Large Petermann factor in chaotic cavities with many scattering channels.

The quantum-limited linewidth of a laser cavity is enhanced above the Schawlow-Townes value by the Petermann factor K, due to the non-orthogonality of the cavity modes. The average Petermann factor K in an ensemble of cavities with chaotic scattering and broken time-reversal symmetry is calculated non-perturbatively using random-matrix theory and the supersymmetry technique, as a function of the decay rate Γ of the lasing mode and the number of scattering channels N. We find for N >> 1 that for typical values of Γ the average Petermann factor K ∝ (N)1/2 >> 1 is parametrically larger than unity.