Broadband Image-Reject Mixing Based on a Polarization-Modulated Dual-Channel Photonic Microwave Phase Shifter

Broadband image-reject mixing using a dual-channel polarization-modulated photonic microwave phase shifter is proposed and experimentally demonstrated. The polarization-modulated photonic microwave phase shifter is realized by an orthogonal circularly-polarized wavelength generator, an optical coupler, two polarizers, and two photodetectors (PDs). When a local oscillator (LO) signal, a radio-frequency (RF) signal, and an image interference are supplied to the polarization-modulated phase shifter, opposite phase shifts would be introduced to the RF- and image-induced intermediate frequency (IF) signals. By carefully adjusting the polarizers to make the two channels of the phase shifter quadrature, a phase difference of & x2212;90 degree is introduced to the RF-induced IF signals in the two channels, while 90-degree phase difference between the image-induced IF signals is produced. With an electrical 90 degree hybrid to combine the two channels, the two RF-induced IF signals will be in phase while those from the image are out of phase. As a result, the image in the downconverted signal is cancelled. An experiment is carried out. An image-reject mixer (IRM) with an image rejection ratio of & x223C;60 dB for a single-frequency signal and 23 dB for a 4-GHz linear frequency-modulated signal is obtained.