This lesson describes how to create a chip-to-fiber coupler. You will create a model of a simple spot-size transformer based on lateral tapering, as reported in [1].

Highly efficient chip-to-fiber coupling with large alignment tolerances is important for applications of optoelectronic integrated semiconductor devices. Such coupling requires a low-loss change of the light beam spot from about 1μm of the mode to the 8-10μm range for the guided mode in the fiber.

A two-layer spot-size transformer is shown in Figure 1. The device consists of a strongly guiding two-layer input waveguide and a weakly guiding output waveguide with a two layer laterally tapered section between them. We will be considering dimensions and materials as given in [1] (Table 7). Details of the production process can also be found in the reference [1].

BPM - Figure 1 Two-layer spot-size transformer

Figure 1: Two-layer spot-size transformer

MaterialParameter
nCladding (InP)3.1662
nSubstrate (InP)3.1662
nTopLayer (InGaAsP_top)3.3832
nBottomLayer (InGaAsP_bottom)3.2416
Ltaper560μm
WInputTop1.2μm
WInputBottom2.4μm
dCladding5μm
dBuffer0.4μm
dSubstrate5μm
dTopLayer0.3μm
dBottomLayer0.11μm
WOutputTop0.08μm
WOutputBottom6.0μm

Table 7: Materials and parameters

Before you start this lesson

  • Familiarize yourself with the procedures in Lesson 1: Getting Started.
  • Familiarize yourself with the procedures in Lesson 2: Create a simple MMI coupler.
  • Familiarize yourself with the procedures in Lesson 3: Create a single-bend device

The procedures include:

  • Defining materials and waveguides for the chip-to-fiber butt coupler
  • Defining the layout settings
  • Creating a chip-to-fiber butt coupler
  • Editing the Input plane
  • Setting the simulation parameters
  • Running the simulation

Defining materials and waveguides for the chip-to-fiber butt coupler

To define the materials and channels for the chip-to-fiber butt coupler, perform the following procedure.

StepAction
1Create the following dielectric material:
Name: InGaAs_bottom
2D Isotropic Refractive Index (Re): 3.2416
3D Isotropic Refractive Index (Re): 3.2416
2Create a second dielectric material:
Name: InP
2D Isotropic Refractive Index (Re): 3.1662
3D Isotropic Refractive Index (Re): 3.1662
3Create a third dielectric material:
Name: InGaAs_top
2D Isotropic Refractive Index (Re): 3.3832
3D Isotropic Refractive Index (Re): 3.3832
4Create the following channel profiles:
Profile Name: InputWgChannel
Layer 1:
Layer Name: BottomLayer
Width: 2.4
Thickness: 0.11
Offset: 0.0
3D Profile definition material: InGaAs_bottom
Layer 2:
Layer Name: TopLayer
Width: 1.2
Thickness: 0.3
Offset: 0.0
3D Profile definition material: InGaAs_top
Profile Name: TopInGaAsChannel
Layer Name: top
Width: 1.0
Thickness: 0.3
Offset: 0.0
3D Profile definition material: InGaAs_topProfile Name: BottomInGaAsChannel
Layer Name: bottom
Width: 1.0
Thickness: 0.11
Offset: 0.0
3D Profile definition material: InGaAs_bottom
Note: The 2D parameters are not going to be used. The structure can be clearly simulated only in 3D.

Defining the layout settings

To define the layout settings, perform the following procedure.

StepAction
1Type the following settings.
a.Wafer Dimensions:
Length: 700
Width: 20
Cladding:
Material: InP
Thickness: 5.4
Substrate:
Material: InP
Thickness: 5.0
2To apply the settings to the layout, click OK.
Note: There is no need to make the simulation space too wide as there should not be a large scattering within the domain.

Creating a chip-to-fiber butt coupler

The input waveguide will be defined as a 2-layer single channel and the output waveguide as a simple one layer channel. To model the tapered part of the coupler, we will decompose it into 2 separate single layer channels positioned on top of each other. Each one of the channels will be a linear taper waveguide with tapering in opposite directions.

To create the chip-to-fiber butt coupler, perform the following procedure.

