5 - Broadband traveling wave modulators in LiNb0₃

Summary

Introduction

Traveling wave electro-optic devices were first proposed in 1963 and demonstrated, in LiNbO₃ waveguide modulators in the late seventies. The basic requirements for broadband operation were known at the outset: velocity matching between the microwave and optical signals, low electrical loss in the microwave guide, and impedance matching between the microwave waveguide and external electrical connectors. However, these conditions were not immediately realized because the dielectric constants and refractive index of LiNbO₃ made velocity mismatch inevitable, within the constraints of existing electrode geometries. Eventually, accurate modeling of the multilayer structure using numerical methods, combined with processing improvements, led to devices which were nearly velocity matched. Electrode thickness increased as the structures evolved, which naturally decreased the microwave loss, further extending device bandwidth.

Impedance matching has been considered a lower priority than velocity matching and low losses and was not achieved in a velocity matched device until etched ridge LiNbO₃ devices were introduced. These devices concurrently satisfy the conditions for broadband operation, and operation beyond 100 GHz has recently been attained.

The development of broadband, low Vπ modulators is a final consideration. While the drive voltage can be decreased by lengthening the electrodes, this also increases electrode loss, resulting in a trade-off between drive voltage and bandwidth. Clearly, requirements for a particular application must be considered in modulator design.

This chapter covers the impressive progress that has been made in broadband LiNbO₃ modulators over several decades, and addresses current challenges which face researchers.