Abstract
Electro-absorptive optical waveguide modulators operating in the 1.5 μm wavelength range are key components for long haul high bit rate fiber links. Large absorption variations can be obtained using the quantum-confined Stark effect (QCSE) in multiple quantum well (MQW) structures1 but it requires large electric fields. This results in large drive voltages which is a major drawback for high-speed modulation. Wannier-Stark localization (WSL) in superlattices (SL) is an alternative to achieve low drive voltage optical waveguide modulation2,3: for photon energies above the SL bandgap the application of low electric fields leads to negative absorption variations ("blue shift") because the resonant tunneling probability decreases dramatically and the zero-field broadened absorption edge sharpens into a step-like edge. However residual absorption is large because of the (-p) oblique transitions connecting electrons and holes localized in wells separated by p periods. Below the SL bandgap these oblique transitions leads to positive absorption variations together with low residual absorption. In our previous work3 we only achieved moderate modal attenuation variations so that relatively long waveguide lengths (500 (μm) were necessary to obtain significant extinction ratios (20 dB) and it could be argued that the ultimate bandwidth of QCSE-based modulators should be higher because of lower device capacitance. In this work we present extinction ratios per unit waveguide length similar to those of the best reported QCSE-based device4, but these have been obtained with much lower electric fields.
© 1991 Optical Society of America
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