Polarization converter using a tapered silicon ridge waveguide

1 Polarization converter using a
tapered silicon ridge waveguide

Tapered waveguides with an adiabatic change with respect to the
propagation direction are very useful for enhancing of the coupling
efficiency between two waveguides with different widths. Mode
hybridization can occur in asymmetrical silicon on insulator (SOI)
waveguides for certain waveguide widths, where the effective indices of
two guided modes match [1-2]. Consequently, the field components of TE
and TM modes are comparable and the polarization conversion becomes
possible. The TE and TM modes refer to the modes with dominant (E_{x}) and (E_{y}) component, respectively.

In this article, OptiFDTD is used to model a polarization converter
(TM0 to TE1) using an SOI taper waveguide connecting two SOI waveguides
with different widths [1].

2 Design

The 3D design of the polarization converter is modelled by three
sections i.e. the input waveguide (width (w_{1})), the taper waveguide, and the
output waveguide (width (w_{2})). All
three sections have an SOI ridge structure, see Fig. 1 and tables below
for further details about the geometrical dimensions.

All boundary conditions in x, y, and z directions are chosen as
absorbing perfectly matched layer (PML).

To create the ridge waveguide, a channel waveguide profile with Si
and thickness h is created and assigned to a linear waveguide with
length and width matching that of the simulation domain.

The input and output waveguides are created on top of the Si slab
using a linear waveguide set to the same channel waveguide profile as
the slab. The taper section is created using an exponential taper
waveguide with the alpha parameter is set to zero.

The optical source was configured using the input plane (positioned
at z = 0.5 (mu)m) set to “Modal”,
see table 3 for further details.

Observation areas (XY) are located at z = 0.7 (mu)m and z = 254 (mu)m for observing the mode profile
inside the input and output waveguides, respectively.

A non-uniform mesh is used to reduce the simulation time. The minimum
and maximum mesh size are chosen as 0.0434 (mu)m and 0.1 (mu)m, respectively, in the x and y
direction. The predefined regions of the minimum mesh size for the x and
y direction are defined from x = -0.75 (mu)m to x = 0.75 (mu)m and y = 0 (mu)m to y = 0.4 (mu)m, respectively. Testing also
confirmed that 50e3 time-steps are required for accurate results.

3 Results

The mode solutions and corresponding refractive indices for the input
and output waveguides using the OptiMode solver are shown in table 4.
Note that the third mode of the input waveguide is TM0, which is the
desired input mode. As OptiMode injects the last mode, the “number of
modes” in table 3 is set to three.

The mode profiles for the first three modes of the input and output
waveguides i.e. TE0, TE1 and TM0 can be seen in Fig. 2.

Figure 3 shows the mode profiles obtained through the observation
areas (XY) when the structure is injected with the TM0 mode. The mode
profile observed at the output waveguide demonstrates the polarization
conversion from TM0 to TE1.