New algorithm for the Guided Lock technique for a high-Finesse optical cavity

A known criticality of optical cavities such as Fabry-Pérot resonant cavities is the presence of non-linear effects in the build-up of the laser fields inside the cavity itself, which can spoil the characteristics of the error signal used to control the cavity length, usually obtained with the Pound-Drever-Hall phase modulation-demodulation technique. The non-linear effects are primarily caused by high cavity speeds prior to acquiring longitudinal control of the cavity (or "lock"), and they are due to the frequency fluctuations of the laser and to the residual seismic motion affecting the system; such effects are amplified with the increasing of the Finesse of the cavity. In order to overcome this limitation, the cavity speed is effectively slowed down before engaging the lock using a non-linear technique, known as "Guided Lock"; here an optimized version of the algorithm will be presented, which relies on a better estimation of the cavity speed based only on optical signals. The application of this technique to the high-Finesse Fabry-Pérot arm cavities of the Advanced Virgo gravitational wave detector will be described. The novel algorithm was applied for the lock acquisition of the Advanced Virgo detector during the O2 Observing Run, in August 2017; the improved algorithm, by dynamically measuring the cavity speed, allowed to implement a predictive capability in slowing down the mirrors, thus improving the efficiency of the lock acquisition procedure for the arm cavities.