Spectral Manipulation of a High-Peak-Power Single-Frequency Nanosecond Pulsed Fiber Laser by Managing Frequency Chirp via Tailoring the Pulse Shape

Spectral linewidth broadening induced by self-phase modulation is one of the main issues hindering single-frequency pulsed fiber amplifiers from high-peak-power operation with narrow linewidth. Gain-saturation induced pulse profile distortion results in the generally used rectangular- and sawtooth-shape pulses inappropriate for high-peak-power narrow-linewidth laser demonstration. Here we proposed a frequency chirp management technique by pre-shaping the seed pulse for spectral manipulation in pulsed laser amplifier. Considering the gain-saturation induced pulse profile evolution during power amplification, the frequency chirp difference among the pulse duration can still be mitigated and narrow linewidth without spectral distortion can be realized at high laser peak power. With a slow-rising part introduced by a designed quadratic curve to the leading edge of the seed pulse, a 7.9-ns pulsed single-frequency laser with the root-mean-square spectral linewidth of only 70 MHz was experimentally demonstrated at a peak power up to 37.5 kW.