Ferrari, E. Allaria, E. Danailov, M.B. De Ninno, G. Di Mitri, S. Gauthier, D. Giannessi, L. Penco, G. Roussel, E. Veronese, M. TUB04 2015 2015-12 Laser-heater systems are essential tools to control and optimize high-gain free electron lasers (FELs), working in the x-ray wavelength range. Indeed, these systems induce a controllable heating of the energy spread of the electron bunch. The heating allows in turn to suppress longitudinal microbunching instabilities limiting the FEL performance. In this communication, we show that a long-wavelength energy modulation of the electron beam induced by the laser heater can be preserved until the beam entrance in the undulators, affecting the FEL emission process. This non-uniform longitudinal heating can be exploited to investigate the electron- beam microbunching in the linac, as well as to control the FEL spectral properties. Here, we present experimental, analytical and numerical studies carried out at FERMI.