Unlike the saturated vapor cell, the HCL generates atomic vapors in a \"sputtering\" way as following [22]: (i) the buffer gas in the HCL, such as Ne, Xe, is ionized by the high voltage between the anode and the cathode of the HCL; (ii) the positively charged ions are then attracted by and accelerate toward the negatively charged cathode, which is made by the metal(s) of interest; (iii) the ions strike the surface of the cathode on arrival at the cathode; (iv) if they have sufficient energy, the ions cause atoms of the cathode to be ejected. In our early work in the strontium HCL-FADOF, the transmittance exceeded 60% at room temperature, corresponding to the strontium atomic density at about 300 \({}^{\circ}\)C in a strontium saturated vapor cell [21]. However, because of the lack of the strontium saturated vapor cell-based FADOF (VC-FADOF) working at the same wavelength, the comparison between the HCL-FADOF and the VC-FADOF has not been completed. Because the 780 nm is the commonly used wavelength of the FADOFs [6; 7], we establish a rubidium HCL-FADOF working at 780 nm to compare with a rubidium VC-FADOF working at the same wavelength. Thanks to the open-source software program ElecSus, which is very accurate in simulating the transmittance spectrum of the ground state rubidium FADOF under weak signal [23; 24], we do not really need to build a new VC-FADOF. The experimental setup is shown in Figure 1. A 780 nm laser passes through an isolator (ISO) and an iris diaphragm (ID). After that, the laser passes through the first Glan-Taylor prism (GT1) to improve the degree of polarization and then passes through the rubidium HCL (HAMAMATSU L2783 series). A pair of permanent magnets (M1 and M2) on both sides of the HCL generate a magnetic field along the laser propagation direction. Because of the Faraday effect, the polarization of the laser near resonance rotates when passing through the HCL. By setting a proper magnetic field and discharge current, the polarization rotates 90\({}^{\circ}\) after passing the HCL. However, the Faraday effect does not occur at other wavelengths, so the polarization of the non-resonant laser does not rotate. The second Glan-Taylor prism (GT2) is used to select the polarization of the laser which rotates 90\({}^{\circ}\). The intensity of the laser after the GT2 is then recorded by a photodetector (D). Unlike the saturated vapor cell, the effective length of the HCL is the cathode length, which is only 20 mm here, because the discharged buffer gas in the HCL limits the range of atoms or ions sputtered from the cathode [22]. It should be noted that the HCL is 1  
The transmittance spectra of the HCL-FADOF at different discharge currents and magnetic fields are measured. The transmittance spectra are calculated as follow (the absorption of optical devices are ignored): (i) when the GT2 is perpendicular to the GT1 and the HCL is on, we record the laser power variation with laser wavelength on the D as \(P_{1}(\lambda)\); (ii) when the GT2 is parallel to the GT1 and the HCL is off, we record the laser power variation with laser wavelength on the D as \(P_{2}(\lambda)\); (iii) when the GT2 is perpendicular to the GT1, the laser is off and the HCL is on, we record the light power of the HCL at different discharge currents on the D as \(P_{3}\); (iv) the transmittance spectra of the HCL-FADOF can be calculated by \(Tr=[P_{1}(\lambda)-P_{3}]/P_{2}(\lambda)\). Some typical transmittance spectra at  
Figure 1: The experimental setup. L: 780 nm laser; ISO: 780 nm isolator; ID: iris diaphragm; GT1 and GT2: Glan–Taylor prisms; HCL: rubidium hollow-cathode lamp; A: the anode of the HCL; C: the cathode of the HCL; M1 and M2: a pair of permanent magnets; D: photodetector.