We report on an efficient mid-infrared thulium (Tm) fiber laser operating on the 3 H 4 → 3 H 5 transition and featuring an upconversion pumping scheme. This laser comprises a heavily T m 3 + -doped (2.50 mol. %) zirconium fluoride glass fiber pumped by a tunable Yb fiber laser around 1.05 µm corresponding to the 3 F 4 → 3 F 2 , 3 excited-state absorption transition. The laser generates 1.24 W at 2269–2282 nm with a slope efficiency of 37% in the quasi-continuous-wave regime. The Tm-glass fiber exhibits a broadband 3 H 4 → 3 H 5 emission with a bandwidth of 173 nm, making it very promising for femtosecond fiber oscillators at ∼ 2.3 µ m .

We report on the first laser operation of a disordered Tm:CaGdAlO₄ crystal on the ³H₄ → ³H₅ transition. Under direct pumping at 0.79 µm, it generates 264 mW at 2.32 µm with a slope efficiency of 13.9% and 22.5% vs. incident and absorbed pump power, respectively, and a linear polarization (σ). Two strategies to overcome the bottleneck effect of the metastable ³F₄ Tm³⁺ state leading to the ground-state bleaching are exploited: cascade lasing on the ³H₄ → ³H₅ and ³F₄ → ³H₆ transitions and dual-wavelength pumping at 0.79 and 1.05 µm combining the direct and upconversion pumping schemes. The cascade Tm-laser generates a maximum output power of 585 mW at 1.77 µm (³F₄ → ³H₆) and 2.32 µm (³H₄ → ³H₅) with a higher slope efficiency of 28.3% and a lower laser threshold of 1.43 W, out of which 332 mW are achieved at 2.32 µm. Under dual-wavelength pumping, further power scaling to 357 mW at at 2.32 µm is observed at the expense of increased laser threshold. To support the upconversion pumping experiment, excited-state absorption spectra of Tm³⁺ ions for the ³F₄ → ³F_2,3 and ³F₄ → ³H₄ transitions are measured for polarized light. Tm³⁺ ions in CaGdAlO₄ exhibit broadband emission at 2.3 - 2.5 µm making this crystal promising for ultrashort pulse generation.

This Letter presents the efficient laser operation of a T m : C a F 2 crystal in-band pumped at 1610 nm by an Er-Yb-codoped fiber laser system. A laser slope efficiency of 55% (versus incident pump power) was achieved in a continuous-wave regime, with a maximum output power of 1.25 W at ∼ 1.88 µ m in a nearly diffraction-limited beam ( M 2 = 1.14 ). We also demonstrated a continuous tuning range of 180 nm, which extends to short wavelengths down to 1773 nm.

We report on a femtosecond thulium laser operating on the ³H₄ → ³H₅ transition with upconversion pumping around 1 µm and passively mode-locked by a GaSb-based SEmiconductor Saturable Absorber Mirror (SESAM). This laser employs a 6 at.% Tm:LiYF₄ laser crystal and a polarization maintaining Yb-fiber master oscillator power amplifier at 1043 nm as a pump source addressing the ³F₄ → ³F_2,3 excited-state absorption transition of Tm³⁺ ions. In the continuous-wave regime, the Tm-laser generates 616 mW at ∼2313 nm with a slope efficiency of 10.0% (vs. the incident pump power) and a linear polarization (π). By implementing a type-I SESAM with a single ternary strained In_0.33Ga_0.67Sb quantum well embedded in GaSb for sustaining and stabilizing the soliton pulse shaping, the self-starting mode-locked Tm-laser generated pulses as short as 870 fs at a central wavelength of 2309.4 nm corresponding to an average output power of 208 mW at a pulse repetition rate of 105.08 MHz and excellent mode-locking stability. The output power was scaled to 450 mW at the expense of a longer pulse duration of 1.93 ps. The nonlinear parameters of the SESAM are also reported.