[Sheila, Paul]

After the recent increase in power it was discovered that IO_AB_PD2 (the Thorlabs PDA55 PD in transmission of the PSL periscope lower mirror) was saturating. We went to the PSL to adjust the transimpedance gain to bring the signal in range again, and also to check the power incident on the PD and the periscope to get an updated calibration factor for power into the IMC.

We measured 276uW on IO_AB_PD2 with VEGA power meter (small head for low power, with the filter off). We measured 8.82W at the lower periscope mirror with the VEGA power meter (large head for high power, no filter). We waited maybe 5mins for the reading to stabilize. This gives us a pick off fraction form the main beam to IO_AB_PD2 of 3.13E-5.

With the PDA55 gain set to its lowest setting (quoted as 15kV/A in the spec sheet), we measured 1.38 V on the output. We can use this to calculate the responsivity of the detector as R [A/W] = 1.38 [V] / (1.5E4 [V/A] x 2.78E-4 [W]) = 0.33 [A/W]. Comparing with the frustratingly gridless plot in the datasheet here: http://www.thorlabs.us/thorcat/2000/PDA55-Manual.pdf this doesn't look far wide of the mark.

I've set the values in the H1:PSL-PERISCOPE_A_DC calibration path accordingly, after some minor adjustments were made to account for the 0.01V offset we observed on the PD.

The H1:PSL-PERISCOPE_A_DC_POWERMON channel shown on the IMC overview screen is now reading ~8800mW, as it should do based on the power meter measurement made before the periscope mirror.

The H1:IMC-PWR_IN_OUTPUT channel which is also shown on the IMC overview screen is, I believe, not currently acquired.