A photon calibrator has been installed in the NE building on February 20th, 2018.

A DAQ-box and a laser have been put on the NE rack.

Figure 7  shows the injection bench where the laser beam is sent to NE mirror through a viewport ; one photodiode (PD1) and a PSD (PSD1) monitor the laser power and the position of the laser beam.

The reflection bench has been set up too, containing a photodiode (PD2) and a PSD (PSD2) to monitor the laser power and the position of the laser beam once reflected on the NE mirror.

We used the opportunity to also install temperature probes on NE and WE PCal benches as reported in logbook entry 40643 : https://logbook.virgo-gw.eu/virgo/?r=40643

Measurements were performed the 20th and the 21st, February :

- The laser dac command (V) vs the laser power on the injection bench (W) (Figure 1) ; the behaviour is linear as expected with a slight offset due to the threshold of the laser command (< 0.1 V).

- The voltage of PD1 (V) vs its bias voltage (V) at max power 3W and at 2W (Figure 2 and 3) ; showing that the response is flat above Vbias ~ 2 V, OK the photodiodes have Vbias = 4.5V.

- We checked the level of noise of PD1 with a laser power 2.1W and the level is not stable in time (same behaviour with local control and remote control of the laser but the 50Hz line appears only in remote control), Figure 4 shows 4 plots of the noise level spaced with ~1 minute in local control ; we noticed that there are sometimes voltage hops in PD1 (see Figure 4, PD1_raw blue 57min06 sec) which increase the noise level.

               -> Those voltage hops do not appear in the laser dac command signal.

- When the laser pump is off, the noise level is around 1e-7 V/sqrt(Hz) (Figure 5).

- The voltage of PD1 (V) vs the laser entrance power (W) (Figure 6) ; the behaviour is linear as expected with little variations due to the time of stabilization of the powermeter with respect to a power laser beam ramp in entrance. The ramp lasts ~4 minutes (up + down) and goes from 0.6W to 3W.

Thus we need to investigate the variations of noise level and the voltage hops in the photodiode, working on a control loop of the laser power.

The photodiodes on the reflection bench have been aligned but NE mirror was unlikely in a good position since the NE arm couldn't be locked. We should check again the alignment once the NE arm will be locked.

We will also check if there are variations of power linked to variations of temperature.