Laser Diode-Pumped Yellow Dy³⁺: GdMgB₅O₁₀ Laser

Visible-light lasers have applications in biology, medicine, and detection. Compared to conventional nonlinear frequency doubling methods for obtaining visible light lasers, it is possible to prepare laser crystals that directly emit a visible light via doping with rare-earth elements such as Pr³⁺, Sm³⁺, Tb³⁺, and Dy³⁺. For yellow light lasers, doping with Dy³⁺ is more suitable. The existing Dy³⁺-doped laser crystals are still at the theoretical stage, with a few experimental cases. Recently, a high-quality Dy³⁺-doped borate crystal GdMgB₅O₁₀ (Dy³⁺:GMB) is prepared to produce a yellow light laser at a wavelength of 580 nm. In this study, a plane-plane cavity laser was assembled using a 3 mm×3 mm×20 mm Y-cut Dy³⁺:GMB crystal with a 444 nm blue LD pump, achieving a laser output at 579.3 nm. The maximum output power reached 161 mW with a slope efficiency of 3.7% as using different output couplers at different transmission rates.

In this experiment, a 444 nm blue LD with a maximum power of 12 W was used as a pump source. The model of the blue LD pump was LSR444CP4-12W. The pump output a spatial light directly without fiber coupling and used air cooling to maintain a temperature of 25 ℃. The beam size at the output of the pump was 4.3 mm horizontally and 4.1 mm vertically. The beam remained parallel within 0.5 m and could be approximated as a parallel light. A plano-convex lens with a focal length of 150 mm was used. A Y-cut Dy:GMB crystal with the dimensions of 3 mm×3 mm×20 mm was used. The doping concentration of Dy³⁺ in the crystal was 6.08%, and the end faces of the crystal were polished but not coated for the blue wavelength band. The Dy:GMB crystal was wrapped in indium foil and placed in a water-cooled copper block, with the copper block’s water cooling set to 20 ℃. The polarization direction of the pump light was vertical. Based on the polarization bsorption efficiency of the Dy:GMB crystal along different axes, the crystal was positioned in an optical path. The pump light absorption efficiency of the crystal was measured under non-lasing conditions. When the crystal was placed, the pump light absorption efficiency was 55%. The laser resonator consisted of two coated plane mirrors. IM was the input mirror for the pump light, with coating arameters that could reduce reflection and increase transmission at 444 nm. The measured transmission at 444 nm was 96%. It also had a high reflectivity at 579 nm with a reflectivity of ≥99.8%. OC was the output coupler mirror, and the coated mirrors with 579 nm transmission rates of 1% and 2% were used. A long-pass edge filter with a cutoff wavelength of 450 nm was placed at the laser output to effectively reduce the impact of pump light on power measurements. After the output power measurement, a Glan prism was used to analyze the polarization of the output laser.

The output wavelength of the experimental laser is 579.3 nm, without other wavelengths detected. In the experiment with an OC transmission rate of 1%, a laser generation begins when the crystal absorbes 1.94 W of pump power, achieving a maximum output power of 161 mW and a slope efficiency of 3.7%. In the experiment with an OC transmission rate of 2%, a laser generation starts at an absorption power of 2.81 W, having a maximum output power of 129 mW with a slightly lower slope efficiency of 3.6%. No saturation in laser output power occurs in either set of experiments. The laser threshold and output power using an OC transmission rate of 1% are slightly greater than those using an OC transmission rate of 2%, which is consistent with the results reported in a previous study. Based on the slope efficiency, it is estimated that under the same pump absorption power, the maximum output power of the laser in this experiment can exceed 628 mW. There are three methods to improve the output power and slope efficiency of the laser via comparing the performance of the crystal in two separate experiments. The first method is to use a higher-power and higher-quality pump to enhance the absorption efficiency of the Dy:GMB crystal. The second method is to employ different types of laser cavities and optimize the coating parameters of the IM and OC, as well as shape the pump beam to achieve better spatial mode matching between the pump light and the laser cavity. The third method is to optimize the growth process to obtain X-cut or Z-cut crystals of sufficient length with varying Dy doping concentrations, as the emission cross-section of the Dy:GMB crystal is maximum at 578.5 nm along the E//Y direction.

A 579.3 nm yellow laser output with a maximum power of 161 mW and a slope efficiency of 3.7% was obtained using a plane-plane cavity laser setup with Dy:GMB crystal pumped by a 444 nm blue LD. An application potential of the crystal could be achieved via comparing the initial laser experiments with the same crystal using blue LDs at different wavelengths. This study could provide insights for future improvements in the output power and slope efficiency of lasers constructed with Dy:GMB crystals.