Electro-optically modulated coupled-cavity VCSELs: electrical design optimization for high-speed operation

Abstract

Vertical-Cavity Surface-Emitting Lasers, due to their properties, are one of the best choices for optical communication purposes. Although direct modulated VCSELs have reached error free operation at speed of 40 Gbit/s, however, their cut-off frequency is limited by the relaxation oscillation phenomenon and is not likely to be further increased. Recently, it has been suggested that properly designed Coupled Cavity VCSEL with one cavity used as a reverse-biased Electro-Optic Modulator, can be only limited by the 3 dB electrical bandwidth cut-off frequency. Therefore, it is important to develop a high-speed electrical design for such VCSELs. In this paper we first present an analysis of an electrical equivalent circuit of EOM CC-VCSEL with lumped electrodes. We base our design on record high-speed structures reported in the literature. We optimize our structure with respect to modulator cavity length, number of top and middle distributed Bragg reflectors, doping levels of layers, radii of both mesas and non-ion implantation region in the DBR as well as the contact pad area. We show that the most influencing parameters are the mesa capacitance, series resistance and polyimide capacitance. The 3 dB bandwidth is enhanced by reducing the contact pad area and modulator cavity diameter, together with increasing the modulator cavity length. Faster operation is provided by pnp structure, instead of npn -one. A realistic structure design that is theoretically able to work with a 90 GHz modulation speed is suggested. Finally we also discuss the possibility of using a design concept of a EOM CC-VCSEL based on traveling wave electrode configuration.