We present a study of 1-D optical microcavities based on a dielectric mirror, a layer of an aligned fluorescent conjugated polymer (poly(9,9-dioctylfluorene) (PFO)) and a metallic mirror. The aligned polymer film is birefringent, with a refractive index difference calculated to be 0.36 between directions parallel and perpendicular to the alignment direction. Because of the large birefringence, two cavity modes are observed within the microcavity stop band. They are split by 32 nm and are polarized parallel and perpendicular to the alignment direction. Photoluminescence from the PFO film correspondingly shows two emission bands with mutually perpendicular polarizations. The polarization ratio of the most intense mode exceeds 300:1. We discuss how this structure might be used to construct highly polarized light-sources.