Influence of the longitudinal position of an aperture inside a cavity on the transverse mode discrimination.

Two aspects of the fundamental mode selection in planoconcave resonators containing a circular aperture are analyzed. The first is the transverse mode discrimination (TMD), and the second is the fundamental mode volume. It is shown that the TMD decreases rapidly when the aperture is moved away from the plane or concave mirror position. Moreover, the half-confocal configuration has been found to be that which optimizes the TMD. For a fixed fundamental mode loss and a fixed TMD, the diffraction-induced increase of the fundamental mode volume is larger when the aperture is against the concave mirror than when it is against the plane mirror.