Refractive index contributions to phase shifting in InP based 30 GHz bandwidth n-i-n Mach-Zehnder Modulators

We present a n-i-n based Mach - Zehnder Modulator with a 30 GHz small signal modulation bandwidth. Fabry-Perot fringes from a straight waveguide structure were investigated as a function of voltage showing a strong positive index shift, which on first sight appears to follow the absolute value of current rather than voltage. However, the refractive index increases with bias, so cannot be a carrier density effect and heating is excluded due to the high bandwidth. Refractive index shifts were measured for both TE and TM polarizations, thereby eliminating the Pockels effect as the major contributing mechanism for index change. From the above considerations of positive index shift and polarization insensitivity we deduce that the dominant mechanism is the second order Quantum Confined Stark Effect, which is quite polarization insensitive at about 100 meV below the bandgap, and that the current does not have a significant contribution to device performance.