We report on the performance of a standard Yb-doped DC-LMA fiber and compare it to a similar core-size chirally-coupled core (3C®) fiber in a co-pumped fiber amplifier configuration. We used Yb-doped 20/400/0.064 DC LMA fiber for the power amplifier and achieved ~2.4 kW of signal power at 2.79 kW of absorbed pump power. However, we observed an onset of TMI at ~2.2 kW. The spectral bandwidth of this amplifier was 20 GHz and there was no sign of SBS at 2.4 kW of output power. We then used an Yb-doped 21.9/400/0.059 DC 3C fiber with a coiling diameter of ~30 cm to test the efficacy of HOM suppression in this fiber with respect to improving TMI threshold. We achieved 2.6 kW of output power (pump combiner limited) without TMI. Further power-scaling experiments are underway and we will report on the latest findings. However, it is clear from these results that 3C fiber has a better HOM suppression capability compared to 10-cm diameter coiled DC-LMA fiber. Even a 30-cm coiled 3C fiber shows no sign of TMI at 2.6 kW; while, a slightly smaller diameter and tightly coiled 10-cm diameter LMA fiber amplifier shows signs of TMI ~ 2.2 kW. We also measured Brillouin shift, gain bandwidth and gain coefficient and they were found to be ~15.3 GHz, ~83 MHz and 0.47 to 0.7 ×10-11 m/W respectively compared to reported values of 16.1 GHz, ~64 MHz and 5 ×10-11 m/W. This significantly lower Brillouin gain and slightly larger gain bandwidth leads to eight times higher SBS threshold for amplifiers using nLIGHT fiber with near single-frequency seed compared to literature values. This is a distinct advantage which will enable optimization of both the LMA and 3C fiber geometry to achieve higher TMI threshold in the future.
M. Kanskar, J. Zhang, J. Koponen, O. Kimmelma, V. Aallos, I-Ning Hu, and A. Galvanauskas, "Narrowband transverse-modal-instability (TMI)-free Yb-doped fiber amplifiers for directed energy applications," Proc. SPIE 10512, Fiber Lasers XV: Technology and Systems, 105120F (Presented at SPIE LASE: January 29, 2018; Published: 26 February 2018); https://doi.org/10.1117/12.2291253.
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