# A High-Sensitivity Cesium Atomic Magnetometer Based on A Cesium Spectral Lamp
## 2 Design and Construction of Cesium Atomic Magnetometer  
### Sensor Assembly  
As depicted in Figure 1a, the sensor assembly mainly consists of the following modules: the cesium spectral lamp (Figure 1b), optical components, an absorption vapor cell containing \({}^{133}\)Cs atomic vapor and buffer gas, and a photodetector (PD). Figure 1c illustrates the working principle of the M\({}_{x}\)-type cesium atomic magnetometer [5]. The light emitted by the cesium spectral lamp is collimated, filtered, and polarized to form a circularly polarized light beam. The main spectral line after the filter is the Cs D1 line (Figure 1d). It polarizes the cesium atomic ensemble in the absorption vapor cell along the light propagation direction by optical pumping, resulting in a macroscopic polarization vector of cesium atomic magnetic moments (M\({}_{0}\)). As shown in Figure 1e, the circularly polarized light from the lamp can pump both hyperfine energy levels of the ground-state Cs atoms, thanks to the broad linewidth of the main emission line of the lamp. In the presence of an external magnetic field B\({}_{0}\) that is not parallel to the light propagation direction, the macroscopic polarization vector M(t) precesses along the direction of B\({}_{0}\) at the Larmor frequency \(\omega_{L}=\gamma_{Cs}\)B\({}_{0}\), where \(\gamma_{Cs}=2\pi\times 3.5\) Hz/nT is the gyromagnetic ratio of \({}^{133}\)Cs. At this point, if a transverse oscillating magnetic field B\({}_{\mathrm{T}}\)(t) is applied perpendicular to the direction of B\({}_{0}\), changing the frequency of the transverse field across the Larmor frequency will alter the magnitude and precession process of the polarization vector in the steady state. In the meantime, the precessing atomic polarization vector alternates the absorption coefficient of the atomic vapor at the transverse field oscillating frequency, thus modulatingthe output light at the same frequency. When the transverse field oscillating frequency matches the Larmor frequency, the modulated absorption coefficient reaches a minimum (\"magnetic resonance\"), resulting in a maximum amplitude of the modulated light signal, which can be detected by the photodetector. The whole process is called optically detected magnetic resonance. By adjusting the oscillating frequency of \(\mathrm{B_{T}(t)}\) through a feedback loop, the magnetic resonance condition can be maintained and the Larmor frequency can be tracked continuously, as well as the external magnetic field of interest (\(\mathrm{B_{0}}\approx 50\)\(\mathrm{\SIUnitSymbolMicro m}\) for the Earth's magnetic field). Choosing the direction of \(\mathrm{B_{0}}\) as the quantization axis (\(z\)-axis), the optical Larmor signal is essentially due to the oscillating \(x\)-axis component of the macroscopic vector (\(\mathrm{M_{x}(t)}\)); therefore, this atomic magnetometer is conventionally called \(\mathrm{M_{x}}\)-type magnetometer. The transverse magnetic field (\(\mathrm{B_{T}(t)}\)) is the component of the feedback magnetic field (\(\mathrm{B_{1}(t)}\)) along the \(x\)-axis, which is generated by the feedback coil and along the light beam direction in the current configuration [11].