  We report on a simple method of locking a laser to a birefringent cavity
using polarization spectroscopy. The birefringence of the resonator permits the
simple extraction of an error signal by using one polarization state as a phase
reference for another state. No modulation of the light or the resonator is
required, reducing the complexity of the laser locking setup. This method of
producing an error signal can be used on most birefringent optical resonators,
even if the details of birefringence and eigenpolarizations are not known. This
technique is particularly well suited for fiber ring resonators due to the
inherent birefringence of the fiber and the unknown nature of that
birefringence. We present an experimental demonstration of this technique using
a fiber ring.
