intensity in these regions. This scenario is represented in Fig. \[Figure\_double\_peak\_principle\]b. For a sufficiently strong seed intensity, the FEL power at the central position almost vanishes and two successive pulses appear, as previously described in [@Labat]. This phenomenon becomes particularly interesting if the seed is chirped i.e., if its frequency depends on the longitudinal position. As shown in Fig. \[Figure\_double\_peak\_principle\]c, in this situation the split temporal pulses have also different frequencies. In other words, two independent pulses with two different frequencies are created [@PRLDP].

Two-colour pulse generation by a seeded FEL was observed for the first time [@talkAllaria] early in 2011 during the commissioning of FERMI@Elettra. A typical measurement of the FEL spectrum as a function of the seed power is shown in Fig. \[Figure\_fork\_seed\_experiment\]. By increasing the seed energy, the portion of electrons getting over-bunched increases, leading to a larger separation of the spectral peaks. As shown in the figure insets, the two-colour spectra are very stable on a shot-to-shot basis with very good repeatability. We obtained very similar pulse-splitting results by increasing the strength of the dispersive section while keeping the seed power constant (this is illustrated later in Fig. \[Figure\_R56\_experiment\]).

The spectro-temporal properties of the FEL emission 