Free-electron lasers (FELs), requiring expensive beam time at large-scale facilities, seem to be fundamentally different from relatively low-cost tabletop lasers. Instead of transitions between energy levels in atomic systems mediating the dynamics of a conventional laser, gain for the optical field in a FEL is controlled by the exchange of energy between an electromagnetic field and a relativistic electron beam circulating in a permanent periodic magnetic field. Often, the relatively inexpensive bench lasers are mode-locked, meaning that many optical modes are forced to constructively interfere to produce a train of short pulses. Further, the dynamics of actively mode-locked lasers typically happens on a very fast time scale, making investigations of pulse dynamics rather difficult, whereas time scales for FEL dynamics can be slower by orders of magnitude, making it relatively easy to record evolution of the spectrum with time.
In a recent publication in Physical Review A, Christelle Bruni from the University of Paris-Sud, France, and colleagues show that FEL oscillators, under the right conditions, can behave very much like mode-locked lasers. Bruni et al. perform a comparative experimental study of the dynamics of the two types of lasers and also establish the similarity in the structure of equations that govern the dynamics of the FEL and mode-locked lasers. This work can potentially open the door to a synergetic study between the low-cost yet high-speed dynamics of a tabletop mode-locked laser and the much larger cost, slower dynamics study of an FEL. – Frank Narducci