Influence of transverse resonator symmetry on excess noise

We demonstrate that the quantum-limited linewidth of an unstable-cavity laser depends on the transverse symmetry of the resonator. This is due to the occurrence of excess noise associated with the nonorthogonality of the eigenmodes of an unstable-cavity laser. We show that square symmetry leads to considerably larger excess noise than circular symmetry, confirming recent predictions. Further we present measurements of the excess noise in lasers with 3, 4, 6 and 8-fold symmetry.