Modal energy transfer by thermally induced refractive index gratings in Yb-doped fibers

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.

Intensity noise as a driver for transverse mode instability in fiber amplifiers

“…However, the same increase of pump-RIN at higher pump-noise amplitudes leads to a significantly weaker decrease of the TMI threshold. This behavior can be understood when taking into account the sensitivity of the refractive index grating (RIG) to phase shifts at different average output powers [10]. As mentioned above, two requirements need to be fulfilled to enable a modal energy transfer in a fiber: a sufficiently strong RIG and a phase shift between the MIP and the RIG.…”

“…Hence, a similar modal energy transfer can be achieved either with a strong RIG and a small phase shift or, alternatively, with a weak RIG and a large phase shift. In more detail, it has been recently revealed, that the strength of the RIG in high-power fiber laser systems increases linearly with the average output power, whereas the modal energy transfer increases nonlinearly [10]. This means that the RIG becomes more and more sensitive to small phase shifts with increasing average output power.…”

“…This is because the strength of these outputsignal modulations determines the heat-load change in the fiber and, thereby, the resulting phase shift between the MIP and the RIG. As mentioned before, this phase shift can eventually induce a modal energy transfer at average powers below the intrinsic TMI threshold [10]. Hence, since pump intensity-noise below 20 kHz generates stronger phase shifts than seed intensity-noise, pump-RIN should provoke a modal energy transfer at lower average powers than seed-RIN.…”

“…Finally, a phase shift between the MIP and the RIG develops, since the RIG cannot follow the MIP instantaneously because it needs time to evolve from its former position [9]. Experimental investigations have supported this theoretical statement by demonstrating that pump-power-induced phase shifts can lead to a modal energy transfer even below the TMI threshold [10]. This finding indicates that such a phase shift is most likely the trigger for TMI.…”

“…This finding indicates that such a phase shift is most likely the trigger for TMI. Furthermore, the experiments in [10] have shown that the fiber becomes more and more sensitive to phase shifts the higher the average output power is. Thus, it was concluded that even extremely small phase shifts are sufficient to provoke TMI at the threshold power.…”