Keita, Daniel, Kiwamu
In response to the too-low-power at HAM6 (see alog 13472), we revisited the IM4 trans power in order to make sure that there is no large loss in the input optics chain.
As a result, we confirmed that the incident power on IM4 is about 1.4 W. This corresponds to a throughput of PSL-IMC-Farday chain of 88%. This is consistent with the number that Giacomo and Paul gave us (see for exmaple, alog 9954). So we conclude that the IO chain is healthy.
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We measured the IM4 trans beam at the east side of HAM2. The power was measured to be 220 uW using a Ophir Vega without the filter on it.
According to the IO documents (E1300756 and E1300160), the relevant optics have the following properties:
- IM4 (SM2): T = 0.23%
- RH4: T = 7.24 %
- LH1: R = 92.23 %
Using these information together with the measured IM4 trans power, we estimated the power incident on IM4,
Power incident on IM4 = 220 uW / T_IM4 / T_RH4 / R_LH1 = 1.432 W
At the time of the measurement, the PSL power was at 1.620 W according to PSL_PERISCOPE_A_DC.
Therefore the throughput of the PSL-IMC-Faraday chain is
throughput = 1.432 / 1.620 = 88.4 %
In HAM6, we measured 3.6mW with the Ophir Vega power meter, with the filter off. (With the filter on, we measured 3.8mW.) The configuration during this measurement was ITMX aligned, ITMY misaligned, 1.620 watts into the IMC.
This is 81% of Daniel's prediction of 4.45mW in HAM6 -- note, the difference between yesterday and today is in the power meter that was used. Kiwamu compared the Vega meter with the Ophir Nova we used yesterday, and found that the Nova, with the filter on, gave bogus measurements. So, yesterday's measurement is not valid.