Performance enhancement of 2.3 μm thulium laser assisted by the resonant excited state absorption of ³ H ₅ → ³ H ₄ transition

The ³H₅ energy level of Tm³⁺ ions is characterized by the behavior of rapid relaxation, making it difficult to effectively utilize this energy level to regulate the laser performance of thulium-doped medium. We report a pumping scheme for improving 2.3 μm thulium-doped laser performance by using ground state absorption (GSA, ³H₆→³H₄) and resonant excited state absorption (RESA, ³H₅→³H₄). The GSA pumping corresponds to the conventional 785 nm laser diode pumping for a 2.3 μm thulium laser. Considering the fact that the emission wavelength band of holmium-doped laser partially matches the RESA band of ³H₅→³H₄ transition in the thulium-doped medium, RESA pumping is realized by the 2.1 μm holmium-doped laser intra-cavity pumped by the same thulium-doped laser medium emitting at 1.9 μm. Besides, the 1.9 μm laser oscillating in the cavity will also consume the population of the ³F₄ level, thereby alleviating the competitive effect on the ³H₄ level caused by the population capture of the ³F₄ level. The GSA-RESA pumping scheme constructed by holmium-doped laser intra-cavity pumped by thulium-doped laser has improved the 2.3 μm laser performance in two media of Tm:YLF and Tm:YAP. The experimental results show that after introducing the 2.1 μm laser RESA pumping, the oscillation threshold of the 2.3 μm thulium-doped laser is significantly reduced, and the output power and conversion efficiency are respectively increased by 34.6% and 74.4%, verifying the effectiveness of the GSA (³H₆→³H₄) and RESA (³H₅→³H₄) dual-wavelength pumping scheme. At the same time, it also verifies the feasibility of the intra-cavity resonant pumping scheme for the effective utilization of population at the energy level with rapid relaxation characteristics. From the perspective of expanding the output wavelength of the laser, the GSA-RESA pumping scheme also realizes the simultaneous dual-wavelength output of 2.1 μm and 2.3 μm, further enhancing the application potential of this type of laser.