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In a laser TE[M.sub.00]-mode selection by the use of a hard intracavity aperture gives rise to losses that are experimentally observable and to an increase in the fundamental-mode volume that is more difficult to observe. We use the parasitic resonances that are characteristic of multicavity lasers to demonstrate this volume enhancement in a He-Ne laser operating at 3.39 [[micro]meter]. The increase in mode volume in the apertured laser can increase the output power by a factor of 3 with respect to that of the nonapertured laser. Key words: Mode volume, regenerative amplifier, intracavity diffraction.

We show that aperiodic coupled cavities may be designed to have a frequency-dependent complex reflectance that is anisotropic in phase (birefringent) and in amplitude (dichroic). The complex mean or average reflectivity may also be frequency dependent. As an illustration we show how such multicavity effects influence the stability of the polarization of light in quasi-isotropic lasers. Possible applications of the properties of aperiodic coupled cavities are suggested.

An intracavity polarimeter is described that is capable of measuring both weak birefringence and dichroism simultaneously. It uses, for the first time to our knowledge, polarization flips in a quasi-isotropic He-Ne laser. The laser operates at 3.39 micrometers. Calibration of the polarimeter is carried out by using the anisotropic properties of a tilted plate. As an illustration of the sensitivity, the diffractive anisotropy of an intracavity slit is measured.

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.

Diffraction patterns and divergence of the fundamental mode of a laser are studied in the case of a plano-concave cavity with two-apertured mirrors. The effects of both apertures are shown to be nonequivalent. The sizes of their diameters control the geometry of the beam and, in particular, control the increase or the decrease of the divergence with respect to that of the nondiaphragmed cavity.