The small-signal \alpha-parameter of a semiconductor Mach-Zehnder modulator depends on the device design and on the bias and modulation voltages applied to the two independent drive electrodes. In particular, for the range of applied voltages which correspond to switching the modulator from the off state to the on state, the \alpha-parameter varies significantly. Effective, large-signal \alpha-parameters provide a convenient and concise description of the modulator chirp. For a multiple quantum well Mach-Zehnder modulator, a comparison is made between four effective \alpha-parameters; a transmission performance \alpha-parameter, an on/off \alpha-parameter, a 3 dB \alpha-parameter and a chirp \alpha-parameter. Both conventional and phase-shifted modulators are considered.
John C. Cartledge, "Comparison of Effective \alpha-Parameters for Semiconductor Mach-Zehnder Optical Modulators," J. Lightwave Technol. 16, 372- (1998)