Quasi-static transverse mode degradation in an erbium–ytterbium co-doped fiber amplifier

Long-term stable operation of high-power fiber amplifiers is required to realize high-speed communication in non-terrestrial networks. Recently, quasi-static transverse mode degradation (QS-TMD) was found in ytterbium-doped fiber amplifiers on a few-hour time scale at 1064 nm. Here we demonstrate experimentally that QS-TMD also occurs in erbium–ytterbium co-doped fibers (EYDFs) that amplify light at 1550 nm. QS-TMD was observed by measuring the power of fundamental and higher-order modes separately. QS-TMD progressed as short-term transverse mode instability increased. A persistent long-period grating also formed in the EYDF. This work will aid design of stable high-power fiber amplifiers.