130 μJ linear-polarized single-frequency 12-μm-core Er/Yb co-doped fiber amplifier based on pre-shaped seed pulse

Stimulated Brillouin scattering (SBS) is the major barrier in the process of energy scaling for pulsed single- frequency fiber master oscillator power amplifier (MOPA). Due to gain saturation effect, the laser pulse profile will be gradually distorted with the increase of pump power, which induces steep leading edge and narrower width for the amplified pulses. The resulting laser peak power would increase rapidly and thus the SBS threshold is reached earlier to limit the amplification of pulse energy. A method to obtain high-energy pulsed single-frequency laser by pulse pre-shaping is demonstrated in this work. By designing the leading edge of the triangular pulse, optimizing its rising trend and the duration of the low-intensity rising part, the pulse width compression phenomenon caused by gain saturation is alleviated effectively. Thereafter, the laser peak power increase process can be retarded to reach the SBS threshold so that higher energy can be amplified for the pulsed single-frequency fiber laser. In the experiment, when the seed pulse is optimized to be a triangular pulse with a low-intensity rising edge of 401 ns and a pulse width of 520 ns, a linear-polarized pulse single-frequency fiber laser of 130.9 mu J is obtained in a 12-mu m-core Er/Yb co-doped polarization-maintaining fiber. The pulse width is broadened to 608 ns at the maximum energy. When it is compared with the triangular pulse seed with a rapidly rising leading edge, its maximum energy is increased by about 25%. The optical signal-to-noise ratio and polarization extinction ratio are measured to be 42 dB and 16 dB at the maximum pulse energy, respectively. The corresponding spectral linewidth measured by a delayed self-heterodyne system is 542 kHz. Higher pulse energy can be anticipated by further optimizing the pulse profile and using large-mode-are gain fibers.