“…There are two main causes of such signals; one is interference between various optical surfaces, usually termed etalons, while the other is residual amplitude modulation (RAM), which in particular can appear from EOMs [25,26]. Numerous efforts have been made to suppress these types of effects, e.g., antireflection coating, alignment of optical components, active feedback control of EOM [25], usage of proton-exchanged EOMs [20] and wedged crystal EOMs [27], application of etalon immune distance (EID) [28], and implementation of a differential NICE-OHMS strategy [24]. However, despite all this, background signals are notoriously difficult to eliminate completely, and the remaining are often the main cause for the restricted performance of the NICE-OHMS system.…”