I revisited a measurement from May 2016 (see LHO aLOG 27155) to determine the amount of watts impinging on the ETM per volts requested of the optical follower servo (OFS) this will allow for a calibration of the excitation point.

To make this measurement, I injected a 2501.3 Hz calibration line on x-arm Pcal, then made a transfer function in DTT H1:CAL-PCALX_TX_PD_VOLTS_OUT / H1:CAL-PCALX_OFS_PD_OUT_DQ = 0.689888 V/V

Using the Pcal force coefficient screen, there is a handy-dandy switch that fixes the input to be 1.0 counts in order to know the calculated force on the ETM in units of N/ct. For the x-arm Pcal TXPD, this value is 7.901e-13 N/ct

Next, we want to convert this to N/V, so we divide by the ADC gain (6.103516e-4 V/ct) which results 1.2945e-9 N/V

Finally, for our purposes, it is useful to recover watts-per-OFS-volt, so we multiply by the watts-per-newton factor c/(2*cos(theta)) = 1.2945e-9 (N/V) * 299792458/2/cos(8.5 deg) (W/N) = 0.1962 W/V

Putting the pieces together, 0.689888 (V/V) * 0.1962 (W/V) = 0.13535 watts/OFS volts

Compared to the May 2016 measurement (=0.0923 W/V), this value is approximately 47% larger.