Mid-infrared lasers at approximately 3 μm can be used in many fields such as medical surgery, trace gas detection, remote sensing, and infrared countermeasures. Such laser source can be obtained via Dy³⁺-, Ho³⁺-, or Er³⁺-doped laser active medium. The development of Dy³⁺- and Ho³⁺-doped lasers is limited because of the lack of a suitable pump source. In contrast, there is a strong absorption band of Er³⁺-doped crystals at approximately 970 nm and they can be pumped by well-developed high-power InGaAs LDs. However, the transition of Er³⁺ ions at approximately 3 μm wavelength is self-terminating because the lifetime of the initial laser level is considerably shorter than that of the terminal laser level. To overcome this “bottleneck”, Er³⁺ ions with a high atomic fraction of 30%-50 % are typically required to depopulate lower laser level ⁴I_13/2. The thermal effects increase with increasing doping concentration. Cascading ⁴I_13/2→⁴I_15/2 near-infrared laser emission can achieve a 3-μm-laser output at an extremely low doping concentration but at the expense of reducing the conversion efficiency. In our recent study, it has been proven that the cyclic cascade can improve the efficiency of a 3-μm-laser in Er∶YAG crystals, with a low doping concentration at room temperature. However, the maximum slope efficiency does not exceed 10%. On the basis of the cyclic cascade, improving the slope efficiency is a meaningful attempt.

In this study, we theoretically analyzed the two main factors influencing the mid-infrared (MIR) laser slope efficiency in Er∶YAG crystals, namely, quantum efficiency and overlapping efficiency. We analyzed the mechanism of improving the quantum efficiency by a cyclic cascade. For the overlapping efficiency, we simulated the distributions of the pump light and laser mode field and analyzed their influence on the overlapping efficiency. Next, the coating parameters of the cavity mirror were designed according to the relevant characteristic wavelengths in the cyclic cascade process. Er∶YAG crystals with doping concentrations (atomic fractions) of 7.5%, 10%, 15%, and 25% were used in the experiment and cut along their〈111〉crystal direction, with cross-sections of 3 mm×3 mm. The lengths of the Er∶YAG crystals with atomic fractions of 7.5%, 10%, and 15% were 10 mm and 2 mm, and the lengths of Er∶YAG crystals with atomic fraction of 25% were 5 mm and 2 mm. The pumping source was a stable wavelength fiber-coupled laser diode with a central emission wavelength of 976 nm. A spectrometer was used to measure the laser output spectrum and determine whether a cascade oscillation was formed. An energy meter was used to measure the output energy and other related parameters. Then, the laser output slope efficiency was calculated to analyze the influence of improving the overlapping efficiency by optimizing the crystal length on the slope efficiency.

In the output spectra of the crystals with four doping concentrations, two emission peaks in near-infrared (1469 nm) and mid-infrared (2937 nm) bands can be observed (Fig. 5), showing that the near-infrared light and mid-infrared light have formed a cascade oscillation in the cavity. For Er∶YAG crystals with lengths of 10 mm and 5 mm, the threshold is greater than 23 mJ, whereas for Er∶YAG crystals with a length of 2 mm, the threshold is lower than 17 mJ. The threshold of Er∶YAG crystals with a larger length is generally higher than 2 mm (Fig. 6 and Table 1). In terms of the 2937-nm mid-infrared laser output slope efficiency, under the same doping concentration, the laser slope efficiency of the 2-mm long crystal is significantly improved compared with that of the longer laser crystal. In particular, for Er∶YAG crystals with an atomic fraction of 10%, when the crystal length is 10 mm, the mid-infrared laser threshold is 23.4 mJ, and the slope efficiency is 23.5%; when the crystal length is 2 mm, the mid-infrared laser threshold is 11.4 mJ, and the slope efficiency is 36.5%. Compared with the 10-mm long crystal, the threshold is reduced by 51.3%, and the slope efficiency is increased by 55.3%. To the best of our knowledge, this is the first time that a slope efficiency exceeding the Stokes limit has been obtained for low-doped Er∶YAG crystals at room temperature (Fig. 6 and Table 1).

A high-efficiency cyclic cascade Er∶YAG mid-infrared pulsed laser at room temperature is reported. Based on the cyclic cascade, using the optimized cyclic cascade cavity, the influence of optimizing the crystal length to improve the beam overlap on the slope efficiency of the mid-infrared laser is explored. The experimental results show that by appropriately shortening the crystal length, the threshold of mid-infrared laser oscillation is reduced considerably, and the slope efficiency is significantly improved. Slope efficiencies of 2937 nm mid-infrared lasers up to 36.5% and 37.2%, exceeding the Stokes limit of 33.2%, are achieved in Er∶YAG crystals with a length of 2 mm and Er atomic fractions of 25% and 10%, respectively. To the best of our knowledge, this is the first time that an Er∶YAG crystal with a low doping concentration (atomic fraction of 10%) is used for a high-efficiency 3-μm mid-infrared laser output exceeding the Stokes limit at room temperature. Moreover, Er∶YAG crystals with an Er atomic fraction of 10% can produce less thermal effect than Er∶YAG crystals with an Er atomic fraction of 25% and is expected to obtain a higher average output power. This study can help to promote the development of 3-μm mid-infrared high efficiency high average power lasers.