# Laser Intensity Noise Suppression for Preparing Audio-Frequency Squeezed Vacuum State of Light
## 1 Introduction  
The first squeezed vacuum state of light based on an optical parametric oscillator (OPO) was realized by [PERSON] et al. [1], since its noise can be lower than the shot noise level (SNL) in a certain quadrature component. The squeezed vacuum state of light has great significance in gravitational wave detection [2; 3; 4] and can be used in quantum communication, quantum storage, and precision measurement [5; 6; 7; 8]. At the analysis frequency of MHz, the influence of laser intensity noise upon producing and detecting the squeezed vacuum state of light can be neglected as its background noise is pretty quiet and the SNL can be achieved easily [9; 10; 11; 12; 13]. The squeezed vacuum state of light at audio frequency [14] plays a vital role in certain areas of research, such as gravitational wave detection [2; 3; 4], magnetic measurements [15; 16; 17; 18; 19], and biological measurement [20], whose sensitivity is limited by the squeezing level due to the coupling of low frequency noise. With the further exploration of audio-frequency squeezed vacuum state of light, it will open a new door for more fields. Traditional methods of suppressing the laser intensity noise are injection locking [21], opto-electronal feedback [22], and mode cleaner [23]. Some new schemes have also been proposed, such as based on the second-harmonic component of the photo-elastic modulator detection [24] and controlling the temperature and mechanical vibration of electro-optical crystal to realize the long-term and wide-band laser intensity stabilization [25]. Recently, a nonclassical way to improve the achievable power stability by the injection of squeezed vacuum field of light has been demonstrated [26]. All theabove-mentioned schemes are used for various applications which cover the laser writing system [27], coherent population trapping clock [28], and other optical devices [29]. Here, we aim to reduce the laser intensity noise to weaken its negative impact on preparation and detection of squeezed light at audio frequency range [30; 31]. For preparation and improvement of audio-frequency 795-nm squeezed vacuum state of light based on a sub-threshold OPO, we designed two sets of feedback loop systems with acousto-optical modulators (AOM) to obtain high-stability 795-nm near infrared (NIR) and 397.5-nm ultra-violet (UV) lasers. Actually, the system will not only play a vital role in the preparation and detection of audio-frequency squeezed vacuum state of light, but also be beneficial to other related optical experiments. In principle, as the laser intensity noise of 795-nm NIR laser is suppressed to SNL, the effect of classical noise on the squeezed vacuum state of light can be eliminated accordingly. However, in practice, a series of nonlinear transformations in the process of frequency doubling will bring extra noise to the second-harmonic wave, no matter how stable the fundamental wave is. This is why the laser fluctuation of 397.5-nm UV laser generated by the second-harmonic generation (SHG) is normally greater than that of 795 nm and we have to use another system to suppress its intensity noise at the same time. Finally, we realized different suppression effects of laser intensity in the case of different feedback bandwidths and different sampling powers. The feedback effects are characterized by a digital multimeter in the time domain and a fast-Fourier-transformation (FFT) dynamic spectrum analyzer in the frequency domain. With feedback control, typical peak-to-peak laser intensity fluctuation is less than 1/100 of the original one in 30 min. With the help of the NIR and UV laser's intensity noise suppression, squeezing level of the squeezed vacuum state of 795-nm light prepared by OPO with a PPKTP crystal is improved from -3.3 to -4.0 dB around 3-9 kHz audio frequency.