Sheila, Paul

Yesterday while in the PSL we aligned the Thorlabs PDA55 known as IO_AB_PD_3. This PD sees the beam picked off from the main beam after the EOM by the wedge IO_MB_WG1. The PD was already on the table and connected to the DAQ, but the final steering mirror steering onto it was not present. The post was in place (labelled as IO_MB_WG1, though I think this should be labeled IO_AB_W1) but there was no mirror mount on top. I borrowed a HR turning mirror already mounted that I found in the IO shelf in the PSL ante-room and mounted this on the post and aligned the beam onto the PD. 

The design actually calls for this PD to be placed behind a window (IO_AB_W1), but I did not find this window so I left it in reflection from an HR mirrror as a temporary setup.

The power incident on the PD was 1.17mW, and it gave a DC reading of 5.56V. The PDA55 switchable gain was at its lowest setting (1.5E4).

This roughly makes sense: using the previously estimated PDA55 responsivity of 0.325A/W we get an expected voltage of 0.325A/W * 1.5E4 V/A * 1.17E-3 W = 5.8V. This looks like the responsivity is a little lower than the other PDA55 (IO_AB_PD_1), so I adjusted the responsivity in the LSC DCPD medm screen until the voltage and power matched the measurement. The channel H1:PSL-EOM_A_DC_POWER is now calibrated in mW incident on this PD. I don't know the pick off fraction of that wedge exactly, so if anyone wants to calibrate it in terms of power leaving the EOM it might be best to measure the main beam power on the PSL table before the IO power control stage.