Abstract

The selection of the fundamental Gaussian mode in optical cavities by a hard or a soft aperture is based on increasing losses with the transverse mode order. This property is directly linked to divergence angles in the far-field θ₀₀, θ₀₁, θ₀₂, … associated to TEM₀₀, TEM₀₁, TEM₀₂, … pure beams of the bare cavity which make a hierarchy expressed as θ₀₀< θ₀₁< θ₀₂<… . Recently, the transverse mode discrimination of apertured resonators has been greatly enhanced by the insertion of a simple diffractive optical element known as π-plate that introduces a π phaseshift in its central portion of radius ρ₁ [1,2]. A first effect of the π-plate is to disturb the hierarchy of angle divergences associated with the transverse modes as shown in figure 1 as a function of parameter Y = ρ₁/W, where W is the incident beam width. A second effect linked to the first one is that the fundamental mode selected by a hard aperture inside the cavity is not necessarly Gaussian as shown in figure 2, depending on the value of Y. We have also found that the fundamental mode volume (FMV), thanks the π-plate, is multiplied by a factor which can be as high as twenty with respect to the FMV of the bare cavity.

© 2001 EPS