A theoretical investigation of an innovative cascade laser source is performed. The main goal of the work is the design of a continuous-wave (CW) photonic crystal fiber (PCF) laser, based on an erbium-doped chalcogenide glass. Due to the comparable lifetimes of the 4I13/2, 4I11/2 and 4I9/2 erbium energy levels, the simultaneous emissions at the wavelengths close to 2.7 μm and 4.5 μm are obtained with a pump wavelength close to 806 nm (direct pumping into the level 4I9/2). This scheme could be useful to develop high efficiency, high beam-quality and compact Near-IR and Mid-IR oscillators with single-mode output for applications not only in surgery but also in spectroscopy, remote sensing, infrared countermeasures. The feasibility investigation is performed by taking into account the optical and spectroscopic parameters measured on a previously fabricated chalcogenide glass sample. A home made numerical code that solves the multilevel rate equations and the power propagation equations has been ad hoc developed. The obtained results constitute a realistic feasibility investigation of the Mid-IR laser performance by varying the length and the transversal cross section of the fiber, the mirrors reflectivity, the erbium concentration.