“…In order to generate ultrashort 2 μm pulsed laser sources, approaches to passively mode-locked thulium-doped lasers have been intensively investigated by making use of different kinds of saturable absorbers (SAs), such as semiconductor saturable absorber mirrors (SESAMs) [6][7][8] , graphene or graphene oxide [9][10][11][12] , and topological insulators [13,14] . With outstanding nonlinear optical properties, such as the low threshold level of saturable absorption, ultrafast recovery time, and high damage threshold, together with the advantages of easy fabrication and broad operation range [15][16][17][18][19] , single-walled carbon nanotubes (SWCNTs) also have been applied as SAs in mode-locked lasers around 2 μm with various laser configurations in the past several years [2][3][4][20][21][22] .…”