Frequency quadrupling using an integrated Mach–Zehnder modulator with four arms

In this paper, a novel technique to realize frequency quadrupling in the radio over fiber system is proposed. The frequency quadrupling is achieved by using an integrated Mach–Zehnder modulator (MZM) consisting of a 1 × 4 multimode interference (MMI) coupler and four optical phase modulator arms. Due to the inherent optical splitting ratio and phase relations between the outputs of the MMI coupler, the optical harmonics at + 1 order and − 3 order are generated corresponding to four times frequency of the microwave drive signal, only by setting the DC bias voltage of the main MZM at the minimum transmission point. That leads to the integrated MZM with reduced complexity compared with the conventional dual-parallel MZM. The effect of the nonideal integrated MZM, including the splitting imbalance and the bias drift, on all the sidebands is also analyzed theoretically. The following simulation results show that the slight deviation of the ideal values would not cause great degradation of the generated optical millimeter-wave signal.

Research highlights
► The integrated Mach-Zehnder modulator is based on the multimode interference coupler.
► The complexity of the proposed MZM is largely reduced.
► Frequency quadrupling is achieved by beating of +1 order and -3 order harmonics.
► The RoF system using our proposed technique has better robustness.