Operating Schemes and Types of Lasers Based on Activated Crystals

Operating schemes for lasers based on activated crystals are divided into three-and four-level systems, the latter having several variants (Fig. 3.1). In one four-level scheme, the terminal level for the induced transition belongs to the ground state of an activator ion and is rather low (Fig. 3.1c); this scheme of operation usually requires deep cooling of the crystal. In another variant of the four-level scheme, the induced transition terminates at the Stark component of one of the excited multiplets (Fig. 3.1b). In lasers based on crystals doped with Cr³⁺, Ho³⁺, V²⁺, Co²⁺, Ni²⁺, or Sm²⁺ ions, stimulated emission at certain frequencies also occurs via phonon-assisted transitions, which terminate on “phonon” levels connected with certain vibronic modes of the host crystal (Fig. 3.1d).