Mode coupling can occur if the gain of a laser is modulated by a frequency near the axial-mode spacing. This modulation is produced by a second laser acting partially upon the same active medium. Mode coupling and intensity pulsations close to the frequency
![c/2 [(1/L_{1}) - (1/L_{2})]](/images/tex/1358.gif)
were observed, in a good agreement with the theoretically calculated waveshapes.