We report the optimization of a 30 GHz COEO by using an all polarization maintaining fiber (PMF) to design its optical loop. We unexpectedly found that polarization mode dispersion (PMD) can have detrimental effects on the COEO performances. Indeed, with a 400 m SMF fiber spool in the optical cavity, a phase noise level as low as-115 dBc/Hz at 1 kHz offset frequency was obtained. Conversely, with a PMF fiber spool, only a phase noise level as low as-100 dBc/Hz and-70 dBc/Hz at 1 kHz offset frequency for a 200 m PMF and 400 m PMF could be obtained respectively. PMD sets therefore an upper limit on the length of the PMF fiber that could be used.