Single-mode Tm ³⁺ -doped germanate-core/silicate-cladding glass fiber for low-threshold multi-wavelength lasing at 1.9  µm

Fiber lasers operating in the near- to mid-infrared region around 2 µm are important for numerous applications, including molecular spectroscopy, medical treatment, and laser radar. Although both silicate and germanate glasses are promising material systems for this spectral window, their large differences in thermo-optical properties often lead to interfacial instability and high propagation loss in multi-material fiber structures. In this work, we demonstrate a fiber design that incorporates a Tm 3+ -doped germanate core within a compatible silicate (ZF) glass cladding, fabricated using the rod-in-tube method. The resulting fiber exhibits a numerical aperture of 0.228, high infrared transmittance (∼80%), and robust bending performance down to a 25 mm radius. Using a compact 1.5 cm long active fiber, multi-wavelength laser emission in the 1.9 µm range was achieved with low-threshold pumps of 28 and 100 mW at 808 and 1570 nm, respectively. These results demonstrate an effective gain medium and a feasible approach for developing compact, low-threshold, multi-wavelength laser sources near 2 µm.