## 2 Experimental Results  
Figure 1 is the schematic of the passively mode-locked Er-doped fiber laser based on CNT-SA. The laser implements a ring cavity configuration, using Er-doped fiber (Liekki, Er110-4/125, 30 cm, nLIGHT, Vancouver, Wash., USA) as the gain medium. A 980 nm laser diode was used as the pump source through a wavelength division multiplexer (WDM, 980/1550 nm, Connet, Shanghai, China). Laser operation direction was ensured by a polarization-independent isolator (PI-ISO, 1550 nm, Connet, Shanghai, China). Cavity polarization and intra-cavity birefringence were regulated by a polarization controller (PC, Connet, Shanghai, China). Output power was controlled by the 10% intensity fiber output coupler (OC, Connet, Shanghai, China). CNT/PVA hybrid film as a SA was assembled between two fiber ferrules by the sandwiching structure. For CNT-SA preparation, single-wall carbon nanotube powder was purchased from XFNANO, and polymer fabrication method was similar to previously reported [36]. The cavity length and net cavity dispersion were 6.5 m and \(-\)0.023 ps\({}^{2}\), respectively. A stable mode-locking was self-started without adjusting the PC when ML CNT-SA inserted into the flange with a pump power of 40 mW. The pulse characteristics are demonstrated in Figure 2. The pulse duration is 439 fs with a sech\({}^{2}\)-pulse assumption as shown in Figure 2a. In Figure 2b, the spectrum is centered at 1560 nm with a 3-dB bandwidth of \(\sim\)5.3 nm, and the Kelly sideband indicates it is typical conventional soliton. Pulse trains and radio frequency (RF) spectrum are correspondingly illustrated in Figure 2c,d. The pulse period is 32.5 ns and fundamental frequency is 28.7 MHz, both of which depend on the cavity length and the signal-to-noise ratio (SNR) of the RF spectrum is about 73 dB, which shows the mode-locking is very stable. The pulse width increases from 439 fs to 628 fs with the increase of pump power. However, limited by the soliton area theory, further increasing the pump power results in the burst of following pulse trains into bound soliton state. These CS behaviors associated with increasing pump power autocorrelation trace (AC) and corresponding spectra evolution are presented in Figure 3a,b. We notice that the two bound solitons always exhibit fixed, discrete temporal separations, and the phase difference is always about \(\pi\) where the interaction optical spectra own a dip in the center of the spectra, as shown in Figure 3b3. Figure 1: Schematic diagram of the mode-locked fiber laser based on polymer CNT-SA. Inserted left, the autocorrelation traces of CS with ML CNT-SA and NLP with BL CNT-SA. Inserted right, the corresponding CS and NLP spectra. Figure 2: Output characteristics of CS using ML CNT-SA with a pump power of 40 mW in experiment. (**a**) The autocorrelation trace; (**b**) spectrum; (**c**) pulse trains; (**d**) RF spectrum, the inset shows the RF spectrum with a span of 180 MHz. In order to confirm the feasibility to switch pulse operation states by significantly changing the SA parameters, we assembled the bilayer CNT-SA in the flange without other modifications. Typical pulse duration and optical spectrum generated by BL CNT-SA are demonstrated in Figure 4a,b. Intriguingly, the spectrum has a triangle shape and the Kelly sideband is not observed. The measured AC trace changes from \(\mathrm{sech^{2}}\)-shape to NLP shape whose AC trace has a unique characteristic of a narrow spike on top of the broad pedestal. The NLP spike widths remain nearly unchanged at 1.75 ps with increasing pump power (inset of Figure 4c) while NLP pedestals broaden from 15 ps to 98 ps. The NLP spectra are centered at 1531 nm as a triangle shape and the 3-dB bandwidth gradually broadens with higher power in Figure 4d. The above results prove the ability to transform the pulse operation states in an Er-doped fiber laser by changing CNT-SA parameters.