Abstract: The precisely controlled spectral characteristics of a novel Sagnac loop filter was studied, which is structured by insertting two segments of high birefringence optical fiber and a fiber polarization controller into a conventional optical fiber Sagnac loop filter. The study results based on the equivalent optical-path analysis and transfer matrix method show that the spectral characteristics of the Sagnac loop filter can be precisely controlled, by changing the polarization controller status, when the birefringence parameters of birefringence fiber are fixed. The Sagnac loop filter can be used to flat the spectra of erbium-doped fiber (EDF) Amplified Spontaneous Emission (ASE) light source, and the experimental results show that the broadband EDF ASE laser spectra is flat, through control the tilt angle of the three PC wave-plates.

Key words: Sagnac loop filter; ASE light source; high birefringence fiber; polarization controller