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.
A time-dependent analytical model is rigorously derived which shows that the thermally induced modal instability in high power rare-earth doped fiber amplifiers is fundamentally a two-wave mixing between fundamental and higher-order modes through a thermally-induced grating imprinted by beating between these modes. We show that previously postulated movement of this grating to phase-match the coupling between the modes naturally occurs due to a finite thermal-response time of a fiber. This theory is consistent with experimental observations in that it accurately predicts the onset-like threshold and temporal instabilities in the kilohertz-frequency range.
3C fiber technology advances the performance frontier of practical, high-pulse-energy fiber lasers by providing very large core fibers with the handling and packaging benefits associated with single mode fibers. First-generation fibers demonstrate scaling to > 240 W average power coincident with 100-kW peak power in 1-mJ, 10-ns pulses while maintaining single-mode beam quality, polarized output, and efficiencies > 70%. Peak powers over 0.5 MW with negligible spectral distortion can be achieved with sub ns, near-transform-limited pulses. In-development second-generation 3C Yb-fiber based on core sizes around 55 μm1 have produced >8 mJ, 13 ns pulses with peak powers exceeding 600 kW.
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