Optical modulators based on interferometric or resonant waveguide structures modulate light by introducing a change in the effective refractive index of the optical material, which provides a shift in the optical phase of a lightwave passing through the modulator structure. Transmission rates of 400 GHz (and higher) are pushing the limits of conventional, LiNbO3-based optical modulators beyond the physical capabilities of the lithium niobate material itself. A LiNbO3 modulator is based on a linear electro-optic effect and exhibits a relatively low degree of modulation as a function of length of the device. As a result, this type of linear electro-optic effect modulator requires either relatively high drive voltages (unwanted heat), or a relatively long length of device (trending away from the “small size” requirement), or both. Lithium niobate is also known to have a limited modulation bandwidth. Current LiNbO3 modulator configurations have a large thermal drift, requiring fast control to stabilize operation.
The continuing migration to smaller package sizes (e.g., CFP to CFP2 to QSFP and beyond) reduces the available space for optical modulators and thus increases component density. There is also an increasing demand to lower power consumption by various ones of these optical devices. Advanced modulation formats (e.g., DQ-PSK and 16QAM) require better extinction ratios (ER), better linearity, lower insertion loss (IL), lower noise and higher modulation bandwidth than possible with LiNbO3-based devices.