Mach-Zehnder Electro-optic Modulator with Multimode Interference Couplers of LiNbO3 Waveguides for Single Sideband Modulation

We designed and evaluated 1×2 and 2×2 optical couplers using multimode interference (MMI) optical waveguides of Ti-diffused LiNbO
3
. A Mach-Zehnder electrooptic modulator (MZM) designed using those two MMI couplers as an optical power splitter and combiner was fabricated. Optical intensity modulation was achieved with a 50 dB extinction ratio. And the experimental results showed that the 2 ×2 MMI optical coupler could provide nearly equal power division with a 90° phase difference as a 90° hybrid. Next, we applied the MZM structure to the single sideband (SSB) modulation. 10 GHz microwave signals of 90° phase difference were fed to the modulation electrodes on the two arms of the MZM. The SSB modulation operation was confirmed from output light spectra, and the sideband suppression ratio of 39 dB was observed by miner adjustment of the optical phase bias.