In this example, we construct a simplified, 5 layer coaxial Bragg fiber to demonstrate
the capabilities of MODE Solutions with respect to microstructured optical fiber (MOF).
First, we locate the mode guided primarily in the low-index air core of the fiber, and
then calculate how the dispersion, group velocity, and the propagation loss of this
mode varies as a function of wavelength. The
far-field radiation profile of this mode is calculated for projection onto a flat
screen 1 mm from the fiber facet.
Finally, we examine how the coupling efficiency
varies as a function of position for injection from a 3 micron diameter tapered fiber
into the MOF.
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Step 1: Construct the microstructured optical, coaxial Bragg fiber with the easy-to-use CAD editor
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The layout editor shows all of the simulation objects, each of which can be
moved and resized with simple mouse movements.
- orange box shows the extent of the computation volume and the boundary conditions
- we take advantage of the known radial symmetry of the mode we are looking for by specifying symmetric boundaries on each of the x and y boundaries of the computation region - this dramatically speeds convergence or allows for greater spatial resolution using a fixed number of points
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Step 2: Sweep over refractive indices to locate the desired mode of the MOF
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MODE Solutions allows you to easily find the mode(s) of interest by scanning through a
specific refractive index range.
- to find the modes of interest, scan over a wide range of effective refractive index values
- each mode found is expressed in terms of a fully-vectorial field profile, propagation loss, and effective index
- for the MOF studied here, the very few layers results in a large propagation loss for the mode; typical devices would be comprised of many more layers in order to reduce the propagation loss
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Step 3: Determine the far-field radiation profile of the microstructured optical fiber mode
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Built-in far-field projection routines enable you to project mode profiles onto a flat screen or onto a hemispherical surface, and integrate the profile over a specified angular cone or plane.
- the particular symmetry of the air-bound mode of microstructured optical fiber results in a characteristic, annular far-field radiation profile
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Step 4: Calculate the dispersion and the propagation loss of the microstructure optical fiber mode as a function of wavelength
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The analysis routines enable the user to perform a
frequency sweep and choose from a pull-down to analyze the propagation
loss, effective index, group index, group delay, group velocity or dispersion as a
function of wavelength or frequency.
- the total dispersion of the microstructured optical fiber increases rapidly as the MOF approaches cutoff at 4.15 microns
- the very large dispersion arises from the slow group velocity of the MOF, which can also be easily calculated using the built-in analysis routines
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Step 5: Automate simulation and analysis - determine the sensitivity of coupling a 3 micron diamater tapered fiber to the MOF mode
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Built-in overlap analysis routines allow the end user to calculate the
overlap and coupling efficiency between
the mode of interest and a Gaussian beam, another
waveguide mode, or data imported from another application such as ASAP 2005.
- using MODE Solutions we calculate the mode profile of a 3 micron diameter tapered fiber and overlap it with the MOF mode as a function of relative displacement
- a peak coupling efficiency of 1.2% results, but
given the radial field symmetry of the MOF, the horizontally-polarized tapered fiber cannot couple to the MOF along the vertical symmetry plane
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