Optical single-sideband polarization modulator based on Sagnac interferometers and its applications in radio-over-fiber systems

A novel scheme to implement an optical single-sideband (OSSB) polarization modulator (PolM) is proposed and theoretically demonstrated in this paper. The proposed OSSB-PolM is based on the phase modulation technique in two consecutive Sagnac interferometers (SIs). It is shown that by applying four equal-power radio frequency (RF) signals with appropriate phases to electro-optic modulators and aligning the polarization state of the modulated signal in the proper direction, an OSSB-PolM signal will be generated. The proposed scheme can find many microwave photonic (MWP) applications. To show this, an OSSB modulator with tunable optical carrier-to-sideband ratio, an OSSB-suppressed carrier modulator/optical frequency shifter, and a full-ranged MWP phase shifter are proposed and demonstrated. The proposed structure includes several important advantages. For instance, the system is free from the limitations caused by optical/electrical filters. Since the propagating lights in an SI traverse the identical optical path in clockwise and counterclockwise directions, the system is immune to environmental perturbations. Furthermore, there is no need for a bias control circuit, so the system has a stable operation. In addition, the proposed scheme can be extended to multichannel applications in array antennas. Theoretically, calculations have been verified by simulation results.