Arrayed Waveguide Grating for Wavelength Division Multiplexing

Arrayed Waveguide Grating (or AWG) is a method for wavelength division multiplexing or demultiplexing. The approach for multiplexing is to use unequal path lengths to generate a phase delay and constructive interference for each wavelength at an output port of the AWG. Demultiplexing is done with the same process, but reversed.

Arrayed Waveguide Gratings are commonly used in photonic integrated circuits. While Ring Resonators are also used for WDM, ring resonators see other uses, such tunable or static filters. Further, a ring resonator selects a single wavelength to be removed from the input. In the case of AWGs, light is separated according to wavelength. For many applications, this is a more superior WDM, as it offers great capability for encoding and modulating a large amount of information according to a wavelength.

Both the design of the star coupler and the path length difference according to the designed wavelength division make up the significant amount of complexity of this component. RSoft by Synopsys includes an AWG Utility for designing arrayed waveguide gratings.

RSoft AWG Utility Guide

Using this utility, a star coupler is created below:

Star Coupler for AWG designed in RSoft using AWG Utility

Methods of Optical Coupling

An optical coupler is necessary for transferring optical energy into or out of a waveguide. Optical couplers are used for both free-space to waveguide optical energy transmission as well as a transmission from one waveguide to another waveguide, although the methods of coupling for these scenarios are different. Some couplers selectively couple energy to a specific waveguide mode and others are multimode. For the PIC designer, both the coupling efficiency and the mode selectivity are important to consider for optical couplers.

Where the coupling efficiency η is equal to the power transmitted into the waveguide divided by the total incident power, the coupling loss (units: dB) is equal to
L = 10*log(1/η).

Methods of optical coupling include:

  • Direct Focusing
  • End-Butt Coupling
  • Prism Coupling
  • Grating Coupling
  • Tapered Coupling (and Tapered Mode Size Converters)
  • Fiber to Waveguide Butt Coupling

Direct Focusing for Optical Coupling

Direct focusing of a beam to a waveguide using a lens in free space is termed direct focusing. The beam is angled parallel with the waveguide. This is also one type of transverse coupling. This method is generally deemed impractical outside of precision laboratory application. This is also sometimes referred to as end-fire coupling.

End-Butt Coupling

A prime example of end-butt coupling is for a case where a laser is fixated to a waveguide. The waveguide is placed in front of the laser at the light-emitting layer.

Prism Couplers

Prism coupling is used to direct a beam onto a waveguide when the beam is at an oblique incidence. A prism is used to match the phase velocities of the incident beam and the waveguide.

Prism Coupling

Grating Couplers

Similar to the prism coupler, the grating coupler also functions to produce a phase match between a waveguide mode and an oblique incident beam. Gratings perturb the waveguide modes in the region below the grating, producing a set of spatial harmonics. It is through gratings that an incident beam can be coupled into the waveguide with a selective mode.

Grating Coupler in RSoft

Tapered Couplers

Explained in one way, a tapered coupler intentionally disturbs the conditions of total internal reflection by tapering or narrowing the waveguide. Light thereby leaves the waveguide in a predictable manner, based on the tapering of the waveguide.

Tapered Mode Size Converters

Mode size converters exist to transfer light from one waveguide to another with a different cross-sectional dimension.

Butt Coupling

The procedure of placing the waveguide region of a fiber directly to a waveguide is termed butt coupling.

Rsoft Tutorials 5. Pathway Monitoring (BeamPROP)

When stringing multiple parts together, it is important to check a lightwave system for losses. BeamPROP Simulator, part of the Rsoft package will display any losses in a waveguide pathway. Here we have an example of an S-bend simulation. There appears to be losses in a few sections.

rsoft6.2

Here, the design for the S-bend waveguide has a few locations that are leaking, as indicated by the BeamPROP simulation.

rsoft6.1

The discontinuities are shown below, which are a possible source of loss:

 

After fixing these discontinuities, the waveguide can be simulated again using BeamPROP. In fact the losses are not fixed. This loss is called bending loss.

rsoft5.9

rsoft5.10

Bending loss is an important topic for wavguides and becomes critical in Photonic Integrated Circuits (PIC).

Rsoft Tutorials 2. Simulating a Waveguide using BeamPROP and Mode Profile

BeamPROP is a simulator found in the Rsoft package. Here, we will use BeamPROP to calculate the field distributions of our tapered waveguides. Other methods built withing Rsoft CAD are will also be explored.

 

Tapered Waveguide

The tapered waveguide that we are simulating is found below. We will use the BeamPROP tool to simulate the field distributions in the waveguide. We will also use the mode calculation tool to simulate the mode profile at each end of the waveguide.

BeamPROP Simulation Results

rsoft3.3

Rsoft CAD

rsoft3.4

Mode Profile Simulation

The mode simulation tool is found on the sidebar:

rsoft3.5

Before choosing the parameters of the Mode Simulator, let’s first take a look at the coordinates of the beginning and end of the waveguide. This dialog is found by right-clicking on the component. The window shows that the starting point along the z axis is 1 and the ending point is 43 (the units are actually micrometers, by the way). We will choose locations along the waveguide close to the ends of the waveguide at z equals 1.5 and 42.5.

rsoft3.6

Parameter selection window:

rsoft3.7

Results at z = 1.5:

rsoft3.72

Results at z = 42.5:

rsoft3.71

Rsoft Tutorials 1. Getting Started with CAD (tapered waveguide)

Rsoft is a powerful tool for optical and photonic simulations and design. Rsoft and Synopsys packages come with a number of different tools and simulators, such as BeamPROP, FullWAVE and more. There are also other programs typically found with Rsoft such a OptoDesigner, LaserMOD and OptSim. Here we focus on the very basics of using the Rsoft CAD environment. I am using a student version, which is free for all students in the United States.

New File & Environment

When starting a new file, the following window is opened. We can select the simulation tools needed, the refractive index of the environment (“background index”) and other parameters. Under dimensions, “3D” is selected.

rsoft1.02

The 3D environment is displayed:

rsoft1.01

Symbol Editor

rsoft1.2

On the side bar, select “Edit Symbols.” Here we can introduce a new symbol and assign it a value using “New Symbol,” filling out the name and expression and selecting “Accept Symbol.”

rsoft1.1

 

 

 

 

 

 

 

Building Components

Next we will draw a rectangle, which will be our waveguide.  Select the rectangular segment below:

rsoft1.2

Now, select the bounds of the rectangle. See example below:

rsoft1.3

Editing Component Parameters

Right click on the component to edit parameters. Here, we will now change the refractive index and the length of the component. The Index Difference tab is the difference in refractive index compared to the background index, which was defined when we created the file. We’ll set it to 0.1 and since our background index was 1.0, that means the refractive index of the waveguide is 1.1. Alternatively, the value delta that was in the box may be edited from the Symbol menu. We also want to use our symbol “Length” to define the length of our waveguide. We also want this waveguide to be tapered, so the ending vertex will be set to width*4. Note that width may also be edited in the symbol list.

rsoft1.4

Here, we have a tapered waveguide:

rsoft1.5