Fig. 1. Schematic of the multimode fiber cavity setup. SPDM, short-pass dichroic mirror; L1 and L2, collimating lens; M, reflective mirror; HWP, half-wave plate; QWP, quarter-wave plate; PBS, polarized beam splitter; ISO, isolator; and SF, spectral filter.

Fig. 2. (a), (b) Typical steady multimode QS pulse train generated in the MMF cavity. (a) QS pulse train with a repetition of 70.14 kHz. The inset shows the corresponding pulse width of 1.88 μs. (b) Corresponding RF spectrum of the QS state (resolution bandwidth: 10 Hz). Left inset: beam profile; right inset: RF spectrum from 0 to 500 kHz. (c) QS output power and (d) repetition rate versus pump power ranging from 6.0 W to 8.0 W. Error bars in (d) shows the repetition rate variation range of the QS pulses.

Fig. 3. Characterization of the multimode QS state using (a)–(c) spatial sampling and (d), (e) spectral filtering. (a) Spectra measured at three different points of the output beam by spatial sampling, which are visualized and marked in (c). (b) Corresponding temporal signals (from top to bottom are pulse trains detected at points A, B, and C, respectively, as indicated by their colors). (d) Spectra of the filtered output beam, showing different spectral components. (e) Corresponding beam profiles of the spectral filtered output light.

Fig. 4. Transition between QS and STML versus pump power changes. (a) Spectra of the laser output with increasing pump power, covering the CW, QS, and ML operating regimes. (b) Corresponding beam pattern with respect to four different pump power levels, as shown in Visualization 1. The operating regimes and the corresponding pump power are notated in the figures.

Fig. 5. Typical multimode QS–ML bistable state, shown in Visualization 2. (a) Spectra of QS and ML states and the corresponding (b) beam profiles and (c), (d) pulse trains, with repetition rates of 66.1 kHz and 32.2 MHz, respectively.