Broadband, velocity-matched, traveling-wave, electrooptic modulator using a novel superstrate cover

Modulators based upon the linear electro-optic effect are the most versatile and fastest commercial modulators for communications available today. They provide a variety of modulation methods, e.g. phase, amplitude and polarization. They are wavelength versatile and can handle relatively high optical power. High driving voltages are their main disadvantage. To reduce driving voltage, traveling-wave designs are employed. By increasing the interaction length of the electrical and optical signals, half-wave voltages are reduced.;This thesis describes a novel gallium arsenide traveling-wave electro-optic modulator that utilizes a resonant-free, broadband, velocity-matching superstrate 'cover' to increase the interaction length, improve electrical and optical wave confinement, reduce radiation loss and most importantly reduce the half-wave voltage. The design, development and testing of a gallium arsenide modulator with and without a superstrate is presented. The superstrate is fabricated by an easily implemented fabrication technique and can be applied to any high-speed integrated circuit device. For gallium arsenide modulator built, it acts as a cladding for the optical waveguide and slow-wave structure for the electrical transmission line. A stable insulating semiconductor superstrate of more than ten micrometers thick was deposited over a coplanar strip transmission line modulator with an optical ridge waveguide situated between the coplanar metal strips. Modeling of the TEM mode of the coplanar strip electrodes shows that 90% of the intensity of the electrical signal's field is contained within the cover. This results in lower attenuation by reducing radiation loss and improved bandwidth by reducing modal dispersion.;Identical modulators with and without a cover were measured for comparison. A 120-fs Ti:sapphire oscillator was used to photo-excite the dc-biased coplanar stripline producing a fast microwave transient, which was then sampled with 120-fs pulses at 1.55 mm from a synchronized optical parametric oscillator. The half-wave voltage was reduced from 82 volts for the modulator without a cover to 54 volts with a cover. Bandwidth was increased by more than 18% from 13.5 to 15.9 GHz.