The 2nd-Harmonic Signal in the Rb Atomic Clock for “Status-of-Health” Monitoring

In the Rb atomic frequency standard (RAFS), the 2^ndharmonic signal, S2, is routinely taken as a “Status-of-Health” indicator for the atomic signal. In part, this role for S2derives from the Quasi-Static Approximation (QSA), where harmonic signals are taken as proportional to derivatives of the static lineshape. However, clock operating conditions violate the assumptions of the QSA, and so justification of the relationship between S2 and $\text{dS}_{1}/\mathrm{d}\Delta\vert_{\Delta=0}$ (i.e., the correction signal's slope on resonance) is questionable. To better understand S2 and its potential as a status-of-health indicator, we are continuing a series of experiments begun in our laboratory. Here, we discuss the role of temperature gradients on S2, and the extent to which temperature gradients might or might not influence S2's ability to monitor clock health.