Controlling output beam profile of multimode fiber amplifier with enhanced threshold for transverse mode instability

By coherently seeding a multimode fiber amplifier with a tailored spatial wavefront, it is possible to generate the target output profile in the presence of thermo-optical nonlinearity and gain saturation. We prove numerically that such wavefront exists with a modified phase-conjugation approach. An optical beam with target profile and power is launched from the distal end to proximal end of a multimode waveguide with absorption, and the exiting wavefront is phase-conjugated and sent back to the waveguide with loss switched to gain of equal magnitude. The target output beam is obtained with enhanced threshold for transverse mode instability due to multimode excitation.