On the Physical Dynamics of Laser Mode Sweeping in a Fabry-Perot Resonator of Varying Optical Length

We present in this paper a basic analysis of the physical mechanism underlying the internal frequency modulation of a laser oscillator which is caused by a variation of the optical path length of its Fabry-Perot resonator. I t is shown how adiabatic variations of the optical resonator length are continuously converted into frequency variations of the electromagnetic field energy trapped between the resonator mirrors by a rapid succession of small Doppler shifts, thus comply­ ing w ith the general principle of adiabatic invariance in oscillatory systems.