Multitap Microwave Photonic Filter With Negative Coefficients Based on the Inherent Birefringence in a $\hbox {LiNbO}_{3}$ Phase Modulator

A novel technique to implement a multitap microwave photonic filter with positive and negative coefficients based on the inherent birefringence in a LiNbO₃ phase modulator is proposed and demonstrated. In the proposed filter, a microwave signal is applied to the phase modulator and an optical polarizer is connected at the output of the phase modulator to perform polarization interference and phase-modulation to intensity-modulation (PM-IM) conversion. Thanks to the inherent birefringence in the LiNbO₃ crystal, a π phase shift is obtained by adjusting the wavelength spacing between two adjacent wavelengths, which leads to the generation of a positive coefficient and a negative coefficient. An equivalent experiment is performed. Four-tap and six-tap microwave photonic filters with positive and negative coefficients are experimentally demonstrated. The reconfigurability of the four-tap and six-tap microwave photonic filters is also investigated.