Microwave structures for traveling-wave MQW electro-absorption modulators for wide band 1.3 mu m photonic links

To implement millimeter wave photonic links using high speed optical modulators, bandwidth and modulation efficiency are important considerations. In this paper we discuss design and fabrication of novel traveling-wave multiple quantum well (MQW) electro-absorption modulator structures which can be used for wide-band applications, covering DC to 40GHz or higher frequencies, that promise to provide better bandwidth and efficiency than conventional lumped modulators. From the microwave point of view, traveling wave modulators are constant impedance transmission lines, and are not limited by the RC roll off associated with modulator capacitance. Their sensitivity can be increased by increasing device length without significantly sacrificing the bandwidth. The principal bandwidth limitation comes from microwave loss. In this work, ridge co-planar waveguide structures were designed and fabricated to achieve good impedance matching, low microwave loss, low dispersion and reasonable phase velocity matching between lightwave and microwave. Two port measurements for these waveguides were performed up to 40GHz with a network analyzer. The results show effective microwave index in the range of 4.5 to 3.6 (which is close to the effective index of the guided light wave), characteristic impedance around 30 Omega, microwave attenuation less than 6dB/mm at 40GHz and low dispersion. These characteristics are all promising for wide band high efficiency traveling wave modulators.