Experimental Study of 5 kW High Stability Monolithic Fiber Laser Oscillator With or Without External Feedback

In this paper, we construct a high power monolithic fiber laser oscillator based on a commercial ytterbium-doped fiber with core/inner cladding diameter of 25/400 μm. The output performances of the fiber oscillator are experimentally investigated in the conditions of either with or without an external feedback. In the presence of external feedback, a very strong stimulated Raman scattering (SRS) (~9 dB below the signal light) emerges at the output power of ~3.56 kW, where the onset of the transverse mode instability (TMI) limits further power scaling. By large angle cleaving the signal arm of the forward combiner to avoid external feedback, the maximum output power of 5.07 kW is achieved with no sign of TMI. The Raman stokes light is ~35 dB below the signal light and the M2 factor is measured to be ~1.6. To directly characterize its power stability and engineering capability, the fiber oscillator is tested for continuous 5-hour operation at ~5.07 kW with power fluctuation less than 0.42%. During the test process, the output power, spectrum, beam quality and temporal signal perform excellent stability. The experimental results reveal that thresholds of SRS and TMI strongly depend on the external feedback in high power fiber oscillators.