With home-made fiber perform and special fiber drawing & coating technique, a new-type of (3 + 1) GTWave fiber theoretically designed for bi-directional pump method, was successfully fabricated and justified of integrating multi-kW pump energy from commercial 976nm laser diodes. This (3 + 1) GTWave fiber amplifier demonstrated uniform absorption of pump light and easy thermal management characteristics along the whole fiber length. This amplifier is capable of simultaneously aggregating 5.19kW pump power at 976nm and finally generating 5.07kW laser output at 1066.5nm with an optical-to-optical efficiency of 74.5%, the first publically-reported multi-kW GTWave fiber directly pumped with commercial 976nm laser diodes to the best of our knowledge. No power roll-over was found at 5kW level and further power scaling can be expected with more pump power. The results indicate that GTWave fiber is a competitive integrated fiber device to collect enough pump energy from low-cost commercial laser diodes for multi-kW fiber laser development.

We experimentally investigate the behavior of the mode instability (MI) threshold in the double cladding Yb-doped fiber amplifier when the amplifier is pumped by broad linewidth laser diodes and narrow linewidth laser diodes respectively. It is found that the MI threshold increases by 26% when the amplifier is pumped by the broad linewidth laser diodes. Experiment results show that the MI threshold is affected by the local heat load rather than the average or the total heat load. The calculation shows that the local heat deposit actually plays the key role to stimulate the MI behaviour. At the MI threshold position in the fiber, the local heat deposit also changes dramatically. The effect of the thermal conductivity on the MI threshold is also studied. Our investigation shows that the MI threshold increases from 1269 W to 1950 W when the thermal conductivity of the fiber amplifier is increased from 0.3 W/(m · K) to 5 W/(m · K). Through optimizing the pump linewidth and the cooling efficiency of the gain fiber, the MI threshold is doubled in our experiment.

A comb-index core fiber (CIF) designed for high-power fiber lasers is proposed, in which the index distribution in the fiber core is comb-like. It is shown that the advantages of the CIFs include few guiding modes, excellent beam quality, large mode area, high optical damage threshold, and high tolerance to fabrication errors. Compared with the conventional single step fiber with the same normalized frequency V , the mode number of CIFs decreases from dozens to only a few ones and single-mode operation is achievable under certain conditions or with the help of fiber bending. The mode profile of LP 01 is close to Gaussian shape, which guarantees the beam quality to be of nearly diffraction limit. Meanwhile, the mode area increases by 50%-100% depending on different V values. The maximum power supported by the CIF increases by more than 100%. Finally, 30% fabrication errors on the refractive index and the ring thicknesses have only a little influence on the mode properties of CIFs, which is favorable in practical application.Index Terms-Fiber lasers, optical fibers, large mode area fibers, fiber design.

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