at the 3-µm transition are attractive for use as pump sources for tunable mid-IR generation using optical parametric oscillation (OPO) or optical parametric generation (OPG) laser systems [4,5], which have broad applications in spectroscopy and atmospheric detection [6]. A simplified Er 3+ energy level diagram is shown in Fig. 1. The 4 I 11/2 upper laser state is directly pumped with a 970-nm laser-diode, and the laser output at 2.79 µm from Er 3+ -doped lasers is because of a transition between the 4 I 11/2 and the 4 I 13/2 energy levels [7]. According to classical laser theory, continuous-wave (CW) operation of the Er 3+ -doped laser is impossible because the upper lasing level has a shorter lifetime than the lower one [8]. However, because of the complex energy transfer processes inherent in these systems and the splitting of the laser levels into manifolds of Stark sub-levels, efficient CW operation for the Er 3+ -doped laser at 2.79 µm was achieved [9].The Er:Y 3 Sc 2 Ga 3 O 12 (Er:YSGG) crystal was demonstrated to be an excellent laser material for a diode-pumped laser at 2.79 µm because of its lower laser threshold and higher laser efficiency compared with other Er 3+ -doped garnet crystals, such as Er:YAG and Er:GGG crystals [10]. Dinerman et al. [9] reported a CW laser output with a power of 511 mW at 2.79 µm from a laser-diode-pumped Er:YSGG crystal. Waarts et al. [11] obtained a 900-mW CW laser output at 2.8 µm using a laser-diode array to pump the Er:YSGG microlaser arrays. Jensen et al. [12] reported a free-running laser with pulse energy of 40 mJ at 20 Hz from a quasi-continuous-wave (QCW) diode end-pumped monolithic Er:YSGG crystal. Arbabzadah et al. obtained a free-running pulse energy of up to 55 mJ at 14 Hz with a QCW diode pumping the Er:YSGG crystal [13]. Recently, we obtained a CW laser output at 2.79 µm with a power of 900 mW from a laser-diode end-pumped composite YSGG/ Er:YSGG crystal [14]. However, because of the thermal Abstract The performance of a laser-diode side-pumped Er:YSGG slab at a wavelength of 2.79 µm was investigated experimentally, and the laser output mode was analyzed using the theory of a thermally induced refractive index planar waveguide. Experimentally, a maximum continuouswave output of 1.84 W with a slope efficiency of 11.2 %, and an optical-to-optical efficiency of 7.5 % at 2.79 µm was obtained with a 970-nm laser-diode dual-side-pumped Er:YSGG slab. To the best of our knowledge, the output power was the highest reported for a laser-diode-pumped Er:YSGG laser with a continuous-wave output at 2.79 µm. The numerical analysis showed that the output power was mainly limited by thermal effects in the thickness direction and the laser output transverse mode in the experiment was the fundamental mode in this direction.