An efficient Er:SrF₂ crystal, lightly Er-doped to a concentration of 4at.%, was successfully grown by the traditional Bridgman method and displayed excellent spectral properties. A diode-end-pumped passively Q-switched dual-wavelength laser, operating at 2.79 μm wavelength, was demonstrated with this crystal by using black phosphorus as the saturable absorber (BP-SA). In the compact passively Q-switched Er:SrF₂ laser, the maximum average output power of 180 mW was achieved at an absorbed pump power of 2.47 W, with a pulse duration of 702 ns and a repetition rate of 77.03 kHz. To the best of our knowledge, this is the first reported application of BP-SA to dual-wavelength pulse laser operation in the mid-infrared region.

The effect of erbium concentration on spectroscopic properties of Er:CaF₂ crystals was investigated. Two highest absorption cross-sections (σ_abs) at 967 nm were achieved in the 4at.% and 8at.% Er:CaF₂ samples with the value of 0.22 × 10⁻²⁰ cm² and 0.23 × 10⁻²⁰ cm², respectively. And the 4at.% and 8at.% Er:CaF₂ samples also had the highest emission cross-section (σ_em) at 2727 nm with the value about 0.67 × 10⁻²⁰ cm². Lifetime of ⁴I_13/2 decreased faster than that of ⁴I_11/2 with the increase of erbium concentration. Under laser diode (LD) pumping, the continuous-wave (CW) laser operations around 2.79 μm were demonstrated in the 4at.%, 8at.% and 11at.% Er:CaF₂ samples. And the 4at.% Er:CaF₂ sample had the best laser performance with a maximum output power of 0.282 W and a slop efficiency of 13.9%.

Bismuth nanosheets (Bi-NSs) were successfully prepared and employed as saturable absorbers to generate a diode-pumped dual-wavelength ${\mathrm{Er}}^{3+}\text{:}{\mathrm{SrF}}_2$ laser in the mid-infrared region. $Q$-switched pulses with a maximum output power of 0.226 W were obtained at an absorbed pump power of 1.97 W. A repetition rate of 56.20 kHz and a minimum pulse duration of 980 ns were achieved. To the best of our knowledge, we present the first application of Bi-NSs in a mid-infrared all-solid-state laser. The results prove that Bi-NSs may be applied as an optical modulator in mid-infrared photonic devices or as a mode-locker and $Q$-switcher.

A mid-infrared Er doped CaF₂ crystal was successfully grown by the bridgeman method. Efficiently continuous wave and Q-switched laser operations were demonstrated at 2.8 μm with a 4 at.% Er doped CaF₂ crystal end-pumped by a fiber-coupled 974 nm diode laser. The continuous wave output power of 295 mW was obtained in a compact linear cavity. A stable 2.8 μm passively Q-switched Er:CaF₂ laser was also demonstrated with a graphene saturable absorber. Under an absorbed pump power of 2.353 W, an average output power of 172 mW was generated with a pulse duration of 1.324 μs and a repetition rate of 62.70 kHz, corresponding to the single pulse energy of 2.74 μJ and the peak power of 2.07 W, respectively. The high-quality Er:CaF₂ crystal and the monolayer graphene are an ideal combination to directly obtain a near 3 μm mid-infrared region pulse laser.

The spectral properties and laser performance of Er:SrF₂ single crystals were investigated and compared with Er:CaF₂. Er:SrF₂ crystals have larger absorption cross-sections at the pumping wavelength, larger mid-infrared stimulated emission cross-sections and much longer fluorescence lifetimes of the upper laser level (Er³⁺:⁴I_11/2 level) than those of Er:CaF₂ crystals. Dual-wavelength continuous-wave (CW) lasers around 2.8 μm were demonstrated in both 4at.% and 10at.% Er:SrF₂ single crystals under 972 nm laser diode (LD) end pumping. The laser wavelengths are 2789.3 nm and 2791.8 nm in the former, and 2786.4 nm and 2790.7 nm in the latter, respectively. The best laser performance has been demonstrated in lightly doped 4at.% Er:SrF₂ with a low threshold of 0.100 W, a high slope efficiency of 22.0%, an maximum output power of 0.483 W.

The authors report on the passive mode-locking and cw lasing performance of Yb³⁺: Sc₂SiO₅ (Yb: SSO) in an x-fold cavity end-pumped by a 978 nm single emitter. The laser produced a maximum cw output power of 2.73 W with a slope efficiency of 70%. Passive mode-locking of Yb: SSO was initiated using a semiconductor saturable absorber mirror (SESAM) while dispersion compensation was introduced using a pair of SF10 prisms. The laser mode-locked at 1041 nm, 1060 nm and 1077 nm with near Fourier transformed limited pulse width of 145 fs, 144 fs and 125 fs, and average output power of 40 mW, 52 mW and 102 mW, respectively. To the authors’ knowledge, this is the first demonstration of femtosecond mode-locking of Yb: SSO.