Mode frequency pulling in He-Ne lasers

In this experiment, the frequency separation between two longitudinal He-Ne -laser modes is. studied. A description is,given of the physics that causes a change in the frequency separation when the modes are observed at variou s positions in the gain profile. The frequency separation is measured with a fast photodetector in combination with a counter as a function of the mod e intensity ratio. The intensity ratio is monitored with a scanning Fabry-P erot interferometer. For a scan of the two modes through the gain profile, a frequency separation change of similar to 30 kHz was observed. (C) 1999 A merican Association of Physics Teachers.