Tunable mode-locked fiber laser based on nonlinear multimode interference with double offset-splicing step-index few-mode fiber

A tunable mode-locked all-fiber Yb-doped laser with a double offset-splicing step-index few-mode fiber (DOS-SIFMF) is demonstrated, to the best of our knowledge, for the first time. The structure of DOS-SIFMF, which constructs a micro Mach-Zehnder interferometer as a consequence of introducing offset splicing, has characteristics of both a saturable absorber and filter and is more accessible to obtain mode-locking operation in an all-normal dispersive region. The results of simulation show that interference with fewer modes is more reliable to acquire mode-locking operation of the fiber laser. The central wavelength, spectrum, and pulse widths are 1032 nm, 6.15 nm, and 28.8 ps, respectively. The output pulse in time and spectrum domains can be tuned in the range of 168.7 ps and 10.7 nm, respectively. This structure has effects of both mode-locking and filtering, showing potential application in communication and sensing. Furthermore, the influence on mode number to interference is generally discussed in the end.