hybrid subtracts the electrical taps from both photodiodes and the signal impinges the vector network analyzer.
In the experiment, all fiber gratings are uniform Bragg gratings fabricated via the phase-mask technique in a nonoptimized procedure and, therefore, they provide an irregular reflectivity profile. In the previous setup, the fiber gratings were fixed in a mechanical stage to be stretched to the desired Bragg wavelengths. The experimental results are shown in Figure 3. The amplitude response of the fiber-grating-based notch filter with fiber spacing of 83 cm is measured. About 33% reflectivity for FBG1 can be obtained at wavelength of 1546.42 nm. When a two-arm delay line with a fiber-length difference of 166 cm is put into the system, the flat-top passband of the combined filter-frequency response can be obtained, and the ripples in the passband are within 0.5 dB above the frequency range of about 50% filter FSR.
CONCLUSION
We have proposed a novel incoherent microwave photonic filter that combines a recirculating notch filter based on a FBG-based resonator and a designed two-arm delay-line filter. This filter can generate a flexible notch filter with wide flat-top passband. The ripples in the passband depend on the coupling ratio of the coupler. The feasibility of the filter has been experimentally demonstrated. The response of the filter is stable in the presence of environmental perturbations, as expected, based on the incoherent optical system.