  We analyse the influence of heat generation by non radiative transitions in
high power 1.55$\mu$m double cladding Erbium-Ytterbium fiber laser. At strong
pumping rates, 1$\mu$m lasing can start due to parasitic reflections. We
present a model including heat generation and its effect on the Stark level
population using the MacCumber relation. Heat generation plays then a
significant role and improves the 1.5$\mu$m laser efficiency by increasing the
1$\mu$m threshold.
