Stihler Christoph, Jauregui Cesar, Tünnermann Andreas, Limpert Jens
1Institute of Applied Physics, Abbe Center of Photonics, Friedrich-Schiller-Universität Jena, Albert-Einstein-Str. 15, 07745 Jena, Germany.
2Helmholtz-Institute Jena, Fröbelstieg 3, 07743 Jena, Germany.
Light Sci Appl. 2018 Sep 5;7:59. doi: 10.1038/s41377-018-0061-6. eCollection 2018.
Thermally induced refractive index gratings in Yb-doped fibers lead to transverse mode instability (TMI) above an average power threshold, which represents a severe problem for many applications. To obtain a deeper understanding of TMI, the evolution of the strength of the thermally induced refractive index grating with the average output power in a fiber amplifier is experimentally investigated for the first time. This investigation is performed by introducing a phase shift between the refractive index grating and modal interference pattern, which is obtained by applying a pump power variation to the fiber amplifier. It is demonstrated that the refractive index grating is sufficiently strong to enable modal energy coupling at powers that are significantly below the TMI threshold if the induced phase shift is sufficiently large. The experiments indicate that at higher powers, the refractive index grating becomes more sensitive to such phase shifts, which will ultimately trigger TMI. Furthermore, the experimental results demonstrate beam cleaning above the TMI threshold via the introduction of a positive phase shift. This finding paves the way for the development of a new class of mitigation strategies for TMI that are based on controlling the phase shift between the thermally induced refractive index grating and modal interference pattern.
掺镱光纤中热致折射率光栅会在平均功率阈值以上导致横向模式不稳定性（TMI），这对许多应用来说是个严重问题。为了更深入地理解TMI，首次对光纤放大器中热致折射率光栅强度随平均输出功率的演变进行了实验研究。该研究通过在折射率光栅和模式干涉图案之间引入相移来进行，这是通过对光纤放大器施加泵浦功率变化来实现的。结果表明，如果诱导相移足够大，折射率光栅在功率显著低于TMI阈值时就足以实现模式能量耦合。实验表明，在更高功率下，折射率光栅对这种相移变得更加敏感，最终会触发TMI。此外，实验结果表明，通过引入正相移可在TMI阈值以上实现光束净化。这一发现为开发基于控制热致折射率光栅与模式干涉图案之间相移的新型TMI缓解策略铺平了道路。
