26 February 2018 Narrowband transverse-modal-instability (TMI)-free Yb-doped fiber amplifiers for directed energy applications
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Proceedings Volume 10512, Fiber Lasers XV: Technology and Systems; 105120F (2018) https://doi.org/10.1117/12.2291253
Event: SPIE LASE, 2018, San Francisco, California, United States
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.
Conference Presentation
© (2018) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
M. Kanskar, J. Zhang, J. Koponen, O. Kimmelma, V. Aallos, I-Ning Hu, 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 (26 February 2018); doi: 10.1117/12.2291253; https://doi.org/10.1117/12.2291253

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