RSoft Enewsletter

August 2015

Efficient AWG Design with the Effective Index Method

The RSoft™ Arrayed Waveguide Grating (AWG) Utility™ has been improved and expanded in v2015.06. New features include:

  • Ability to easily and self-consistently switch between 2D, 3D, 3D Effective Index Method (EIM)
  • Ability to simulate AWGs in any material system, including pre-defined silica and silicon
  • Improved and simplified simulation flow making advanced simulation possible
  • Increased layout robustness

In this example, we illustrate a 3D EIM simulation of a Si3N4-based AWG.

As with most optical devices, it is highly desirable for AWGs to have low loss, especially for low power applications. Typical losses include fiber-to-chip coupling loss, material loss, bending loss, waveguide scattering loss, and the transition loss between the FPR (free propagation region) and the arrayed waveguides. An ultra-thin core waveguide AWG1,2, as illustrated in Fig. 1, uses a wide single-mode waveguide which reduces scattering losses due to sidewall imperfections, and increases the coupling from a typical fiber mode due to the large mode size. 

The cross-section of the low-loss buried optical waveguide
mode profiles

Figure 1: a) The cross-section of the low-loss buried optical waveguide; 
b) and c) mode profiles

The AWG design used here (shown in Fig. 2) is based on these parameters: a waveguide width and height of 5.5 μm and 50 nm respectively, a center wavelength of 1550 nm, 150 arrayed waveguides, and 16 channels with spacing of 1.6nm. The minimum bending radius in the design is 4000 μm, which corresponds to a bending loss of 0.02 dB/cm as calculated by Synopsys’ FemSIM™ software. The input and output ports are separated by 125 μm to match the typical fiber diameter.

designed AWG (de)multiplexer

Figure 2: The layout of the designed AWG (de)

Since this structure is quite large compared to the operating wavelength, approximately 1-2 cm on each side, it is impossible to simulate it using a highly memory-intensive simulation algorithm such as FDTD (Finite-Difference Time-Domain). This example uses the BPM (Beam Propagation Method), which has been proven to be a very suitable and efficient method for simulating AWGs in both 2D and 3D.

For this example, we utilize 3D EIM to reduce this 3D structure into an equivalent 2D structure. While this is the default mode of the AWG Utility, it is also simple to switch to a full 3D simulation. The 3D EIM results are shown in Fig 3; this is a well distributed, low loss, and low crosstalk AWG design.

EIM results

Figure 3: a) 2D EIM simulation results for 16 channels, 
b) Comparison of the full 3D and 2D EIM results.

A full 3D simulation is also performed for one wavelength channel, the comparison can be seen in Fig. 3. The 3D results, which match well with the 3D EIM results, take approximately 100 times more to compute.

In summary, RSoft’s AWG Utility provides a highly efficient, accurate, and easy-to-use tool to model AWG devices. If you have any questions regarding the use of the AWG utility, please contact us at


  • [1]  J. F. Bauters, M. J. R. Heck, D. John, D. Dai, M.-C. Tien, J. S. Barton, A. Leinse, R. G. Heideman, D. J. Blumenthal, and J. E. Bowers, “Ultra-low-loss high-aspect-ratio Si3N4 waveguides,” Opt. Express 19(4), 3163–3174 (2011).
  • [2]  Daoxin Dai,* Zhi Wang, Jared F. Bauters, M.-C. Tien, Martijn J. R. Heck, Daniel J. Blumenthal, and John E Bowers, ”Low-loss Si3N4 arrayed-waveguide grating (de)multiplexer using nano-core optical waveguides,” Opt. Express 19(15), 14133 (2011).