Analysis and Suppression of the Detection System Polarization Error Caused by Residual Angle in a K–Rb–21Ne Comagnetometer

This article investigates the detection system polarization error (DSPE) of the optical rotation detection system (ORDS) in a K–Rb–21Ne comagnetometer operating in the spin-exchange relaxation free (SERF) regime. The residual angle will couple the DSPE into the output signal, which will seriously degrade the performance of the ORDS. By establishing the mathematical model of scale factor, scale factor nonlinearity, and sensitivity coefficient of probe laser intensity, the polarization error introduced by residual angle is analyzed. The experimental results show that after eliminating the residual angle, the scale factor of the system is effectively improved, the coupling effect between the probe laser intensity fluctuation and output signal is decreased, DSPE is effectively suppressed, and the sensitivity performance of the K–Rb–21Ne comagnetometer is improved. Then, an azimuth closed-loop device (ACD) is proposed, which utilizes the characteristics of liquid-crystal variable retarder (LCVR) and a quarter-wave plate (QWP) to overcome the precision limit of manual adjustment and realize the long-term closed-loop elimination of residual angle, and suppress the DSPE. After adding ACD, DSPE is further suppressed, and the sensitivity at 1 Hz is improved more than 20% higher than that in open-loop mode. Moreover, the DSPE suppression method proposed in this article also has the potential to be applied to other atomic sensors.