Microwave Photonic Filters Based On Phase Modulation

We propose and demonstrate three microwave photonic filters (MPFs) based on phasemodulation (PM). The first one is based on the concept of using the Brillouinselective sideband amplification to achieve effective PM-to-AM conversion. Onlywhen one of the phase modulated sidebands lies in the stimulated Brillouin scattering(SBS) gain spectrum, can PM-to-AM conversion occur and the corresponding PMfrequency be detected. As a result, a widely tunable single bandpass filter centered ata special frequency has been obtained, with narrow bandwidth of about35MHz and atuning range of20GHz. The second one is based on a fiber-Bragg-grating-basedFabry-Perot (FBG-FP) cavity etched in a high-birefringence fiber. It can switchbetween continuously tunable single notch filter and bandpass filter operation. Theprinciple of the tunable notch filter is based on the reflection response of the FBG-FPto break the phase balance of PM sidebands and achieve effective PM-to-AMconversion, while the principle of the bandpass filter is based on the transmissionresponse of the FBG-FP to break the amplitude balance of the PM sidebands. Atunable notch filter with a3dB bandwidth of about40MHz and a bandpass filterwith a3dB bandwidth of about100MHz have been obtained. The third is anincoherent complex coefficient microwave photonic filter based on a multiwavelengthfiber laser and a high birefringence fiber loop mirror (Hi-Bi FLM). A multiwavelengtherbium-doped fiber laser (EDFL) provides multiple taps, while the Hi-Bi FLMperforms PM-to-AM conversion. Taps with positive, negative, or complex coefficientscan be generated by control of the multiwavelength input at different positions of theHi-Bi FLM response with positive and negative slopes. Therefore, the filter withpositive, negative, or complex coefficients has been achieved by adjustment of theEDFL output.