A Novel Method for Improving the Performance of InP/InGaAsP Multiple-Quantum-Well Mach-Zehnder Modulators by Phase Shift Engineering

Semiconductor external modulators are increasingly employed in high speed and long haul optical fiber communication. A highly desirable feature of an external light modulator is the ability to generate a signal with a negative frequency chirp. Mach-Zehnder (MZ) III-V multiple-quantum-well (MQW) modulators are thus increasingly attractive relative to other modulator designs because of their adjustable frequency chirp, low drive voltage, small size, long term reliability and potential for integration with laser sources[l, 2, 3, 4], Unfortunately, in III-V MQW MZ modulators where the two arms of the interferometer are of equal length and the power splitting ratio 50%, the nonlinear electrooptic effect imparts a small positive chirp under symmetric push-pull operation. While negative chirp can be induced by designing the Y-junction to inject less optical power into the more deeply biased modulator arm and employing a small amount of overdrive, [5] the back-to-back extinction ratio is degraded. In this paper we demonstrate that by increasing the length of one of the modulator arms such that the transmitted optical signal is shifted by π relative to the other arm, an optimal chirp can be obtained while maintaining a high extinction ratio.