Sheila, Craig We retook the ALS COMM frequency noise measurement using the IR PDH signal of REFL_9_I at an out-of-loop witness. In my old measurement, I hadn't removed the 42 Hz IR arm pole from the spectrum. This time it's removed using a DTT calibration filter which is just a zero at 42 Hz. This time we requested 10 watts input power while resting on the fringe to try and suppress REFL_9 sensor noise, and we were successful.Out-of-loop ALS_COMM Frequency Noise RMS = 1.5 Hz
Measured Input Power August 2 = 9.6 W Measured Input Power July 29 = 2.6 W REFL_9_I_ERR_DQ Calibration August 2 = 2.0 Hz/cts REFL_9_I_ERR_DQ Calibration July 29 = 18.0 Hz/ctsIt seems our broadband suppression above 1 Hz goes linearly with power, which suggests we're limited by dark noise. More investigation of the REFL_9 signal chain required. I was only able to integrate RMS starting at 900 Hz since we used the REFL_9_I_ERR_DQ channel this time. Still unclear what's happening above 1 kHz, probably REFL_9 noise was killing our measurement there as well, but it could also be COMM sensor noise starting to take over.
Comparing the noise to Class. Quantum Grav. 31 (2014) 245010, one can see that the noise is significancy lower. From the new ETMs with the corrected green transmission we expected a factor of ~5 reduction of the end station noise. This includes the PDH sensing noise of the green locking (curve VI), the fiber noise (VII) and the laser noise (VIII). We still expect a contribution of about 2 Hz rms from these, concentrated at frequencies near 1 kHz.
The surprise is that the noise due to acoustic (curve V) and fringe wrapping (III) is also much lower. We didn't notice this in the past with the higher noise coming from the end stations. I suspect this is due to improvements in the acoustic couplings we made around the PSL. The early ALS measurements were done before we recognized its importance.
The above plot also misses the VCO noise which contributes another ~2Hz at frequencies below 0.01 Hz. This would indicate that the new common ALS noise is around 3-4 Hz rms. This is almost an order of magnitude better than the original ALS system.
PEM seems to explain some of the peaks we're seeing in the 10 watt input power ALS COMM spectrum. The periscope accelerometers is strongly coherent with a bunch of high frequency peaks (330 Hz, 154 Hz, 200 Hz, mess above 500 Hz). The ISCT1 accelerometer is coherent with the broad peak around 70 Hz.
