Message ID: 16127
Entry time: Fri May 7 11:54:02 2021
In reply to: 16117
Reply to this: 16129

Author:

Anchal, Paco

Type:

Update

Category:

LSC

Subject:

IMC WFS noise contribution in arm cavity length noise

We today measured the calibration factors for XARM_OUT and YARM_OUT in nm/cts and replotted our results from 16117 with the correct frequency dependence.

Calibration of XARM_OUT and YARM_OUT

We took transfer function measurement between ITMX/Y_LSC_OUT and X/YARM_OUT. See attachment 1 and 2

For ITMX/Y_LSC_OUT we took calibration factor of 3*2.44/f^{2} nm/cts from 13984. Note that we used the factor of 3 here as Gautum has explicitly written that the calibration cts are DAC cts at COIL outputs and there is a digital gain of 3 applied at all coil output gains in ITMX and ITMY that we confirmed.

This gave us callibration factors of XARM_OUT: 1.724/f^{2} nm/cts , and YARM_OUT: 4.901/f^{2} nm/cts. Note the frrequency dependence here.

We used the region from 70-80 Hz for calculating the calibration factor as it showed the most coherence in measurement.

Inferring noise contributions to arm cavities:

For converting IMC frequency noise to length noise, we used conversion factor given by where L is 37.79m and lambda is wavelength of light.

For converting MC1 ASCPIT OUT cts data to frequency noise contributed to IMC, we sent 100,000 amplitude bandlimited noise from 25 Hz to 30 Hz at C1:IOO-MC1_PIT_EXC. This noise was seen at both MC_F and ETMX/Y_LSC_OUT channels. We used the noise level at 29 Hz to get a calibration for MC1_ASCPIT_OUT to IMC Frequency in Hz/cts. This measurement was done in 16117.

Once we got the calibration above, we measured MC1_ASCPIT_OUT power spectrum without any excitaiton and multiplied it with the calibration factor.

Attachment 3 is our main result.

Page 1 shows the calculation of Angle to Length coupling by reading off noise injects in MC1_ASCPIT_OUT in MC_F. This came out to 10.906/f^{2} kHz/cts.

Page 2-3 show the injected noise in X arm cavity length units. Page 3 is the zoomed version to show the matching of the 2 different routes of calibration.

BUT, we needed to remove that factor of 3 we incorporated earlier to make them match.

Page 4 shows the noise contribution of IMC angular noise in XARM cavity.

Page 5-6 is similar to 2-3 but for YARM. The red note above applied here too! So the factor of 3 needed to be removed in both places.

Page 7 shows the noise contribution of IMC angular noise in XARM cavity.

Conclusions:

IMC Angular noise contribution to arm cavities is atleast 3 orders of magnitude lower then total armc cavity noise measured.