short pulses usually suffer from temporal broadening during the amplification process caused by gain-narrowing and gain bandwidth.Several possibilities can be implemented to shorten the pulse duration at the output of the laser. Briefly, the spectrum is broadened by non linear process in various media (gas, bulk, crystal, fiber) and the accumulated phase is compensated to reach an ultra-short pulse duration near the Fourier Transform limit. 

Pulse compression by filamentation

S Driever, D Bigourd, N Fedorov, M Cornet, M Arnold, F Burgy, S Montant, S Petit, D Descamps, E Cormier, E Constant, A Zaïr, "Tunable 1.6-2 µm near infrared few cycle pulse generation by filamentation", Applied Physics Letter 102,191119 (2013)