Transmission through a (100)-oriented single crystal of bismuth ferrite (BiFeO3) 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.