Design and analysis of low-chirp electroabsorption modulators using bandstructure engineering

The design principle for completely negative-chirp operation of an electroabsorption (EA) modulator in the 1.55 μm window is studied theoretically in InGaAsP strained quantum well (QW) structures for strain ranging from compressive to tensile. The small-signal chirp parameter for TE polarization is evaluated from calculated EA spectra based on k·p theory and their Kramers-Kronig transformed refractive index changes. It is found that both compressive and tensile strain in the well layer reduce the chirp parameter The compressive-strained QW lowers the chirp parameter to nearly zero or negative values, the amount of the reduction being proportional to strain amount. In a tensile-strained QW, almost continuously negative values irrespective of applied electric fields occur at an optimized amount of strain