Experimental demonstration of frequency pulling in single-pass   free-electron lasers
  Frequency pulling is a well-known phenomenon in standard laser physics, leading to a shift of the laser frequency when the cavity and maximum gain frequencies are detuned. In this letter we present the first experimental demonstration of frequency pulling in single-pass free-electron lasers. Measurements are performed using the single-pass free-electron laser installed on the Elettra storage ring. 
E. Allaria, C. Spezzani, G. De Ninno
physics.optics