We report on fundamental mode content measurements performed on single-frequency large mode area fiber amplifiers based on Er:Yb-codoped multifilament-core (MFC) and step-index pedestal-design fibers. The mode content of the amplifier output was analyzed with respect to free-space TEM modes using a nonconfocal scanning ring cavity. At power levels of several watts, the commercial step-index pedestal-design fibers with respective core diameters of 23 and 25 μm showed a TEM<sub>00</sub> mode content exceeding 80%. However, a TEM<sub>00</sub> mode content of up to 95% could be obtained with an MFC fiber also with larger core diameter of 31/28 μm in hexagonal shape. To the best of our knowledge, we utilized the nonconfocal scanning ring cavity technique for the first time in the wavelength region of 1.5 μm and additionally for the first time on fibers with pedestal as well as MFC design.
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