## 3 Optically Pumped Probe Design
### Probe Front End
#### 3.1.1 Cesium Atomic Lamp Design  
The cesium atomic lamp comprises a cesium bulb, excitation structure, and non-magnetic heating wire. The design aims to improve the discharge uniformity and sensitivity of the cesium lamp while reducing the thermal consumption of radiofrequency (RF) excitation [19]. The structural model of the cesium atomic lamp is shown in Figure 4, and the physical object is shown in Figure 5. The cesium bulb is spherical and has a diameter of 8 mm, and it contains 800 \(\upmu\)g of cesium, filled with 5 Torr of xenon gas as a buffer gas. The sealing tail end is used to fix the cesium bulb and collect excess cesium metal as the cold end. The excitation structure includes an excitation coil, a parallel capacitor, and a shielding copper shell. Combined with the cesium bulb, the structure forms a load that matches the impedance of the RF excitation source, with a resonant frequency of  
Figure 3: The cesium optically pumped probe.  
Figure 2: The hardware of the optically pumped probe.  
approximately 165 MHz and a standing wave ratio of less than 1.5, thereby minimizing the thermal consumption of RF excitation. The shielding copper shell is wound with non-magnetic heating wires, using twisted pairs to cancel the additional magnetic field generated by the heating current. A resistance of 45 \(\Omega\) is used to heat the cesium lamp in low-temperature environments, increasing the initial concentration of cesium atoms in the bulb and achieving rapid ignition under low-temperature conditions.  
Additionally, the cesium absorption chamber is another key component that is responsible for providing the working substance, ensuring effective interaction between the pump light and cesium atoms. The optical components, including lenses, filters, and polarizers, are designed to optimize the optical path and improve the accuracy of the optically pumped probe. The photoelectric detector is responsible for receiving the optical signal and transmitting it to the signal processing unit, employing a non-magnetic design to avoid magnetic interference. The overall design considers the heating efficiency of the cesium bulb, temperature gradient, and minimization of magnetic field interference to enhance the performance of the probe.  
The cesium absorption cell significantly impacts the overall volume of the probe owing to its relatively large size. Therefore, to achieve the goal of miniaturizing the cesium optically pumped probe, it is essential to develop technology for manufacturing smaller atomic absorption cells. When the size of the absorption cell cavity is significantly reduced, the actual filling pressure of the absorption cell is highly susceptible to the sealing temperature of the cavity, causing substantial pressure changes. For each specification of the atomic absorption cell, we precisely measured key parameters, such as the intrinsic linewidth and absorption line frequency shift, maintaining the pressure control accuracy of the absorption cells within 10% across different specifications. The model of the cesium absorption cell is shown in Figure 6. The cesium bulb has a cylindrical design, containing a small amount of high-purity cesium element, with 60 Torr of helium used as the buffer gas. Non-magnetic heating wires are wound around the outer wall of the cesium bulb, with a resistance of 46 \(\Omega\), designed as twisted pairs to cancel the additional magnetic field generated by the heating current, thus avoiding interference with magnetic measurements. The outermost layer of the cesium absorption cell contains a non-magnetic thermal insulation material,  
Figure 4: A three-dimensional model of the cesium atomic lamp.  
Figure 5: A physical picture of the cesium atomic lamp.  
designed using high-temperature resistant insulation cotton, which improves the temperature uniformity inside the cesium bulb, thereby minimizing the resonance signal linewidth of the cesium absorption cell.