# Laser Intensity Noise Suppression for Preparing Audio-Frequency Squeezed Vacuum State of Light
## 5 Conclusions  
We used the zero-order diffraction light of AOM instead of the first-order diffraction for sampling feedback, and successfully suppressed the 795-nm NIR and 397.5-nm UV lasers' intensity fluctuation involved in the preparation of squeezed vacuum state of light. After stabilizing, typical peak-to-peak laser intensity fluctuation is less than 1/100 of the original one. The relationship between the bandwidth, sampling power, and feedback effect is investigated. The narrower the bandwidth is, the more obvious the suppression effect is; and the larger the sampling power is, the better the feedback effect will be on the premise that the optical power reaching the detector is not saturated. By effectively suppressing intensity noise, not only the squeezing level can be improved in the feedback bandwidth, but also  
Figure 4: Relative noise power spectra normalized to the shot noise level (SNL) for the squeezing phase and the anti-squeezing phase. The two cases with and without both the 795-nm NIR and the 397.5-nm UV intensity noise feedback loops are compared. The traces (a) and (e) indicate the anti-squeezing of \(+\)5.0 dB and the squeezing of \(-\)4.0 dB for the case with feedback. The traces (b) and (d) indicate the anti-squeezing of \(+\)4.3 dB and the squeezing of \(-\)3.3 dB for the case without feedback. The trace (c) shows SNL. Clearly in the analysis frequency range from \(\sim\)3.3 to \(-\)9.3 kHz the squeezing is improved from \(-\)3.3 to \(-\)4.0 dB with help of both the 795-nm NIR and the 397.5-nm UV intensity noise feedback loops. In fact, there is a significant loss in the path from the OPO output coupling mirror to the BHD, including the round trip losses (\(\sim\)3%) of light and limited quantum efficiency (\(\sim\)97%) of BHD we utilized, which directly lead to an underestimate of the squeezing level. Therefore, we can infer that the true squeezing level of audio-frequency squeezed vacuum state of light is 0.2-0.3 dB higher than we detected. The noise power spectra are measured with a spectrum analyzer (Agilent 4396B, USA) and averaged for 60 times. The resolution bandwidth (RBW) is 100 Hz, and video bandwidth (VBW) is 10 Hz.  
the stable continuous output of the squeezed vacuum state of light can be guaranteed. It is expected to achieve higher squeezing level by reducing the loss of OPO cavity and improving the detection system, which has great application value in precise measurement. In addition, further stabilizing the laser intensity and phase to close SNL and improving the detector's performance will yield the squeezed vacuum state of light at much lower analysis frequency.  
[PERSON], [PERSON] and [PERSON] setup the AOM feedback system, performed the experiment, and collected all data; [PERSON] and [PERSON] proposed and designed the zero-order AOM diffraction beam sampling and whole AOM feedback system, optimized the feedback loop's parameters, and analyzed experimental data; [PERSON] and [PERSON] wrote and revised the manuscript. All authors have read and agreed to the published version of the manuscript.  
This research is partially funded by The National Key R&D Program of China, grant number 2017 YFA0304502, the National Natural Science Foundation of China, grant number 11974226, 61905133, 11774210, and 61875111, and the Shanxi Provincial 1331 Projects for Key Subjects Construction.  
There is no conflict of interest in this article.