A robust phase-locking strategy for optimal applications of various optical feedback cavity-enhanced spectroscopies in harsh environments

In various optical feedback cavity-enhanced spectroscopies (OF-CESs) based on absorption or scattering, conventional phase-locking methods are constrained by their ability to handle only minor phase deviations. This limitation is due to the source of an error signal for phase adjustment. This paper introduces a robust approach for phase-locking, which combines the shape and intensity of cavity transmission profiles to identify phase deviations. The advantage of this combination is that it can always generate a suitable error signal, irrespective of the phase's position in the entire 2 pi period. The outstanding performance of the corresponding servo loop under severe airflow shocks demonstrates that our approach significantly increases the feasibility of applying various OF-CES setups for real-time, in situ gas detection in harsh environments.