LiNbO3 optical SSB modulator using microwave branchline coupler miniaturized by inter-digital capacitors

Optical single sideband (SSB) modulation can be realized by using a Mach-Zehnder (MZ) interferometer with the branch-line coupler (BC) circuit as a 90 degree hybrid. In this paper, a miniaturized configuration of the BC circuit with installed series capacitors of the inter-digital structure was designed onto an electro-optic (EO) crystal, LiNbO₃, substrate. Compared with a conventional BC, an area of the miniaturized BC was reduced by about 60% and the operation as the 90 degree hybrid was confirmed at around 10 GHz. The BC circuit directly connected with modulation electrodes was designated onto an EO modulator substrate for a single input type optical SSB modulator with a compact configuration. In the experiment, the modulator was fabricated and the optical SSB signal with the sideband suppression ratio of more than 30 dB was observed using light-waves of the 1550 nm region.