Propagation of Spontaneous Emission from an Active Laser Medium in an Unstable Resonator during Resonance Field Formation

The correlation function of output laser radiation has been investigated as a function of the spontaneous emission passes number of an active medium in an unstable resonator. The initial incoherent spontaneous emission is represented as a sum of spatially δ-correlated fields formed by point sources. The characteristics of focused output laser beam have been investigated within the Fresnel approximation. The beam width and Strehl ratio have been analyzed as a function of the number of spontaneous-emission passes and resonator parameters. The number of intra-resonator passes and the time that are necessary for spontaneous-emission transformation into a stationary resonance field of specified optical quality have been estimated. For large unstable resonators with a Fresnel number of 100 to 200 and a geometric magnification factor of 1.5 to 2, the formation time of a resonance beam with a Strehl ratio of 0.8 is about 90 to 170 ns.