Photonic generation of millimeter-wave signals with tunable phase shift

A novel photonic approach to generating a frequency-quadrupled millimeter-wave (mm-wave) signal with a tunable phase shift is proposed and demonstrated. Two second-order optical sidebands are generated by using a Mach-Zehnder modulator (MZM) that is biased at the maximum transmission point and an optical notch filter. A polarization-maintaining fiber Bragg grating (PM-FBG) is then utilized to make the two sidebands orthogonally polarized, which are then sent to a polarization modulator (PolM). By aligning the two orthogonally polarized sidebands with the two principal axes of the PolM, complementary phase modulation is thus achieved. By beating the two phase-modulated sidebands at a photodetector, an mm-wave signal is generated and its phase is continuously tunable by tuning the bias voltage to the PolM. An experiment is performed. An mm-wave signal with a frequency tunable from 36 to 52 GHz is generated and its phase is tunable over 360° by tuning the bias voltage.