Spot-defect mirrors were fabricated by focusing laser pulses on the surface of conventional dielectric mirrors. These mirrors were used as rear mirrors of a He-Ne laser cavity for generating a vortex beam. The intensity distribution of the beam generated from the cavity with a spot diameter of 50？ m was in excellent agreement with theory. Comprehensive analysis of the intensity distribution, the beam quality factor, and the interference pattern revealed that the beam obtained was a purely single transverse mode LG01 beam. 1. Introduction Laguerre-Gaussian (LG) beams have been highlighted due to its dominant feature of carrying optical angular momenta in singular optics [1]. They have brought about many applications such as optical spanners for driving micromachines, quantum information processing [2], super-resolution microscopy [3], and microneedle fabrication [4]. To enhance the validity of these applications, the generation of a stable and high-quality LG mode beam is inevitable. LG beams can be produced by extracavity methods such as a mode converter composed of a pair of cylindrical lenses [5], computer-generated holograms [6–8], diffractive optics [9], an adaptive helical mirror [10], and a spatial light modulator [11]. However, the misalignment and the imperfection of the optical elements gave rise to the degradation of LG beams. By contrast, direct generation from a laser cavity is expected to improve the beam quality due to an inherent feedback effect in the cavity. Although many attempts have been reported, for example, a ring-shaped beam pumping [12], the thermal lensing effect in a diode-side-pumped bounce laser cavity [13], the insertion of a spiral phase plate [14], and the use of a circular absorber [15–17], most of them have merely reported the generation of the LG beam without a detailed evaluation of the beam purity and quality. Recently, we have demonstrated the generation of LG beams from a Nd？:？yttrium aluminum garnet (YAG) laser cavity using a spot defect mirror [18]. This method is simple, robust, and applicable to many laser systems. In addition, a Gaussian mode will be strongly suppressed by properly choosing a diameter of the defect leading to the generation of a pure single transverse mode. In this paper, we demonstrate the generation of a purely single transverse mode LG beam from a He-Ne laser cavity with a spot defect mirror. For mirrors with different spot diameters, comprehensive analysis was performed for the intensity distribution, the beam quality factor, and the observed interference pattern. The generation of a
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