Depressed cladding waveguides have been formed in laser crystals by a tightly focused beam of a femtosecond laser. A laser based on a depressed cladding waveguide in a neodymium-doped YAG crystal has been demonstrated for what is believed to be the first time.

A depressed cladding waveguide with record low loss of 0.12 dB/cm is inscribed in YAG:Nd(0.3at.%) crystal by femtosecond laser pulses with an elliptical beam waist. The waveguide is formed by a set of parallel tracks which constitute the depressed cladding. It is a key element for compact and efficient CW waveguide laser operating at 1064 nm and pumped by a multimode laser diode. Special attention is paid to mechanical stress resulting from the inscription process. Numerical calculation of mode distribution and propagation loss with the elasto-optical effect taken into account leads to the conclusion that the depressed cladding is a dominating factor in waveguide mode formation, while the mechanical stress only slightly distorts waveguide modes.

A wideband near-IR (NIR) luminescence centered at $1080\mathrm{nm}$ was found in a $\mathrm{Rb}{\mathrm{Pb}}_2{\mathrm{Cl}}_5:\mathrm{Bi}$ single crystal grown by the Bridgman technique. Absorption, luminescence, and excitation of luminescence spectra were investigated at room and cryogenic temperatures. The luminescence was proposed to be due to the ${}^3\mathit{P}_1\to {}^3\mathit{P}_0$ transition in ${\mathrm{Bi}}^{+}$ ion.

Dependencies of optical transmittance and luminescence of ${\mathrm{Y}\mathrm{A}\mathrm{G}:\mathrm{C}\mathrm{r}}^{4+}$ crystals upon intensity of excitation at 1064 nm are investigated, when absorption of tetrahedrally coordinated ${\mathrm{Cr}}^{4+}$ ions $({\mathrm{Cr}}^{4+}$ centers) is saturated. The spectroscopic model of ${\mathrm{Cr}}^{4+}$ center in YAG crystals is proposed. Absorption saturation of ${\mathrm{Y}\mathrm{A}\mathrm{G}:\mathrm{C}\mathrm{r}}^{4+}$ crystals at 1064 nm have to be described by two absorption cross sections ${\sigma }_{0\pi }=(3.9\mathrm{}–5.0)\times {10}^{-18}{\mathrm{cm}}^2$ and ${\sigma }_{0\sigma }=(1.5\mathrm{}–1.9)\times {10}^{-19}{\mathrm{cm}}^2,$ arising from the $D_{2d}$ local symmetry of ${\mathrm{Cr}}^{4+}$ centers. The cross sections slightly vary within the pointed range in dependency of level of the Cr dopant and a type of the ${\mathrm{Me}}^{2+}$ dopant $({\mathrm{Mg}}^{2+}$ or ${\mathrm{Ca}}^{2+}).\mathrm{}$ The absorption band centered at 1000 nm is basically due to ${}^3{B}_1{(}^3{A}_2{)}^3{A}_2{(}^3{T}_1)$ transition. Energy positions of ${}^3{A}_2{(}^3{T}_1)$ and ${}^3{E(}^3{T}_2)$ states are very close to each other, and the absorption band due to ${}^3{B}_1{(}^3{A}_2{)}^3{E(}^3{T}_2)$ transition is masked by the absorption due to ${}^3{B}_1{(}^3{A}_2{)}^3{A}_2{(}^3{T}_1)$ transition. The proposed model allows us to explain the dependence of the extent of luminescence polarization on the intensity of light exciting the ${\mathrm{Y}\mathrm{A}\mathrm{G}:\mathrm{C}\mathrm{r}}^{4+}$ crystal. Absorption from the metastable excited state ${}^3{B}_2{(}^3{T}_2)$ is negligible; its absorption cross section is less than $2\times {10}^{-20}{\mathrm{cm}}^2.$ Linear losses from other centers are found in the crystals with a high level of the chromium dopant and with the calcium dopant.

Oscillation at $5.5\mathrm{\mu }\mathrm{m}$ wavelength has been investigated in the low-phonon, moisture-resistant ${\mathrm{RbPb}}_2{\mathrm{Cl}}_5:{\mathrm{Dy}}^{3+}$ crystal in free-running mode at room temperature. In line with our knowledge this is the longest laser wavelength for a rare-earth doped crystal that does not require any special precautions for survival. Spectra and kinetics of luminescence of the new laser crystal were investigated under pumping to $1.3\mathrm{\mu }\mathrm{m}$ absorption band, corresponding to lasing experiments. Emission cross sections were determined for mid-IR transitions. It was found that emission cross section on the ${}^{6}H_{9∕2}+{}^{6}F_{11∕2}\to {}^{6}H_{11∕2}$ laser transition at $5.5\mathrm{\mu }\mathrm{m}$ wavelength is as high as $0.6\times {10}^{-20}{\mathrm{cm}}^2$.