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

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.