UBC Theses and Dissertations
Integrated-optic electrorefraction modulators Ristic, Sasa
The large electroabsorption (EA) and electrorefraction (ER) effects that can be achieved in coupled quantum well (CQW) structures give these structures great potential for use in intensity, phase, and polarization modulators for future optical links. This thesis focuses on designing structures that will result in state-of-the-art performance of CQW-based modulators. In this thesis, a design for a push-pull polarization-modulation scheme is presented, and it requires placing multiple repetitions of two different CQW structures in the optical waveguide of the proposed polarization modulator. The main function of one of the two structures is to provide a large increase of the refractive index for the TM polarization, without a proportionate increase of the refractive index for the TE polarization. It is shown that this effect can be achieved by anticrossing light holes. The main function of the other structure is to provide a large decrease of the refractive index for the TE polarization, without a proportionate decrease of the refractive index for the TM polarization. It is shown that, although this effect can be efficiently achieved by anticrossing heavy holes, the CQW structures that achieve this effect by anticrossing electrons are less sensitive to the layer thickness and compositional variations that may be expected during the growth process. The large EA and ER effects in CQW structures are typically seriously compromised by the layer variations, and, it is shown in this thesis that decreasing the confinement of the anticrossing wave functions can decrease the sensitivities of the CQW structures to these variations. In addition, the very small confinement of light holes in the InGaAlAs-based material systems together with the small sensitivity of the light-hole band edge to the compositional variations, make the anticrossing of the light holes the most robust-to-growth and, therefore, the most promising mode of operation for CQW-based ER intensity, phase, and polarization modulators. The absorption spectra of the novel CQW structures designed in this thesis are obtained using the variational method for solving the 1S exciton equation in the effective-mass, envelope-function approximation, and the electric-field-induced refractive index changes are obtained using the Kramers-Kronig relations.
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