Transient response following frequency or amplitude switching in a cesium beam tube

The transient responses of an optically pumped cesium beam tube to square wave frequency and amplitude modulation is considered. The frequency transient is computed assuming a phase difference phi of either 0 or pi between the two oscillatory fields. We present theoretical and experimental data showing that, contrary to the frequency transient, the amplitude transient depends on the direction of switching. The knowledge of this property is useful for the design of the servo-loop controlling the amplitude of the microwave signal applied to the atomic resonator. A justification of this asymmetrical behavior is given. Experimental results confirm the theoretical predictions in the case phi=pi.