Frequency stabilization of 0.633-μm line with the aid of 3.39-μm line locked to CH4

0.633- and 3.39-μm lines were brought into simultaneous oscillation with a He-Ne laser. The 3.39-μm line was locked to the saturated absorption line of CH4 to control the cavity length, as a result of which the 0.633-μm line was also frequency stabilized. The frequency stability of the 0.633-μm line was better than 4.2×10−9/h, which is the stability of a reference laser HP 5500C.