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An Er 3+ -doped Gd 2 SiO 5 single crystal with high optical quality has been grown by the Czochralski method. Polarized absorption and fluorescence spectra and fluorescence lifetime of the crystal were measured at room temperature. Intensity parameters, spontaneous emission probabilities, fluorescence branching ratios, and radiative lifetimes were estimated on the basis of the Judd–Ofelt theory. Besides, potentiality of 1.55 μm laser emission in an Er 3+ –Yb 3+ co-doped Gd 2 SiO 5 crystal was evaluated.

Yb 3+ :Gd 2 SiO 5 single crystal with Yb 3+ concentration of 6.33 at.% was grown by the Czochralski method. Polarized absorption and fluorescence spectra were measured at room temperature. Intrinsic fluorescence lifetime of the 2 F 5/2 multiplet was measured to be 0.87 ms when the powder of Yb 3+ :Gd 2 SiO 5 crystal with low Yb 3+ concentration of 0.5 at.% immersed in refractive index-matching fluid was adopted to weaken the influence of radiation trapping and concentration dependent fluorescence quenching. Furthermore, diode-pumped continuous-wave laser around 1090 nm with a slope efficiency of 52% and maximum output power of 1.2 W was demonstrated in the crystal.

A Sm 3+ -doped LiLuF 4 single crystal was grown by the vertical Bridgman–Stockbarge technique. Polarized absorption spectra, polarized fluorescence spectra, and fluorescence lifetime of the Sm 3+ :LiLuF 4 crystal were recorded at room temperature. Based on the Judd–Ofelt theory, spectral parameters of the Sm 3+ :LiLuF 4 crystal were calculated. Emission cross sections for the 4 G 5/2 → 6 H J ( J = 5/2, 7/2, 9/2, and 11/2) transitions with special interest for visible laser application were obtained by the Fuchtbauer–Ladenburg formula. The results indicate that the Sm 3+ :LiLuF 4 crystal may be a potential laser gain medium operating in visible region pumped by diode lasers around 401 nm.

Room-temperature polarized spectral properties of the Er:CaGdAlO 4 crystal are reported. The Judd–Ofelt theory was applied to analyze the polarized absorption spectra and then calculate the spontaneous emission probabilities, radiative lifetimes, and branch ratios. Room-temperature fluorescence lifetimes of the 4 I 13/2 , 4 I 11/2 , 4 F 9/2 , and 4 S 3/2 multiplets for Er 3+ ions were measured. Stimulated emission cross-sections of the 4 I 13/2 → 4 I 15/2 transition obtained by the Fuchtbauer–Ladenberg formula and the reciprocity method were compared. The results show that the Er:CaGdAlO 4 crystal may be a potential gain medium for a low-threshold 1.55 μm laser.

Ho 3+ /Tm 3+ co-doped β ′-Gd 2 (MoO 4 ) 3 single crystal was investigated as gain medium for Ho 3+ laser around 2.0 μm. Polarized absorption and fluorescence spectra of the crystal were measured. Polarized spectral parameters of Ho 3+ ions in the crystal were calculated and the gain curves around 2.0 μm were estimated. The fluorescence decay curves for Tm 3+ and Ho 3+ ions related to the 2.0 μm laser were measured and analyzed. The distinct coupled decay behavior for the Ho 3+ /Tm 3+ co-doped system embodies the existence of energy transfer between Tm 3+ and Ho 3+ ions.

Undoped and Er 3+ -doped Sr 3 Yb 2 (BO 3 ) 4 crystals were grown by the Czochralski method. Room temperature polarized spectral properties of the Er:Sr 3 Yb 2 (BO 3 ) 4 crystal were investigated. The efficiency of the energy transfer from Yb 3+ to Er 3+ ions in this crystal was calculated to be about 95%. End-pumped by a diode laser at 970 nm in a hemispherical cavity, a 0.75 W quasi-CW laser at 1.5–1.6 μm with a slope efficiency of 7% and an absorbed pump threshold of 3.8 W was achieved in a 0.5-mm-thick Z-cut crystal glued on a 5-mm-thick pure YAG crystal with UV-curable adhesive.

Efficient 1.9 ?m monolithic micro-laser was realized in a 0.9-mm-thick Tm3+:NaLa(MoO4)2 microchip coated with cavity mirrors directly. End-pumped by a fiber-coupled quasi-continuous-wave 795 nm diode laser, an average output power of 140 mW was obtained when the average incident pump power was 340 mW. Slope efficiencies calculated with respect to incident and absorbed pump power were 42 and 60%, respectively. The spectra and spatial profiles of the monolithic Tm3+:NaLa(MoO4)2 micro-laser were also measured and analyzed.

