Ultimate Stability of Methane-Stabilized Lasers

Random frequency fluctuations of the 3.39 µm He-Ne lasers stabilized on an absorption line of methane are theoretically investigated. The minimum detectable frequency deviations are calculated for the method of linear absorption in an external cell and for the method of saturated absorption in an internal cell, and compared with each other. The method of saturated absorption in an internal cell of 1 Pa (7.5 milli-torr) pressure requires a few milli-watts of power flow in a beam of 1 mm radius. The minimum detectable frequency fluctuation for saturated absorption is found to be almost equal to that for linear absorption with a power of 1 mW through the cell. With an ideal detector of 100 percent quantum efficiency the calculated stability is ±7 × 10 -15 t -1/2 in either method, where t is the averaging time in seconds. By employing a larger power through an external cell, still higher stabilities will be achieved, if the gas and the detector are kept at low temperatures.