We coarsely tuned the loop parameters (gain and offset of error signal) to make the loop stable without measuring the TF. Then we tuned the demodulation phase in order to maximize the effect of anti-squeezing on DARM noise. Detected anti-squeezing is now aroung 5.5 dB, higher than yesterday because of the reduced phase noise. We then changed the phase by 90 deg to inject phase squeezing.
Fig 1 shows the DARM and Hrec spectra in three different conditions: with the SQZ fast shutter closed (purple), with injected anti-squeezing (orange) and with injection of squeezing (blue). Differently from the effect of anti-squeezing, the effect of squeezing is barely visible, probably due to a combination of the large residual phase noise and of possible extra misalignment losses.
We also characterized the 7 MHz PLL which locks the B1 demodulation frequency to the local oscillator for the SQZ PLL.
Fig 2 shows
- the CC error signal (SQZ_B1_PD1_7MHz_I),
- the derivative of the phase extracted from SQZ_PLL7 monitoring (SQZ_PLL_7MHz_raw_dFreq), which is used as error signal for the 7MHz phase loop
- the coherence between these 2 signals
in the following conditions
- Blue: CC error signal gain equal to 1 and 7MHz phase loop closed
- purple: CC error signal gain equal to 0.5 and 7MHz phase loop opened
One can see that
- with a gain of 0.5 the bump above 100 Hz decreases
- with 7MHz loop open there is no major modification of coherence and the coherence remains at the same high level in the 7-30Hz region : to be investigate
We spent the remaining time to characterize the alignment error signals from B1p quadrants. We closed the SQZ fast shutter at 4:32. ITF is left in LN3, and the SDB1 Faraday shutter is left open to avoid unlocking.
Searching for the time with lowest level of noise using band rms in the 2.0-2.1 kHz, here is the spectrum for 1 second of data (blue), compared to a time when the squeezed beam was not injected (purple line). On this very specific 1 second of data (lowest level of noise floor), there is maybe a 10% reduction of the shot noise level.
