Miniature optical atomic magnetometers are small, low-power, and highly sensitive. They have broad application prospects in geological exploration, geomagnetic navigation, and underwater target detection. To realize a miniature optical atomic magnetometer, it is necessary to accurately measure the temperature of the cell and achieve high-precision temperature control of the cell. In various existing temperature measurement methods, such as infrared, grating, and optical refractive index temperature measurements, the temperature measurement structure is complex and difficult to miniaturize. Therefore, in this study, a spectral absorption method is proposed to measure the internal temperature of the cell. After stabilizing the power and frequency of the incident light, the cell temperature is controlled by projecting the light from the cell. This method provides an alternative scheme for the temperature control of the cell of a miniature optical atomic magnetometer.

In this study, the atomic absorption spectrum was used to measure the temperature of the cells. First, the feasibility of using the absorption spectrum to control temperature was theoretically analyzed. The theoretical analysis shows that when the incident laser intensity is locked and the laser frequency is locked at the center frequency of the spectral line, the light intensity transmitting the cell is only related to the temperature. Subsequently, an experimental platform was developed (Fig. 1). In the experimental setup, a laser was produced using a vertical-cavity surface-emitting laser (VCSEL). First, the laser output power was detected by a photodetector, and a laser power servo was applied to control the VCSEL-injection current, thus locking the optical power. Subsequently, the atomic absorption spectrum line was detected and converted to a laser frequency-discriminating signal by synchronous modulation and demodulation technology. A laser frequency servo was applied to control the laser temperature, consequently locking the laser frequency at the center of the absorption spectrum line. Finally, the signal amplitude at the center of the atomic absorption spectrum was used to measure the cell temperature and realize cell temperature control. We achieved cell-temperature control using this scheme. The temperature control scheme proposed in this paper and the temperature control realized using the thermistor 1 measurement were used, and thermistor 2 was used to evaluate the control effect.

It was observed that when the laser intensity and frequency were locked, the laser noise power spectral density decreased in the low-frequency area (Fig. 2), and after locking, the light intensity before and after the cell decreased with the increase in cell temperature (Fig. 3). For our experimental parameters, the optimal working temperature of the cell for our scheme is 65-70 ℃. A fitting curve was created using the theory, and the error of the cell temperature calculated by the fitting curve was not more than ±0.3 ℃ within the range of 61-75 ℃. The variation in temperature with time in the two control modes was recorded, as shown in Fig. 4(a). The Allan variance calculated according to Fig. 4(a) is shown in Fig. 4(b). It can be observed that the Allan variances for the two temperature control methods are almost equivalent, indicating that the temperature control effect is equivalent.

Because the laser intensity and frequency are locked, the produced laser can be easily employed to polarize the atoms. The assembly in the scheme is easily miniaturized, and this scheme is suitable for fabricating miniature atomic magnetometers. When applied to a miniature atomic magnetometer, the temperature sensitivity of the magnetometer can be reduced, and the performance of the magnetometer deteriorates slightly after long-term operation.

In this study, we proposed a scheme to measure and control the temperature of a cell using the atomic absorption spectrum. We used this scheme to achieve laser power and frequency locking, as well as temperature control of the cell. After the laser power and frequency locking were realized, the laser intensity noise was reduced compared to the free-running state. We also used a thermistor to measure the temperature of the cell to realize the temperature control of the cell. The temperature control effect of the scheme proposed in this study is nearly equivalent to that achieved using a thermistor. This study provides a feasible scheme for realizing laser frequency locking, power locking, and cell temperature control of miniature optical atomic magnetometers.