Broadband high-gain Tm3+/Ho3+ co-doped germanate glass multimaterial fiber for fiber lasers above 2 µm

Broadband 1.0 µm emission in Nd3+/Yb3+ co-doped phosphate glasses and fibers for photonic applications

In this work, the spectroscopic properties of 1.0 µm emission in Nd3+/Yb3+ co-doped phosphate glasses were systematically investigated under 808 nm excitation. Notably, broadband 1.0 µm emission with a full width at half maximum (FWHM) of 96 nm was obtained in the phosphate glass doped with 2 mol.% Nd2O3 and 1 mol.% Yb2O3. In addition, the energy transfer microscopic parameter and transfer efficiency were analyzed. What is more, multimaterial fibers with Nd3+/Yb3+ co-doped phosphate glass core and silicate cladding were successfully drawn by using the molten core method. An intense 1.0 µm amplified spontaneous emission (ASE) can be realized in a 3 cm long multimaterial fiber. More importantly, the FWHM of the ASE can reach as large as 60 nm when excited at 976 nm. These results demonstrate that the Nd3+/Yb3+ co-doped phosphate glasses and fibers are promising gain materials for amplifier and laser applications in photonics.

Improved 2 µm broadband luminescence in Tm3+/Ho3+ doping tellurite glass

Tm³⁺/Ho³⁺ doping tellurite glasses (TeO₂-ZnO-La₂O₃) were prepared by applying melt-quenching technique, and the ∼2.0 µm band luminescence characteristics were examined. A broadband and relatively flat luminescence at 1600 to 2200 nm was observed in the tellurite glass co-doped by 1.0 mol% Tm₂O₃ and 0.085 mol% Ho₂O₃ under the excitation of 808 nm laser diode (LD), which is the result of spectral overlapping of 1.83 µm band of Tm³⁺ ions and 2.0 µm band of Ho³⁺ ions. Further, about 103% enhancement was acquired after the introduction of 0.1 mol% CeO₂ and 7.5 mol% WO₃ at the same time, which is primarily caused by the cross-relaxation between Tm³⁺ and Ce³⁺ ions together with the enhanced energy transfer from the Tm³⁺:³F₄ level to Ho³⁺:⁵I₇ level due to the increase in phonon energy. Spectral characteristics associated with the radiative transition of Ho³⁺ and Tm³⁺ ions on the basis of Judd-Ofelt theory, and the fluorescence decay behaviors after the addition of Ce³⁺ ions and WO₃ component were analyzed to understand the broadband and luminescence enhancement. The findings in this work indicate that tellurite glass with optimal Tm³⁺-Ho³⁺-Ce³⁺ tri-doping combination and appropriate amount of WO₃ is a prospective candidate for broadband optoelectronic devices operated in the infrared bands.

Efficient three-level continuous-wave and GHz passively mode-locked laser by a Nd3+-doped silicate glass single mode fiber

Nd³⁺-doped three-level (⁴F_3/2-⁴I_9/2) fiber lasers with wavelengths in the range of 850-950 nm are of considerable interest in applications such as bio-medical imaging and blue and ultraviolet laser generation. Although the design of a suitable fiber geometry has enhanced the laser performance by suppressing the competitive four-level (⁴F_3/2-⁴I_11/2) transition at ∼1 µm, efficient operation of Nd³⁺-doped three-level fiber lasers still remains a challenge. In this study, taking a developed Nd³⁺-doped silicate glass single-mode fiber as gain medium, we demonstrate efficient three-level continuous-wave lasers and passively mode-locked lasers with a gigahertz (GHz) fundamental repetition rate. The fiber is designed using the rod-in-tube method and has a core diameter of 4 µm with a numerical aperture of 0.14. In a short 4.5-cm-long Nd³⁺-doped silicate fiber, all-fiber CW lasing in the range of 890 to 915 nm with a signal-to-noise ratio (SNR) greater than 49 dB is achieved. Especially, the laser slope efficiency reaches 31.7% at 910 nm. Furthermore, a centimeter-scale ultrashort passively mode-locked laser cavity is constructed and ultrashort pulse at 920 nm with a highest GHz fundamental repetition is successfully demonstrated. Our results confirm that Nd³⁺-doped silicate fiber could be an alternative gain medium for efficient three-level laser operation.

Toward low-loss mid-infrared Ga2O3-BaO-GeO2 optical fibers

The development of efficient and compact photonic systems in support of mid-infrared integrated optics is currently facing several challenges. To date, most mid-infrared glass-based devices are employing fluoride or chalcogenide glasses (FCGs). Although the commercialization of FCGs-based optical devices has rapidly grown during the last decade, their development is rather cumbersome due to either poor crystallization and hygroscopicity resilience or poor mechanical-thermal properties of the FCGs. To overcome these issues, the parallel development of heavy-metal oxide optical fiber from the barium-germanium-gallium oxide vitreous system (BGG) has revealed a promising alternative. However, over 30 years of fiber fabrication optimization, the final missing step of drawing BGG fibers with acceptable losses for meters-long active and passive optical devices had not yet been reached. In this article, we first identify the three most important factors that prevent the fabrication of low-loss BGG fibers i.e., surface quality, volumic striae and glass thermal-darkening. Each of the three factors is then addressed in setting up a protocol enabling the fabrication of low-loss optical fibers from gallium-rich BGG glass compositions. Accordingly, to the best of our knowledge, we report the lowest losses ever measured in a BGG glass fiber i.e., down to 200 dB km-1 at 1350 nm.

Enhanced 2-µm and upconversion luminescence properties regulated by network structure in Ho3+/Yb3+ co-doped germanate laser glasses

Rare-earth (RE) ions doped laser glass has attracted the interest of many researchers because of its numerous potential applications in planar waveguides and fiber lasers. In this work, the 2-µm and upconversion luminescence properties of Ho³⁺ are simultaneously enhanced through the design of components used to regulate the network structure of the germanate glass. Furthermore, the thermal, structural, and spectroscopic properties of the Ho³⁺/Yb³⁺ co-doped germanate laser glass are systematically investigated. It is noted that the calculated gain coefficient of the Nb₂O₅ modified germanate laser glass can reach as high as 3.05 cm^-1 at 2047 nm. These results suggest that the prepared germanate laser glass with superior performances is a promising candidate for 2-µm mid-infrared laser materials applications.