Optical Cavities: Free-Space Laser Resonators

Abstract

This chapter reviews the fundamental aspects of laser resonators and discusses some of the techniques of laser operation. Optical resonators have resonant modes and these modes are important to the analysis of laser operation. An optical resonator mode is a field distribution that is resonant with the structure and that is reproduced in phase and in relative intensity after a round-trip transit of the resonator. The Gaussian beams of stable open resonators can be characterized with analytical expressions and provide a widely used approach to the discussion of laser resonators. A resonator is called ‘stable’ if a ray of a geometrical optical analysis always remains within the resonator rather than wandering off an increasing distance from the resonator centre with an increasing number of cavity transits. Resonators simultaneously have axial modes in addition to their transverse modes. Unstable resonators are discussed here both from the geometrical optics and diffraction points of view.