Tunable Multimode Optical Delay Line for Single-Wavelength Microwave Photonic Transversal Filter

Most existing microwave photonic transversal filters are implemented in the incoherent regime using multiple optical carrier wavelengths to avoid optical interference. The system therefore becomes complicated and expensive. In this work, we present a novel single-wavelength microwave photonics transversal filter design in the coherent regime using a single multimode fibre. Filter taps are generated from spatial modes excited by multiple narrow optical beams with different incident angles. Therefore, optical interference is eliminated due to space demultiplexing. Time delays between filter taps are obtained due to large modal dispersion and can be simply tuned using spatial slots. In a proof-of-concept experiment, a modal dispersion enabled optical delay line module for a two-tap microwave photonic transversal filter has been demonstrated with avoided optical interference.