Lithium Niobate Modulators
A¥er the Czochralski process for single-crystal growth had been established, research on lithium niobate (LiNbO3) optical modulators started in the 1960s. LiNbO3’s large electro-optic (EO) coešcient led to the development of bulk-type optical modulators, in which a LiNbO3 crystal is sandwiched by two metal plates. ™e optical beam propagating in the crystal is modulated by applying a voltage of several hundred volts to the electrode plates. In the 1970s, fabrication technologies for optical waveguides in LiNbO3 substrate were developed by exploiting the out-diŸusion of lithium oxide (Li2O), where the optical wave is con˜ned and propagates in the high-index guiding layer and so the drive voltage (half-wave voltage) was reduced to 24 V . A ridge waveguide with a cross section of several square micrometers was fabricated by ion-beam etching as shown in Figure 6.1 . ™is advance made it possible to restrict the interaction region of the external electrical signal and optical waves near the optical waveguide and to keep the interaction length at several centimeters. As a result, the ešciency of the EO interaction increased and drive voltages were reduced to about 4 V.