Introduction to the Physics of Free Electron Laser and Comparison with Conventional Laser Sources

The Free Electron Laser (FEL) can be considered a laser, even though the underlying emission process does not occur in an atomic or a molecular system, with population inversion, but in a relativistic electron beam, passing through the magnetic field of an undulator. Several parameters (such as gain, saturation intensity, etc.) are common to both devices and this allows a unified description. The origin of Laser traces back to the beginning of the last century, when Planck derived the spectral distribution of the radiation from a "blackbody" source, a device absorbing all the incident radiation and emitting with a spectrum which only depends on its temperature. A perfectly reflective cavity, with a little hole, can be considered a blackbody source; the radiated energy results from the standing wave or resonant modes of this cavity. The Planck distribution law, yielding the equilibrium between a radiation at a given frequency  and matter at a given temperature T, is the following: