Photonic generation of widely tunable and background-free binary phase-coded radio-frequency pulses.

We present a novel photonic approach to generating widely tunable and background-free binary phase-coded radio-frequency (RF) pulses by cascading a polarization modulator (PolM) and a phase modulator (PM). The PolM is used to produce an optical carrier and two sidebands with orthogonal polarization states. The phase shift θ between the optical carrier and the sidebands is controlled by the electrical driving signal applied to the PM. For θ>π/2 or <π/2, the phase of the detected RF signal is 0 or π, respectively. For θ=π/2, there is no RF signal recovered in the photodiode (PD). In this way, binary phase-coded RF pulses can be generated, while the optical power launched to the PD keeps constant. The proposed technique is therefore background free by eliminating the baseband frequency components. Moreover, the carrier frequency of the RF pulses is widely tunable and the π phase shift of the RF signal is independent of the amplitude of the electrical driving signal. The proposed scheme is theoretically analyzed and experimentally verified.