craig.cahillane@LIGO.ORG - posted 02:46, Sunday 13 January 2019 - last comment - 18:32, Wednesday 16 January 2019(46386)
Calibration of True Input Power
I have been wondering about the true input power, i.e. the input power incident on PRM, ever since the SRCL dither arm power measurement turned out so low.
Conclusion
With an input power of 1.87 watts according to the PSL photodiode, we probably have around 1.55 watts incident on PRM, assuming an addition 21% unaccounted losses in the REFL A/B LF PD path.
Power Measurement Methods
I aligned PRM and the input optics, and misaligned the rest of the corner with 1.87 W reported from the PSL input power PD going into the IMC.
IM4 Trans
The easiest way would be to get Pinput would be via IM4 TRANS. IM4 TRANS is calibrated into microwatts, IM4 (aka SM2) transmission is reported to be 2400 +- 200 ppm, and there's what I think is a 90:10 BS (ROM LH1) allowing 10% of the light going to IM4. However, if we turn the crank here we end up with only 0.2 W, which is not possible. This means that IM4 TRANS is not calibrated correctly, or the IM4 transmission path is actually around 8 times less transitive that I though.
REFL A/B LF
We believe that the REFL A/B LF PDs are correct based on the calibrated CARM shot noise. We can use them to calibrate the correct input power. The main issue here is if we use the REFL optical path (4 50:50 beamsplitters, 1 90:10 beamsplitter, one input faraday isolator, and one PRM) and assume no losses, we only get 1.19 W of true input power on the PRM. This could be the case, but it's more likely that there are some unaccounted for losses on the REFL path.
PSL and IMC Transmission
If we trust the old numbers from P1500076 on the PSL and IMC transmission, we can make an estimate of the true input power. We find that by varying the PSL and IMC transmission, we cannot reasonably recover the REFL A/B power levels we see.
PD Powers and Optic Transmissions Assumed
# Most taken from P1500076
# PD calibrated powers
PSL_INPUT = 1.87 # W
REFL_A = 7.3e-3 # W
REFL_B = 6.72e-3 # W
IM4_TRANS = 50.24e-6 # W
# PD uncertainties
dREFL_A = 0.04e-3
dREFL_B = 0.03e-3
dREFL_SUM = np.sqrt(dREFL_A**2 + dREFL_B**2)
# Transmissions
T50 = 0.5
T10 = 0.1
T_PSL = 0.953 # +- 0.013 PSL to MC1 transmission
T_IFI = 0.977 # +- 0.004 https://dcc.ligo.org/DocDB/0119/P1500076/003/Mueller16rsi-AdvIO3.pdf
T_IMC = 0.92 # +- 0.031
IMC_ModeMatching = 0.984 # 0.001
T_PSL_to_IFI = T_PSL * T_IMC * IMC_ModeMatching
T_IM4 = 2400e-6 # +- 200e-6 E070092
T_PRM = 0.031 #
# Trans Uncertainties
dT50 = 0.01
dT10 = 0.01
dT_PSL = 0.013
dT_IFI = 0.004
dT_IMC = 0.031
dIMC_ModeMatching = 0.001
dT_PSL_to_IFI = np.sqrt( T_PSL_to_IFI**2 * ((dT_PSL/T_PSL)**2 + (dT_IMC/T_IMC)**2 + (dIMC_ModeMatching/IMC_ModeMatching)**2) )
dT_IM4 = 200e-6
Images attached to this report
Comments related to this report
This calibration is now implemented in H1:IMC-IM4_TRANS_{SUM,PIT,YAW} FM10.
The IM4_TRANS is no longer calibrated in µW due to a gain change in the whitening. The calibration dates back to alog 9716, where the whitening gain was 36dB. It is 18dB now, so the "µW"-readbacks are 18dB too low.
We added NSUM channels to the IM4 and MC2 Trans QPDs to facilitate an additional calibration of the incident power onto these mirrors.
With this additional 18 dB of whitening gain, we can calibrate true power input the simple way using IM4 TRANS:
Requested Laser Power = 2 W
PSL Reported Power = 1.87 W
T_IM4 = 2400 ppm
True Input Power = 1.66 W
If we let T_IFI = 0.977, T_PSL = 0.953 and IMC_ModeMatching = 0.984, then T_IMC = 0.968.
And the mystery REFL path losses are 28.4% (see attachment).
Images attached to this comment
