Laser-based Mid-infrared Sources and Applications

2Solid‐state Crystalline Mid‐IR Lasers

Crystalline mid‐IR lasers are direct sources of coherent light in the sense that they require a minimal number of energy conversion steps. When combined with laser diode pumping, these lasers are efficient, simple, and compact. The gain medium of a crystalline laser is a host crystal doped with active ions. These active ions (also referred to as impurity ions) doped into a crystalline matrix acquire, due to energy‐level splitting, characteristic set of energy levels, not present in free ions. For rare earths, the primary cause of energy‐level splitting is the interaction of electron spins of the dopant ion with the orbital angular momentum of electrons (spin–orbit interactions), while in transition metal ions, it is mostly due to the interaction of the optically active electron with the crystalline electric field of the host (the Stark effect).

The most common active media for mid‐IR crystalline lasers are based on triply ionized rare‐earth thulium (Tm), holmium (Ho), and erbium (Er) ions in yttrium aluminum garnet (Y₃Al₅O₁₂ or YAG), yttrium lithium fluoride (LiYF₄ or YLF), yttrium‐scandium‐gallium garnet (Y₃Sc₂Ga₃O₁₂ or YSGG), or other crystalline hosts. Alternatively, transition‐metal‐doped (Cr²⁺, Fe²⁺) II–VI zinc chalcogenide crystals (ZnSe, ZnS) or other chalcogenides (CdSe, CdS, ZnTe, and CdMnTe) can serve as active media for mid‐IR lasers with an extremely broad gain bandwidth.

2.1 Rare‐Earth‐based Tm³⁺, Ho³⁺_, and Er³⁺ Lasers