Lithium Niobate Traveling-wave Optical Modulators To 50 GHz

Modulation of optical signals is necessary for both high-speed digital and analog fiber-optic systems. Direct modulation of laser diodes is possible in some of these systems, but linewidth broadening due to chirp can lead to dispersion penalties. In addition, the highest modulation rates have been achieved with external modulators, with performance up into the millimeter-wave frequency range. The most mature external modulator technology is based on waveguide devices in lithium niobate.¹ Earlier problems of high optical insertion loss in the devices have largely been overcome. Much of the recent work has concentrated on increasing the bandwidth and reducing the drive power requirements in the external modulators.