On Monday 14/06 the squeezer was switched to squeezing mode around 6:30 a.m. UTC. On first we did not find the 4 MHz beat note at the homodyne detector, and we realized that the delay line was misaligned. We recovered the coarse alignment using the subcarrier beam, and we performed a fine tuning by observing the CC beam on the HD cameras, then by engaging the HD AA loop.
After the recent tuning of the working point for the QNR lasers, we scanned the OPA and observed the transmission of the CC laser on the SQZ_EQB1_IR_PD_MONI photodiode to check the single mode operation as already done in the past. We do not see evidence for multimode operation, see Fig 1.
We measured the OPA parametric gain by observing the magnitude of the 4 MHz LO-CC beat at the homodyne detector while scanning the LO optical path length, see attached note CC_scan.pdf. We repeated the measurement for three different values of the green pump power, with the following MZ offset and timing:
12:10 + 2min - MZ Offset = 0V
12:15 + 2min - MZ Offset = 0.25V
12:20 + 2min - MZ Offset = 0.5V
Fig 2 shows the 4 MHz magnitude for MZ offset = 0V (magenta) and MZ offset = 0.5V (blue). The measured parametric gain from the data, i.e. the ratio of maximum and minimum magnitude of the CC-LO beat, is g=1.9 for MZ offset = 0V and g=2.3 for MZ offset = 0.5V. We also observed the effect of pump power tuning on the amplitude of the pump-phase error signal, see Fig 3 with MZ offset = 0V (magenta) and MZ offset = 0.5V (blue). By comparison with the SQZ_MachZ_PD_DC signal we confirm that both the amplitude of the pump-phase error signal and the parametric gain measured from HD RF channel scale as the square root of the green pump power, as expected.
With the lower pump power setting (MZ offset = 0V) we engaged the HD CC loop with DSP filter 1 at gain 1500, and we changed the CC phase between 0.9 rad, corresponding to maximum anti-squeezing, to 0.9 rad corresponding to squeezing, then we open the CC loop and closed the LO shutter. Fig 4 shows the rms of HD audio channel vs time: the three successive plateau values correspond to anti-squeezing, squeezing, and shot noise respectively. The measured anti-squeezing is about 2.3 dB and the squeezing is about 0.7 dB, while the expected produced squeezing from the measured parametric gain should be around 3.2 dB, suggesting the presence of extra losses and large phase noise.
While extra losses can be attributed to possible clipping along the EQB1 delay line limiting the accuracy of the AA loop, we investigate the phase noise issue by performing noise injections on the CC loop. Data were taken first with the fast CC loop alone using DPS filter 1 at gain 1500.
- A stream of clean data without injected noise injection but with the angular dither lines on (0.005 mV amplitude) starts at 2021-06-14-23h35m03-UTC.
- Dither lines were switched off at 2021-06-15-05h35m45-UTC
- White sensing noise injection with 50 uV amplitude at 2021-06-15-06h43m39-UTC
- Coarse CC loop added with gain = 1 at 2021-06-15-06h51m11-UTC
- Coarse CC loop gain increased to 3 at 2021-06-15-06h53m19-UTC
- Coarse CC loop gain increased to 10 at 2021-06-15-06h53m54-UTC
Fig 5 shows the measured TF with the fast CC loop only; the structure around 230 Hz prevents to increase the loop gain.
Fig 6 show the TF with the addition of the coarse CC loop at gain = 10.