Poster Summary: Lock Acquisition & Commissioning of the Advanced Virgo Detector
(by Diego Bersanetti) Advanced Virgo is an interferometric gravitational wave detector, whose optical layout is depicted in Figure 1 (see bel0w); its detection technique is to measure the phase difference between laser light travelling through the two perpendicular arms of a Michelson interferometer; its basic sensitivity is enhanced by subsituting the arms with resonant Fabry-Perot cavities, which keep the light trapped in the interferometer, increasing the effective arms’ length which translates into a higher sensitivity; another improvement is provided by the Power Recycling mirror, placed before the beam splitter, which allows to reuse the light that would be lost at the symmetric port, thus increasing the circulating power and, therefore, the sensitivity.
In order to work properly, all optical cavities need to be locked, which means that their length must be very accurately controlled. This is done using the Pound-Drever-Hall technique, which uses electro-optical modulation to generate a radio-frequency error signal which can be used in a linear feedback control loop, as shown in Figure 2 for an arm cavity. A very important part of the Commissioning of the detector is the development of the Lock Acquisition sequence, which is a sequential series of steps where all the optical cavities and their several degrees of freedom are brought to their optically resonant working point.
For Advanced Virgo, this sequence starts with the lock of the arm cavities, using the Guided Lock algorithm to estimate the relative velocity of a cavity (Figure 3, bottom-left panel) and then, if necessary, slow it down with calibrated impulses (Figure 3, bottom-right panel) in order to use a PDH error signal (Figure 3, top-left panel) to lock the cavity and bring the power (Figure 3, top-right panel) to its resonance condition. After all other longitudinal degrees of freedom are locked (common and differential arm motion, Michelson’s length and Power-Reycling length), the interferometer’s working point is reached by balancing the central Michelson interferometer, which means to bring the length difference between the arms of the short Michelson towards zero (“MICH offset reduction”, as shown in Figure 4); this allows to reach the “Dark Fringe” condition, where the light going towards the detection port of the interferometer is virtually zero, allowing a more sensitive detection of an incoming gravitational wave signal.