Experimental observation of spectral oscillations in frequency modulation laser operation

The experimental observation of transient coherent oscillations in the optical frequency spectrum of an Er-Yb solid-state laser with internal frequency modulation (FM) is reported. These oscillations, which arise in the presence of a slight detuning between the modulation frequency and the cavity axial mode separation, are transiently observed when the modulator is switched on and appear as a damped breathing of laser spectral width superimposed to the relaxation oscillation dynamics of the laser intensity. The experimental results are in good agreement with the theoretical predictions which take into account both the fast dynamics of spectral oscillations and the slow dynamics of relaxation oscillations for the codoped Er-Yb laser system.