Delay Lines
Design delay lines in a layout environment and simulate the full device
Full Delay Line Simulations
A delay line is a long and coiled waveguide path introduced to a PIC to add a time delay to a signal while taking up as little space as possible. Given their size, delay lines have been a challenging simulation in their entirety but can be approached with MT-FIMMPROP.
This delay line simulated in MT-FIMMPROP has an optical path length over 22 milometers but a rigorous 3D simulation of its scattering matrix can be done in less than 20 seconds on an average workstation. Creating a field plot of a full delay line in a single tool has not been viable until MT-FIMMPROP’s release.
Advantages of This Design
Exploit Repetition, Cut Runtime
All of the bends in this delay line are copies of each other meaning sub devices can be used to copy the simulation output (scattering matrix) of one bend and reuse that result for all of the bends. This cuts the 44 mode list simulations down to just a single simulation.- Minimise Number of Cross-Sections
EME simulation runtime scales with the number of modes calculated and therefore the number of unique cross-sections in a device. This design uses exclusively straight waveguides (a single mode list) and constant curvature sections (also a single mode list of bend modes). As such, this design is very efficient for EME simulations.
Archimedes Spiral
Another popular design is the Archimedes spiral, where the waveguide’s curvature increases until reaching a central Euler s-bend and spiral out again. This can be simulated in FIMMPROP where a path can be described as a function of its curvature.
Key Points of Delay Line Design
- Bend Loss
Delay lines should aim to be lossless so the minimum radius of curvature at which there is no loss should be found. To find this radius FIMMWAVE’s parameter scans could be used; increasing waveguide curvature until loss is found using the absorbing boundary conditions.
- Waveguide Coupling
The separation of waveguides in a design should be chosen to remove nearest neighbour coupling between waveguides. Neighbouring waveguides can be simulated at varying separations in MT-FIMMPROP to find the distance at which waveguides are decoupled. This should be investigated between the highest curvature waveguides as the decoupling length depends on the waveguide’s curvature.