Conventional Soliton and Noise-Like Pulse Generated in an Er-Doped Fiber Laser with Carbon Nanotube Saturable Absorbers  
[PERSON]  
1 College of Applied Sciences, Beijing University of Technology, Beijing 100124, China; [EMAIL_ADDRESS] (Z.D.); [EMAIL_ADDRESS] (J.T.); [EMAIL_ADDRESS] (Y.C.) Department of Chemistry, National University of Singapore, Singapore 637551, Singapore; [EMAIL_ADDRESS] 1  
[PERSON]  
1 College of Applied Sciences, Beijing University of Technology, Beijing 100124, China; [EMAIL_ADDRESS] (Z.D.); [EMAIL_ADDRESS] (J.T.); [EMAIL_ADDRESS] (Y.C.) Department of Chemistry, National University of Singapore, Singapore 637551, Singapore; [EMAIL_ADDRESS] 1  
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the saturable absorption effect (SAE) and reverse absorption effect (RAE) arisen from the third-order susceptibility under the incident light field [16; 17; 18; 19]. Their differences lie in the reduction of absorption at high intensities for SAE whereas RAE increases. SAE is also a process of filling near the stages of the conduction and valance bands which is cataloged as accumulative nonlinearity [20]. In contrast, RAE is an instantaneous nonlinearity (e.g., two-photon absorption effect) that is always triggered in relatively high intensity compared to SAE [21]. The correlation between SA properties and USP behaviors has been revealed in several studies. For instance, [PERSON] et al. reported the reduction of USP pulse duration with increasing SA modulation depth, indicating the stabilization and regulation of SAE on the USP behaviors [18]. [PERSON] et al. achieved the transformation between DS and NLP in a CNT-SA fiber laser operating in normal dispersion regime, owing to the trigger of RAE in higher pump power and the restriction on further USP peak power increasing [22]. The switch between DS and DS resonance using RAE is also possible in the normal dispersion regime. These indicate a nanomaterial-based SA with RAE in relatively low intensity can switch the USP operation states in a fiber laser oscillator.  
Recent advance on nanoscience has offered a wide range of selection for such saturable absorber, from the classical graphene [20], carbon nanotubes (CNT) [23], topological insulators [24], black phosphorus [25], transition-metal dichalcogenides [26; 27], to more recent MXene and bismuthene [28; 29]. Among these, CNT possesses a two-photon absorption (TPA) effect to respond to RAE in high peak power conditions, as well as a substantial nonlinear scattering loss from its quasi-one-dimensional cylindrical nanostructure. Based on both nonlinear effects, CNT is highly competitive to introduce RAE into one fiber laser system with relatively low peak power [30; 31]. Introducing RAE through CNT-SA into a fiber laser is practical to transform the USP operation states between CS and NLP that has never been explored. To properly integrate CNT-SA in a fiber laser, side polishing and tapered fiber integration methods are both utilized in the evanescent field, which decays exponentially to interaction distances [32; 33; 34]. Comparatively, the SA integration method with free-standing polymers is more suitable since it acts directly on the laser and enhances the RAE in relative low peak power conditions [35].  
In this article, we demonstrated the production of conventional soliton and noise-like pulse with monolayer and bilayer CNT-SAs in an Er-doped fiber laser in anomalous dispersion regime, for the first time. The saturable absorption effect and reverse absorption effect enabled the transformation of USP operation states. Further numerical simulations verified the essential correlation between nonlinear behaviors and operation state transformations.