comsol simulates processors for fiber optics communication
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08/07/2013 COMSOL simulates processors for fiber optics communication
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COMSOL simulates processors for fiber opticscommunication
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Serge Bidnyk, Enablence Technologies Inc., Ottawa, ON, Canada
In fiber optics systems, information from an electronic source is converted
into light signals. The light is guided into a glass fiber and travels down its
optical core. The light remains within the fiber because the refractive indexof materials surrounding the optical core of the fiber is far lower than that
of the optical core itself. When the light signal arrives at its destination, it is
converted into the electronic form for further processing.
Optical processors, such as those that Enablence designs, manipulate the
light at critical points along its path. At the source, a processor may
multiplex light signals to a denser form. At the destination, optical
processors can demultiplex the dense signal into fundamental wave lengths,
which eases the conversion back to electronic form. And so on.
Virtual test environment
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Dr. Serge Bidnyk uses COMSOL Multiphysics in an iterative design strategy to develop optical processors
for Enablence. To avoid unnecessary constraints on the design, he chooses materials and manufacturingmethods that are as non-restrictive as possible, including innovative deposition techniqu es, advanced oxide
etching, and metallization.
What the COMSOL Multiphysics simulations offer the design process is a "virtual testing" ground, he makespredictions for (and learns from) unusual designs, extreme conditions, limiting behaviors, impacts fromcoupled processes, to name a few (e.g. Fig.1).
Required for the physics-based predictions is the capacity to handle: arbitrary geometries; individual andcoupled processes (e.g., Maxwell's equations, heat transfer, stress/strain, chemistry); user-defined governingequations; vector equations; inhomogeneous and anisotropic material properties; complex numbers in material
properties; freely prescribed boundary and initial conditions; flexible postprocessing, and full access to thesolvers via a scripting program. Dr. Bidnyk says he chose COMSOL Multiphysics because of its unique
ability to meet all these analyses requirements. When questioned about the user flexibility aspects, he explains,"No other package would let me get anywhere near as close to the simulation engine as would COMSOL
Multiphysics".
An iterative design case
Consider the DWDM (dense wavelength-division multiplexing) optical processor designed by Dr. Bidnyk
using COMSOL Multiphysics simulations. The DWDM typically consists of a substrate and a series ofwaveguide cores with either a surficial cladding or some filling material to encase the core. How a given
frequency signal travels through the core, or the mode number, is determined by the way the light bounces offthe channel walls. There can be, for instance, 20 or 30 modes for just one color of light, each with its own
intensity profile and characteristic speed. A 10 mm x 30 mm device will handle 40 optical waveguide cores.With each channel carrying 40 GHz of information bandwidth, a total bandwidth of 1.6 terabits per opticallink is obtained, this level of information transfer eclipses anything possible with electronic systems.
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Dr. Serge Bidnyk of Enablence uses COMSOL Multiphysics to optimize photonic processor design.
In a given design loop, Dr. Bidnyk would use COMSOL Multiphysics to address the same well-definedquestion for incrementally different DWDM configurations. Typical questions are: What is the intensitydistribution in the waveguide core? What is the passband performance for this waveguide? Which
Eigenmodes are sustainable in the waveguide? or What is their speed of propagation? Once the question isdecided, the simulations are conducted in a rapid fire batch mode. To minimize computational time and
duplication of effort, he finds the simplest geometry that represents the system without losing valuable detail.The results of the batch simulations can be used to cull geometries that do not work and identify the
interesting workable ones that should receive further attention and analysis.
The biggest problem in optical processing is birefringence, which plagues the ability to processlight.
For just one DWDM configuration, a simulation procedure is established, typically with the COMSOL
Multiphysics graphical user interface (GUI). The appropriate governing equation is chosen, for instance,
Maxwell̀ s vector wave equations for a complex permittivity tensor. Then the geometry is defined, materialproperty values are entered, and boundary conditions are set. The material properties may involve complex
numbers. The boundaries typically are some combination of absorbing, periodic, metallic and magnetic
boundary conditions. Once the problem is set up, the appropriate solver is chosen. Where possible, Dr.Bidnyk uses the adaptive meshing algorithm, which instructs COMSOL Multiphysics to automatically zoom in
on areas where the solution changes rapidly. During postprocessing he may generate graphics and evaluate
certain quantitative results.
