Single-mode fiber is often used in the construction of femtosecond fiber laser pointer. Its small core diameter is accompanied by strong nonlinearity, which is easy to cause pulse splitting, thus limiting the increase of laser power. Using the space-time mode locking of a graded-index multimode fiber, theoretically high power can be obtained, and the self-cleaning effect of the beam discovered in recent years is expected to improve the quality of the output beam. This article reports that a multimode mode-locked fiber laser with self-cleaning characteristics of Kerr-induced beams in the cavity can generate femtosecond pulses with high energy and good beam quality.
In the multi-mode resonator simulation, the author considered the coexistence of five low-order modes. The system is shown in Figure 1. The initial simulation condition is a quantum noise field, and the pulse width and spectral width changes in the pulse evolution process are shown in Figure 2. The shortest pulse can be obtained at the end of the gain fiber, the pulse width is 450 fs, which also corresponds to the highest peak power in the cavity, and the pulse can reach the Kerr-induced beam self-cleaning threshold in the GRIN MMF part.
Because the production of Yb-doped gain GRIN MMF is very complicated, commercial step index gain MMF is used in this experiment. Compared with the step index MMF, the inter-mode dispersion of the GRIN MMF segment in the cavity can be ignored. In the experiment, the group used nonlinear polarization evolution (NPE) to achieve passive mode locking, as shown in the experimental setup.
The power of the diode pump source is 20W, the wavelength is 976nm, and the 1.3-meter-long MMF gain fiber is fusion spliced through the pump combiner. In order to excite the higher-order modes in the GRIN MMF, an offset of 5 μm is deliberately introduced when the gain fiber is spliced with the 1.4-meter GRIN MMF. After the first GRIN MMF, the beam passes through a set of wave plates, polarizing beam splitter (PBS), isolator, grating pair (600/mm) and spatial filter (small holes randomly placed) after being collimated.
The spatial filter is used to limit the modalities in the cavity. In the experiment of a multi-mode green laser pointer cavity, without modal filtering, the mode locking will produce oscillation during long-term operation. In the above cavity, randomly placed pinholes can ensure stable clamping. It is found in the experiment that for different pinhole positions, the spatial beam distribution remains unchanged. The grating pair in the cavity introduces a large negative dispersion, which causes the pulse entering the GRIN MMF to have a certain negative chirp, so the pulse will be compressed in the GRIN MMF and the gain MMF.
The pulse evolution from continuous output to mode-locked output in the experiment is shown in Figure 4. It can be seen that when passive mode-locking occurs, along with the formation of femtosecond pulses in the laser cavity, the Kerr-induced self-cleaning effect greatly improves the beam quality.