Broadband and low driving-voltage LiNbO3 optical modulators

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Abstract
Travelling-wave type LiNbO3 (LN) optical modulator structures employing thicker coplanar waveguide (CPW) electrodes are presented, for achieving a lower driving-voltage and broader bandwidth characteristic with an impedance and velocity match condition. To design a high-performance device-structure, a figure of merit p is introduced for travelling-wave type optical modulators, which can make an excellent modulator with a smaller value of p. Microwave characteristics of the modulators with a ridged type LN-substrate are theoretically compared to those of planar-type modulators. As a result, the characteristic-impedance and the product of driving-voltage and interaction-length of the electrodes in modulators are shown to be almost determined only by the buffer-layer thickness for any modulator-structure under a velocity-matching condition. By utilising a wider gap and accordingly thicker electrodes with a ridged LN-substrate, LN modulator structures having a 40 GHz-band and far less than a 3 V driving-voltage have been clarified under a 50 Ω characteristic-impedance system.