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Removal of the undesired time-energy correlations in the electron beam is of paramount importance for efficient lasing of a high-gain free-electron laser. Recently, it has been theoretically and experimentally demonstrated that the longitudinal wakefield excited by the electrons themselves in a corrugated structure allows for precise control of the electron beam phase space. In this Letter, we report the first utilization of a corrugated structure as a beam linearizer in the operation of a seeded free-electron laser driven by a 140 MeV linear accelerator, where a gain of ~10000 over spontaneous emission was achieved at the second harmonic of the 1047 nm seed laser, and a free-electron laser bandwidth narrowing by 50% was observed, in good agreement with the theoretical expectations. [ABSTRACT FROM AUTHOR]

We study theoretically and experimentally the subharmonic entrainment (SHE) breather soliton in mode-locked lasers for the first time, in which the ratio of the breather period to the round-trip time is an integer. We build a non-Hermitian degeneracy map of breather soliton, and illustrate that SHE arises between the two exceptional points (EPs). We obtain SHE at the ratio of 20, observe the evolution of breather soliton when tuning the gain and/or cavity loss, and prove that this phenomenon can improve the stability of breather soliton. Our research brings insight into the EP physics of ultrafast lasers and makes the mode-locked laser a powerful test bed for non-Hermitian degeneracy, which may open a new course in ultrafast laser research. [ABSTRACT FROM AUTHOR]