“…Up to now, the FADOFs have been realized on different atomic transitions, mostly with transmittance between 40% and 100%, and equivalent noise bandwith (ENBW) around 1 GHz, such as Na 589 nm (90%, 5 GHz) 18 , Rb 780 nm (83%, 2.6 GHz) 19 , Rb 795 nm (70%, 1.2 GHz) 20 , Cs 459 nm (98%, 1.2 GHz) 21 , Cs 852 nm (88%, 0.56 GHz) 22 , Cs 894 nm (77%, 0.96 GHz) 23 , Sr 461 nm (63%, 1.19 GHz) 24 , etc 25 – 28 . An ultra-narrow optical filter based on Faraday effect has been demonstrated in 2012 29 , of which the bandwidth is 6.2 MHz.…”