Microstructured optical fiber - Optical design, dispersion calculation and coupling efficiency calculation with MODE Solutions

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 hemispherical surface 1m from the fiber facet. Finally, we examine how the coupling efficiency varies as a function of position for injection from a 3 micron diameter fiber into the MOF.

Step 1: Construct the microstructured optical, coaxial Bragg fiber with the easy-to-use CAD editor

The layout editor shows all of the simulation objects, each of which can be moved and resized with simple mouse movements.  An extensive library of simulation objects and materials exist to ease the creation of complicated fiber and waveguide device models.

• 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

Step 2: Sweep over refractive indices to locate the desired mode of the MOF

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

Step 3: Determine the far-field radiation profile of the microstructured optical fiber mode

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

Step 4: Calculate the dispersion and the propagation loss of the microstructure optical fiber mode as a function of wavelength

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

Step 5: Automate simulation and analysis - determine the sensitivity of coupling a 3 micron diamater tapered fiber to the MOF mode

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.

• using MODE Solutions we calculate the mode profile of a 3 micron diameter 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