Ultra-broadband emission in Er³⁺/Tm³⁺/Ho³⁺ triply-doped germanate glass and double-clad optical fiber

In this paper 1400 nm–2200 nm broadband emission under 808 nm and 976 nm laser diodes excitation in germanate glasses co-doped with (0.35–0.7) Er₂O₃/(0.35–0.7) Tm₂O₃/(0–0.15) Ho₂O₃ (mol%) are presented. Flattened luminescence profiles were achieved as a result of RE content optimization in glasses co-doped with 0.35Er₂O₃/0.7Tm₂O₃/0.15Ho₂O₃, (λ_exc = 808 nm) and 0.7Er₂O₃/0.35Tm₂O₃/0.07Ho₂O₃, (λ_exc= 976 nm). The second, glass characterized by the widest emission range, has been used for the fabrication of double-clad optical fiber with 15 µm/250 µm (core/outer cladding) diameters. The optimization of broadband spectrum in glass and optical fiber was performed as the result of the superposition of 1550 nm (Er³⁺: ⁴I_13/2 → ⁴I_15/2), 1800 nm (Tm³⁺: ³F₄ → ³H₆), and 2000 nm (Ho³⁺:⁵I₇ → ⁵I₈) emission bands. The influence of optical fiber length and pump wavelength on near-infrared amplified spontaneous emission (ASE) features have been determined. As a result, the broadest bandwidth of ASE (415 nm - 3 dB and 731 nm – 10 dB) was obtained in 5 m length optical fiber under 808 nm pumping. It is significantly broader than in silica and tellurite fibers, which showed the advantage of RE triply-doped germanate optical fiber.