The signal to noise ratio in atomic magnetometry is fundamentally limited by the optical depth of the atomic ensemble. During the last decade, different technics for optical depth enhancement has been developed by several groups, some of them involving the implementation of an optical cavity. This thesis evaluates the performance of such an implementation by the polarization read out noise analysis of an optical cavity with similar characteristics as the already implemented by other groups. We obtained an estimated value for the signal to noise ratio enhancement with a cavity of 814 Finesse, and we studied the behaviour of the different sources of polarization noise in this system.