Super free spectral range tunable narrow linewidth fiber laser based on a droplet-shaped fiber microbubble resonator

The full tunability of the fiber laser is a highly attractive feature and has been rarely reported. In this article, a super free spectral range tunable narrow linewidth fiber laser based on a high-quality factor droplet-shaped fiber microbubble resonator is proposed and demonstrated for the first time. The prepared fiber microbubble resonator with a quality factor of 6.5 x 107 is utilized as a narrowband filtering feedback element of the fiber laser. Subsequently, a stabilized single-mode laser with a linewidth of similar to 1.5 kHz is excited by non-resonant pumping in the 976 nm band. It is noteworthy that the fiber microbubble resonator designed in this article has a hollow confined space inside, which provides a unique advantage in thermo-optic tuning. In addition, the strong thermooptic effect of iron oxide nanoparticles is utilized to achieve efficient and controllable full tuning with a tuning efficiency of 53.2 pm/mW, and the tuning range exceeds the free spectral range as high as an astonishing 4.14 nm. This fiber laser has the advantages of low cost, high tuning efficiency and extensive tuning range. It shows promise for application in high-precision fiber sensors, coherent optical communication, and high-resolution spectral measurement.