Stabilized, single-frequency output from a long laser cavity

The problem of obtaining single-frequency output from a long laser is considered, and two methods are investigated experimentally. The first method consists of using an external filter to select one of a number of oscillating modes. The second method consists of suppressing internally the unwanted resonances so that the laser oscillation can only take place at a single frequency. It is shown that with the second method one can in many cases obtain greater power, and experiments are reported in which single-frequency output power of 15 mW was obtained from a 6328 Å He-Ne laser, A simple feedback circuit is described for stabilizing the frequency of the laser oscillation.