High Performance Si AWG With Geometrically Improved Interface Between Slab And Waveguide Array, Sep 2020 – Sep 2021
Fig1 - Overall simulation flowchart
Master's Thesis in optoelectronics and silicon photonics in the scope of the T.I.M.E. Double Degree program between UCLouvain (Belgium) and Keio University (Japan).
Tasks: Design of an optical (de)multiplexer (arrayed waveguide grating), aiming at both higher transmission performances and reducing the Si photonics drawbacks with respect to what is currently used in the industry.
Used in this context:
- RSoft CAD Environment™ for circuit layout design
- FemSIM™ using Finite Element Method (FEM) for waveguide/slab mode solving
- FullWAVE™ using Finite-Difference Time-Domain (FDTD) method for light propagation at sensitive material interfaces
- Python for data processing, optimizations and overall characteristics computation.
Framework: Arrayed Waveguide Gratings (AWG), also called PHASARS in the literature, are optical communication devices which play a key role in Wavelength Division Multiplexing (WDM), a technique considerably improving networks transmission capacity and flexibility. Taking light as an input and by the successive use of Fourier Optics transformation as well as a waveguide array, which provides the focusing and dispersive properties, the AWG are able to operate as wavelength multiplexers, demultiplexers and routers.
Firstly built in silica which resulted in too bulky structures and with the advance of Si photonics technology, Si was therefore used for the miniaturization of AWGs. This has permitted to reach dimensions of one or two order of magnitude smaller.
Issues: Si AWG’s waveguides enhanced optical confinement induces larger phase error and scattering/losses in transition regions. This is typically resulting in deteriorated performances of crosstalk and insertion loss. The most sensitive region located at the abrupt transition, with a high index difference between Si and SiO2, between the planar slab region and the arrayed WGs region, which yields to optical field mismatch in addition to scattering.
Problem solving strategy:
- Historical review, state-of-the-art study and industry benchmarks
- New improvements to the structure
- Step-by-step optimization techniques design
- Simulations and optimization results
- Overall characteristics computation and final parameters values
Results: The study demonstrate improvements compared to simple transition shapes usually used, evaluate the device overall performances and computed the final parameters values for an operating device.
Fig - Layout and metrics
| FEM meshing | waveguide fundamental mode |
|---|---|
 |
 |
Fig2. - FEM used for waveguide mode solving in the scope of the project
Finite-difference time-domain (or FDTD) is a method directly derived from Maxwell’s curl equations. It is one of the most famous numerical methods for solving for fields in mediums. FDTD works even in the case of complex geometries with quickly varying envelope or backward reflections, something that cannot be done by some other famous simulation methods such as the BPM, which suppose a slowly varying envelope. This is mainly the reason why it was chosen for the computation of the field across the irregular interface domain.
FDTD simulation allows to take into account complicated
Fig3. - FDTD simulation used for fields propagation in the scope of the project
Multiple optimization techniques are used along the project. Here the final results are presented for one case.
Fig4. - Taper first stage optimization final results
Step-by-step simulation methods and results for the overall characterictics computation. The sensitive interface optimized structure was used at the boundary between the free-propagation region and the waveguide array. A unit power is used at the input beam.
Fig5. - Overall simulation flowchart
Step1
Fig6. - Input beam
Step3
Fig7. - Coupling to array
Step4
Fig8. - Field in the waveguide array
Step 6,7 and 8
Fig9. - Field out of array displayed on the top right picture and far in the second free-propagation region using FFT (far-field) on the bottom right picture
| Interface optimal design and values | AWG final design parameters and characteristics |
|---|---|
 |
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Fig10. - Final values for interface optimization and global parameters
Most of the files, PDF and Powerpoint presentation as well as thesis paper on Github.