Abstract <p>A laboratory experimental setup based on a three-mirror ring resonator (traveling-wave interferometer) designed for the visual demonstration and studying the spectral features of helium–neon laser radiation in several lasing modes is presented. In particular, this installation makes it possible to measure the parameters of the interferometer mirrors, including the transmission and reflection coefficients. The installation can be used in laboratory workshops for students who are starting to study the physical foundations and operating principles of gas lasers. The installation contains elements of single-mode fiber optics.</p>