Direct measurement of the transverse excess noise factor in a geometrically stable laser resonator

The excess noise factor due to the nonorthogonality of the transverse modes of a geometrically stable cavity subject to large diffraction losses is measured. This transverse excess noise factor is isolated from other sources of modification of the laser linewidth due to a differential measurement method, leading to the experimental evidence of values as large as 13 in cylindrical symmetry. Moreover, it is shown experimentally that the introduction of a well-chosen second diffracting aperture inside the cavity permits one to drastically reduce the overall laser excess noise factor, without altering the laser losses and intracavity power.