## 4 Results  
To optimize the power of the LO, when the pump and CCF was blocked, the intensity noise of the LO was measured by the BHD system firstly. The measured noise power of the LO at analysis frequencies from 2 kHz to 200 kHz was recorded by SA with a resolution bandwidth (RBW) of 51 Hz, a video bandwidth (VBW) of 110 Hz and the sweep time of 1 s. When the total power of the LO was 1 mW, that is, two photodiodes (FD500, Fermionics Opto-Technology) were illuminated by the power of 500 \(\upmu\)W, respectively, the measured noise power of the LO is shown in Figure 4. Curve (i) is the SNL; Curve (ii) is the intensity noise of the LO; and Curve (iii) is the electronic noise level (ENL). The intensity noise of the LO was more than 25 dB above the SNL and the SNL was 18 dB above the ENL at analysis frequencies from 2 kHz to 200 kHz, respectively. At the audio frequency of 5 kHz, the intensity noise of the LO was 30 dB above the SNL. The intensity noise of the LO at analysis frequencies from 0.2 MHz to 5 MHz was recorded by SA with an RBW of 110 kHz, a VBW of 110 Hz and the sweep time of 250 ms, as shown in the Figure 4 insert. Curves (i) and (ii) in the inset of Figure 4 are the SNL and the intensity noise of the LO, respectively. The intensity noise of the LO reached the SNL for analysis frequencies higher than 1.8 MHz. According to the theoretical calculation, when the intensity noise of the LO is 30 dB above the SNL at the analysis frequencies of 5 kHz, the value of CMRR of detectors should be higher than 54 dB to avoid the contamination of the squeezed light by intensity noise of the LO. For this purpose, the CMRR of photodetectors was measured firstly. The noise spectrum of the unbalanced detector, which was the common mode signal, was recorded by the SA when one photodiode was blocked and the other was illuminated at the power of 40 \(\upmu\)W. The maximum allowed illuminated power was 50 \(\upmu\)W, since the photodetector was easily saturated when photodetectors worked at unbalanced state with the self-subtraction photodetector scheme (one photodiode was blocked). However, when the squeezed vacuum states were measured, i.e., photodetectors worked at balanced state, the maximum allowed illuminated power was 5 mW. The illuminated laser was amplitude modulated using an EOM at different analysis frequencies. The noise spectrum of the balanced detector, that was the differential mode signal, was recorded when two photodiodes were illuminated at the power of 40 \(\upmu\)W, respectively. The measured noise power from unbalanced detector and balanced detectors as a function of analysis frequencies is shown in Figure 5. The CMRR of photodetectors in the BHD system can be determined by the difference of noise power from balanced detectors and that from unbalanced detector. Dots were the measured data and curves were connected lines. It can be seen that the CMRR of photodetectors was higher than 60 dB at the range from 4 kHz to 20 kHz. The maximum CMRR of 67 dB was measured at the analysis frequency of 5 kHz. The CMRR was 54 dB at the analysis  
Figure 4: Intensity noise of the LO as a function of analysis frequencies: Curve (i), the SNL; Curve (ii), the intensity noise of the LO; and Curve (iii), the ENL. The inset is the intensity noise of the LO at analysis frequencies from 0.2 MHz to 5 MHz. frequency of 3 kHz. A typical example of the measurement for the CMRR of photodetectors at analysis frequency of 5 kHz is shown in the inset in Figure 5. Curves (i) and (ii) in the inset of Figure 5 are the photodetectors worked at unbalanced and balanced states, respectively. Curve (iii) in the inset of Figure 5 is the ENL. The CMRR was 67 dB at 5 kHz. Dots in Figure 5 were measured in this way at different audio-band analysis frequencies. The generated vacuum squeezed states could be effectively measured at audio-band frequencies with homemade high CMRR photodetectors.