Gas discharge modulation noise in He-Ne lasers

This paper reports measurements on laser light noise and discharge current noise in a 6328 Å He-Ne laser. A cross-correlation experiment shows a strong correlation between the two noise phenomena, indicating that the laser light noise is caused by the modulation of the light beam by the discharge current noise. The discharge current noise has a spectrum with a sharp resonance at 40 kc/s that does not show up in the laser light noise. The noise is represented by an equivalent circuit. By measuring the two noise spectra and the absolute value and the phase of the cross-correlation coefficient as functions of frequency, it is possible to evaluate some of the components of the equivalent circuit. The values thus obtained turn out to be quite reasonable, which supports the assumptions on which the equivalent circuit is based.