We investigated the frequency noise coupling mechanism of a 2 μm polarization-maintaining single frequency fiber laser (SFFL) theoretically and experimentally. The coupling of pump's relative intensity noise (RIN) to frequency noise of a single-frequency high-gain silica fiber laser is shown experimentally to be consistent with a theoretical model where thermal expansion and thermo-optic effect mediate the coupling. The measured and theoretical frequency noise of the 2 μm SFFL with three pump sources is compared. We find using a 1550 nm single frequency laser pump source produces the lowest frequency noise, less than 100 / at frequencies higher than 100 Hz.
