Abstract
A femtosecond soliton erbium-doped fiber ring laser with the symmetrical saturable absorber (SA) of graded index multimode fiber-step index multimode fiber-graded index multimode fiber (GIMF-SIMF-GIMF) is proposed and proved. Based on the nonlinear multimode interference (NL-MMI) effect of SA, the fiber laser achieved femtosecond soliton mode-locking operation and repetition rate tuning. The duration of soliton was 364 fs at 1562.5 nm and repetition rate was tunable from 10.29 MHz to 763.36 MHz. In the structure of SA, a short step index multimode fiber (SIMF) was inserted between two graded index multimode fibers (GIMFs) to generate more high-order modes, adjust the self-focusing length, and improve operation stability of mode-locked fiber laser. The SA also has the advantages of immunity to the external environment variation, high damage threshold and inoxidizability, which make the laser operate stably for a long time.
Highlights
The ultrafast soliton fiber lasers with adjustable repetition rate have widely been paid attention by researchers due to their meaningful applications in fiber optic sensing, nonlinear optics, optical communications, biomedical applications, etc. [1]–[3]
The saturable absorber (SA) was removed from the cavity, the laser only operate at a continue wave (CW) output state, no matter how to adjust the pump power and beam polarization states, which excludes the possibility of the nonlinear polarization rotation or self-mode locking
An interesting experimental phenomenon was found that the laser always started mode locking at a high pump power level
Summary
The ultrafast soliton fiber lasers with adjustable repetition rate have widely been paid attention by researchers due to their meaningful applications in fiber optic sensing, nonlinear optics, optical communications, biomedical applications, etc. [1]–[3]. The passively mode-locked laser, with a simple and compact structure, no modulator, narrower pulse, and higher repetition frequency, is an effective scheme for generating adjustable repetition rate femtosecond soliton [4]–[6]. The above SAs are generally integrated into the fiber end face in the form of a thin film, or deposited on the surface of a tapered micro-nano fiber, and the thickness (or molecular/atomic layer number) of the two-dimensional materials needs to be precisely controlled. Due to they are oxidized and damped, their long-term stability is poor. Researchers in the field of ultrafast photonics have never stopped looking for
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