stefan.ballmer@LIGO.ORG - posted 19:21, Monday 24 August 2015 - last comment - 19:41, Monday 24 August 2015(20829)
ASC-AS_C as RIN sensor
Daniel, Dan, Stefan I used the 45MHz modulation depth reduction in alog 20777 to fit the amount of 45MHz sideband on the ASC-AS_C_SUM diode: reduction ASC-AS_C_SUM cts 0dB 38430 -1dB 32630 -2dB 27460 -3dB 23680 This suggests that we have the equivalent of 8796cts of power that doesn’t respond to the 45MHz modulation index reduction, plus 29735cts of 45MHz sideband power (at 0dB reduction). Thus 29735/38430 = 77% of the total light is 45MHz sideband, and 23% is something else (mostly carrier). For the ASC_AS-C_SUM calibration I get: 4e-4 fraction of HAM6 light on AS_C (alog 17738, super-seeds 15431) x 800V/W PD gain (alog 15431 -> 200V/W, but in the digital system we do not divide by 4) x 10^(36/20) whitening gain x 1638.4cts/V ADC gain = 33080 cts/Watt_HAM6 I get under nominal conditions (23W, Gamma=0.3, 0dB reduction): 899 mW of 45MHz light (compare that to an expected 23W(INPUT)*(.3)^2/2(MOD)*0.88(IMC)*0.89(NOM IFO TRANSMISSION) = 811mW) and 266 mW of other light (carrier junk?). 20mA of light on the OMC_DCPD transmission, corresponding to about 25mW of "good" carrier light - 10 times less than the junk. Also, if I use the calibration of the ASC-AS_C-SUM diode, normalizing it by the 29735cts of 45MHz SB, I should get a calibrated (power) RIN sensor. I can look at the driven oscillator amplitude noise transfer function to x-check this: I would expect the power RIN / amplitude RIN TF to be equal to sqrt(2). (Note: Daniel's RIN sensor calibrated in Vrms/rtHz / Vrms - a sqrt(2) of my previous alogs, which all quote RIN as Vrms/rtHz / V_pk, being equivalent to a rad_rms/rtHz number. ) I indeed get 1.4. (plot 1). Plot 2 shows the same transfer function but using only the seg1, with the IOP channel (H1:IOP-ASC0_MADC6_TP_CH11). Again, 1.4. Now things hang together...
Images attached to this report
Comments related to this report
I can now use ASC-AS_C to check whether the amplitude noise on the 45MHz SB has changed before vs after the EOM driver change. Plot 1 shows ASC-AS_C_SUM in cts (the data is from seg1 only, but a factor of 4 is included to mimic ASC-AS_C_SUM counts). - Red vs blue (-3dB vs 0dB reduction in Gamma) shows a factor of 2 reduction in the noise level, consistent with a Gamma^2 scaling of the noise. - Green shows the noise level before we installed the EOM driver. From the ASC_AS-C counts I estimated that the modulation index was slightly lower, and the noise is consistent with that. Plot 2 shows all 3 traces calibrated in (power) RIN, using the calibration from the main entry, but either way, it looks like the (power) RIN has not changed during the EOM driver installation - very puzzling.... Finally, plot 3 shows the noise calibrated in Watt into HAM6.
Images attached to this comment
