Quantum intensity noise of laser diodes and nonorthogonal spatial eigenmodes

Laser systems with nonorthogonal eigenmodes have a linewidth that is broader than the usual Schawlow-Townes value, by a factor that is known as the Petermann excess noise factor. In a recent quantum analysis, this excess noise was attributed to loss-induced coupling betwen the laser modes. Using the same approach, we show here that the Petermann excess noise also appears on the laser quantum intensity noise. The calculation is shown to be in good agreement with an experiment using a laser diode with two contributing transverse modes.