# Laser Intensity Noise Suppression for Preparing Audio-Frequency Squeezed Vacuum State of Light
## 3 Experimental Setup of Feedback System for Laser Intensity Noise Suppression  
In the experiment of preparing and detecting audio-frequency squeezed vacuum state of 795-nm light, the laser source we used is a continuous-wave single-frequency tunable Ti:Sapphere laser (Model SolsTiS, M Squared, UK), which can be tuned to 795-nm NIR and the output power is about 2.5 W. As shown in Figure 1a, the output beam is divided into two parts, one part needs to be stabilized by the 795-nm NIR laser feedback system and then used as the lock beam of OPO and the LO beam. The other part is used to realize 397.5-nm UV laser for pumping OPO via cavity-enhanced SHG cavity with a LBO bulk crystal while two high-reflectivity mirrors (F) serve as switch of the feedback system.  
Figure 1: (**a**) Schematic diagram of experimental setup for preparing the squeezed vacuum state of 795-nm light at audio frequency. The dotted line path means without feedback and the solid line represents noise suppression via feedback. (**b**) Diagram of experimental setup of the AOM feedback system. Both 795-nm NIR and 397.5-nm UV laser’s AOM feedback systems are similar. ISO: Optical isolator; EOM: Electro-optical modulator; NIR: Near infrared; UV: Ultraviolet; HWP: Half-wave plate; PBS: Polarization beam splitter cube; BS: Beam splitter plate; L: Lens; AOM: Acousto-optical modulator; D: Dump; PMF: Polarization-maintaining fiber; PD: Photo-detector; HR: High-reflectivity mirror; SHG: Second-harmonic generation cavity; OPO: Optical parametric oscillator cavity; SV: Squeezed vacuum state of light; LO: Local oscillator beam; S: S polarization; P: P polarization; BHD: Balanced homodyne detectors.  
The feedback device consists of two parts, in-loop and out-loop parts, which are respectively used for servo control and monitor, shown in Figure 1b. The in-loop part of the device mainly includes AOM, polarization-maintaining fiber (PMF), polarization beam splitter (PBS) cube, beam splitter (BS) plate, sampling photodetector (PD1), and servo control, for details see reference [33]. According to the Bragg diffraction mode of AOM, there is only a first-order diffraction beam and a zero-order beam after passing through AOM and its temperature is controlled at room temperature accurately by an oven to prevent thermal instability produced by the interaction between crystal and laser from increasing the intensity fluctuation of laser. The first-order diffraction beam is blocked with dump and the zero-order diffraction beam with no frequency shift is injected into a short PMF fixed by ultra-stable mounts, which not only can act as a filter to clean the spatial mode, but also reduce the pointing instability of the beam. In order to reduce the fluctuation of polarization, a PBS with an extinction ratio of 3000:1 was used. The laser reflected by BS1 is utilized as in-loop laser sampling, and the transmitted laser is used for monitor and later use. The sampling laser with intensity fluctuation is transformed into an electrical signal via PD1 and a servo control to produce a signal to drive AOM. The PD2 is used to characterize the noise of laser intensity.