aidan.brooks@LIGO.ORG - posted 13:40, Wednesday 30 September 2015 (22116)
Calculation of CO2X coupling to displacement noise
[Aidan, Alastair]
We ran a calculation of the coupling of intensity noise to displacement noise for the CO2X laser at LHO. The details are as follows:
"The absolute coupling is very low right now due to the small amount of power being used to heat the test masses. The transfer function has not been directly measured but we have an estimate for it in the following document:
dz = 1.2E-15 m (100Hz / f) * (P / 1Watt)* RIN(f)
f = frequency,
P = DC power onto the CP
RIN = relative intensity noise
In this case, you want eqn 5 which describes the central heating intensity noise coupling.
The power levels currently used for the ITMs are:
L1X: 0.21W L1:TCS-ITMX_CO2_LSRPWR_MTR_OUTPUT
L1Y: 0.60W L1:TCS-ITMY_CO2_LSRPWR_MTR_OUTPUT
H1X: 0.226W H1:TCS-ITMX_CO2_LSRPWR_MTR_OUTPUT
H1Y: 0.054W H1:TCS-ITMY_CO2_LSRPWR_MTR_OUTPUT
For a standard RIN > DN estimate, we pulled the TCS-ITMX_CO2_ISS_IN_AC and _ISS_IN_DC from LHO overnight (last night) to calculate the RIN. For some reason, the anti-whitening filter banks are not engaged at LHO right now. However, to get the RIN, we do the following calculations to get the signals from the ISS PD referenced back to the photodiode (but in counts, not volts and assuming that counts_AC/counts_DC is approximately equivalent to intensity_AC/intensity_DC).
WF = abs(f./(f - i*20))*177,800 % whitening filter of ISS photodiode AC channel
counts_AC = TCS-ITMX_CO2_ISS_IN_AC_OUT_DQ / WF
DCgain = 510;
counts_DC = TCS-ITMX_CO2_ISS_IN_DC_OUT_DQ / DCgain
RIN = counts_AC/counts_DC
And then we apply this to equation 5 to get the estimated displacement noise from the CO2 laser. The results are plotted in the attached PDF.
Non-image files attached to this report
