Liquid level-tuned mode-locked fiber laser based on the refractive index characteristics of no-core fiber

We propose and demonstrate a scheme for tuning a nonlinear polarization rotation mode-locked fiber laser using the liquid level height. A multimode interference filter is constructed by splicing a section of no-core fiber between two sections of single-mode fibers. When the no-core fiber is covered with liquid, the optical properties of the filter change, thereby tuning the peak wavelength of the filter. In the experiment, a liquid with a refractive index of 1.41 is used with a maximum coverage length of 70 mm, and a tuning range from 1032.00 nm to 1042.30 nm is achieved. Simulations have been conducted to model the operating characteristics of the filter and the impact of the filter's transmission curve on the mode-locked pulse characteristics. Compared with the performance of other structural filters in mode-locked Yb-doped fiber lasers, this filter is characterized by its simple structure, low cost, fast response, and excellent repeatability.