Observation of excess quantum noise in a geometrically stable laser

We have observed excess quantum noise in a geometrically stable He-Xe laser. This was achieved by insertion of an aperture in front of one of the laser mirrors, the aperture diameter being considerably smaller than the beam waist of the lowest-loss Gaussian mode. The measured excess noise is as large as a factor of 15 for the smallest aperture. A simple model is introduced to calculate the transverse excess noise factor using a far-field approximation. We have studied the square and circularly shaped aperture case, both theoretically and experimentally. Our experimental data demonstrate that the overall excess noise factor is given by the product of the transverse and longitudinal excess noise factors, except for the smallest circular apertures, where the results suggest that possible effects of gain guiding and waveguiding need to be considered.