The nuclear magnetic resonance gyroscope (NMRG) has attracted wide interest in recent years because of its small size, low power consumption and high precision. The alkali metal atoms, as an important component in the vapor cell of the NMRG, can be used to polarize noble gas atoms through spin-exchange optical pumping and are sensitive to the magnetic field generated by the nuclear magnetic moment of noble gas. The performance of embedded magnetometer directly determines the measurement accuracy of the NMRG, and therefore the parameter optimization and selection for the embedded magnetometer are critical. At present, most researchers focus on the influence of parameters such as the frequency and power of pump light on the zero-bias stability of the NMRG, and the research on the probe light mostly focuses on the frequency stability methods. In contrast, the research on the performance of the embedded magnetometer affected by the frequency of the probe light is rarely reported. As an important parameter, the frequency of the probe light plays an important role in improving the performance of the embedded magnetometer. In this study, a theoretical model of the probe light frequency influencing the embedded magnetometer signal is established, and the asymmetry of the positive and negative frequency responses of the experimental D1 line and the phenomenon of local extreme value near the central frequency by introducing hyperfine structure correction are explained. The simulation and experiment results match well, which proves the reliability of the theoretical model. The proposed theoretical model can provide design rules for improving the performance of the embedded magnetometer and the accuracy of the NMRG.
First, the theoretical model of the relationship between the Faraday rotation angle and the probe light frequency is established without considering the hyperfine structure. Second, the corresponding experimental system is established, the signal amplitude and transmitted light power of the embedded magnetometer are measured under different probe light frequency detunings and different cell temperatures. Third, the experimental results are compared with the theoretical results to analyze the errors in the theoretical model. At last, the hyperfine structure is taken into consideration to modify the theoretical model, and the simulation results are compared with the experimental results to verify the reliability of the modified theoretical model.
Without considering the hyperfine structure, the signal amplitude first increases and then decreases with the decreasing absolute value of the frequency detuning under the same cell temperature. On the other hand, the signal amplitude continues to increase when the cell temperature increases from 90 ℃ to 120 ℃, and starts to decrease at 130 ℃, while the absolute values of the frequency detunings corresponding to the maximum values under different temperature conditions gradually increase. The experimental and theoretical results match well in the negative frequency part of the D1 line, while the experimental results are significantly lower than the theoretical results in the positive frequency part of the D1 line, which may due to ignoring the hyperfine structure (Fig.3). After considering the hyperfine structure, the theoretical results and the experimental results match well. The asymmetry of the positive and negative frequencies is explained and it is caused by the asymmetry of the hyperfine structure. The results show that the signal amplitude can be optimized by about 3 orders of magnitude relative to the minimum by selecting the probe frequency detuning of around -17 GHz from the D1 line under a suitable gas chamber temperature (120 ℃) (Fig.4). The transmittance distribution at each temperature is basically consistent with the theoretical result, showing a Voigt distribution, and the transmittance curve gradually decreases with the increase of the temperature. Furthermore, there exists a small local extreme point near the central frequency, which is caused by the hyperfine structure and more obvious at lower temperatures (Fig. 5). The signal amplitude distribution at each temperature has also such an extreme point near the central frequency. As the temperature increases, the rotation angle curve with dispersion form is almost close and gradually decreases, while the extreme point position remains basically unchanged. The discrepancy may due to the weak signal amplitude near the central frequency which may be caused by the non-ideal 85Rb isotope in the vapor cell. The small extreme point near the central frequency is also caused by the hyperfine structure (Fig.6).
An optimization model that describes the influence of the probe light frequency on the embedded magnetometer is established. The theoretical and the experimental results match well, which show that the signal amplitude can be optimized by about 3 orders of magnitude relative to the minimum by selecting the probe frequency detuning of around -17 GHz from the D1 line under a suitable gas chamber temperature (120 ℃). By introducing the hyperfine structure correction, the asymmetry of the positive and negative frequency responses of the D1 line and the local extreme point near the central frequency are explained, which verifies the reliability of the theoretical model. The proposed theoretical model can provide design rules for improving the performance of the embedded magnetometer and the accuracy of the NMRG.
