The resonator can be considered the real heart of any laser system, the key element that determines the properties of laser radiation, including mode structure or temporal and spatial characteristics. The theory of different laser types has been well-developed in the last few decades of the 20th century, starting with the pioneering papers of Fox, Li, Body and Gordon. But today, due to the development of new types of lasers (fibre, diode), new optical elements (adaptive mirrors, phase conjugation techniques, graded phase correctors), the development of optical technology, and new needs for industrial lasers, novel types of resonators are under investigation. Unlike some monographs on laser resonators, this work does not present classical theory and the derivation of the basic equations of laser mode generation and so on. Instead, based on the well-known earlier literature, new results in the field of laser resonators are presented. Although it is not a textbook, it outlines the novel trends in the development of laser resonators science, shows what has already been achieved in this field, and indicates directions for research and applications.
