A femtosecond mode-lock all-fiber laser based on the nonlinear multimode interference (NL-MMI) effect is demonstrated. The saturable absorber (SA) in this laser is a graded-index multimode fiber-step index multimode fibergraded-index multimode fiber (GIMF-SIMF-GIMF) structure. The laser can work in mode-lock or soliton mode-lock two states. Through finely turning the polarization controller (PC), the laser will change the operation state. When the laser is running in the mode-locked state, the pump threshold is 51.4 mW. The center wavelength of the mode-lock pulse is 1559.71 nm with a spectral bandwidth of 10.73 nm, 354 fs duration and the fundamental repetition rate of 10.8 MHz. When the laser working in the soliton mode-lock state, the pump threshold is 54.1 mW. The center wavelength of the soliton pulse is 1561.85 nm, and the spectral bandwidth is 4.01 nm. The lasers have environmental stability. After a slight disturbance, the laser can stabilize itself to the mode-locked state.
We demonstrated an experimental observation of bright-dark pulse pair and soliton bunch in mode-locked erbium-doped fiber ring laser with the saturable absorber (SA) of graded index multimode fiber-step index multimode fiber-graded index multimode fiber (GIMF-SIMF-GIMF), which utilizes nonlinear multimode interference inside the multimode fiber to realizing mode locking of fiber laser. During the experiment, we observed a fascinating phenomenon of dualwavelength bright-dark pulse pair, in which the wavelengths of the two wavelengths are 1558.5 nm and 1560.1 nm, respectively, and the wavelength interval is 1.6 nm. The PBS filter was used to verify that the bright and dark pulses are orthogonal to each other in polarization. We also obtained multiple set of soliton bunches running at the fundamental frequencies of 100.5 ns, and when we increase the pump power we could find that the number of optical solitons increases in a single soliton bunch, which experimental phenomenon is in appropriate agreement with the theory. Based on the observation and understanding of these experimental phenomena, it is beneficial to explore the rich nonlinear effects in the mode-locked fiber laser.
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