Results and Discussions On the basis of 6S_1/2 (F=4)-6P_3/2 transition line of ¹³³Cs, the FADOF is demonstrated using a commercial-type HCL in the experiment. The number density of the atomic sample can be changed by adjusting the working current of HCL (Fig. 3) instead of a temperature controller. Line-center FADOF with single-peak characteristic is realized in a working current of 1-4 mA due to the existence of buffer gas in HCL (Fig. 4). When the working current is 6-10 mA, the line-wing FADOF similar to popular FADOF in a ¹³³Cs vapor cell is also obtained (Fig. 4). These two kinds of FADOFs have potential applications in frequency stabilization laser and radar remote sensing systems. Under the optimized experimental parameters, the FADOF has a peak transmission rate of up to 77% and an equivalent noise bandwidth ENBW of less than 1.8 GHz (Fig. 8). In addition, a narrower ENBW usually indicates a stronger system ability to resist noise interference.

Faraday anomalous dispersion optical filter (FADOF) has many advantages such as narrow bandwidth, high transmittance, and excellent background rejection, and it has been widely used in optical communication, radar remote sensing system, long-term frequency-stabilized laser system, and so on. In order to improve the transmittance of the optical filter, it is necessary to apply a certain intensity of magnetic field along the propagation direction of the signal light to rotate the polarization plane of the linearly polarized signal light according to the Faraday magneto-optic rotation effect. Besides, it is necessary to heat the temperature of the atomic vapor cell, so as to increase the number density of atoms, enhance the interaction between light and atoms, and thus improve the transmittance of a FADOF. However, for some atomic media with a high melting point, it is often inconvenient to heat the atomic vapor cell to a high enough temperature to obtain atomic samples with high number density. Some scholars have noticed that some light sources commonly used in atomic absorption spectrometers, such as hollow cathode lamps (HCLs) and electrodeless discharge vapor lamps, are excited by the collision between atoms. In the process of collision, the number density of the atomic samples in the lamp correspondingly increases. According to this point, we control the number density of atomic media in the lamp by adjusting the working current of the HCL and realize a FADOF with a wavelength of 852 nm based on the 6S_1/2-6P_3/2 transition line of ¹³³Cs, and the wavelength is located in a transparent window of the atmosphere, which is helpful for free space laser communication.

Experimental setup for the FADOF system based on the commercial-type HCL is shown in Fig. 2. The laser beam emitted from an external cavity diode laser (ECDL) at 852 nm with its frequency tuned to the 6S_1/2-6P_3/2 transition line of ¹³³Cs first passes through an optical isolator (OI), and then is divided into two beams through a half-wave plate (HWP) and a polarizing beam splitter cube (PBS). Specifically, one beam is used for the saturated absorption spectrum experiment, and spectral signals are obtained at detector PD1 and taken as a frequency reference. The other beam is used as the signal light in the FADOF experiment, and a FADOF transmission spectral signal is obtained at the detector PD2 via a ¹³³Cs HCL located between a pair of orthogonal Glan-Taylor prisms. The number density of atoms in the HCL is controlled by adjusting the working current of the lamp. The magnetic field is provided by a pair of permanent magnet rings (H1 and H2), and the intensity of the axial magnetic field is controlled by changing the distance between the two magnetic rings.

A FADOF working in the line-center and line-wing operations at a particular working current is demonstrated in a commercial-type HCL based on the 6S_1/2-6P_3/2 transition line of ¹³³Cs. The number density of the atomic samples in the lamp can be controlled by adjusting the working current of HCL. The ¹³³Cs atoms have a lower melting point (about 28.4 ℃), and FADOF can be realized in the temperature-controlled vapor cell. Furthermore, higher atomic density can be obtained at a lower temperature, which results in a relatively high transmission of FADOF. Therefore, it is valuable to use the commercial-type HCL to realize FADOF based on the atom with a high melting point, and this HCL is expected to realize FADOF operating on the transition between two excited states for simplifying the experimental system, without an extra pumping laser.