Dynamic range improvement of a microwave photonic link based on bi-directional use of a polarization modulator in a Sagnac loop

A novel microwave photonic link (MPL) with an improved spurious-free dynamic range (SFDR) based on a bidirectional use of a polarization modulator (PolM) in a Sagnac loop is proposed and demonstrated. The PolM in the loop functions, in conjunction with a polarization controller and a polarization beam combiner, as a Mach Zehnder modulator (MZM), which only modulates the incident light wave along the clockwise direction, leaving the counter-clockwise light wave unmodulated due to the velocity mismatch. Two clockwise intensity-modulated signals along two paths (Path 1 and Path 2) are generated, with one (Path 2) combined with the non-modulated light wave from the counter-clockwise direction to suppress part of the optical carrier. By controlling the power relationship between the two paths, the third-order intermodulation distortion (IMD3) can be fully suppressed, and thus an MPL with improved dynamic range is achieved. A theoretical analysis is presented, which is validated by an experiment. The IMD3 can be suppressed by 50 dB, giving an improvement in SFDR of 16 dB.