Extinction ratio limitations in GaAlAs electroabsorption light modulators

This paper reports experimental and theoretical results concerning absorption and scattering losses in GaAlAs electroabsorption (EA) light modulators. Light coupled from the outer confining layers into the central guiding layer is shown to limit the extinction ratio. If a is the EA coefficient and l the modulator length, scattering results in a low background level in the detected light which decreases as a−1 for sufficiently large al product rather than as the usual exp(−al) for smaller al. Scattering effects occur in all the layers, with many modulators showing large local increases in the density of scatter centers in the confining layers.