For telecom applications, high-speed optical modulators with small size & low operating voltage are highly desired. The electro-absorption modulator (EAM) is one technology commonly used to achieve these goals. Compared to an electro-optic modulator, an EAM can operate with much lower voltages (only a few volts), and can operate with a modulation bandwidth ~50 Ghz. In addition, an EAM can be integrated with a laser diode onto a single chip to form a photonic integrated circuit (PIC) data transmitter.
An EAM is a high-speed modulator design based on the Quantum Confined Stark Effect (QCSE). QCSE describes the effect that an externally applied electric field has upon the light absorption spectrum of a quantum well structure. In a QW, the absorption spectrum is very sharply peaked at a specific wavelength, known as the absorption edge. This is due in part to the fact that in a QW, exciton formation is greatly enhanced as compared to bulk media, due to the quantum confinement of electrons & holes in the QW. If an electric field is applied to the QW, the wavelength at which the absorption edge occurs will be redshifted compared to the unbiased case. The ability to suddenly and dramatically increase the absorption of the QW at a specific wavelength, by applying an electric field to the QW, is the principle upon which an EAM operates.
Most EAMs are made in the form of a waveguide with electrodes applying an electric field in a direction perpendicular to the modular light beam; this configuration allows for efficient absorption due to the long interaction length of light with the absorbing QW region. This application note demonstrates the use of the Synopsys’ RSoft Multi-Physics Utility to simulate this type of EAM.