An Er:Yb:NaCe(WO4)2 crystal was grown by the Czochralski method. Spectral properties of the crystal were investigated and compared with those of the Er:Yb:Ce:NaGd(WO4)2 crystal. The green up-conversion was not observed when the Er:Yb:NaCe(WO4)2 crystal was excited by a diode laser at 970 nm. The efficiency of energy transfer from Yb3+ to Er3+ in the Er:Yb:NaCe(WO4)2 crystal was calculated to be about 93%. End-pumped by a diode laser at 970 nm in a hemispherical cavity, 0.95 W quasi-CW laser at 1.5–1.6 μm was achieved in a 1.47-mm-thick c-cut Er:Yb:NaCe(WO4){ia2} crystal, the slope efficiency was 11%, and the threshold was 2.83 W.

Trivalent thulium-doped K5Bi(MoO4)4 single crystals were grown by the Czochralski method. Its polarized absorption and fluorescence spectra and fluorescence decay curves were recorded at room temperature. On the basis of the Judd–Ofelt theory, the spectral parameters of the Tm3+:K5Bi(MoO4)4 crystal were calculated. The cross relaxations between Tm3+ ions were analyzed. The emission cross sections of the 3F4 → 3H6 transition were obtained by the Fuchtbauer–Ladenburg formula and then the gain cross sections around 1.9 μm were calculated. The peak emission cross section and width of emission band around 1.9 μm are comparable to those for Tm3+:YAG and the tunable range is about 280 nm for the potential ∼1.9 μm laser operation via the 3F4 → 3H6 transition.

A Ho3+-doped NaLa(MoO4)2 single crystal was grown by the Czochralski method. The polarized absorption spectra, polarized fluorescence spectra, and fluorescence decay curves of the crystal were measured at room temperature. The spontaneous emission probabilities, radiative lifetimes, and fluorescence branching ratios of the typical fluorescence multiplets of Ho3+ ions were calculated. The polarized stimulated emission and gain cross-sections of the 5I7 → 5I8 transition were obtained. The results show that the Ho3+:NaLa(MoO4)2 crystal is a promising gain medium for tunable and ultrashort pulse lasers operating around 2.0 μm.

End-pumped by a single-stripe 805-nm diode laser, a maximum continuous-wave output power of 290 mW with slope efficiency of 42% and 90 mW with slope efficiency of 44% around 2 µm were achieved in a 1.6-mm-thickness, unprocessed 7.2 at.% Tm3+:BaGd2(MoO4)4 cleavage plate in a hemispherical and plano–plano cavities, respectively, when the absorbed pump power was 830 mW. The dependence of the polarization of output laser on the output coupler transmission was investigated. The influence of upconversion on the laser performance was analyzed. The results show that the unprocessed Tm3+:BaGd2(MoO4)4 cleavage plate is a promising 2-µm laser gain medium pumped by diode laser around 808 nm.

Detailed spectral properties of uniaxial Tm 3+ :LiLa(MoO 4 ) 2 crystal have been investigated, including the polarized absorption spectra, polarized emission spectra, and fluorescence decay. Especially, the fluorescence spectra and decay curve of the upper level for ∼2.0 μm laser, 3 F 4 multiplet, were measured. The re-absorption of the emission from the 3 F 4 multiplet and the cross-relaxation between Tm 3+ ions were analyzed. The emission cross-sections of the 3 F 4 multiplet were obtained by the reciprocity method and then the gain cross-sections around 2.0 μm were calculated. The maximum absorption cross-section of the pump band is 4.04 × 10 −20 cm 2 at 795 nm with a bandwidth of 8 nm, which means this crystal is suitable for laser diode pumping. All the results show that Tm 3+ :LiLa(MoO 4 ) 2 crystal is a promising candidate for gain medium of ∼2.0 μm laser.

A 1.2 mm thick microchip cleaved from a 1.2 at.% Nd3+: BaGd2(MoO4)4 crystal was used as the laser gain medium directly. The laser cavity mirrors were coated on the microchip without cutting and polishing. In the 1.06 μm quasi-CW laser operation, the microchip was pumped by a Ti:sapphire laser with a duty cycle of 10%, the threshold is 10 mW, and the slope efficiency is 50%. In the CW laser operation pumped by a diode laser, the threshold and slope efficiency are 140 mW and 33%, respectively. The results demonstrate that the unprocessed surfaces of the cleaved microchip have good enough flatness and parallelism for laser operation.

