Passively Mode-locked Erbium-doped Fiber Laser Based on Nonlinear Fiber Loop Mirror

In this paper, a passively mode-locked erbium-doped fiber laser with tunable wavelength of the "8" cavity based on a nonlinear fiber loop mirror is studied. The laser uses a 2x2 coupler with a coupling ratio of 30/70 to control a section of the laser with polarization controller. And a Sagnac ring is constructed at the right end of the entire resonant cavity to form a filter. When the pump power up to 270 mW, we tune the polarization controller to achieve a stable mode-locked state, the fiber laser outputs a traditional soliton with a center wavelength of 1 555.7 nm, the 3-dB bandwidth of the spectrum is 4.2 nm and the repetition frequency is 21.1 MHz, respectively. The results match the cavity length built in the experiment, the signal-to-noise ratio of the output laser is 68 dB, after fitting the autocorrelation curve of the output pulse, the pulse width is 0.759 ps, and the time bandwidth product is 0.397. Collecting the spectrum every 2 hours in the experiment, observe the changes with time of the two peaks of the left and right sidebands closest to the center wavelength in the spectrum and the change of the output power with time during the whole process, and analyze the results to calculate the output . The fluctuation of the power in the course of more than 20 hours is only 0.11 mW, which explain the excellent stability of this fiber laser. In addition, we only increase the pump power without moving other components in the cavity, due to the stress of the polarization controller, the single-mode fiber causes the internal unbalanced pressure of the fiber to introduce stress birefringence, and together with PC-2, which forms a filter with a smaller bandwidth and realizes the continuous tunable output of the mode-locked fiber laser. The tuning range of the wavelength interval is 1.5 nm. After increasing the pump power to 360 mW, we adjust the polarization controller to introduce excessive nonlinearity in the laser cavity, the soliton become unstable. Due to the peak power clamping effect, pulse splitting can occur and the modulation period of the spectrum is 0.467 nm. According to the relationship between the time domain spacing and the spectral spacing, the pulse spacing of the two sets of pulse output terminals is calculated to be about 17.23 ps. Further collect its autocorrelation graph, which can be seen from the autocorrelation graph. The pulse interval is obviously greater than 5 times the pulse width, so it is concluded that the fiber laser produces a bound state soliton at this time, and it is in a loosely bound state. The mode-locking threshold of this laser is 270 mW at the pump power is 5.67 mW. From 270 mW to 360 mW, the overall mode-locked output power increases linearly, and the overall output power slope is 2.42%. The low light-to-light conversion efficiency is caused by the loss of the fiber fusion splice in the cavity. The fiber laser has a simple structure, easy tuning and has good stability. It provides a technical method for realizing wavelength tuning and traditional soliton mode locking, it can also be used as a seed light source in optical communications.