Tunability of a seeded free-electron laser through frequency pulling

Frequency pulling is a well-known phenomenon leading to an output frequency shift in a conventional laser, when the cavity and the maximum gain frequencies are detuned. We demonstrate that a similar mechanism is at play in a seeded free-electron laser (FEL), when the seed frequency is out of resonance. Frequency pulling thus may give the possibility of fine-tuning the FEL frequency. On the basis of numerical simulations, we provide a general formula for the FEL output frequency. Such a formula generalizes the one normally used when treating the frequency pulling in conventional lasers.