Theoretical research of multiple-pass fiber optical frequency shifter

All-fiber frequency shifter of polarisation maintaining fiber is of great importance in fiber optics. Previous researchers generally studied one-pass all-fiber frequency shifter of polarisation maintaining fiber. This paper discusses multiple-pass all-fiber frequency shifting of polarisation maintaining fiber theoretically. The solutions of coupled-mode equations for polarisation maintaining fiber when the two orthogonal modes exist is discussed, and the multiple-pass frequency shifting is analyzed. The special issue of the two-pass frequency shifter where the linearly polarized light launching into one polarisation axis of the polarisation maintaining fiber is discussed.