End-pumped by a continuous-wave 976 nm diode laser, efficient 1522 nm laser operation was demonstrated in an Er:Yb:GdAl(3)(BO(3))(4) crystal when a sapphire crystal was used as a heat diffuser. A continuous-wave 1522 nm laser with a maximum output power of 750 mW and slope efficiency of 36% was realized at an absorbed pump power of 4.1 W. The pulse performances of an acousto-optic Q-switched laser with various repetition frequencies were investigated in detail. In a repetition frequency range of 1-10 kHz, 1522 nm pulse lasers with a slope efficiency of about 10%, peak output power at the kilowatt level, and width of about 50 ns were first obtained in an Er:Yb:GdAl(3)(BO(3))(4) crystal pumped by a continuous-wave diode laser. The results indicate that the crystal is a promising gain medium for an actively Q-switched 1.5 μm laser.

Based on the long fluorescence lifetime of upper laser level 4I13/2 of Er3+ ions and high 1.5−1.6 μm laser operation efficiency, an Er3+:Y b3+:NaCe0.43Gd0.57(WO4)2 crystal was used to generate a Q-switched pulse laser with high energy and low threshold. End-pumped by a 970 nm diode laser, a passively Q-switched 1.5−1.6 μm pulse laser was first realized in an Er3+ and Y b3+ co-doped tungstate crystal, when a Co2+:Mg0.4Al2.4O4 spinel crystal was used as saturable absorber. A 1.53 μm pulse laser with about 22 μJ energy, 76 ns duration and 21 kHz repetition rate was obtained at an absorbed pump power of 12 W in a c-cut 1.7-mm-thick Er3+:Y b3+:NaCe0.43Gd0.57(WO4)2 crystal.

Using an Er:Yb:YAl3(BO3)4 crystal as the gain medium and a Co2+:Mg0.4Al2.4O4 spinel crystal as the saturable absorber, an efficient passively Q-switched 1520 nm pulse laser was realized in a plano–plano cavity end-pumped by a 970 nm diode laser. At an absorbed pump power of 14.8 W and output mirror transmission of 4.6%, a 1520 nm pulse laser with 11.1 μJ energy, 16 ns duration and 27 kHz repetition rate was obtained, which resulted in about 0.7 kW output peak power. Furthermore, the influence of the output mirror transmission on the pulse energy, duration and repetition rate was also investigated.

By using mirrors with specially designed transmissions in a diode end-pumped hemispherical cavity, laser oscillations except for 1580 nm were suppressed and an enhanced pulse performance of an acousto-optic Q-switched laser at a single wavelength of 1580 nm was realized for Er:Yb:RAl3(BO3)4 crystals. When the absorbed pump power was 13.5 W and the pulse repetition frequency was 1 kHz, 250 μJ Q-switched pulse energy with a width of 215 ns and 350 μJ Q-switched pulse energy with a width of 125 ns were achieved in Er:Yb:YAl3(BO3)4 and Er:Yb:LuAl3(BO3)4 crystals, respectively. The pulsed laser performances of both the crystals were also compared and investigated in detail.

Er:NaY(WO4)2 and Er:Yb:Ce:NaY(WO4)2 crystals were grown by the Czochralski method. Er:Yb:NaY(WO4)2, Er:Ce:NaY(WO4)2, and Yb:NaY(WO4)2 crystal powders were synthesized by means of the solid-state reaction. The spectroscopic properties of the crystals were investigated, including the absorption and fluorescence spectra and fluorescence decay. The energy transfer efficiency from Yb3+ to Er3+ in Er:Yb:NaY(WO4)2 and Er:Yb:Ce:NaY(WO4)2 was calculated and analyzed. End-pumped by a diode laser at 970 nm in a hemispherical cavity, a 1.84 W quasi-continuous-wave laser around 1.55 μm with a slope efficiency of 17% and an absorbed pump threshold of 1.60 W was achieved in a 1.04 mm thick c-cut Er:Yb:Ce:NaY(WO4)2 crystal. The results show that the Er:Yb:Ce:NaY(WO4)2 crystal with high Ce3+ doping is a candidate for gain medium of low threshold 1.55 μm laser.