Benefiting from the maturity of 0.8 μm laser diode pump sources and a diverse range of laser materials, thulium-doped lasers serve as the mainstream solution for achieving 2 μm laser sources. However, the 0.8 μm pumping scheme suffers from a large quantum defect. Even with the use of concentration-dependent cross-relaxation processes to enhance the populating efficiency of the upper energy level, the slope efficiency of 0.8 μm laser diode (LD)-pumped thulium-doped lasers generally remains below 50%. According to the absorption spectra of thulium-doped media, these materials exhibit strong ground state absorption (³H₆→³F₄) at 1.6?1.7 μm, which enables an in-band pumping scheme and thereby enhances the operation performance of 2 μm thulium-doped lasers. Although using 1.6 μm fiber or solid-state lasers as pump sources can precisely match the strongest absorption peak of thulium-doped media in the 1.6?1.7 μm band, it inevitably increases the complexity and the cost of 2 μm thulium-doped laser systems. Additionally, 1.6 μm Er-doped fiber or solid-state lasers typically utilize pump sources based on 0.8?1 μm laser diodes. If the conversion efficiency of the 1.6 μm pump source is also taken into account, the overall conversion efficiency of the 2 μm thulium-doped laser with 1.6 μm laser pumping will also be significantly reduced. Emerging application scenarios like transparent plastic welding have driven the commercialization of InP-based long-wavelength laser diodes at 1.7 μm, which provide cost-effective in-band pump sources for 2 μm thulium-doped solid-state lasers.

The laser experiment employs LD with a central wavelength of 1700 nm (linewidth of 5.3 nm) as the in-band pump source. The output fiber of LD has a core diameter of 400 μm and a numerical aperture (NA) of 0.22. Significant absorption of thulium-doped media at 1700 nm is a prerequisite for achieving in-band pumping. Therefore, Tm∶YAG and Tm∶YAP crystals are selected as laser media. Tm∶YAG crystals cut in the [111] crystal orientation and Tm∶YAP crystals cut in the b-axis are used. The doping atomic fractions of both crystals are 2%, and the dimensions are both 4 mm×4 mm×8 mm. Anti-reflection films at laser wavelength are coated on the two light-transmitting surfaces of 4 mm×4 mm. The resonant cavity has a length of 15 mm. The side of the input mirror (IM) facing away from the resonant cavity is coated with an anti-reflection (AR) layer at 1650?1750 nm, and the other side is coated with a high-reflection (HR) layer at 1900?2150 nm (reflectivity of >99.5%). The output coupler (OC) has a transmission of 5% across the 1900?2100 nm band.

For the 1700 nm LD-pumped Tm∶YAP laser, oscillation initiates at the absorbed pump power of 1.56 W. When the absorbed pump power is increased to 10.7 W, a maximum output power of 7.13 W at 1987 nm is achieved, corresponding to a slope efficiency of 80.5% (Fig. 7), which is increased by 78.5% compared to that achieved in 785 nm LD pumping. The beam quality of the 1700 nm LD-pumped Tm∶YAP laser at the maximum output power is measured with the beam quality factors being Mx2=1.73 and My2=1.87. The 785 nm LD-pumped Tm∶YAP laser exhibits degraded beam quality factors of Mx2=2.71 and My2=2.52 at its maximum output power of 5.22 W. For the Tm∶YAG laser, a maximum output power of 4.47 W at 2015 nm is achieved with an absorbed pump power of 7.51 W, yielding a slope efficiency of 69.2% (an 86.0% efficiency enhancement over that of 785 nm LD pumping). Considering the effectiveness of the in-band pumping scheme for enhancing the operation performance of a thulium-doped laser, we further construct a 1700 nm LD-pumped Tm∶YAP/Ho∶Y?O? laser. Laser oscillation at 2117 nm begins at an absorbed pump power of 1.56 W. A maximum output power of 3.37 W is attained at a 7.64 W absorbed pump power, corresponding to a slope efficiency of 56.8%.

In this paper, a 2 μm thulium-doped all-solid-state laser with in-band pumping is constructed using 1700 nm LD as the pumping source. The maximum output powers corresponding to Tm∶YAP and Tm∶YAG are 7.13 W and 4.47 W, with slope efficiencies of 80.5% and 69.2%, respectively. The two efficiencies respectively represent 78.5% and 86.0% improvements over that of the conventional 0.8 μm LD pumping scheme. The beam quality factors of the 1700 nm LD-pumped Tm∶YAP laser measured at the maximum output power are Mx2=1.73 and My2=1.87. As an application demonstration of the in-band pumped thulium-doped laser, the 1700 nm LD-pumped Tm∶YAP/Ho∶Y₂O₃ laser is further configured, generating an output power of 3.37 W at 2117 nm with a slope efficiency of 56.8%. The experimental results show that the 1.7 μm LD in-band pumped thulium-doped laser has significant comparative advantages, such as a more compact structure, higher conversion efficiency, better beam quality, and wider application scalability.