Spin exchange relaxation free (SERF) atomic inertial measurement instruments have become important in paving the direction for the potential development of ultra-high-precision inertial measurement instruments by virtue of their ultra-high theoretical accuracy and easy integration. The realization of the SERF state requires a stable and uniform atomic density, which is closely related to temperature.Therefore, it is necessary to accurately measure the temperature and its gradient inside a cell. Among the various existing methods for temperature measurement, such as the platinum resistance method, the fiber grating method, the technique of infrared temperature measurement, and the spectral absorption method, only the last one can accurately measure the temperature inside the cell. However, all existing research on the spectral absorption method focuses on the accuracy of temperature measurement and the analysis of influencing factors. Thus, relevant research on the temperature gradient that exists inside a cell is lacking. Therefore, in this study, the spectral absorption method is proposed to measure the temperature field inside the alkali metal cell of the serf inertial measurement system. A longitudinal temperature gradient is obtained inside the cell by the continuous control of the laser orientation in the incident chamber. This method can accurately evaluate the cell temperature and the temperature gradient of high-precision quantum sensors. It provides important technical support for optimizing the design of electric heating without a magnetic system or an oven structure.
In this study, the spectroscopic absorption is used to measure the temperature inside a cell. First, the relationship between the temperature and the ratio of the intensity of linearly polarized light before and after passing through the gas cell is theoretically analyzed. Simultaneously, the influence of the frequency of the linearly polarized light on the accuracy of temperature measurement is also analyzed, and the linearly polarized light frequency under the highest temperature sensitivity is obtained. Subsequently, an experimental platform is built. The temperature at the center of the cell is measured and compared with that of platinum resistor at the monitoring point. After that, the temperatures at five points at equal interval in the longitudinal direction of the cell are also measured by moving the light source on the stage, and the results thus obtained are compared with the temperature simulation results to evaluate the accuracy of the method.
When the parameters of the cell are determined through theoretical analysis, it is observed that the ratio of the intensity of light before and after entering the cell is affected jointly by the cell temperature as well as the frequency of the linearly polarized light. The ratio decreases with an increase in cell temperature (Fig. 1). When the frequency of the linearly polarized light is close to the transition frequency at the D1 line of the Rb atom, the ratio increases with the temperature change rate of the gas cell, thus indicating that it is more sensitive to the temperature change (Fig. 2). At the optimum laser frequency, the temperature at the center of the cell is measured using an experimental setup. The result obtained is less than that at the oven walls, and this difference increases as the temperature increases (Fig. 6). The study shows a loss in heat conduction from the oven to the cell, and this loss increases as the oven temperature is increased, which is consistent with the actual situation. Subsequently, four points are selected at equal interval in the longitudinal direction of the cell, and the test results show that the longitudinal temperature gradient of the cell is 0.744 ℃/mm (Fig. 7). For comparison, the simulation results at the same location indicate that the temperature gradient of the cell is 0.725 ℃/mm (Table 1). The two results are comparable with the difference lying in the allowable error range, thus showing the accuracy of this method in evaluating temperature gradients.
In this study, an online method for measuring the temperature and its gradient in alkali metal cells based on spectral absorption is proposed. This method can obtain accurate temperature field information using the intensity ratio of the linearly polarized light before and after entering the cell in the SERF inertial measurement instrument. At the optimal laser frequency, the temperature gradient of the cell in the current heating structure is 0.744 ℃/mm. This result is similar to the simulation result, which shows the accuracy of this method. Thus, this method is suitable for the accurate evaluations of the cell temperature and the temperature gradient of high-precision quantum sensors, including SERF inertial measurement instruments, and provides important technical support for the optimization of the design of electric heating without a magnetic system or oven structure.
