For the TEM00mode in a near-hemispherical resonator with variable mode-selecting aperture, we have investigated the diffraction losses versus mirror spacing. The experiments are done with a low gain 633 nm He-Ne laser and two situations are considered. At first the aperture is located at the concave mirror and the experimental values agree very well with those computed by Li for a passive cavity. Then the aperature is placed near the active medium and discrepancies are observed between the experimental results and the proposals of various authors.
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