StepAction
1Draw and edit the straight input waveguide.
a.Start offset:
Horizontal: 0
Vertical: 0
b.End Offset:
Horizontal: 40
Vertical: 0
c.Width: 2.4
d.Depth: 0.4
e.Label: Linear Input Waveguide
f.Profile: InputWgChannel
2Draw and edit the bottom tapered waveguide.
a.Start Offset:
Horizontal: 40
Vertical: 0
b.End Offset:
Horizontal: 600
Vertical: 0
c.Start Width: 2.4
d.End Width: 6.0
e.Depth: 0.4
f.Label: Taper Linear Bottom
g.Profile: BottomInGaAsChannel
3Draw and edit the straight output waveguide.
a.Start Offset:
Horizontal: 600
Vertical: 0
b.End Offset:
Horizontal: 700Vertical: 0
c.Width: 6.0
d.Depth: 0.4
e.Label: LinearOutput
f.Profile: BottomInGaAsChannel

Note: Notice the Depth Parameter values are set to 0.4. This will place the bottom of the waveguides 0.4 micrometers above the substrate, leaving a space for the buffer layer. As the buffer is of the same material as the cladding and substrate, there is no need for further concern.
The resulting layout should look similar to Figure 2.

BPM - Figure 2 Layout design

Figure 2: Layout design

4Draw and edit the top tapered waveguide.
Note: To ensure that the top waveguide is on top of the bottom one, make sure the depth of the top waveguide is set to (buffer + bottom layer thickness) 0.4 + 0.11 = 0.51.
a.Start Offset:
Horizontal: 40
Vertical: 0
b.End Offset:
Horizontal: 600Vertical: 0
c.Start Width: 1.2
d.End Width: 0.08
e.Depth: 0.51
f.Label: Taper Linear Top
g.Profile: TopInGaAsChannel
5 Insert the input plane.

Editing the Input plane

To edit the input plane values, perform the following procedure.

StepAction
1Double-click the input plane.
The Input Plane dialog box appears (see Figure 3).
2Type the following values:
a.Starting Field: Mode
b.Z Position: 0
3Click the Input Fields 3D tab.
4Click Edit.
The Input Field dialog box appears (see Figure 4).
5In the window under Waveguides, select the check box (see Figure 4).
6Click Add.
The item under Waveguides moves into the window under Fields.
7To return to the Input Plane dialog box, click OK.
8To return to the layout window, click OK.

BPM - Figure 3 Input Plane dialog box

Figure 3: Input Plane dialog box

BPM -Figure 4 Input Field dialog box

Figure 4: Input Field dialog box

The resulting layout should be similar to the one found in Figure 5.

BPM - Figure 5 Layout design with top waveguide

Figure 5: Layout design with top waveguide

Setting the simulation parameters

When setting the simulation parameters, adjust the number of mesh points so that the resolution of the results is satisfactory (in this case, 201×201).You may also want to adjust the View Cut coordinates so that you can observe cuts through the areas of interest during the simulation. In this example, the viewing planes cross both the waveguide in the vertical direction (x~0.0) and the bottom waveguide in the lateral direction (y~0.5).

To set the simulation parameters, perform the following procedure.

StepAction
1From the Simulation menu, select Simulation Parameters.
The Simulation Parameters dialog box appears.
2Click the Global Data tab and type the following settings:
a.Reference Index: Modal
b.Wavelength: 1.550000
c.Number of displays: 100
3Click the 3D Isotropic tab and type the following settings:
a.Polarization: Semi-Vectorial, TE
b.Number of points: 201 x 201
c.View cut:
i.X Mesh Pt: 101 (x-position ~ 0)
ii.Y Mesh Pt: 107 (y-position ~ 0)
d.Propagation Step: 1.55

Running the simulation

To run the simulation, perform the following procedure:

StepAction
1From the Simulation menu, select Calculate 3D Isotropic Calculation.
The Simulation Parameters dialog box appears.
2Click the 3D tab and check the settings (see Figure 6).
3Start the simulation.
The results appear in the layout. See Figure 7, Figure 8, and Figure 9 for result examples.

BPM - Figure 6 Simulation Parameters dialog box

Figure 6: Simulation Parameters dialog box

BPM - Figure 7 X-Z Ex vs RIdx results

Figure 7: X-Z Ex vs RIdx results

BPM - Figure 8 Y-Z Ex vs RIdx results

Figure 8: Y-Z Ex vs RIdx results

BPM - Figure 9 Ex XY results — 2D

Figure 9: Ex XY results — 2D

BPM - Figure 10 RIdx-XY results — 2D

Figure 10 RIdx-XY results — 2D

References

[1] R. Zengerle, O. Leminger, W. Weiershausen, K. Faltin, and B. Huebner: Laterally tapered InP- InGaAsP waveguides for low-loss chip-to-fiber butt coupling: A comparison of different configurations, IEEE Phot. Techn. Lett. 7, (May 1995): 532-534.