Fluorescence and site-selective excitation spectra of chromium-activated forsterite at 10 K are reported. It is found that a high-field ${\mathrm{Cr}}^{3+}$ site in inversion symmetry emits sharp ${\mathit{R}}_1$ line fluorescence, while a low field ${\mathrm{Cr}}^{3+}$ site in mirror symmetry produces ${}_{}{}^4{}_{}{}^{}$ ${\mathit{T}}_2$ broadband emission. The dominant absorption in the visible is shown to be the ${}_{}{}^3{}_{}{}^{}$ ${\mathit{A}}_2$ ${\to }^3$ ${\mathit{T}}_1$ ${(}^3$F) transitions of ${\mathrm{Cr}}^{4+}$ in tetrahedral sites. In the near-infrared region, three sharp lines from ${\mathrm{Cr}}^{4+}$ were observed and are assigned to the transition from the ${}_{}{}^3{}_{}{}^{}$ ${\mathit{A}}_1$ ${(}^3$ ${\mathit{T}}_2$) spin triplet to the ${}_{}{}^3{}_{}{}^{}$ ${\mathit{A}}_2$ ground state. The energy-level splittings of ${\mathrm{Cr}}^{4+}$ due to an orthorhombic-distorted field with spin-orbit coupling are discussed.

We report on passively Q-switched operation of a diode-pumped Er–Yb:glass laser with a Co²⁺:MgAl₂O₄ plate as a saturable absorber. Optical pulses with peak power exceeding 2 kW and a pulse length of 2.3 ns have been generated. Single-longitudinal-mode Q-switched operation at 1.53 µm has been obtained by use of the Co²⁺:MgAl₂O₄ plate as an intracavity etalon. A discussion of the optimal Er³⁺ concentration as well as optimization of the cavity design is included.