Characterization and mid-infrared laser operation in Er:CaF2 single crystal fiber grown by the laser heated pedestal growth method

Abstract

Er:CaF₂ crystal, characterized by low doping and high efficiency, is a suitable material for single-crystal fiber (SCF). 3 at.% Er:CaF₂ SCFs were grown using the laser heated pedestal growth (LHPG) method. Significant oxidation-induced whitening phenomena were observed during growth. Increasing the growth rate helped mitigate further deterioration due to oxidation. This is likely because higher growth speeds allow the SCF to quickly move away from temperature ranges conducive to oxidation. As oxidation progressed, the phonon energy of 3 at.% Er:CaF₂ SCF increased, causing the emission intensity at ∼ 1 μm to decrease from 77 % in the initial source rod to 6 % in the fully whitened state. Additionally, the lifetime of the ⁴I_11/2 level decreased by approximately 10 times, from 8.988 ms to 0.822 ms. Continuous laser output at ∼ 2.8 μm was achieved using the transparent portion of 3 at.% Er:CaF₂ SCF. With an output mirror transmission of 2 %, a maximum output power of 251 mW and a slope efficiency of 15.9 % were obtained. The laser experiment demonstrated the potential application of LHPG-grown Er:CaF₂ SCF in ∼ 2.8 μm lasers, but further performance enhancement requires additional strategies to address oxidation issues. This work provides valuable insights into the growth of Er:CaF₂ SCF using the LHPG method.