'L. Fröhlich'
- 'D. Gauthier'
- 'L. Giannessi'
- 'N. Mahne'
- 'G. Penco'
- 'L. Raimondi'
- 'S. Spampinati'
- 'C. Spezzani'
- 'C. Svetina'
- 'M . Trovò'
- 'M . Zangrando'
title: 'Two-colour generation in a chirped seeded Free-Electron Laser'
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Introduction {#intro .unnumbered}
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Chirped pulse modulation is a widely used technique, with relevant applications in several fields of research, ranging from digital communication [@DC] to radar detection [@RD], and from optics [@OP] to laser physics. In a broadband laser, frequency chirping is employed to stretch a short pulse prior to amplification. This stretching and the associated lower intensity mitigates the problem of phase distortion in the amplification medium. After pulse amplification, the pulse is recompressed to recover a short duration and, hence, high power [@CPA]. A similar method has been proposed [@YuPRE; @NIMPoletto; @NIMFeng] to enhance the power generated by a single-pass seeded free-electron laser (FEL) working in the so-called coherent harmonic generation (CHG) regime [@YuPRA]. After amplification and removal of the chirp, one in principle can obtain a high power, nearly Fourier-transform-limited pulse.

In this paper, we characterize the performance of a seeded FEL where frequency chirping on the seed laser is instead exploited 