# A High-Sensitivity Cesium Atomic Magnetometer Based on A Cesium Spectral Lamp
## 4 Optimization Processes for Improving the Magnetometer's Sensitivity
### Optimization of Cesium Spectral Lamp  
According to the characteristics of extreme temperature in winter and summer over China [31], the minimum and maximum surface temperatures are roughly \(-50\)\({}^{\circ}\)C and \(55\)\({}^{\circ}\)C, respectively. Thus, the range \(-50\)\({}^{\circ}\)C to \(70\)\({}^{\circ}\)C was chosen for investigating the cesium magnetometer's performance, which could cover the operating temperature range of outdoor applications. Current commercial atomic magnetometers have a common problem that prevents them from working properly: when the ambient temperature is below \(-20\)\({}^{\circ}\)C, the cesium lamp does not ignite due to the low output power of the RF excitation source and the low density of cesium atoms inside the lamp bulb. In this study, a temperature feedback mechanism was introduced to improve the stability of the cesium spectral lamp output. The improved prototype of the cesium spectral lamp is shown in Figure 1. The fluctuations in the RF excitation frequency, RF output power, and optical output power of the optimized cesium spectrum lamp are shown in Figures 7-9, respectively, before and after the improvement. Prior to the improvement, as the ambient temperature changed from \(-50\)\({}^{\circ}\)C to \(+70\)\({}^{\circ}\)C, the output power increased but the RF excitation frequency declined. When the temperature fell below \(-20\)\({}^{\circ}\)C, the cesium spectral lamp's optical output power was less than \(2\) mW, resulting in insufficient pump light from the cesium D1 line [32] for the sensor to function. The RF frequency and power variation patterns with temperature are in agreement with the nominal specifications of voltage-controlled oscillators and power amplifiers. When the temperature deviates from room temperature, detuning between the RF excitation source and the cesium lamp occurs, which ultimately results in a reduction in output optical power. After the improvement, the fluctuations in RF excitation frequency and output power significantly decreased as the ambient temperature changed from \(-50\)\({}^{\circ}\)C to \(+70\)\({}^{\circ}\)C, and the optical output power remained steady at \(3\) mW or higher. Because the improved RF excitation circuit, as depicted in Figure 4, includes a temperature feedback mechanism, the resistance value of the thermistor varies with temperature, which affects the voltage division of the voltage-controlled oscillator and power amplifier. The change in voltage division alters the frequency and power of the RF excitation, and its change trend is opposite to the effect of temperature on frequency and power; in other words, the thermistor functions as negative feedback to the temperature effect. Thus, the frequency and power stability of RF excitation could be improved, thereby enhancing the stability of the output power of cesium lamps. The fractional fluctuation was used to evaluate the stability of the Cs lamp; this is defined as standard deviation/mean value \(\times\)100%. Table 1 compares each output parameter's fractional fluctuations before and after the improvement. Within the designated operating temperature range of \(-50\)\({}^{\circ}\)C to +70 \({}^{\circ}\)C, the fractional fluctuations of the RF excitation frequency increased from 0.80% to 0.07%, the RF output power improved from 61.97% to 2.05%, and the optical output power improved from 42.92% to 14.68% These enhancements indicate a substantial improvement in the output performance of the cesium spectral lamp, which promises a more rigid and stable operational status when confronting a variety of unpredictable geomagnetic survey scenarios. Figure 8: Comparison of RF output power variation for cesium lamp. Figure 7: Comparison of RF frequency variation for cesium lamp. In addition, we show the measured noise floor of the detected signal (with the optimal optical power) near the Larmor frequency in Figure 10, which is primarily dominated by the photon shot noise of the output optical power from the cesium spectral lamp constructed in this study.