## 1 Introduction  
Free-space communication [1], light detection and ranging (LIDAR), or gas detection systems [2; 3] would take advantage of laser systems operating at 2 \(\upmu\)m because they are \"eye safe\" compared to conventional systems operating at shorter wavelengths. The strong water absorption in this wavelength range also makes these laser systems ideal for medical applications [4; 5], such as tissue ablation [6], dermatology [7], and scalpels [8]. These laser systems have also shown promise for low-latency interconnection using hollow-core fibers [9], as well as for cutting and welding polymers [10].  
There are already some proposals for all-fiber modelocked laser systems that emit below [11; 12; 13; 14; 15; 16] or above [17; 18; 19; 20; 21; 22; 23; 24; 25] 2 \(\upmu\)m and use a thulium doped fiber (TDF) as the medium gain. These proposals for a mode-locking mechanism primarily rely on nonlinear polarization rotation (NPR) [11; 20; 25], nonlinear optical loop mirrors (NOLM) [14; 15; 16; 17; 18; 19], nonlinear amplifying loop mirrors (NALM) [12], non-semiconductor saturable absorbers (NSA) [13; 21; 22; 23], or, much less frequently, semiconductor saturable absorbers (SESA) in either transmission or reflection (SESAM, semiconductor saturable absorber mirrors) [22; 24]. In an ultralong cavity, NPR, NOLM, and NALM are prone to overdrive, and all depend heavily on the polarization and phase evolution of the optical pulse. For this reason, most of these proposals use polarization controls that require multiple adjustments and the uncertainty of self-start. In addition, the operating wavelength often varies depending on the polarization of the resonator and pump power, which could be a serious problem in some applications. Single-walled carbon nanotubes [13], iron-doped carbon nitride nanosheets [21], and MoS\({}_{2}\)[23] are examples of NSAs that are still at an early stages of technology development and whose behavior has not yet been tested under challenging conditions. SESAMs are a mature technology that are inherently compatible with polarization-maintaining (PM) cavities, unlike other proposals based on polarization evolution, nonlinear optical loop mirrors, and similar solutions.  
Although there are other proposals that use holmium-doped fibers [26], in this work, we focus on laser systems operating in the 2 \(\upmu\)m range whose medium gain is a thulium-doped fiber. However, there are some differences between the emission cross sections of the two medium gains, with a maximum for TDFs at 1.85 \(\upmu\)m [27] and holmium doped fibers (HDFs) at 2.02 \(\upmu\)m [28]. TDFs can be pumped at 0.79 or 1.56 \(\upmu\)m [29], making pumping sources easily accessible. In contrast, HDFs require less accessible pumping sources with emissions at 1.1 or 1.9 \(\upmu\)m [30]. Since the emission and absorption bands strongly overlap, reabsorption is a second problem for HDFs that does not occur for TDFs.  
Therefore, our goal in this work is to compare the passively modelocked emission using a TDF resonantly pumped in C-band as the medium gain and a SESAM as the mode-locker at both the common emission wavelength of 1.95 \(\upmu\)m and at the less common (for a TDF) emission wavelength of 2.07 \(\upmu\)m. Since robustness to environmental effects is a must for practical applications, we designed both lasers in a fully polarization-maintaining cavity. Both lasers have been properly characterized, and we have tested both the long-term stability of this setup in 24-h operation and the short-term stability in a simulated harsh environment.