A novel radiative optical isolator and a circulator are discussed in this paper. It is numerically estimated that with =1.3 m, 45-dB isolation can be achieved with an isolator of 8 mm in length. For a 21.5-mm-long 4-port circulator, the achievable isolation is about 45 dB, with approximately 1-dB insertion loss.
Kang Xie and Lionel E. Davis, "Radiative Optical Isolator and Circulator," J. Lightwave Technol. 19, 1028- (2001)