The principles of the double resonance method applied to gas cell frequency standards

In recent years, gas cell frequency standards, using optical pumping and optical detection of the O-O hyperfine transition in alkali metal vapors, have been developed with good performances regarding stability, reliability and ease of operation. In this paper the physical principles of the double resonance method used in this device are described. The factors affecting the accuracy of the gas cell frequency standard are discussed, together with the solutions adopted for optimum results. The possibility of developing an optically pumped rubidium maser oscillator of very high spectrum purity at microwave frequencies is also briefly discussed.