Authors: Bertrand Morasse, Stéphane Chatigny, Éric Gagnon, Jean-Philippe de Sandro, Cynthia Desrosiers
Pulseshaping is important in high energy pulsed fiber MOPA system to mitigate non-linear effects and optimize the processing of different materials. However, pulseshaping is greatly limited by the spectral features of the semiconductor seed source commonly used as the master oscillator. Through the appropriate design of an external fiber Bragg grating (FBG) and adequate current modulation, the spectrum of the fiber-coupled seed laser was broadened to suppress stimulated Brillouin scattering occurring in the amplifier chain and the central emission wavelength and bandwidth were controlled. Pulseshaping is also quickly limited by the saturation energy and doping level of standard aluminosilicate ytterbium doped fibers used in the power amplifier even with large core diameter. Co-doping the fiber with phosphorus greatly increases the saturation energy of the system, which gives smoother pulseshape and significantly lower stimulated Raman scattering (SRS). It is shown that going from 1060 nm to longer emission wavelength at 1090 nm with this fiber increases further the pulseshaping capabilities and reduces SRS. The phosphorus codoping also allows higher ytterbium doping level without photo-degradation, which decreases nonlinear effects generation during the amplification while giving more flexible pump wavelength choice and efficiency.