Compensation of optical nonlinearities in a femtosecond laser system in a broad operation regime.

• A method of tunable nonlinear phase pre-compensation in CPA laser system is presented. • Compact pre-compensation is achieved by tunable CFBG used also as a pulse stretcher. • Nonlinear phase between 0 and 14 rad is efficiently pre-compensated. • Detailed FROG analysis is performed to characterize output laser pulses. • The method is applied to a femtosecond fiber laser with pulse energies up to 70 µJ. We present here a detailed study of a compact and tunable nonlinear phase pre-compensation method based on a tunable chirped fiber Bragg grating (TCFBG) applied to an ultrafast fiber laser system. Frequency resolved optical gating (FROG) measurements reveal a significant variation in the output pulse shape due to the variation in accumulated nonlinear phase for varying pulse energy and repetition rate (even if the pulse energy is kept constant) of the laser system. The TCFBG, used as a stretcher and a pre-compensating element in this chirped fiber amplification (CPA) system, eliminates the need for additional pulse shaping components which greatly simplifies the system for tunable pre-compensation. We demonstrate an efficient pre-compensation of the accumulated nonlinear phase between 0 and 14 rad resulting in pulses with a constant pulse duration and Strehl ratio over a broad operating regime of the laser with repetition rates ranging from 100 kHz up to 30 MHz and pulse energies spanning from 1 µJ up to 70 µJ. [ABSTRACT FROM AUTHOR]