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20 February 2007 Low-loss low-voltage AlGaAs/GaAs high-speed optical switch with doping and composition graded heterojunction interfaces
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Carrier-injection-type high-speed semiconductor optical switches have been of interest in recent years due to their nanosecond switching times, their immunity to variations in temperature, wavelength, polarization, etc, and the ease with which they can be monolithically integrated with other optoelectronic components and electronic circuitry. Their drawbacks, however, have been high insertion loss and excessive power dissipation. To overcome these limits, a novel, large cross-section, single-mode AlGaAs/GaAs optical switch has been designed and fabricated. The switch's strip-loaded waveguide uses a five-layer W-shaped heterostructure and a 1.7&mgr;m-thick core layer, which provides high fiber-coupling efficiency. Since the constituent heterojunction band discontinuities can impede the current across the junction, the addition of 20nm-40nm thick, compositionally graded interfaces significantly reduces the switching voltage. In addition, using a lightly doped core layer can reduce the series resistance of the switch, which is important in heat reduction. The core doping needs to be low otherwise it will cause increased free-carrier absorption, which contributes to high insertion loss. We have fabricated switches with different core doping levels using both abrupt and graded heterojunctions. The measured on-chip optical propagation losses are 0.3dB/cm for unintentionally doped core, 1.5dB/cm for n = 1x1016cm-3 doped core, and 2.7dB/cm for n = 5x1016cm-3 doped core. The measured I-V curves show that the switching voltage can be reduced by changing abrupt heterojunctions to graded ones. The calculated theoretical band structure for switches with abrupt/graded heterojunctions based on thermionic emission clearly demonstrated the advantages of applying grading in semiconductor optical switches.
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Liping Sun, Julian Noad, Robert James, David Coulas, Shaochun Cao, Glendon Lovell, and Erle Higgins "Low-loss low-voltage AlGaAs/GaAs high-speed optical switch with doping and composition graded heterojunction interfaces", Proc. SPIE 6469, Optical Components and Materials IV, 64690Q (20 February 2007);

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