Output characteristics’ static fluctuations versus the pump power in 1018 nm fiber oscillators

Abstract

This paper investigates the static fluctuating behavior of output parameters in 1018 nm fiber lasers using 20/400 µm and 25/400 µm ytterbium-doped fibers (YDFs). It is seen that by increasing the pump power, some static fluctuations is induced in the output characteristics of the lasers, such as output power, back-reflected power, and beam quality factor (M2). The growth of these parameters fluctuates versus the pump power, without any modulation frequency in the temporal behavior of the output beam profile. This effect, which to the best of our knowledge is reported for the first time, occurs at powers much lower than the threshold for dynamic transverse mode instability (TMI). It was found that the static mode-coupling occurs between two lowest-order modes and causes these fluctuations in the lasers’ output parameters. Conducting the experiment for 1080 nm fiber lasers with different lengths of YDF, in addition to confirm the descriptions about how the static fluctuations occurs, shows that this effect occurs in other wavelengths as well.