Transmission through a (100)-oriented single crystal of bismuth ferrite was studied at frequencies of 0.485–0.765 THz with a continuous-wave source and in the temperature range of 30–350 K as a function of polarization angle. The fine splitting of spin-wave modes that could be detected with our vector network analyzer-based spectrometer is shown to depend on polarization angle. The crystal shows strong, temperature-dependent, linear dichroism. The interference pattern produced in the sample slab revealed a huge birefringence. The fine splitting dependence on polarization angle suggests correlation with the observed dichroism and the birefringence.