_Article_  
A Faraday Anomalous Dispersion Optical Filter  
Based on Rubidium Hollow-Cathode Lamp  
[PERSON] 1,2,3, [PERSON] 1,2, [PERSON] 1,2, [PERSON] 1,2,*, [PERSON] 3, [PERSON] 1,2, [PERSON] 3 and [PERSON] 3  
State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Beijing 100876, China; [EMAIL_ADDRESS] (L.S. ); [EMAIL_ADDRESS] (R.M. ); [EMAIL_ADDRESS] (L.Y. ); [EMAIL_ADDRESS] (S.Y.) School of Electronic Engineering, Beijing University of Posts and Telecommunications, Beijing 100876, China  
State Key Laboratory of Advanced Optical Communication Systems and Networks, Department of Electronics, and Center for Quantum Information Technology, Peking University, Beijing 100871, China; [EMAIL_ADDRESS] (D.P. ); [EMAIL_ADDRESS] (J.C.); [EMAIL_ADDRESS] (H.G.) Corresponding author: [EMAIL_ADDRESS]  
Received: 5 September 2020; Accepted: 4 October 2020; Published: 12 October 2020  
Using a hollow-cathode lamp (HCL) to build a Faraday anomalous dispersion optical filter (FADOF) is a new method to realize narrow linewidth optical filters. In contrast to other atomic optical filters based on saturated vapors, which work at a relatively high temperature to maintain the atomic density, the HCL device using sputtered particles can work at a much lower temperature. In this work, a rubidium HCL-based FADOF (HCL-FADOF) working at 780 nm is established and carefully tested. With 20 mm cathode length, the transmittance can reach 29% under 18 mA discharge current and 260 G magnetic field at room temperature, which is equivalent to the performance of a saturated vapor-based FADOF (VC-FADOF) at more than 60 \({}^{\circ}\)C. This work provides a direct comparison of the performance of the HCL-FADOF and the VC-FADOF, which is of great benefit to further studies of atomic filters at normal temperature. hollow-cathode lamp; atomic filter; atomic density  
The atomic filter is a kind of ultra-narrow bandwidth optical filter, which takes advantage of the narrow linewidth of the atomic resonance line. Up to now, most atomic filters are realized in alkali metal saturated vapor cells and cover some specific wavelengths from visible to near infrared, such as 589 nm in sodium [1]; 532 nm, 770 nm in potassium [2,3]; 776 nm, 780 nm, 1529 nm in rubidium [4, 5, 6, 7, 8, 9]; 455 nm, 852 nm in cesium [10,11]. Such atomic filters exhibit important applications in LiDAR [12,13], optical communication [14,15] and quantum physics [16,17]. Methods of atomic filters develop from early types of atomic resonance filters (ARFs) based on frequency conversion [18], to recent Faraday anomalous dispersion optical filters (FADOFs) structure based on magneto-optical birefringence [19] and other types. However, what remains unchanged is the use of atomic saturated vapor cells and the requirement of the high atomic density. The higher atomic density means the higher pressure and therefore the higher temperature. This leads to two problems: (i) it is necessary to heat the saturated vapor cell, sometimes higher than 200 \({}^{\circ}\)C even for alkali metals [16,20], which could be very inconvenient in some cases; (ii) the saturated vapor pressure for lots of elements, such as alkali earth metals, is so low that it is necessary to heat them to an extremely high temperature. For example, the rubidium atomic density in a saturated vapor cell is about \(1.56\times 10^{16}/\mathrm{m}^{3}\) at 27 \({}^{\circ}\)C. However, strontium vapors must be heated to about 280 \({}^{\circ}\)C to achieve the same atomic density. Since the requirement of the high temperature for saturated vapors of many elements is so hard to meet, the temperature problem of atomic saturated vapor cells becomes one of the main difficulties in expanding wavelengths of atomic filters. Therefore, other methods beyond saturated vapor cells are proposed. In 2016, we proposed a new kind of atomic filter based on a hollow-cathode lamp (HCL) and demonstrated a strontium HCL-based FADOF (HCL-FADOF) working at 461 nm [21].