Gravitational wave detectors require linearly polarized single-frequency laser sources with a high fractional TEM 00 mode content. We investigated the modal decomposition of a polarization maintaining photonic crystal fiber with a mode field diameter of 29 µm, operating in a single-frequency master-oscillator power-amplifier scheme, with respect to the TEM nm modes. Low degradation of the beam quality with increasing pump power could be observed, while a maximum power in the TEM 00 mode of 203 W was achieved.