Gravitational wave detectors require linearly polarized singlefrequency laser sources with a high fractional TEM00 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 TEMnm modes. Low degradation of the beam quality with increasing pump power could be observed, while a maximum power in the TEM00 mode of 203 W was achieved.