Rare earth and transition-metal ions that are doped in a solid host have been operated successfully as solid state lasers for several decades. Solid state lasers based on ions such as Nd3þ, Ho3þ, Tm3þ, and Er3þ (and many other rare earth ions doped in crystals such as Nd:YAG, Nd:YVO4, Nd:YLF, or various glasses) are currently standard products of the laser industry. These lasers possess narrow emission linewidths and therefore emit coherent radiation at very specific wavelengths. The spectral range of these lasers can be extended by using nonlinear optical elements in two ways: by a frequency conversion technique, e.g., second harmonic generation (SHG) or third harmonic generation (THG), or by frequency shifting, e.g., optical parametric oscillation (OPO) or stimulated Raman scattering (SRS). Both methods require complex optical and mechanical systems as well as high peak-power density in the nonlinear optical element. On the other hand, tunable lasers with a broadband emission in the near-IR (800–1100 nm) or mid-IR (1100–1600 nm) may provide a solution to the narrow bandwidth limitation of the currently used lasers based on rare earth ions. Room-temperature tunable lasers that cover the visible and mid-IR spectral range are Ti:sapphire (790–1100 nm), Cr:alexandrite (600–810 nm), Cr:LiSAF or Cr:LiCAF (750–950 nm), Cr4þ:Mg2SiO4 (forsterite; 1100–1300 nm), Cr4þ:YAG (1200–1550 nm), and both Cr2þ:ZnSe and Cr2þCdSe, which are tunable within the 2–3-mm spectral range.
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