Research On Fundamental Mode Selection By A Ircular Aperture In He-ne Lasers Made Of Zerodur

The method of fundamental transverse mode selection in He-Ne lasers made of zerodur is usually by inserting an aperture into the resonator. Research was carried out to find out the aperture-diameter's influence on the oscillation modes, output power and beam quality of this kind of laser, which could provide important information for the laser design.Based on Fresnel-Kirchhoff diffraction integral equation and self-reproducing principle, the transit matrix of the resonator after insertion of the aperture is obtained by finite element method. Thus, optical field distribution on the plane mirror in plano-concave resonator and field loss of each transverse mode could be acquired by solving the eigenvalue and eigenvector of the transit matrix. The calculating results show that when the fundamental mode is acquired exactly with the largest output power, the ratio of aperture-diameter to the laser width is obtained.Software is programmed for acquiring and analysis of laser spot,based on virtual instrument. A system for analyzing laser spot is established and an experiment platform is built up. The resonator was extended gradually and after each extension several apertures with diameters in 0.05mm increments are put against one of the mirrors one by one. With the laser spot pattern and spectrum, the transverse modes can be distinguished accurately and some beam information can be obtained, and with the power meter the output power is measured. Therefore, the most suitable aperture is found out. The experimental results indicate that when the fundamental mode is acquired exactly with the largest output power, the ratio of aperture-diameter to the laser width fluctuates around some value. Under this addition, the output beam is very close to Gaussian beam with good quality.