Microwave Band-pass Filters With Negative Coefficients Based On Photonics

A novel and simple technique for obtaining negative coefficients for the implementation of all-optical microwave band-pass filters is presented.Digital electronics is the most widely used approach nowadays for microwave signal processing. However, its speed is normally less than several gigahertzes, limited by the electronic sampling rate. it is extremely unconvenient since there is need to perform opto-electronic and electro-optic conversions. Microwave photonics make it a promising alternative for processing the limitation above. Most of the proposed filters are operating in the incoherent regime with only positive coefficients. All-optical filters with all positive coefficients can only function as low-pass filters or equivalent band-pass filters. Positive coefficients filters are impossible to improve their performance. So it's very important to implement the negative coefficients.the positive and negative coefficients are obtained by letting two phase-modulated optical carriers pass through an unbalanced Mach-Zehnder interferometer which serves as a PM-IM conversion device. Because the center wavelengths of the two carriers are tuned at the quadrature frequencies of the positive and negative slopes of the transfer function of the unbalanced Mach-Zehnder interferometer. The PM-IM converted signals are then applied to a length of single-mode fiber (SMF) serving as a dispersive device to achieve different time delays. When the signals are accepted by the photodetector, the envelopes of the recovered microwave singals will beπout of phase.Based on the theory above, a two-tap microwave band-pass filter with negative coefficients is demonstrated. The intensity transfer function of the proposed filter is simulated and the tunability is also investigated.The frequency response of the negative microwave filter has equal FSR from baseband to a few Ghz. The proposed approach has the potential to implement the arbitrary conversion of positive coefficients to negative coefficients or on the opposition by selecting proper optical carriers.