Instrumented baffle for the Advanced Virgo input mode cleaner end mirror

A novel instrumented baffle surrounding the suspended end mirror in the input mode cleaner cavity of the Virgo interferometer was installed in Spring 2021. Since then, the device has been regularly operated in the experiment, and the data collected show good stability. The baffle will operate in the upcoming O4 observation run, serving as a demonstrator of the technology designed to instrument the baffles in front of the main mirrors in time for O5. In this paper, we present a detailed description of the baffle design, including mechanics, front-end electronics, and data acquisition as well as optical and vacuum compatibility tests, calibration and installation procedures, and performance results.

Figure 1

Simplified, not to scale, optical layout of the Advanced Virgo interferometer. Each interferometer 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, located in front of the interferometer arms, is shown highlighted.

Figure 2

Design of the instrumented baffle for the end mirror of the IMC. The laser light reaches the photosensors through holes in the stainless steel plates. On the right side, the fully assembled baffle is shown, while on the left side, the top cover has been removed to reveal the PCB board equipped with photosensors.

Figure 3

Rear side of the baffle. The thickness of the green, orange, and blue areas are 2, 4, and 6 mm, respectively.

Figure 4

Report of the metallic coating provided by Optimask [6]. The arrow shows the reflectance for 1064 nm incident light at an angle of 8 deg.

Figure 5

Half-baffle being bolted to the PTFE support with handles.

Figure 6

Edge of a hole seen with a $5\times$ magnification lens.

Figure 7

Sketch of the location of the photosensors in the baffle in the XY plane.

Figure 8

Optical setup at EGO to measure the sensor’s TIS. Left: in the vertical position, a small sample with two holes and photodiodes behind them, with the same geometry as those of the instrumented baffle, attached to a support. Right: sample placed in front of the integrating sphere.

Figure 9

Cross section of the PCB showing the type, material, and thickness of the different layers.

Figure 10

Top: top layer of the PCB containing the photodiodes. Bottom: bottom layer of the PCB, which contains all components except the photodiodes.

Figure 11

Antennae for the wireless communication connected perpendicularly to the surface of the PCB.

Figure 12

Left: photosensor being removed from the wafer. Right: Photosensor placed on the PCB with the help of a template.

Figure 13

Scheme of the DAQ system of the instrumented baffle.

Figure 14

GUI for the operation of the IMC end mirror instrumented baffle. The central green button turns the baffle on and off with a click. The purple circles show the temperature of the 16 ADC, and the readings from all 76 photodiodes are displayed in a color proportional to the ADC counts.

Figure 15

Optical setup used in the laboratory to calibrate the photodiodes. A laser is attached to a collimator via an optical fiber. This collimator directs the laser beam toward the photodiodes on the PCB. The triangular mirror visible in the picture was used to help in the alignment of the laser.

Figure 16

The minimum, the maximum, and the average value of the ADC counts read in a period of $500\mathrm{\mu }\mathrm{s}$ for a given sensor are shown as a function of the nominal input power. The measurements are repeated more than one hundred times for each laser power value.

Figure 17

Top: transportation box containing the instrumented baffle. Bottom: the instrumented baffle before its installation inside the transport box.

Figure 18

Top: old noninstrumented baffle before being removed from the payload. Bottom: supporting structure ready for the installation of the instrumented baffle. The installation took place in the IMC end mirror cavity.

Figure 19

Instrumented baffle after its installation in the payload of the IMC end mirror.

Figure 20

Temperature evolution of the 16 sensors for a period of five days. The sensor with the highest readings corresponds to that closest to the controller, dissipating most of the heat.

Figure 21

Temperature evolution of three representative temperature sensors for a period of four months. The sensor with the highest readings corresponds to that closest to the controller, dissipating most of the heat.

Figure 22

Pedestal subtracted sensor output (normalized to the injection laser power) averaged for each of the four rings of sensors for a period of nine months. The sensors are grouped into rings according to their distance to the center of the mirror.

Figure 23

Top: baffle temperatures. Middle: average sensor signals for each of the four rings. Bottom: input laser power over a four-day period.