The Instrumented Baffle at the Input Mode Cleaner of Advanced Virgo Plus: Four years of Successful Operation as a Monitor of Stray Light

Abstract

The suspended end mirror of the input mode cleaner cavity in the Advanced Virgo Plus interferometer was equipped with an instrumented baffle in spring 2021, serving as a demonstrator of the technology in preparation for the installation of large instrumented baffles in the main arms of the interferometer. This baffle includes tens of sensors positioned near the mirror to enable monitoring of stray light within the cavity. In this contribution, we assess the performance and stability of the instrument after four years of operation. After introducing the main characteristics of the baffle, we study the distribution of stray light and show that the instrumented baffle can be used to monitor laser stability and alignment within the cavity. Finally, we assess the noise level during the final stages of the O4b commissioning to monitor the impact of the baffle, and conclude that the baffle does not introduce any additional disturbance to the normal operation of the interferometer.

Figures (13)

• Figure 1: Simplified, not to scale, optical layout of AdV+. Each arm is formed by an input mirror (North input (NI) or West input (WI)) and an end mirror (North end (NE) or West end (WE)). The IMC cavity is located between the laser source and the Power Recycling Mirror (PRM), prior to the beam splitter (BS). The instrumented baffle is shown in yellow.
• Figure 2: Fraction of time during which AdV+ was in a locked state (as defined in the text) over the studied period of IMC instrumented baffle operation, from April 2021 to November 2025. The plot differentiates between O4 AdV+ commissioning time and AdV+ participation in the O4 observing run (including O4b and O4c).
• Figure 3: Two-dimensional diagram of the instrumented baffle design with labelled sensors, where the colours correspond to the distinct rings and the numbers indicate the sensor indices.
• Figure 4: Two-dimensional diagram of the baffle indicating the average power measured by each sensor over October 2025.
• Figure 5: Power measured by the sensors in the four rings of the baffle and averaged over one month of stable operation in October 2025. The error bars show the $3 \rm \sigma$ confidence level over one month.