SPICE model
generation
Best Fit
Laser
Toolkit
dN I N ––– = –– – –– – G·S dt q n
dS S N ––– = G·S – –– + –– dt p n
Threshold current
Slope effciency
Etc.
SPICE simulation
Laser datasheet
Optical system simulation
Rate-equation
model parameters
i(t)i(t)
βτ τ
τ
01001 LD
RX
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Software&Computing
September 2013 www.laserfocusworld.com Laser Focus World 32
and self-heating. If desired, parasitic effects can be includ-
ed to account for packaging. Results from device model-
ing in LaserMOD can be used for the generation and ex-
traction of system-level model parameters that can then be
used for end-to-end fiberoptic system simulation in the com-
mercial system simulator RSoft OptSim, as well as genera-
tion of an analog circuit representation for laser-driver de-
sign (see Fig. 1). 4
One of the advantages of this grounds-up approach is that
the system designer has full device-level knowledge of the
laser and does not need to rely on third-party datasheets.
Therefore, the designer can build Monte Carlo performance
boundaries for the whole system based
on stochastic variations in the geometrical and physical properties of the laser.
Datasheet-driven laser modeling
Within OptSim, different types of semiconductor lasers are modeled by using
rate equations. 5 Solving these equations
requires knowledge of a large number of
physical parameters to accurately model the electrical and optical behaviors of
the laser. While mixed-level simulation
is an excellent approach for obtaining
these parameters, system designers often only have access to laser datasheets
from component vendors. Furthermore,
these datasheets do not report many of
the parameters specific to a rate-equation model. The Best Fit Laser Toolkit is a built-in utility
within OptSim that helps bridge this gap by extracting the
physical parameters required to solve the rate equations using a limited number of datasheet parameters.
For the Best Fit Laser Toolkit’s parameter fitting process,
a designer starts with a laser datasheet and inserts the electrical and optical parameters (such as slope efficiency and
threshold current) found in the datasheet into the Toolkit (see
Fig. 2). 6 Using the methodology discussed in the next section, and taking into account the accuracy settings defined
by the user, the Toolkit then performs numerical optimization to determine the physical parameters required by the
laser rate equations. These parameters can then be used to
model the laser within an OptSim simulation, or the Toolkit
may be used to generate an equivalent circuit model that includes the p-n junction and packaging effects.
Measurement-driven laser modeling
There may be cases when none of the above approaches
can help a system designer in modeling a laser of interest.
Sometimes, for example, the laser design is unknown or a
datasheet is not available. Furthermore, even when physical parameters are known, there can be wide variations
in the performance of lasers within the same batch due to
imperfections in the laser manufacturing process. As a result, the behavior of a laser model parameterized from device-level simulation or datasheets may differ from the measured behavior of a particular fabricated device. In these
cases, measured characteristics of the laser can be used to
FIGURE 2. OptSim’s Best Fit Laser Toolkit can generate model parameters for circuit- and
system-level simulation.
