## 1 Introduction  
The optically pumped magnetometer (OPM) is a total field measurement instrument that is able to detect an extremely weak magnetic field of fT level in uncooled conditions. The OPM's working principle is based on the responses of the Larmor frequency (LF) to an external magnetic field. The LF occurs due to the atomic precession of a gas in an isolated cavity that is filled with the sensing and buffer gas; usually, the type and fraction of the contents is artificially determined to maximize the sensitivity. Compared to the SQUID (Superconducting Quantum Interference Device) magnetometers, the OPMs are able to operate under room temperature, to provide a high resolution including a bandwidth from DC (direct current) to several hundred/thousand Hz [1; 2; 3]. Thus, the OPMs are widely used in geophysical explorations, aerospace developments, information receiving, bio-magnetism researches, etc., [4; 5; 6; 7; 8; 9; 10; 11].  
There has been much research on the limit of detection (LOD) of OPMs in low environmental magnetic field conditions [12; 13]. [PERSON] et al. developed a laser pumped cesium magnetometer with an equivalent magnetic noise level (EMN) of 15 fT/VHz under an external field of 2 uT [14; 15]. [PERSON] et al. realized a miniaturized OPM in weak magnetic field with a high LOD of 42 fT/VHz [16; 17; 18]. [PERSON] et al. demonstrated a spin-exchange relaxation free (SERF) magnetometer with an EMN of 13 fT/VHz, which worked only in near zero field condition [19].  
The noise performance of typical commercial optically pumped magnetometers used in the geomagnetic field are about several hundred fT/Hz. For example, the Cs-OPM \"CS-3\" that developed by Scintrex Company has an EMN less than 600 fT/VHz [20]. The \"G-824A\" Cs-OPM developed by Geometrics Company can realize an EMN of 350 fT/VHz [21]. The \"GSMP-35\" potassium-OPM from the GEM Company has an EMN of 200 fT/VHz [22]. QuSpin's total-field magnetometer is a sensor in small size and with low power consumption; however, its sensitivity is only a little bit lower than 1 pT/VHz [23]. These commercial magnetometers are widely accepted by the market, but there is little information about their optimizations in design, especially the noise improvements in the associated detection circuits. [PERSON] et al. investigated the low frequency noise of the OPMs [24]. However, their studies included neither the condition in high environmental magnetic fields, nor the optimization of the parameters in OPMs. Although, in some investigations the parameter optimizations were involved [25; 26; 27; 28], only the shot noise contribution induced by the incident light was considered, excluding the noise sources from the pre-amplifier.  
In this paper, the noise source of the pre-amplifier circuit of the Cs-OPM is analyzed. By means of the signal characteristics in the photoelectric circuit for the proposed OPM, the signal-to-noise ratio (SNR) for the circuit is derived. By analyzing the components that affect the SNR, the amplifier parameters are optimized. As such, a low-noise circuit for the Cs-OPM is developed. The variation of the detection performance due to the lamp power and the radio frequency (RF) power, as well as the cell temperature, is investigated and discussed.