A comparison of power requirements of traveling-wave LiNbO3optical couplers and inteferometric modulators

It is shown that a coupler and interferometer may be considered two extremes of a generalized two-guide coupler device in which electrode length is less than a coupling length. For electrode lengths less than half of a coupling length, the generalized device looks more like an interferometer than a coupler. The impedance and electrical loss of asymmetric-stripe and coplanar-waveguide transmission lines on LiNbO3for thick as well as thin electrodes are then presented. The effects of the electrical loss on the bandwidth and drive requirements of traveling-wave LiNbO3couplers and interferometric devices are discussed, and it is shown that electrical loss in the transmission line is more detrimental to a coupler than to an interferometer. These results are then used to compare the drive power requirements of coupler and interferometric devices, and it is shown that for the same bandwidth and same optical guide dimensions, an interferometric device with a coplanar transmission line can require a lower drive power than a coupler with an asymmetric-stripe line.