All-optical microwave bandpass filters implemented in a radio-over-fiber link

An all-optical microwave bandpass filter implemented in a radio-over-fiber (RoF) link is proposed and demonstrated. The filter consists of an optical phase modulator, a length of high birefringence (Hi-Bi) fiber, a 25-km singlemode fiber and a narrow linewidth laser source. Different time delays are achieved when the two orthogonal polarization modes are traveling along the Hi-Bi fiber. The baseband resonance is eliminated by use of the optical phase modulator in combination with the 25-km single-mode fiber serving as a dispersive device. The proposed filter is immune to optical interference because of the orthogonality of the two polarization modes. A two-tap all-optical microwave bandpass filter with a null-to-null bandwidth of 8.7 GHz and a notch rejection level greater than 30 dB implemented in the 25-km RoF link is demonstrated.