Manipulating the heat load distribution by laser gain competition in TMI-limited fiber amplifiers

Supported by both experimental and simulated results, this contribution demonstrates the heat load distribution in a co-pumped, ytterbium (Yb)-doped fiber amplifier seeded with two different wavelengths can be significantly changed depending on the seed power ratio. Longitudinal temperature measurements in a Yb-doped 10.5 m 20/400 mm fiber confirm a significant shift of the heat load maximum by 3.5 m towards the fiber output when decreasing the seed power ratio from P1030nm/P1080nm = 1.7 to 20. In single-tone operation with a seed power of P1080nm = 3.5 W, the amplifier is limited by the onset of transverse mode instabilities at a power-level of 1950 W. However, dual-tone seeding with a seed power ratio up to P1030nm/P1080nm = 10 reduces the TMI-threshold dramatically down to 1050 W. Additionally we show, that the modal instability threshold is very susceptible to 1030 nm seed noise in the frequency regime up to 10 kHz.