Meaning
Optical phase shifts can be generated dynamically within a waveguide structure by altering the localized concentration of free carriers. Dynamic index modulation utilizes this principle to alter the phase of propagating light in real time. This mechanism operates as the basis for high-speed electro-optic switches and silicon modulators.
Refractive Shift
The change in the refractive index of the semiconductor material is governed by the plasma dispersion effect. By injecting or depleting charge carriers in a p-n junction, the optical path length experienced by the light shifts rapidly. This shift alters the interference pattern in a Mach-Zehnder interferometer, thereby converting phase modulation into amplitude modulation.
Control over this refractive change determines the signal quality and extinction ratio.
Carrier Injection
Electrical injection of electrons and holes relies on forward-biased diode architectures. While this method yields large changes in refractive index, it is inherently limited by the carrier lifetime of the semiconductor. Passive recovery techniques or reverse-biased depletion modes are used when faster transition times are required.
Modulation Speed
Operating frequencies of dynamic index modulation are limited by carrier dynamics and parasitic capacitance. High-frequency designs utilize traveling-wave electrodes to match the phase velocity of the electrical drive signal with the optical signal. This alignment extends the modulation bandwidth into the gigahertz range.
