Self-heterodyne mixing method of two inter-mode beat frequencies for frequency stabilization of a three-mode He-Ne laser

We present a robust self-heterodyne mixing method of two inter-mode beat frequencies suitable for frequency stabilization of a three longitudinal-mode (3-mode) He-Ne laser at 633 nm. A high-contrast frequency discrimination signal with a broad locking range of 244 MHz is obtained by using a self-heterodyned double balanced mixer operating at the inter-mode beat frequency of 607 MHz. We show that the central-mode frequency of the 3-mode He-Ne laser could be stabilized to the center of the gain profile with a frequency fluctuation less than ± 5 MHz for more than 12 h.