CODE V Photonics/Telecom FAQ
What do you mean by photonics
systems?
We use the term photonics systems more or less interchangeably with
"optical telecommunication systems." This includes a number
of optical devices that typically interface with fiber optics, such
as connectors, multiplexors, isolators, couplers, and filters.
Where does CODE V fit in? What
sorts of photonics systems can CODE V handle?
CODE V was originally developed as a tool for the design and analysis
of image-forming optical systems such as microscopes, cameras, laser
scanners, and scientific instruments. Many photonics systems have
components that are fundamentally similar to conventional image-forming
optics. CODE V is a powerful tool for working at the optical
component level, but it does not have features related to complete
systems, networks, etc.
What are the advantages of CODE V over other codes for this type of design work?
CODE V has an extremely large range of types of systems it can model,
as well as good analysis capabilities, powerful optimization, tolerance
analysis, and a powerful macro language for extending its calculations.
It can handle systems with many surface types, diverse materials
(including gradient index), general diffraction gratings, multi-wavelength
systems, polarization properties (including birefringence), multi-layer
coatings, and even user-defined properties. Analysis includes many
types of geometrical calculations, diffraction based evaluations
(e.g., point spread function, encircled energy, fiber coupling efficiency,
and even diffraction beam propagation).
How do you model optical fibers
in CODE V?
In most cases, the optical fibers themselves are not modeled in
CODE V, and usually do not need to be modeled. The reason is that
CODE V is typically used to design components that interface to
or from optical fibers. For this, you need a way to define properties
of the light emerging from a fiber (which is then processed by the
device you are modeling). For certain analysis such as coupling
efficiency, you also need to define the input properties of a fiber.
CODE V has these capabilities, which will be explained in the examples
later in this document. CODE V has a feature called non-sequential
surface modeling that can be used to model straight segments of
certain types of fibers, particularly large-diameter multi-mode
fibers or light pipes. This type of model is useful for some special
purposes, but is not normally used in the design of photonics components,
which typically utilize single-mode fibers.
Can CODE V model integrated optics
or wave guide-type systems?
No. CODE V is used to model discrete components consisting of such
things as lenses, prisms, mirrors, filters, gratings, gradient index
rods, and polarizing elements. Simulation of integrated optics requires
modeling of the electromagnetic field on a microscopic scale. This
is the domain of other software packages, though it is possible
to take the output of an integrated device (in the form of amplitude
and phase distributions) and use this as the input to a beam propagation
calculation in CODE V. This is a specialized capability that would
typically require some macro programming.
What about optical coatings?
Yes, CODE V includes a coating design and analysis module that can
be used to define and even optimize multi-layer coatings. Although
it is a powerful utility program, its design features are not as
extensive as those of certain dedicated coating design programs,
since CODE V’s purpose is primarily optical system design, not coating
design. Once the coating prescription is defined and saved in a
file, the coating can be attached to any surface, where it will
properly affect the polarization state and transmission of rays
traced through it. Polarization can be very important in certain
diffraction analyses, including point spread function and coupling
efficiency calculations.
What sorts of analysis can CODE V do that apply to photonics systems?
Geometrical calculations such as simple ray tracing, spot diagrams,
and encircled energy are useful in the early stages of component
design. To evaluate system performance, the diffraction point spread
function (PSF) is used, or more often the coupling efficiency (CEF)
which is derived from the PSF. Macros allow study of CEF vs. wavelength,
important for WDM systems. You can also use the beam propagation
feature (BPR) to examine the amplitude and phase of a beam anywhere
in the system. BPR is a powerful feature but requires careful attention
to issues of sampling to get the most accurate results. Tolerance
analysis is also available (see below).
What can CODE V do to optimize
photonics system?
CODE V has a powerful optimization engine called AUTO (for automatic
design). It systematically varies system parameters to reduce the
size of a specified "error function" that correlates with
image quality. Default error functions based on spot size or RMS
wave front area are normally used at first, but user-defined error
functions are also possible, and a single mode-profile command can
be added to AUTO input to define a coupling efficiency error function.
CODE V’s ability to precisely control boundary conditions (constraints)
is unmatched in the industry.
What do you mean by tolerance
analysis, and why is it important?
Tolerance analysis refers to the study of how the system will perform
when it is constructed with manufacturing errors within a specified
range – these are the tolerances. Simulation of how the system will
perform with errors in construction is essential in order to know
if the design is truly practical or not. CODE V has a number
of tolerance methods available, including the ability to tolerance
on coupling efficiency, on user-defined quality measures, and even
with user-defined tolerance parameters. The effects of assembly-time
adjustments (known as compensators) are also simulated. Tolerance
sensitivity is a major driver on the cost of manufacturing any optical
device. As photonics systems become more complex and handle more
closely-spaced channels, tolerance analysis will become even more
essential.
