An approximate expression for the output beam from a confocal resonator, into which a pinhole for mode selection is inserted at an arbitrary position near the mirror at one end, is studied experimentally. The output beam consists of the fundamental component, in which the diffraction from the pinhole edge is not considered, and a field of the higher order component including the diffraction. In the region of the pinhole aperture larger than the beam width of the confocal resonator before the insertion of the pinhole, the diffraction component can be neglected. The output beam then may be approximated by the beam of the fundamental component, which is expressed simply by the field of the confocal resonator with unequal-sized mirrors of Boyd and Kogelnik. These results are confirmed by both experimental and theoretical examinations.
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