Measurement of the Effect of Thermal Noise on the Amplitude of Oscillation of a Maser Oscillator

Experimental verification of the effect of thermal noise on the oscillation amplitude of a maser is described. Two different masers are considered, the H maser with an homogeneous atomic line and the Rb maser with an inhomogeneous line. The effect is characterized by the spectral density of the relative amplitude fluctuations and this parameter is measured with a conventional superheterodyne AM receiver for various maser saturation factors. Experimental results show good agreement with theoretical predicted values. At high saturation factors, a peak appears with a maximum at a frequency close to the Rabi frequency. Observed values at high Fourier frequencies allow a measurement of the atomic power by the knowledge of the absolute temperature of the cavity and the prediction of the oscillator short-term frequency stability. Furthermore, the effect on amplitude and on frequency noise of an external feedback loop used to increase the cavity quality factor is discussed.