The He-Ne laser serial experimental system (LEOI-53) is designed with an adjustable He-Ne laser cavity. By changing the cavity length or the radius of curvature of the cavity output mirror, corresponding change in lasing mode can be observed. The use of a confocal spherical scanning interferometer further allows students to directly observe the frequency distribution pattern of both longitudinal and transverse modes of a He-Ne laser. The scanning interferometer is also used to measure the frequency spacing between laser modes. Other optical parameters such as the spot size and the beam divergence angle of a He-Ne laser, and the free spectral range and the finesse of a scanning Fabry-Perot interferometer can be also measured with this system.