《Table 1.Saturable Absorption Properties for Different Nanomaterial-Based SAs at 1.5μm and Mode-Lock

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《Q-switched and mode-locked Er-doped fiber laser using PtSe_2 as a saturable absorber》

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Due to the polarization-dependent characteristics of SA，the above-mentioned laser can be switched from a Q-switched state to a mode-locked state by adjusting the polarization state.Mode-locked pulses are generated when the pump power increases to about 80 m W.The stable mode-locked state maintains at even pump power up to a maximum of 450 mW，and the output single pulse energy can be up to 0.53 nJ.Figure 6shows typical output characteristics of the mode-locked laser with a pump power of 250 mW.As presented in Fig.6（a），the pulse repetition rate is about 23.3 MHz，which matches the cavity length well and indicates that the laser is working in the mode-locked state.Figure 6（b）is the pulse autocorrelation trace that can be well fitted by the hyperbolic secant function with a width of 1.58 ps，which indicates the real pulse width of approximately 1.02 ps.Figure 6（c）is the corresponding optical spectrum，whose 3 dB bandwidth is～6 nm，and the center wavelength is 1563 nm.There are four groups of sidebands on both sides of the center wavelength，which shows a clear characteristic of the traditional negative dispersion soliton and is consistent with our calculation of the net dispersion in the resonant cavity.The radio-frequency（RF）spectrum of the mode-locking state is measured and shown in Fig.6（d）with a resolution bandwidth of 300 kHz.The signal-to-noise ratio（SNR），which is up to 61 d B，shows that the fiber laser operates in a highly stable regime.To confirm the long-term stability of the mode-locked operation，fluctuation of the 3 d B spectral bandwidth is measured and shown in the inset of Fig.6（c）.The results show that the mode-locked pulses can be stable for hours.The durations of the mode-locked laser pulses are comparable to those of the reported lasers mode-locked with other nanomaterials as shown in Table 1，which indicates that PtSe2is a strong competitor for acting as an SA.To confirm the contribution of the PtSe2-based SA to mode-locking operation，we remove the PtSe2film covering the microfiber and just cover the microfiber with a piece of PDSM，which is the same as the one supporting the PtSe2film.By carefully adjusting the PC and pump power，no laser pulses can be observed in this case.Therefore，the mode-locking operation is caused by the PtSe2-based SA.Owing to the PtSe2-covered-microfiber structure，PtSe2interacts with the evanescent field of light waves outside the microfibers.Therefore，this PtSe2-based SA has relatively high damage threshold.Also，the PtSe2was not damaged during our experiment.