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kiwamu.izumi@LIGO.ORG - posted 18:55, Thursday 07 May 2015 - last comment - 19:03, Thursday 07 May 2015(18298)
low cavity pole study; comparison of RF sidebands and some DC powers between two data

In response to Jeff's (re)discovery of low DARM cavity pole (alog 18283), I looked at past data of various DC and RF sideband power signals to see if there was something anomalous. Specifically speaking, I made a comparison between the data from May-1-2015 and the one from May-6-2015.

  May-1-2015 18:22:59 May-6-2015 11:37:31  Ratio  Notes
DARM cavity pole 355 Hz  270 Hz  -  
IMC input 2364 mW  11432 mW  4.83  
MC2_TRANS  160.27 uW  771.2 uW  4.81  good
TRX_NORM  1147  1239  -  
TRY_NORM  1250  1314  -  
POP 18 I  148 uW  425 uW  2.87  why so low when 11 W ? rf saturation ?
POP 18 Q  6 uW  -5 uW  -  
POP 90 I  37 uW  200 uW  5.405  SRC misaligned ?
POP 90 Q  6 uW  3 uW  -  
AS 90 I  1500 cnts  7100 cnts  4.73  good
AS 90 Q  140 cnts 1375 cnts  9.8   why Q signal ?
DARM offset  3x10-5 cnts  2 x 10-5 cnts  -  should not impact on the cavity pole
Recycling gain (carrier)  37  40  -  

 

A first impression I had is that, looking at the AS 90 in-phase signals from the two data, the DRMI alignment looked very good to me and therefore I imagined that losses due to misalignment in SRC have been small. Since the DRMI for the 45 MHz sidebands is (almost) critically coupled, I can do an order estimation for SRC intra cavity losses. First, I simplifed the DRMI transmissivity as

  (amplitude transmissivity of DRMI to AS) = Ts / (1 + Ts + L) ~ 1 - L/Ts

where Ts and L are SRM power transmissivity and intracavity loss (or half of the round trip loss) respectively. AS 90 is made of beatnote of the upper and lower 45 MHz sidebands. So the AS90 should be proportional to the square of the above equation. In order to explain a 2 % reduction in the AS90 as shown in the above table, L needs to be about 350 ppm. This sounds too small in order for loss to reduce the DARM cavity pole as low as 270Hz (see the plot in alog 17889).

Perhaps, I should do a similar analysis for POP90 which showed some increase -- this is usually an indication of misalignment in SRC. However, on the other hand, POP18 behaved anomalously in the sense that it dropped by 40 % while the carrier recycling gain increased slightly. This shounds to me some rf saturation, beam clipping or that sort of things and therefore I am not so keen to analyze the POP signals of the 11 W data.

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kiwamu.izumi@LIGO.ORG - 19:03, Thursday 07 May 2015 (18318)

Trends attached.

Also, forgot to mention that the POP90 I showed slow drift as shown in the first attachement when the power was at 11 W.

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