Generation of triangular waveforms based on a microwave photonic filter with negative coefficient.

We report a novel approach to generating full-duty-cycle triangular waveforms based on a microwave photonic filter (MPF) with negative coefficient. It is known that the Fourier series expansion of a triangular waveform has only odd-order harmonics. In this work, the undesired even-order harmonics are suppressed by the MPF that has a periodic transmission response. A triangular waveform at fundamental frequency can be generated by setting the bias of a Mach-Zehnder modulator (MZM) at quadrature point. However, it is found that a broadband 90° microwave phase shifter has to be used after photodetection to adjust the phases of odd-order harmonics. Alternatively, a frequency doubling triangular waveform can be generated by setting the bias of the MZM at maximum or minimum transmission point. This approach is more promising because the broadband microwave phase shifter is no longer required in this case but it is more power consuming. The proposed approach is theoretically analyzed and experimentally verified.