We　investigated　the　frequency　noise　coupling　mechanism　of　a　2　mu　m　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　mu　m　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　Hz/　root　Hz　at　frequencies　higher　than　100　Hz.　(C)　2021　Optical　Society　of　America　under　the　terms　of　the　OSA　Open　Access　Publishing　Agreement