Research on 32-tupling frequency terahertz wave generation based on Mach-Zehnder modulators cascaded

Abstract

A novel scheme to generate 32-tupling frequency terahertz waves by four Mach-Zehnder modulators (MZMs) cascaded is proposed. Four MZMs are divided into two groups linked in series and each group includes two MZMs connected in parallel. The output of the first group and second group of MZMs are ± 4nth and ± 8nth-order sideband signals, respectively. The 0th-order sideband can be cancelled by adjusting the splitting ratio of coupler; The ± 8th-order sidebands can be suppressed by adjusting the modulation index m of MZMs; The ± 8nth (n＞2) sidebands are negligible because of their very small amplitudes. Then the main components of output signal from the second group MZMs are ± 16th-order sidebands, which can generate 32-tupling frequency terahertz signal on the photodetector. The operation principle and simulation experiment of the proposed scheme is performed, and the optical sideband suppression ratio (OSSR) and radio frequency spurious suppression ratio (RFSSR) are obtained to be 53.7 dB, 53.34 dB and 47.7 dB, 47.39 dB, respectively. Simulation experiment values are very close to theoretical values and this demonstrates the feasibility of our scheme. The effects on the OSSR and RFSSR deviated from their designed values are investigated. When the deviations of modulation index, extinction ratio, DC bias, half-wave voltage, the insertion loss of MZM reach about ± 0.42 %, 82 %, ± 8.4 %, ± 1 %, 0.14 %, respectively, and when the deviations of initial phase of RF and splitting ratio of coupler reach about ± 1.3 % at 45°, ± 1.1 % at 90°, ± 2.27 %, respectively, the RFSSR are ≥ 15 dB, which satisfies the engineering applications