## 1 Introduction  
Carbon nanotubes, the one-dimensional form of graphite, maintain substantial popularity among the scientific community. Since their \"re-invention\" in 1991 [1], their unique electrical, mechanical, and optical properties have been extensively investigated and applied to a diverse range of devices, including transistors [2], photodetectors [3], optical switches [4], and gas sensors [5]. A report on the presence of saturable absorption in single-walled carbon nanotubes [6] led to the first demonstration of a nanomaterial-based ultrafast erbium-doped fiber laser in 2003 [7]. Since then, the family of nanomaterials exhibiting nonlinear optical absorption with ultrashort recovery times has expanded quickly. A pioneering two-dimensional material, graphene [8, 9], was soon joined by graphene oxide [10], black phosphorus [11, 12], transition metal monochalcogenides [13], transition metal dichalcogenides such as molybdenum disulfide [14, 15] or tungsten disulfide [16, 17], and topological insulators including bismuth telluride [18] and antimony telluride [19]. More recently, MXenes [20], bismuthene [21], and antimonene [22] were also found to show the desired nonlinear optical properties.  
Despite the rapid growth in the field of nanomaterial-based saturable absorbers [23], carbon nanotubes maintain their established position due to their versatility, low cost, and ease of fabrication, as complex epitaxial processes are not required. Notably, free-standing films offer increased reliability and long-lasting performance in comparison to polymer-based composites [24]. A large number of diverse ultrafast laser setups based on carbon nanotube saturable absorbers present the material as a nearly universal mode locker. To date, they have been applied to fiber lasers operating in a broad range of wavelengths: From visible [25], through the entire near-infrared [26, 27, 28], to \(\sim\)3 um generated from ZBLAN fiber lasers [29, 30]. Their broad transmission bandwidth allows to design lasers tunable over a 200 nm span [31], and the high damage threshold supports high energy (up to 34 nJ) picosecond pulse generation at \(\sim\)1.5 \(\upmu\)m [32]. The outstanding performance of the material enables the operation of more demanding ultrafast systems, such as stretched-pulse lasers [33; 34] or resonators operating at gigahertz repetition rates [35]. Reports on mode-locking solid-state lasers that include sub-200 fs pulse generation at 1 \(\upmu\)m [36] and 2 \(\upmu\)m [37] present carbon nanotube-based saturable absorbers as a potential alternative to semiconductor saturable absorber mirrors (SESAMs).  
The great majority of carbon nanotube-based ultrafast lasers demonstrated so far utilize either a naturally occurring mixture of metallic and semiconducting nanotubes, or purely semiconducting nanotubes. In the past, we have shown that a purely metallic single-walled carbon nanotube (m-SWCNT) thin film with interband transition M\({}_{11}\) at a maximum around 700 nm also exhibits effective saturable absorption in the \(\sim\)2 \(\upmu\)m band [34]. In this work, we extend our previous findings by demonstrating that the saturable absorption outside the M\({}_{11}\) transition is broadband and supports ultrashort laser pulse generation in a \(>\)500 nm wide spectral range. For this purpose, a fully fiberized saturable absorber based on m-SWCNT thin film was fabricated. Erbium, thulium, and holmium-doped fiber lasers were mode-locked with a single saturable absorber with a 300 nm thick m-SWCNT layer, yielding sub-800 fs pulse generation in three wavelength regions: 1.5, 1.9, and 2.1 \(\upmu\)m, respectively.