Plot 1 shows the dark noise in LSC-REFL_A_LF and REFL_B_LF (yellow/black), with 10W laser input with ISS second loop (red/blue) and w/o ISS second loop (magenta/cyan). The photocurrent in REFL_A and REFL_B was about 16mW each for the 10W measurements. It will turn to about 10mW in full lock. So, we are now about a factor 4 above dark noise.
Plot 2 shows the RIN of the 2 REFL PDs and their average, together with the ISS second loop sensors. The ISS inner and outer sensors have about 7-8mW of light each, so the measurements are limited by the shot noise of the inner PD. In full lock these sensors see about 60mW.
Here are 2 plots when the interferometer is locked. The ISS second loop sensors see about 60mW of light, whereas for this time LSC-REFL_A/B see about 8mW each.
The first plot shows a large excess in REFL power fluctuations below ~200Hz. EVen the flat part 300Hz shows some excess. It should be about 70% of the 16mW measurements, but shows a very similar level. Looking at the coherence between REFL_A and B indicates that this is a real signal and not noise.
The second plot shows relative intensity noise. To get the curves calibrated correctly one should match the peak near 4.5kHz since this seems rela intensit noise from the laser. (There is a factor of 0.3 in the calibrations of the RIN of REFL_A/B to acocunt for the interferometer reflectivity at DC. This factor should be 1 when the interferometer isn;t locked.)
Here is comparison between early in the lock and after 4 hours.
The hump in the reflected power is clearly getting larger as time progresses, and is its coherence with PRCL. The input power as measured by the ISS second loop outer sensor doesn't have a large correlection with the reflected power (some is expected due to the shot noise of the inner sensor).
Q1: Why is PRCL coherent with the power in reflection? If theer is a couplinh, shouldn't it be at least second order?
Q2: What's the flat noise above 300Hz that we see in the reflection power?
Here is the power trend during this lock.
And these are the plots more than 7 hours into a 60W lock. The REFL PD now seems to be shot noise limited above 100Hz.
Here is a comparison between the noise measured in reflection at 75W and 60W and against the dark noise. Some observations:
- The power in reflection reduced about 2.5, much more than the scaled input powers.
- The noise also scaled down more than just shot noise, indicating that there was additional noise at 75W (also seen in the coherence).
- The measured noise at 60W is only a factor of 1.8 above dark noise, somewhat marginal.
- If we subtract the dark noise from the measured noise we are a factor of 1,5 above ADC noise.
- Assuming this is all shot noise, the shot noise limited power becomes 2.8mW.
The outer loop RIN is always reported about 8% higher than the innner loop one. This is not real. In the PSL ISS model of the second loop ISS both detector values are divided by the DC value of the inner loop detector. Since the outer loop detector sees about 8% more light, the RIN in the outer loop detector is overestimated by this amount. To get a better value multiply by 0.922. With this correction both RIN spectra agree with each other.
A better calibration of the REFL/ISS PDs measured with 10W input and all TMs misaligned.
| Measured | Calibration | |
| IMC-PWR_IN | 9.855 W | 1 W/W |
| PDSUMINNER | 7.477 mA | 0.7588 mA/W |
| PDSUMOUTER | 8.115 mA | 0.8235 mA/W |
| REFL_A_LF | 16.70 mW | 1.694 mW/W |
| REFL_B_LF | 15.59 mW | 1.582 mW/W |
Here is a 60W trend for completness.
