Pulley Coupler
Use MT-FIMMPROP to simulate the fabrication error tolerant pulley coupler in seconds
The Pulley Coupler is an alternate design to conventional ring resonators with great advantages for fabrication tolerance. Simulation of the entire coupler can be performed in MT-FIMMPROP, which would typically require multiple tools (component simulation, circuit simulation, and layout).
Why Use Pulley Couplers? - Fabrication Error Tolerence
A conventional ring resonator has a short coupling region so requires high coupling between its waveguides. This means a small gap is needed between the waveguides which is prone to fabrication errors (vertical and constant etches are more challenging to achieve with small separations).
A pulley coupler can vastly lengthen the coupling region by having the input waveguide follow the ring's circumfrence. This means a weaker coupling is required allowing the designer to separate the waveguides to a distance less prone to fabrication error.
How to Build a Pulley Coupler in MT-FIMMPROP
MT-FIMMPROP performs multiple eigenmode expansion (EME) simulations and natively combines them to provide a simulation of the full device. There are two important parts in the pulley coupler
1. Constant Separation Region
A computational region can be drawn that follows the curvature of this coupling region. This uses the ‘sub-path’ feature where one path is defined at a constant offset to another.
MT-FIMMPROP needs only a single mode list to describe this section as it consists of a constant curvature cross section.
2. Decoupling Region
A spanning computational region models the coupling between the waveguides as they separate until the waveguides are fully decoupled. This section requires discretisation with FIMMPROP’s taper algorithm so is the most computationally demanding simulation of the device. Using sub-devices the results of this section can be copied and flipped for the other side to reduce simulations time by essentially half.
These results and the other decoupled waveguides are snapped together to define the full coupler.
Design Tip - Waveguide Width
Similar to a directional coupler, the pulley coupler relies on the phase matching relationship between symmetric and anti-symmetric supermodes in the coupling region. As the two coupled waveguides have different curvatures, modes in each waveguide have different effective indices. This can be compensated for by using waveguides of different thicknesses.
- This can be easily investigated using a FIMMPROP variable sweep natively in the software.
Racetrack Resonator
A ‘Race-Track’ resonator couples its input to a loop over a straight coupling region for a similar reason to the pulley; weaker couplings over long distances are more tolerant to fabrication error. These designs are also possible in MT-FIMMPROP, resembling a directional coupler connected to a loop. Such designs have a larger footprint to the pulley coupler but their straight sections can also have advantages such as easy tuning and modulation.
