Reports until 17:26, Monday 10 February 2014
H1 ISC (ISC)
lisa.barsotti@LIGO.ORG - posted 17:26, Monday 10 February 2014 (9954)
Summary about missing power in REFL beam (ignore previous entry)
Kiwamu, Lisa


There has been some confusion about the power we were measuring in the REFL path, compared to the expected one. Most of the confusion was due to a BS in the in air path which was forgotten to be taken into account, and the assumption of getting 10 W back from PRM, while it is significantly less than that. The previous entry was right in the final message, but a couple of things were wrong...so I post another summary here.
  • Pin = 8.8 W entering the IMC, measured on the PSL table;
  • I checked with Paul and Giacomo that a good number for the transmission PSL - IMC - PRM back to the Faraday is something like 83%, so I will use T = 80% as a round number for the transmission from the PSL back to HAM1 the PRM is aligned;
  • given the HAM1 path, we expect 1.25% of the REFL power which arrives on HAM1 to reach the LSC RF diode P_exp = 8.8 * 0.8 * 1.25e-2 = 88 mW;
  • in counts, this means: 88e-3 [W] x 0.76 [A/W] x 200 [Ohm] x 1638 [counts/V] = 21900 counts;
  • Measurement: LSC IN VAC REFL A (LSC RF PD) (1.25% of REFL power) = 15900 counts (maximum found by moving RM2) This corresponds to 64 mW, by using signal chain calibration;
  • Measurement: LSC REFL AIR A (LSC RF PD) (0.625% of REFL power) = 7955 counts 32.5 mW measured directly in front of the diode on table (30.4 mW using signal chain calibration)
  • Measurement: LSC REFL AIR B (BBPD) (0.625% of REFL power) = 21770 counts 34 mW measured directly in front of the diode on table
Conclusions
  • From the in air LSC REFL A diode we see that the signal chain calibration agrees with the direct measurement of the power;
  • The total amount of light in the in AIR REFL path is 66.5 mW, which is the same power we see on the in vac diode, as expected;
  • The absolute value of the power we see is 25% - 30% less than what we expect, but we don't know exactly how much light makes it through the Faraday, and we don't have a direct measurement of the 1.25% transmission through the HAM1 optics, so this doesn't seem a big deal;
  • Based on these numbers, we don't think we have a problem.
P.S: Still need to calibrate these numbers for the WFSs, measurements were taken all at the same time. WFS in vac REFL A (0.625% of REFL power) = 34200 counts WFS in vac REFL B (0.625% of REFL power) = 36400 counts