Filter-free photonic frequency sextupler operated over a wide range of modulation index

An integrated dual-polarization modulator is proposed to generate frequency-sextupled microwave signal without an optical filter. The dual-polarization modulator can achieve even-order optical sidebands suppression modulation along the orthogonal polarization directions with different modulation index. By properly adjusting the power relationship of the orthogonal polarization directions, two optical first-order sidebands will cancel each other out. Thus, a pure frequency-sextupled signal with an optical sideband suppression ratio (OSSR) exceeding 38 dB and a radio frequency spurious suppression ratio (RFSSR) higher than 32 dB is generated, which match well with the theoretical analysis. Since there is no requirement to carefully adjust the modulation index to achieve correct operation of the sextupler, it is much easy to implement. Furthermore, it is also proved to be valid that even if the extinction ratios of the MZMs, the polarization angle of the polarizer, and the direct current (DC) bias phase shift deviate from the ideal values to a certain degree, the performance of the generated frequency-sextupled microwave signal is still acceptable.