Abstract
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.
Highlights
Fiber laser technology has earned a solid reputation as a power-scalable laser concept with excellent beam quality
In this article, we have experimentally proven the existence of a thermally induced refractive index grating in active fibers under a high thermal load, which leads to modal energy transfer
By introducing a phase shift between the modal interference pattern (MIP) and the refractive index grating (RIG), we demonstrated that the latter is sufficiently strong to couple energy between transverse modes at powers that are significantly below the transverse mode instabilities (TMI) threshold
Summary
Fiber laser technology has earned a solid reputation as a power-scalable laser concept with excellent beam quality. Fiber lasers deliver the highest diffractionlimited average power of any solid-state lasers and, have enabled a wide range of applications in industry, medicine, defense, and science[1]. The TMI effect occurs once an average power is exceeded and manifests itself as a dynamic energy coupling between transverse modes. Since this leads to an abrupt degradation in the quality and stability of the output beam at high average output powers, the TMI phenomenon prevents the development of new fields of applications. To develop new effective mitigation strategies, it is important to obtain a deeper understanding of the phenomenon
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