Potential chirp-free characteristics (negative chirp parameter) in electroabsorption modulation using a wide tensile-strained quantum well structure

The possibility of nearly chirp-free electroabsorption (EA) characteristics in intensity modulation at an operating wavelength of 1.55 μm is studied theoretically. The TE-polarized EA spectra, their Kramers–Krönig (K–K) transformed refractive index changes, and the chirp parameters derived under both small-signal and large-signal conditions are calculated including excitonic effect within the framework of the effective mass theory. We investigate on the basis of the simulated EA results how to keep chirp parameters always negative. This can be accomplished by following two successive steps: (1) Widen the well width to increase the Stark shift. This makes it easier for the chirp parameter to turn negative at smaller applied electric field. (2) Apply tensile strain to the wide QW to induce a negative refractive index changes even in the vicinity of zero electric fields, indicating that there exists a desirable condition where the chirp parameter is negative independently of applied fields.