Full 3D Mach Zehnder Modulator (TFLN)

This example shows how a full 3D simulation can be performed on a Mach-Zehnder Modulator including how the effects of an applied voltage will vary optical outputs. 

This example uses electrical modulation of X-Cut thin film lithium niobate* (TFLN) but the same design flow could be created thermal modulators using FIMMWAVE’s thermal module, or using silicon modulators with a model found with software Harold.

FIMMWAVE’s electro-optic (EO) solver simulates the Pockles effect on our TFLN cross section; how the effective index of the simulated fundamental mode changes under an applied voltage.

The full MZM example uses information from the EO solver to inform a surrogate model. Instead of lithium niobate, a surrogate material is used in the modulator’s arms. By varying the surrogate material’s refractive index, we produce the same effective index changes found with the EO solver; this allows us to calibrate a surrogate model.

This modulator example uses a ‘push-pull’ configuration with one arm under forward bias and the other under reverse bias. This allows a pi phase difference to be achieved with a modulator half the length as if just a single arm were modulated.

MT-FIMMPROP connects together rigorous EigenMode Expansion (EME) simulations which construct a simulation of a full MZM. This means the GDS-II export contains the full MZM to be easily imported to layout tools.

MMI Design
EME allows for incredibly fast 3D simulations of the MMIs used in this design including near instant parameter sweeps of how their output coupling changes with length. For example, the length of the recombining 2x2 MMI in this example is tuned such that:

• Inputs in phase (i.e no applied voltage) maximises one output
• Inputs in anti-phase (i.e applied voltage) maximises the other output

Deciding Modulator Length
The arm length of the interferometer is chosen such that when a decided voltage is applied, the output of the MZM switches from maximising output ‘a’ to output ‘b’. MT-FIMMPROP can perform a length scan of the arms to find the length required for a pi phase shift between the arms.

Circuit simulator PICWave provides a time evolving simulation of the MZM including a travelling wave electrode model to create the GHz modulators. Using both FIMMPROP and PICWave we can:

• Export rigorous simulation models of designed components, MMIs, Y splitters, Bends (from FIMMPROP) to circuit simulator PICWave.
• Create modulator section informed from FIMMWAVE’s model.

Top: PICWave Circuit of MZM including travelling wave electrodes
Left: Hybrid SOI/ TFLN optical mode propagated along MZM
Right: Eye Diagram at 220 GHz NRZ input

In this example, X-Cut TFLN is used for the straight waveguide sections but an approximation is currently used for the bends. As an anisotropic material, X-Cut TFLN poses challenges for most EME solvers with its refractive index varying with orientation. 

Speak with Photon Design's team to learn how this can be solved, allowing TFLN designers to switch from time consuming FDTD. 
Read more on the comparison between the methods, including relating to bends, here