Back-reflection effects in a frequency-stabilized two-mode He-Ne laser

We investigated the behavior of a frequency-stabilized 1 mW two-mode helium-neon laser in the presence of optical feedback. In this paper, we presented how the amplitude and frequency shift of the returning beam influence operation of the laser. We showed the conditions under which the He-Ne laser work is unstable and stable in frequency and where it is insensitive to the returning beam. We also showed how much the frequency of the returning beam has to be shifted in order to reduce the sensitivity of the laser on the back-reflection.