Solid state and gas laser amplifier and oscillator

Abstract

Rate equation and population inversion In a material which has only two energy levels, χ ″ is always positive because ρ 11 > ρ 22 for E 1 E 2 at thermal equilibrium. Prior to the invention of lasers, there was no known method to achieve ρ 22 > ρ 11 . However, we now know that a negative χ ″ in Eq. (5.41a) (i.e. ρ 22 > ρ 11 ) can be achieved by pumping processes that are available in materials that have multiple energy levels, as described in the following. Let there be many energy levels in the material under consideration, as shown in Fig. 6.1. Let there be a mechanism in which the populations at E 1 and E 2 , i.e. N 1 = Nρ 11 and N 2 = Nρ 22 , are increased by pumping from the ground state at pump rates R 1 and R 2 . In solid state lasers, the pumping action may be provided by an intense optical radiation causing stimulated transition between the ground state and other higher energy states, where the particles in the higher energy states relax preferentially into the E 2 state. In gas lasers, the molecules in the ground state may be excited into higher energy states within a plasma discharge; particles in those higher energy states then relax preferentially to the E 2 state. Alternatively, collisions with particles of other gases may be utilized to increase the number of particles in the E 2 state. Various schemes to pump different lasers are reviewed in ref. In order to obtain amplification, it is necessary to have R 2 ≫ R 1 .