## 1 Introduction  
Two-micrometer single-longitudinal-mode (SLM) lasers have become an attractive research topic due to their dramatic advantages. For example, it is safe for eyes and it has fine transparency in the atmosphere [1; 2]. The 2 \(\upmu\)m SLM lasers can serve as efficient sources in optical measurements, such as wind measurement, as well as the measurement of the concentration of atmospheric atoms [3]. The narrow width and the high quality of the laser are also needed in spectroscopic measurements. There are several traditional methods to obtain SLM lasers, such as microchip lasers [4], torsion mode cavities [5], non-planar ring cavities [6], and coupled cavities with intra-cavity high-loss Fabry-Perot (F-P) etalons [7], which are the easiest methods of obtaining tunable SLM laser output. The intra-cavity F-P etalons can also provide a higher output power and a narrower width due to its mechanism [8]. Compared with SLM microchip lasers, the SLM lasers with F-P etalons in laser cavities are more likely to gain a higher efficiency and a higher output power due to the high gain [9]. The efficiencies of the SLM lasers with intra-cavity F-P etalons are similar to those of the torsion mode cavity and non-planar ring cavity SLM lasers [10; 11]. However, the torsion mode cavity and non-planar ring cavity SLM lasers are too complicated to realize. Initial work regarding 2 \(\upmu\)m SLM lasers primarily focused on Tm, Ho co-doped crystals. In 1990, [PERSON] et al. researched Tm, Ho:YAG lasers with intra-cavity etalons and realized SLM laser output with a maximum output of 58 mW [12]. In 2010, [PERSON] et al. demonstrated an SLM Tm, Ho:YLF laser with F-P etalons and an output power up to 118 mW was realized [13]. In 2015, [PERSON] et al. demonstrated an SLM Tm-Ho:YVO\({}_{4}\) laser, and the maximum output power was 95 mW when the central output wavelength was 2051.3 nm [9]. However, Tm, Ho co-doped lasers need additional cryogenic cooling devices to keep the laser efficient, especially for high-power laser operation [14]. The cooperative up-conversion losses in Tm, Ho co-doped systems are significant, which make it unsuitable for room temperature operation [15]. One attractive way to avoid the losses associated with co-doped systems is to separate Tm and Ho ions into different crystals [16]. Then, a 2 \(\upmu\)m Ho single-doped laser can be obtained by a resonant pumping system with laser diode (LD)-pumped Tm-doped lasers. Compared with Tm, Ho co-doped lasers, the Ho single-doped lasers have obvious advantages on account of their high efficiencies and low quantum losses [17]. Thus, pumping the Ho-doped laser with a Tm laser is a good way to obtain the 2 \(\upmu\)m SLM lasers, which has become an attractive issue in the 2 \(\upmu\)m SLM laser research field. As we know, the characteristics of the host materials can affect the output properties of the solid-state laser to a great extent because the host materials determine the thermal, mechanical, and spectral features of the active media. In this paper, Sc\({}_{2}\)SiO\({}_{5}\) (SSO) is considered to be a potential Ho single-doped laser gain medium. Compared with the common Ho-doped crystals, such as Ho:YLF and Ho:YAG [18, 19], there are several outstanding superiorities for the Ho:SSO crystal host, such as large energy splitting (\(\Delta E\) = 712 cm\({}^{-1}\)) and high thermal conductivity. Additionally, the widths of the absorption and the emission spectra are very large, the full width at half maximum (FWHM) of which are 59 nm and 193 nm, respectively [20]. It is easier to obtain the high power output for an Ho:SSO laser crystal due to its high thermal conductivity. On the other hand, the SSO crystal is a special laser crystal, which has a negative refractive coefficient. It is obvious that the thermal lens effect, birefringence effects, and crystallographic site distortions can be limited by the negative refractive coefficient [21]. In this paper, we report an SLM Ho:SSO laser with two intra-cavity F-P etalons, which was pumped by a Tm:YAP laser.