We report here on a spectroscopic study of Dy3+-doped and Tm3+-Dy3+ doped CaF2 as promising candidates to develop crystalline waveguide laser sources around 3 μm. The advantages of Tm3+ ions as sensitizers to improve the excitation of Dy3+ ions in CaF2 is demonstrated: an energy transfer efficiency from Tm3+ to Dy3+ ions of 99.6% has been reached for Dy3+ concentration as high as 2 at.%, by considering a Tm3+ ratio set at 5 at.%. Moreover, the behavior of such doped crystals in a laser waveguide configuration has been modeled and the modeling results show that it seems possible to achieve promising laser perspectives around 3 μm, with laser thresholds in the watt level for singly doped Dy3+:CaF2 and around 0.2–0.3 W for codoped Tm3+-Dy3+:CaF2, presenting both laser efficiencies in the order of 30%. Finally, the saturation of the absorption which is observed in the modeling for such codoped Tm3+-Dy3+:CaF2 waveguide at Dy3+ concentration below 1% is discussed and its origin is explained.