Fig. 2. A semiconductor wafer can contain thousands of photonic processors. In this photo you can
see a dozen separate devices, each a rectangle of less than 0.2 square centimeters. Each device is
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capable of processing far larger amounts of information compared to anything possible with
electronic systems.
The set up for Eigenvalue analysis of the DWDM is straightforward (Fig. 3). Since the device has 40
waveguide cores lying side by side and sandwiched between substrate and fill, the geometry is symmetric, the
same slice is repeated 40 times. Only a representative zone of the DWDM need be modeled. Periodic
boundaries are placed where the solution would repeat. With roughly 4000 elements to the cross-section,finding the Eigenvalue solution requires approximately two minutes. The results (Fig. 1) reveal that the
periodic boundaries are effective at simulating the desired symmetry condition.
Fig. 3. A finite-element mesh of a typical photonic device shows the major divisions. The waveguide
cores in the center are surrounded by a cladding or filling material that has a substantially lower
index of refraction, thus containing lightwaves within the optical core channels. Each such channelcan accommodate optical transmission rates as high as 40 GHz.
What if you have hundreds of variations on one DWDM design? Learning how to perform a little automation
with a Matlab routine suddenly is extremely important. First, open the DWDM model (above) in theCOMSOL-with-Matlab GUI, and save it as an M-file. Now to run the COMSOL Multiphysics model from
the Matlab command line, simply enter the filename. All that remains is to modify the M-file with a Matlab
script so that it repeats the COMSOL Multiphysics commands for each geometry.
With an eigenvalue solution in two minutes and a Matlab automation routine, Bidnyk can analyze hundreds of
geometries per day. "I couldn't really consider using any other tools," says Bidnyk. "The fact that COMSOL
Multiphysics can be run from the Matlab command line results in enormous time savings," he adds.
Advanced analyses
Results from batch mode simulations can be used to identify which designs merit in-depth analysis, like
coupling between thermal, mechanical, electromagnetic, and/or chemical impacts. Consider birefringence, forexample. Birefringence is an optical property whereby the refractive index of a material differs with the
polarization of the light. Inherent birefringence arises during the manufacturing process because materials are
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stressed as the hot wafer cools down. Form birefringence occurs when the refractive index varies withgeometry. Comments Dr. Bidnyk, "The biggest problem in optical processing is birefringence, which plagues
the ability to process light." Because the polarization of light is unknown during transmission in the cable, it is
critical for a device to operate identically at any polarization.
To examine birefringence with a numerical model requires vector analysis. Only a very few software
packages offer this capability, and COMSOL Multiphysics is among them. Dr. Bidnyk uses COMSOL
Multiphysics to account for inherent and form birefringence in separate steps. For analyzing inherent
birefringence, he adds a simulation of mechanical stresses and assigns inhomogeneous and anisotropic opticalproperties to the waveguide. Now the model can handle different growth and annealing temperatures and
yield precise values for the stress-induced birefringence. For form birefringence, he uses the inherent
birefringence analysis (as above) and then finds a core geometry with a form birefringence that compensatesfor the inherent birefringence, resulting in a device that is essentially polarization-insensitive.
COMSOL's unusual capacity to do both vector analysis and accept refractive indices built with complex
numbers (as above) also allows Dr. Bidnyk to analyze other difficultto-address issues. Consider what dopingtypes and levels will achieve desired amplification and attenuation rates. "Some packages just won't accept
complex numbers," explains Bidnyk, "but COMSOL Multiphysics does, and my algorithm can thereby
calculate attenuation for any core geometry." This capability is very helpful for considering advanced design
options. Device engineers, for example, could dope core material during manufacturing to alter itsamplification or attenuation properties. With the complex part of the refractive index, he can determine to
what degree the doping will change how the core attenuates or amplifies the light passing through it.
Dense yield, lowered cost
Clever design strategies, such as Dr. Bidnyk's, are making it possible for Enablence to produce high-
performance chips at dramatically lower cost. Careful miniaturization is increasing the density of photonic
devices that will fit on a single wafer: common photonic technologies produce 5 to 30 chips on a wafer, but
Enablence packs as many as 3000 chips on each one (Fig. 2). These small chip sizes bring high manufacturingyields as they limit the impact of wafer defects and nonuniformities. Moreover, the high density lowers
packaging costs, which are a significant factor in the cost of photonic and optoelectronic products.
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