Eleonora, Matteo and Yuhang

When we measure squeezing, we need to lock phase of LO with squeezing. Since the phase of squeezing is influenced a lot by suspended mirrors, the correction signal we send to IR phase shifter is quite large. We want to know how it will influence the measurement of squeezing.

On one hand, the power of IRMC transmission will be modulated. As we reported in entry 1883, we had 10% of IRMC transmission reduction when we drive IR phase shfiter with whole range. We did calculation, if we have 10% power reduction, we will have shot noise reduction of 10log10(0.9), which is 0.46 dB of reduction. This is the maximum reduction while the reduction in the real case should be half of 10log10(0.97),  which corresponds to 0.066dB reduction of shot noise.

On the other hand, there are frequency components of noise sent to IR phase shifter(attached figure 1). We measured the spectrum of homodyne when only lock CC2. (OPO is locked with p-pol, no pump sent to OPO, CC is on resonance inside OPO by tuning PLLp-pol, then let field goes to FC and reflects to homodyne, the 7MHz on homodyne is used to lock CC2) The result is shown in the attached figure 2. We also tried to send sine wave to IR phase shifter, and measured homodyne spectrum. This result is shown in the attached figure 3. These two measurement proves that the low frequency bump has contribution from IR phase shifter noise.