# Continuous-Wave and Mode-Locked Operation of an In-Band Pumped Tm,Ho,LuCaGdAlO4 Laser
## 1 Introduction  
The tetragonal calcium aluminate CaInAlO4 crystals where Ln3+ denotes Gd3+ or Y3+[1; 2; 3] represent one of the most interesting recently studied laser host materials families. The Gd compound is abbreviated in the literature as CALGO (or CGA) and the Y compound as CALYO (or CYA). Both crystallize in a K2 NiF4-type structure with the Ca2+ and Ln3+ cations statistically distributed over the same Wyckoff site (4e, C4+-symmetry) leading to structural disorder [4; 5]. Doped with laser-active trivalent rare-earth (RE3+) ions, such as Yb3+, Tm3+, Ho3+, etc., these crystals exhibit inhomogeneous broadening of the absorption and emission spectral bands [6]. In contrast to the well-known disordered cubic garnets, however, disordered CaInAlO4 crystals possess relatively high thermal conductivity (~6.7 Wm-1K-1 for CALGO) with moderate dependence on the RE3+ doping level [7]. The emission of such lasers is polarized [8] due to the natural birefringence of the host crystals [9]. RE3+ doping is relatively easy due to the presence of passive Ln sites and both compounds melt congruently which facilitates large crystal growth by the conventional Czochralski (Cz) method [10]. The spectroscopic properties of RE\({}^{3+}\)-doped CaLnAlO\({}_{4}\) render these materials excellent candidates for femtosecond pulse generation and amplification [11; 12] with power scaling [13], including thin-disk geometries [14; 15], facilitated by the good thermal conductivity. While most of the past ultrafast laser research focused on Yb\({}^{3+}\)-doping with emission in the \(\sim\)1 \(\upmu\)m spectral range [11; 12; 13; 14; 15], more recently the interest is shifting towards Tm\({}^{3+}\) and Ho\({}^{3+}\) dopant ions with emission around 2 \(\upmu\)m [16]. Co-doping of the CaLnAlO\({}_{4}\) crystals with Tm,Ho [17] is of special importance because the emission of Tm alone in this type of relatively weak ligand field crystals lies below 2 \(\upmu\)m, in a wavelength range of structured water vapor absorption, which complicates the spectral support of broad bandwidths. In fact, Tm,Ho:CALGO and Tm,Ho:CALYO have been some of the most successful materials employed so far in femtosecond 2 \(\upmu\)m lasers [16]. Lu\({}^{3+}\) is another passive host-forming cation in many oxide materials and due to the close ionic radii of Lu\({}^{3+}\) and Yb\({}^{3+}\) or Tm\({}^{3+}\) ions, such crystals are more easily doped and the thermal conductivity is less sensitive to the doping level compared to the Gd-containing counterparts, which are more suitable for doping with Nd\({}^{3+}\). However, CaLuAlO\({}_{4}\) could not be synthesized as a single crystal [18], and the obvious remaining option is to grow \"mixed\" or passive ion doped host crystals. The choice of Lu\({}^{3+}\):CaGdAlO\({}_{4}\) as a passive host crystal, with a partial substitution of Gd\({}^{3+}\) by Lu\({}^{3+}\), is justified because due to the larger difference in the ionic radii (compared to Lu\({}^{3+}\) and Y\({}^{3+}\)), one can expect more inhomogeneous spectral line broadening as a result of the additional compositional disorder. Such mixed host crystals were employed for the first time in [19] with Yb-doping and in [20] with Tm-doping. Note that Lu doping levels of 5.2-5.5% as used in [19; 20] have a similar disordering effect as the active ion doping itself. However, especially in the case of Tm\({}^{3+}\), with its complex multi-level energy scheme, excessively increasing the active ion doping concentration may lead to undesirable excited state interactions and fluorescence quenching. Very recently, we carried out a comprehensive characterization of Cz grown Tm,Ho,Lu:CALGO crystals [21; 22] and demonstrated continuous-wave (CW) [22] and mode-locked (ML) [23] laser operation under conventional \(\sim\)800 nm pumping of the Tm\({}^{3+}\) ion followed by energy transfer to the Ho\({}^{3+}\) dopant.