Wide-bandwidth high-efficiency electroabsorption modulators for analog fiber-optic links

Semiconductor electroabsorption modulators (EAMs) are attractive modulation devices due to their compact size, high modulation efficiency and potential of integration with other components. Wide-bandwidth and high-efficiency EAMs are desirable for both analog and digital fiber-optic links. In this work, it is found that to achieve maximum modulation efficiency in analog links, the optimal waveguide length for EAMs is 200–300 μm, primarily limited by the optical propagation loss. Large-optical-cavity (LOC) waveguide offers higher link RF gain than the 3-layer waveguide due to better optical coupling efficiency and higher optical saturation power. Two novel self-bias-control approaches are proposed for EAMs to maintain the best modulation efficiency under changing operating conditions.; An RF circuit model has been developed for lumped-element EAMs (L-EAMs), including the effect of photocurrent. It is found that the EAM photocurrent can limit the optical saturation power, change the EAM impedance, improve the modulation bandwidth, degrade the link noise figure and spurious free dynamic range. Various approaches are analyzed for broadening the L-EAM bandwidth. Analysis indicates that bandwidth up to 30 GHz can be achieved for L-EAM without compromising much modulation efficiency.; Traveling-wave electroabsorption modulator (TW-EAM) is investigated for ultra-wide bandwidth and high modulation efficiency. A theoretical model is established to calculate the TW-EAM frequency response, including the effects of impedance mismatch, velocity mismatch and microwave loss. It is shown that traveling-wave design can improve performance for short-length EAM. Low-impedance-termination turns out to be the simplest approach for achieving 50–100 GHz bandwidth for TW-EAM.; The fabricated TW-EAM devices demonstrate excellent performance, including 11.3 dB optical insertion loss without anti-reflection coating, 0.65 V −1 slope efficiency (corresponding to 2.4 V equivalent V _π), modulation bandwidths of 35 GHz to more than 40 GHz (limited by the measurement equipment), 25–45 mW optical saturation power under 18 GHz modulation and −35 dB high frequency RF gain. These combined performances are considered the best results to date. It is proved that the theoretical modeling for the TW-EAM agrees very well to the experiments.