Differential Electro-Optical Equivalent Circuit Model of a Vertical-Cavity Surface-Emitting Laser for Common-Mode Rejection Ratio Estimation

Vertical-cavity surface-emitting lasers (VCSEL) can be used to achieve galvanic isolation of systems operating in environments with high levels of electromagnetic noise. A small-signal differential electro-optical equivalent circuit model of a VCSEL is presented, in contrast to the usual single-ended models. The electro-optical and opto-electrical conversion of the signal is modelled, as well as the electrical characteristics of the VCSEL chip. The circuit model is based on an approach combining Y-parameters and mixed-mode S-parameters. Compared to conventional models that are aimed at describing the internal laser dynamics, the presented model is primarily focused on modelling the asymmetric parasitics in a differential VCSEL configuration. The parasitics between the 5-lead VCSEL TOSA package and the test printed circuit board are modelled. The impact of the asymmetry of the parasitics from the laser anode and cathode towards the ground is examined. The differential VCSEL model is used to estimate the transfer coefficients for both differential-mode and common-mode input signals. The ratio between the wanted and unwanted signal is expressed as the common-mode rejection ratio (CMRR). The model covers the frequency bandwidth up to 4 GHz required for integrated circuit EMC and ESD measurements.