Design & Analysis of Photonic Devices:
Software Plays a Key Role in Overcoming Network Bottlenecks
The demand for ever-increasing band-width in communications networks
is the driving force in the rapidly growing
photonics industry. The development of
low-loss optical fibers and wavelength division
multiplexing techniques during the past
decades has brought technology to a point
where mere electronic switches and gates
are the bottlenecks in communication systems.
In the development of all-optical replacements
for such electronic components,a combination
of modeling and experiments is required.
Modeling of photonic devices involves the modeling of electromagnetic
wave
propagation in heterogeneous, anisotropic,
nonlinear, dispersive, and lossy media.
Material parameters typically depend on
mechanical stresses and thermal conditions
as well as electric and magnetic fields.
Mode analysis of a single step optical fiber made of
silica glass. The full vector hybrid mode
formulation computes the HE11 mode (left)and
the HE21 mode (right)without inherent approximations.
Here the axial field components
Ez and Hz are plotted as color and contour plots,respectively.Arrows
indicate the direction of the in-plane electric field.
Mode Analysis of Optical Fibers
One of the winning
devices of modern communication systems has been the single mode
silica glass (SiO2) fiber,
having a step index profile with a higher
refractive index in the center core, and
a lower index in the outer cladding. Numerical
modeling is playing an important role
in the design of single mode waveguides
and fibers.In the figure, a single step
index waveguide is studied. The inner core
is made of pure silica glass with a refractive
index of 1.4457 and the cladding is doped,with
a refractive index of 1.4378. These values
are valid for free space wavelengths of
1.55 µm. The radius of
the cladding is field of confined modes
is zero at the outer boundaries. For a
confined mode there is no energy flow in
the radial direction, thus the wave must
be evanescent in the radial direction
in the cladding. This is clearly seen in
the simulation of the fiber.
Light Propagation in Photonic Crystals
The study of photonic crystals involves
electromagnetic modeling of periodic
structures of alternating layers of materials
with different refractive indexes. Depending
on the type of structure and scale, a
photonic band gap of forbidden wavelengths
is obtained for the device. By destroying
the periodic structure in a limited region
of the crystal, a waveguide can be created.
Such waveguides can be designed having
very sharp bends without significant
loss of radiation. This may enable an
increase in integration density in photonic
circuits by several orders of magnitude.
Stress-Optical Effects in a Silica-on-Silicon Waveguide
Planar photonic waveguides in silica (SiO2)
have great potential for use in
wavelength routing applications. A major
problem with this type of wave- guide is
birefringence resulting in splitting of
the fundamental mode and pulse broadening.
One source of birefringence is thermally
induced stresses originating in the manufacturing
process posited on a silicon (Si) wafer.
After annealing at high temperature (approximately
1000 °C), a mismatch in thermal expansivity between the silica
and silicon layers results in thermally
induced stresses in the structure at the
operating temperature (typically room temperature,
20 °C). The
stresses affect the refractive index, and
the material becomes birefringent. The
design goal is to minimize birefringence
effects by adapting materials and manufacturing
processes. In order to examine the shape
and effective index of the fundamental
mode, it is critical to use prototyping
software that allows for full coupling
of the heat transfer, structural, and optical
analysis.
Design of a Photonics Micro-Prism
Another way to reduce radiation losses in photonics waveguide bends
is to use a micro-prism. If a micro-prism
is placed between two waveguides forming
a sharp bend, light will be guided between
the waveguides, through the prism. For
a certain refractive index of the prism,
the light propagating through the prism
will couple to the respective mode under
just the appropriate resonance angle. If
the initial field distribution does not
diffract
while propagating through the prism, the
coupling from the prism to the guide is
the inverse to the transfer of the light
from the guide to the prism. Therefore,
the efficiency of the process is very high.
The prism must be sufficiently long to
allow almost all of the power to exit into
the prism and vice versa. However, to avoid
diffraction,the size of the prism should
be kept as small as possible. The trade-off
between coupling and diffraction effects
is readily studied in a numerical model.
This article was submitted by COMSOL, Inc., 8 New England Executive
Pk., Burlington, MA 01803. For more information
e-mail info@comsol.com or call
(781) 273-3322. The Los Angeles, CA office
can be reached at (310)689-7250. To
find out more about FEMLAB software visit
COMSOL online at www.comsol.com.
Images
were created using FEMLAB software from
COMSOL. |
