ABSTRACT
This chapter discusses the advantages unstable resonators offer the laser designer, how the basic mode properties can be analysed using a simple geometrical model, and to what extent agreement with experimental results is obtained. A model for mode evolution in short-pulse lasers is then presented which illustrates the useful role that unstable resonators can play for such systems. In carefully designed systems, with the appropriate quality gain medium, the far-field pattern approaches 'diffraction-limited' with discrepancies that can be accounted for by non-uniform irradiance in the annulus rather than significant phase-front errors. Injection locking with a narrow frequency input beam to excite only one axial-mode of the unstable resonator is possible, leading the high-power, single-frequency, output. Unstable resonator mode properties have been studied extensively using both analytical and computational solutions of the Fresnel–Huygens integral. The chapter concludes with a brief consideration of injection controlled, novel geometry, and variable–reflectivity–mirror unstable resonators.
