Observed factorization of excess quantum noise that is due to both polarization and spatial mode nonorthogonality

When the eigenmodes of a laser cavity are nonorthogonal, the quantum-limited linewidth of the laser is larger by an excess-noise factor than the standard Schawlow-Townes expression. Mode nonorthogonality can exist in the spatial domain, as in unstable-cavity resonators, but also in the polarization domain when the two polarization eigenmodes are nonorthogonal. We show experimentally that these two contributions are independent of each other, i.e., that the excess-noise factor factorizes as a product of the spatial and polarization excess-noise factors. (C) 1998 Optical Society of America.