Technical Papers and Presentations

Directional couplers are used for the purpose of redirecting, splitting, and combining light in silicon photonic circuits. They are used extensively in data communication applications such as wavelength-division-multiplexing and signal switching [1]. Their functionality is an integral building block for devices such as Mach-Zehnder interferometers (MZIs) , polarization filters , and distributed Bragg reflector (DBR) based wavelength filters [2]. Although the directional coupler has a flexible design, resulting power splitting ratios are very sensitive to variations in the operating wavelength [3]. In this work, the power splitting ratio of a symmetric 2 x 2 directional coupler based on the silicon-on-insulator (SOI) platform is explored by varying the coupling length (l_c) and the waveguide gap (g) as shown in Figure 1. This work also serves as a comparative study between the modeling capability of the Wave Optics Module in COMSOL Multiphysics^® simulation software and Lumerical MODE Solutions^® in simulating directional couplers with various power splitting ratios.

The 2-D 4-port directional coupler configuration consists of two parallel SOI waveguides, an S-bend transition region, and a coupling region where the waveguides are a distance g apart. The power coupling between waveguides is simulated using COMSOL’s Electromagnetic Waves, Beam Envelopes physics interface in the Wave Optics Module [4]. To study the wavelength sensitivity of the directional coupler, a parametric sweep over the wavelength range of is included. The refractive index of the Si core and SiO₂ cladding are defined as wavelength dependent functions. Simulation results show the normalized electric field coupling power at a wavelength of 1550 nm while operating in the TE mode. As the wavelength is swept, power splitting ratios show the fraction of output power at the cross-port and through-port.

The significance of a comparative study is to primarily validate COMSOL’s ability to reproduce results from a popular integrated photonics modelling platform in order to extend the complexity of the modeling problem. Lumerical is limited in scope to the field of optics-based modeling and for example does not allow users to include contributions to phenomena such as the stress-optic effect with ease [5]. Alternatively, a COMSOL Multiphysics^® user can simply couple the output from the Structural Mechanics Module as an input for the Wave Optics Module and analyze how fabrication induced stresses have an impact on the optical performance of directional couplers.  

[1]    Z. Lu et al., “Broadband silicon photonic directional coupler using asymmetric-waveguide based phase control,” (eng), Optics express, vol. 23, no. 3, pp. 3795–3806, 2015.

[2]    R. K. Gupta, S. Chandran, and B. K. Das, “Wavelength-Independent Directional Couplers for Integrated Silicon Photonics,” J. Lightwave Technol., vol. 35, no. 22, pp. 4916–4923, 2017.

[3]    G. F. R. Chen et al., “Broadband Silicon-On-Insulator directional couplers using a combination of straight and curved waveguide sections,” (eng), Scientific reports, vol. 7, no. 1, p. 7246, 2017.

[4]    COMSOL Multiphysics, "The Wave Optics User's Guide", 2019

[5]    Lumerical Solutions," Stress and Strain", 2019.