Etude du champ proche d'un laser diaphragmé

A circular aperture is generally used inside a metrologic gas laser to suppress transverse modes. The resonant field inside the laser or the near field observable at the output display longitudinal or transverse oscillations due to diffraction. These fields are different from the Gaussian modes usually obtained from nondiaphragmed cavities. This article studies the near field of a diaphragmed laser in both experimental and theoretical aspects. For this purpose we use a field expansion on the basis of geometrical Laguerre Gauss modes and we calculate the field diffracted by an aperture, inside a resonant cavity. This allows calculation of the measurable field at the laser output. Numerical applications and corresponding experiments are done with a single frequency laser working on the 3.39-microm Ne line. Excellent agreement is obtained between experimental and theoretical curves.