Frequency discriminator based on ring-assisted fiber Sagnac filter

We build and experimentally investigate the characteristics of a frequency discriminator based on a ring-assisted fiber Sagnac filter. By adding an intracavity fiber ring resonator to a Sagnac filter, we introduce additional phase shifts which linearize the transfer function around the quadrature point under proper polarization conditions. Compared to a standard Sagnac filter frequency discriminator, the proposed device improves the dynamic range of the filter by 3 dB. This corresponds to a 4-dB improvement in the dynamic range for the recovered radio-frequency signals.