V I S T A S
VCSEL Integrated Spatio-Temporal Advanced Simulator

VISTAS (VCSEL Integrated Spatio-Temporal Advanced Simulator) is a 2D, multimode, time-domain, VCSEL model.

Fig. 1. The animation above shows the time-domain response of a cavity with 12 transverse modes to an applied rectangular pulse. The perturbations introduced by noise and feedback are evident. It is obvious that these effects significantly affect the modal distribution in the active layers and the BER of VCSEL-based transmission systems.

VISTAS accounts dynamically for the spatial interactions between the optical field and carrier distributions in the active layer (see Fig. 1). Such effects as Spatial Hole Burning (SHB), gain (mode) competition, or carrier diffusion losses can therefore be studied in details (see Fig. 2). VISTAS is based on 2D spatially-dependent rate equations that were mathematically transformed so as to remove any explicit spatial dependency, resulting in a numerical efficiency improved by several orders of magnitude, without noticeable influence on the accuracy. Very efficient algorithms for computing the steady-state characteristics of VCSELs were also derived from the transformed equations (see Fig. 3), and will be included in future versions of VISTAS.

A variety of additional effects that influence the performance of VCSEL-based optical links are included as well. Among them, thermal effects, noise, and optical feedback show the most significant influence. Special attention is paid to their self-consistent implementation. An efficient method is proposed, which allows dynamic computations of the coupling efficiency into waveguides. Resulting spatial filtering and mode partition significantly alter the performance of the entire link.

Fig. 2. Instantaneous near-field, carrier, and far-field profiles simulated using VISTAS

• VISTAS basic:
• basic algorithm for computing the dynamic behavior of VCSEL cavities, with detailled description of the interactions between the inhomogeneous carrier and field distributions. A mode solver (LP modes) is included in the package.
• VISTAS advanced:
• same as VISTAS basic, plus noise, phase, feedback (external cavity), far-field transformation and fiber coupling computation.
• VISTAS expert:
• same as VISTAS advanced, plus thermal model, electrical parasitics, and longitudinal carrier transport into the quantum wells.

The exact mathematical derivation and formulation of the core model as well as of the advanced mechanisms can be found here.

Fig. 3. Steady-state response of a multimode cavity. The lowest plot depicts the evolution of the radial carrier profile.

Due to the very efficient nature of the implemented algorithm, the required time for running a typical simulation on a standard computer (or even a laptop), is always kept within acceptable limits (typically 1s-10min). VISTAS is therefore distributed in the form of Matlab m-files, which are optimized for readability and flexibility, not for speed. The m-files are also easy to understand and modifiy, and Matlab provides powerful tools for debugging and visualization. Finally, this allows optimal portability. However, real system simulations require simlating the laser together with the electronic elements of the system (driver, detector,...), preferably in a circuit simulation environment. The mathematical formulation of VISTAS in the form of ordinary differential equations makes it possible, and an initial implementation of VISTAS in the circuit simulation environment of Cadence based on the behavioral language Verilog-A is already available.

Marc Jungo, March 2003 (e-mail me)

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