When two or more modes oscillate in a laser, beat frequencies develop because of nonlinearities in the medium. Specifically, the population difference Paa — Pbb contains pulsations at multiples of the intermode frequency. We encountered similar pulsations in Van der Pol’s double-tank circuit (Sec. 4-1). The pulsations act very much like a modulator generating side bands on im­ pressed signals. The process is analogous to the Raman effect; in fact, the physics is sufficiently similar that much of the algebra encountered here carries over with little modification to the Raman effect.