Simple pulse preshaping method for realizing self-similar amplification efficiently with a narrow bandpass filter

A careful choice of input pulse characteristics is critical for realizing self-similar amplification in a Yb fiber amplifier. With fixed input pulse energy and amplifier parameters, there exists an optimal input pulse duration ∆ T in,opt which allows for the most efficient convergence to the parabolic regime. Most existing self-similar amplifiers with practically achievable parameters have a ∆ T in,opt of several hundred femtoseconds or even longer than 1 ps. However, femtosecond Yb fiber lasers, which are commonly used as input of self-similar amplifiers nowadays, usually have a sub-200 fs compressed pulse duration, owing to the broad gain bandwidth of Yb fiber. Here, we propose a simple pulse preshaping method to optimize the input pulse duration. A narrow bandpass filter (NBF) is used to clip the overbroad spectrum, so the pulse duration can be lengthened several times to approach ∆ T in,opt . We have verified the method by clipping the spectrum of pulses delivered from a mode-locked Yb fiber oscillator with a 3.9-nm NBF and achieving self-similar amplification in the following fiber amplifier. Amplified pulses have a spectrum of 42.6 nm and can be dechirped to 72.7 fs. We believe the spectrum-filtering method is useful to design a self-similar amplifier for generating parabolic-shaped pulses or dechirped sub-100 fs short pulses.