Objective With advancements in quantum manipulation and optoelectronic detection technologies, the atomic magnetometer, a novel quantum extremely- weak magnetic sensor, has experienced rapid development. Operating in the spin exchange relaxation free (SERF) state, atomic magnetometers offer numerous advantages, including non- cryogenic operation, compact structure, low maintenance costs, and high sensitivity, making them widely applicable in various fields. However, the phenomenon of optical frequency shift, induced by off- resonant circularly polarized pumping light, critically affects the orthogonality of measurement and output response in SERF atomic magnetometers. This study proposes a method for suppressing optical frequency shift based on the analysis of the non- sensitive axis response using a self-made compact single- beam SERF 87 Rb atomic magnetometer. The curve of optical frequency shift, combined with the curve of light absorption, is precisely plotted and analyzed to determine the central resonant frequency of the SERF atomic magnetometer. The analysis indicates that employing this suppression method significantly reduces coupling crosstalk between the measurement axes of the magnetometer, thereby enhancing the orthogonality and output response of the sensitive axis. Experimental results validate the effectiveness of this suppression method. Further comparison reveals that the performance of the self-made compact SERF atomic magnetometer is enhanced by suppressing optical frequency shift, resulting in a sensitivity of 13 fT/root Hz within a bandwidth of 140 Hz and a dynamic range of approximately +/- 3 nT. Methods The optical frequency shift direction is regarded as the photon spin direction, which is equivalent to the existence of an equivalent virtual magnetic field in the direction of the pumping light (x-axis). Due to this fictitious magnetic field, the sensitive axis (z-axis) of the SERF atomic magnetometer reacts to signals from the non- sensitive axis (y-axis). Applying a weak oscillating magnetic field at 40 Hz to the y-axis and a direct current compensation magnetic field to the x-axis reveals a trend where the response of the y- axis varies with the compensation magnetic field along the x-axis. When the response of the y-axis reaches its minimum value, it indicates completion of compensation for the fictitious magnetic field. Results and Discussions As a result of the suppression method, the response of the 40 Hz signal from the y-axis notably decreases (Fig. 2) after compensating for the magnetic field along the x-axis. The variation curve of the compensation magnetic field applied along the x-axis with the laser frequency (Fig. 3) indicates that the magnetic field along the x-axis mainly consists of the fictitious magnetic field of optical frequency shift due to the constant residual magnetic field. After analyzing and compensating for the residual magnetic field along the x-axis, approximately 0.2 nT, based on the principle of optical frequency shift, the optical frequency shift curve [Fig. 4(a)] is plotted, confirming its existence. Coupled with the light absorption curve [Fig. 4(b)], the resonant frequency of 87 Rb atoms is determined to be 377109.23 GHz. By optical frequency shift suppression, the measured coupling coefficient is about 4.5%, which can be used to evaluate more accurately the orthogonality between the measured axes and the response of the sensitive axis(Fig. 5). With potential coupling crosstalk between the y and z axes accurately eliminated, the response of the z-axis significantly improves after optical frequency shift suppression (Fig. 6). As a result of this operation, the self-made compact fiber- coupled SERF atomic magnetometer achieves high sensitivity of 13 fT/ Hz within a-3 dB bandwidth of 140 Hz, and its dynamic range is approximately +/- 3 nT. Conclusions This study proposes a method for suppressing optical frequency shift based on analyzing the response of the non- sensitive axis using a self-made single- beam compact SERF Rb atomic magnetometer. By utilizing the proposed suppression method, the phenomenon of optical frequency shift and the existence of its equivalent fictitious magnetic field are verified. The central resonant frequency of the SERF atomic magnetometer is determined from the plotted curves of optical frequency shift and light absorption. Moreover, suppressing optical frequency shift significantly enhances the orthogonality between measurement axes, reduces crosstalk,and improves the response of the sensitive axis of the SERF atomic magnetometer, thus demonstrating the efficacy of the suppression method. The study results indicate that after suppressing optical frequency shift, the bandwidth, sensitivity, dynamic range, and other performance indicators of the self-made SERF atomic magnetometer improve. Its sensitivity can maintain a level of 13 fT/ Hz within a bandwidth of 140 Hz, with a dynamic range of approximately +/- 3 nT.
