Atomic optical filters have a broad range of applications in several areas, including atomic clocks, free-space optical communications, and laser remote sensing systems. The Faraday anomalous dispersion optical filter (FADOF) is one of the most popular optical filters because of its narrow bandwidth, high transmission, fast response, and high noise rejection. As a result, it has been intensively studied both theoretically and experimentally. The FADOF is based on the rotation of the polarization direction of a linearly polarized light signal when it passes through an atomic medium in a magnetic field. Most previously published studies have focused on the FADOF of the atomic transition between the ground and excited states; consequently, the selectivity of the operating wavelength of the FADOF is often limited. Some scholars have further investigated the FADOF between two excited states (ES-FADOF), owing to their abundant transitions. However, the bandwidths of the FADOF and ES-FADOF are usually of the order of ~GHz. Currently, the investigation of atomic optical filters with ultranarrow bandwidths remains a focus.

Based on a ¹³³Cs 6S_1/2-6P_3/2-6D_5/2 (852 nm+917 nm) ladder-type atomic system, we present an experimental study on a nonlinear optical filter with an ultra-narrow bandwidth, as shown in Fig.1. A circularly polarized laser with a wavelength of 852 nm was used as the pump light to populate the atoms from the ground state 6S_1/2 to the intermediate excited state 6P_3/2 and to polarize the atomic medium. The polarization direction of the 917 nm linearly polarized laser as the signal light, with a frequency in the vicinity of the 6P_3/2-6D_5/2 transition, was rotated when it passed through the polarized atomic medium. The experimental setup is shown in Fig.2. The temperature-controlled ¹³³Cs vapor cell was placed between a pair of Glan-Taylor prisms with perpendicular polarization directions; the extinction ratio of the prisms reached 100000∶1. The 852 nm pump and 917 nm signal lights overlapped in the ¹³³Cs vapor cell and were then separated by two dichroic mirrors. Subsequently, the signal light passed through an interference filter and reached a photodetector, enabling the realization of the induced dichroism excited atomic line filter (IDEALF) operating on the 6P_3/2-6D_5/2 transition with an ultra-narrow bandwidth.

The influences of parameters such as the temperature of ¹³³Cs vapor cell and the power of the 852 nm pump light on the peak transmittance and equivalent noise bandwidth (ENBW) of the IDEALF, are measured and analyzed. In particular, the difference in the IDEALF between the two experimental configurations is investigated when the 852 nm pump light is co-propagating or counter-propagating with the 917 nm signal light in the atomic medium. Notably, the Autler-Townes splitting phenomenon in the IDEALF spectral signal is observed for the counter-pumping configuration when the power of the 852 nm pump laser is relatively high (>4 mW), as indicated in Fig.6, which is in good agreement with the theoretical calculation result using a simple model, as shown in Fig.7. As a typical result, the IDEALF in the counter-pumping configuration has a higher peak transmission and narrow ENBW in comparison to that of the IDEALF in the co-pumping configuration (Fig.5, Fig.8, and Fig.9). This is because the counter-pumping configuration is Doppler-free with an atomic coherence effect in a ladder-type atomic system, which has been confirmed in many other experiments, whereas the co-pumping configuration is an incoherent experimental system. The difference between the two experimental configurations causes a significant difference in the ENBW of the IDEALF. In our experimental parameters, the ENBW is in the range of ~7?60 MHz for the counter-pumping configuration and the ENBW in the range of ~90?140 MHz for the co-pumping configuration, and thus, the ENBWof the former is approximately half of the latter, as shown in Fig.9.

We demonstrate an IDEALF with ultra-narrow bandwidth in a ladder-type atomic system and compare its properties under two different experimental configurations. Under the optimized experimental parameters, the peak transmission of the IDEALF reaches ~20%. The ENBW of the IDEALF is at least one order of magnitude narrower than that of the FADOF (~GHz). The narrowest bandwidth of ~7 MHz of the IDEALF is realized, which is close to the natural linewidth of 5.2 MHz of the intermediate excited state 6P_3/2. Under certain experimental conditions, the IDEALF signal exhibits two distinct profiles: a line-center filter with a single transmission peak is obtained for a co-propagating experimental configuration, and a line-wing filter similar to the popular FADOF is also realized in the case of a counter-propagating experimental configuration. Both of these profiles have significant application value in detecting weak light signals and eliminating the influence of background noise light, particularly in the case of a non-magnetic environment.