Polarization-dependent nonlinear optical properties of aligned single-walled carbon nanotube arrays for actively controlled pulsed thulium-doped fiber lasers

Active control of laser operational parameters within a single fiber laser cavity remains a significant challenge due to the complex interplay between intracavity nonlinearities and loss dynamics. Here, well-aligned arrays single-walled carbon nanotubes (SWCNTs) were synthesized within the one-dimensional nanochannels of an AlPO-41 (AFO) zeolite template. The high degree of structural alignment enables the first direct experimental observation of intrinsic polarization-dependent nonlinear optical (NLO) absorption in SWCNTs using polarization-resolved Z-scan spectroscopy. The NLO absorption coefficient β exhibits a remarkable 4.2-fold variation from 0.84 × 10 −5 cm/W (E⊥C) to 3.51 × 10 −5 cm/W (E||C). Furthermore, by integrating this polarization-sensitive SWCNTs saturable absorber (SA) into a thulium-doped fiber laser, active reversible switching between Q-switching and mode-locking, reversible soliton wavelength tuning over 21.6 nm, and efficient Kelly sideband suppression were all achieved simply by adjusting the intracavity polarization state at a fixed pump power. The zeolite-confined synthesis not only ensures perfect carbon nanotube alignment and prevents aggregation but also provides a robust protective framework, endowing the device with long-term stability. This work establishes aligned arrays SWCNTs as a promising anisotropic NLO material and offers a robust, versatile platform for next-generation intelligently controlled ultrafast fiber lasers.