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 lo sses is measured. This transverse excess noise factor is isolated from other sources of modification of the laser linewidth due to a differe ntial measurement method, leading to the experimental evidence of valu es as large as 13 in cylindrical symmetry. Moreover, it is shown exper imentally that the introduction of a well-chosen second diffracting ap erture inside the cavity permits one to drastically reduce the overall laser excess noise factor, without altering the laser losses and intr acavity power.