Diffraction losses reduction in multiapertured non-Hermitian laser resonators

The behavior of diffraction losses in multiapertured laser resonators is theoretically and experimentally investigated. In the case of geometrically unstable resonators, it is shown that the combination of several apertures whose diameters have been independently optimized to reduce the laser diffraction losses leads to a still lower value for these losses. This puzzling fact is verified experimentally for a negative branch unstable resonator laser containing up to five intracavity apertures. In the case of geometrically stable resonators, it is shown that the introduction of a first intracavity aperture that selects the fundamental mode leads to modifications of the fundamental mode shape which, besides being relatively small, are sufficient to make the diffraction losses oscillate with the diameters of the following extra intracavity apertures. In particular, the diameters of these apertures can be optimized to reduce the diffraction losses below the value obtained with the first aperture only. These predictions are experimentally verified in the case of a dynamically stable cavity high power Nd:YAG (yttrium aluminum garnet) laser. In all cases, the calculations based on the Huygens-Fresnel principle exhibit a good agreement with the experimental results.