A gain-enhanced 10/125 µm polarizationmaintaining Yb-doped fiber (PMYDF), with the cladding absorption of 7.5 dB/m at 976nm, for narrow linewidth linearly polarized fiber oscillator or amplifiers is presented. By utilizing the gain-enhanced PMYDF in an all-fiber laser oscillator, an output power of 123 W is achieved with applying appropriate bending strategies and thermal management, corresponding to the slope efficiency of 86.7%. By applying the gain-enhanced PMYDF in seed, an increase of 34% in the longitudinal mode interval is effectively achieved compared to the commercial PMYDF, while the number of longitudinal modes is only 75%. According to the main oscillation power amplification (MOPA) system, the total amount of spectral broadening applying the seed with the gainenhanced PMYDF are only 67%, 64% and 54% of applying the seed with the commercial PMYDF, corresponding to co-pumping, bi-pumping and counter-pumping schemes, respectively.
In this work, a narrow-linewidth linearly polarized fiber amplifier with a record output power of 3.2 kW was achieved based on a homemade polarization-maintaining Yb-doped fiber corresponding to a slope efficiency of 79% and a 3 dB linewidth of 0.2227 nm. By examining various numerical aperture (NA) PMYDFs, the experimental investigation on expanding mode instability (MI) threshold in PM fiber amplifiers was put on display. And the results reveal that the MI threshold is enhanced by more than 370 W for every 0.004 decrease in core numerical aperture. Increasing the seed linewidth from 0.0454 nm to 0.0976 nm by adding 200 m polarization maintaining Ge-doped fiber the stimulated Brillouin scattering threshold increased from 805 W to above 3.2 kW. By applying the MI suppression method, a double-eight-shaped aluminum plate was adopted to coil the gain fiber, and the MI threshold increased by more than 1100 W.
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