VCSEL SPICE model
This application note to describes a quasi-dc model of a Vertical Cavity Surface Emitting Laser (VCSEL) for use in a circuit analysis tool such as SPICE. The model contains the electrical and optical characteristics of the VCSEL. The designer can call this model as a subcircuit to an existing design, and determine the optical output as a function of the VCSEL device parameters and the ambient temperature. Typical low frequency applications of a VCSEL include encoders, reflective/transmissive sensing, and scatterometry. For a more complete description of several applications, and the VCSEL itself, see the application note titled VCSELs: An Enabling Technology. This note is organized into two major sections. The first describes the model used in SPICE, and the second gives a design example using the model to predict power variation over temperature.
The electrical and optical properties of VCSELs have been described previously in the application note Modulating VCSELs. The reader is referred to that paper for physical descriptions of the VCSEL threshold current, slope efficiency, series resistance, and emitted power.
As with any laser system, there are classes of safety depending on the emitted optical power and the laser wavelength. There are several standards, with different test methods and requirements, but there is always a maximum allowable average power for each classification. For purposes of this application note, we will describe mostly class IIIb lasers, and we do not (and cannot) provide specific guidance for achieving any particular eye safety classification.
The VCSEL behavior defined here is based on testing and analysis, but it must be considered typical, not assured. Some data is shown here for instruction and may not be representative of VCSELs available for sale; refer to data sheets for actual limits. Finally, while described behaviors may fit many VCSELs, they are specifically applicable to ~850-nm, protonimplanted devices.
Some simplifying assumptions are built into the model, and it should be used with some caution. In particular, when extrapolating results over large temperature ranges.
