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High power peripheral coupled waveguide electroabsorption modulator for analog fiber-optic link applications

Abstract

Semiconductor electroabsorption modulator (EAM) has found its way in fiber-optic communications thanks to its small size, good modulation efficiency, and promising integration with other semiconductor optoelectronic devices. High power handling capability and low insertion loss are desirable for EAM to achieve high link gain, low noise figure (NF), and high spurious-free dynamic range (SFDR) in analog fiber-optic link applications. By applying peripheral coupled waveguide (PCW) structure to EAM, the optical mode is buried down below the absorption layer with small confinement factor [Gamma], leaving only an evanescent tail for modulation. Consequently, the coupling between the EAM waveguide and fiber along with the waveguide propagation loss is improved, resulting in a reduced fiber-to-fiber insertion loss that is comparable to low loss LiNbO₃ Mach-Zehnder modulators (MZMs). A record low loss of 4 dB was demonstrated by a PCW EAM. Besides, small [Gamma] results in low absorption and low photocurrent densities, which enables EAM to survive high optical power and delays the onset of saturation. Devices withstanding 590 mW input optical power and generating 222 mA photocurrent were made and measured. Link gain of within -10 dB was commonly achieved with PCW. PCW with tapered waveguide structure was also investigated to further increase EAM power handling capability. At high optical power, multiple physical mechanisms kick in to play roles in the performance of EAM, among which are carrier screening, band-gap shrinkage due to temperature increase, material index of refraction change due to large photocurrent, and most prominently the junction resistance reduction as a feedback effect on the input microwave signal. An analysis based on EAM equivalent circuit model was presented for the photocurrent feedback effect with experimental support, yielding a gain limit for EAM at high power. The analysis also led to an alternative conjecture of using blue-shift QCSE material with negative differential resistance to overcome the EAM gain limit. In addition, the linearity of the EAM at high power was theoretically investigated, showing improvement with the presence of photocurrent feedback. An SFDR of 135 dB/Hz²/³ was estimated to be obtainable at 700 mW input optical power

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