In the mid-infrared region, erbium-doped lasers at 2.8 μm have attracted significant attention owing to their wide applications in the medical treatment, detection, and military fields. The phonon energy of erbium-doped crystalline oxides (e.g., Er∶YAG and Er∶YSGG) is high (860 cm-1 and 728 cm-1), which causes the self-termination phenomenon. The phonon energy of erbium-doped sesquioxides, such as Er∶Lu2O3 and Er∶Y2O3, is low (618 cm-1 and 597 cm-1) and the thermal conductivity of these materials is high. However, the fabrication of these active laser materials is complicated and expensive mainly due to their high melting temperature. Erbium-doped fluorides, such as Er∶YLF, Er∶CaF2, and Er∶SrF2, show lower phonon energy (560 cm-1, 322 cm-1, and 280 cm-1) than sesquioxides, which can effectively suppress the non-radiative transition. Especially, the doped Er3+ ions prefer to form clusters in CaF2 and SrF2 crystals. These spontaneous clusters can achieve strong energy transfer between Er3+ ions with very low doping concentration (approximately 1%), accordingly obtaining a high-efficiency and high-power continuous-wave (CW) laser at 2.8 μm. The emission spectrum of Er∶CaF2 and Er∶SrF2 crystals is wide, approximately 250 nm (from 2600 nm to 2850 nm), around 2.8 μm. Therefore, spectral tuning of the Er∶CaF2 laser output wavelength is viable.
To obtain high-power CW laser, a dual-end pumped Er∶CaF2 laser is demonstrated [Fig. 1(a)]. The two pumping sources are wavelength-stabilized 976 nm fiber coupled laser diodes (LD) with a fiber core diameter of 105 μm and a numerical aperture of 0.22. The pumping radiations are focused into the laser crystal by the optical coupling systems with a coupling ratio of 30∶60 (L1∶L2) and 30∶75 (L4∶L3), respectively. A plane-concave resonator with a cavity length of 23 mm is formed by a concave dichroic mirror (DM1) and a plane output coupler (OC). The DM1, with a radius of curvature of 50 mm, is AR coated (T=95%) for 960?980 nm and HR coated (R=99.8%) for 2.65?2.85 μm. An OC with T=3% at 2.65?2.85 μm is used. The DM2 is placed between the OC and the L3 lens to separate the laser from the pumping radiations. With a dimension of 2 mm×2 mm×20 mm, an uncoated 2%-doped Er∶CaF2 crystal is wrapped with indium foil and placed in a water-cool copper block with a temperature of 18 ℃ for heat dissipation. In addition, a tunable Er∶CaF2 laser is also demonstrated [Fig. 1(b)]. Uncoated MgF2 birefringent filters (BRF), with a diameter of 15 mm and three different thicknesses, 1, 2, and 4 mm, are mounted on a rotating frame and inserted between the Er∶CaF2 crystal and OC at a Brewster angle (θB=53.7°), respectively. The cavity length is approximately 67 mm. The spectral tuning is achieved by rotating the angle between the BRF
In the single-end pump scheme, the laser exhibits a saturation trend when the pump power reaches 19.7 W, and a CW output power of 3.54 W with a slope efficiency of 19.6% is obtained (Fig.2). In the dual-end pump scheme, there is still no saturation trend when the pump power reaches 32.5 W. However, to protect the crystal, the pump power is not further increased. The maximum achieved CW output power is 5.04 W with a slope efficiency of 16.5%, a central wavelength of 2799.27 nm, and a beam quality factor of
In this work, high-power CW and tunable Er∶CaF2 lasers are demonstrated. In the single-end pump scheme, the output power reaches 3.54 W when pump power is 19.7 W. In the dual-end pump scheme, the CW output power reaches 5.04 W with a slope efficiency of 16.5% and a central wavelength of 2799.27 nm when pump power is 32.5 W. To the best of our knowledge, this is the highest CW output power among all reported LD-end-pumped erbium-doped fluoride crystal lasers at 2.8 μm. Er∶CaF2 lasers with tuning ranges of 168.89 nm, 148.87 nm, and 141.17 nm are achieved by using MgF2 birefringent filters with thicknesses of 1 mm, 2 mm, and 4 mm, respectively. To the best of our knowledge, this is the widest spectral tuning range among all reported erbium-doped fluoride crystal lasers.