Spectral oscillations in a frequency-modulation laser operation

We show the existence of coherent oscillations in the optical frequency spectrum of a laser with internal frequency modulation. Three oscillations arise in presence of a slight detuning between the modulation frequency and the cavity axial mode separation and are analogous to collective oscillation modes of a chain of weakly coupled pendula with resonance frequencies slightly varying along the chain. Although spectral oscillations are damped due to the finite gain bandwidth of the laser cavity, they can be observed in the transient dynamics during switch-on of the modulator, or can be resonantly excited by an external forcing.