Traveling Wave Electrode – TFLN Modulator

Circuit simulator PICWave is uniquely positioned for modulator design with time domain simulations, an advanced electrical model including traveling wave electrodes, advanced physics to simulate non-linear effects.

The result of this design is transmission of a 220 GHz signal with bit error rate orders of magnitude within industry accepted standard including simulation of losses due to non-linear effects.

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

With PICWave’s traveling wave electrode model, engineers can tune the impedance of electrical contacts such that electrical and optical signals have the same propagation speed; this allows for faster modulation speeds for a given error tolerance. This can be shown in this thin film lithium niobdate modulator from [1].

The applied voltage and the modulator length are chosen to induce a pi phase shift so destructive interference can be seen at the output.

A longer modulator requires a smaller applied voltage to provide this phase shift. This example demonstrates however the bandwidth of frequencies the modulator can operate at decreases for longer modulators as microwave modes carrying electrical signals are attenuated over a longer length.

The simulation results also show the optimum bandwidth modulator is a result of engineering electrical contacts to minimise group index mismatch between microwave and optical modes. [1] Engineers this with corrugated electrical contacts.

The mode solvers available in PICWave are able to simulate microwave/ RF mode group indices for straight microstrips, informing the traveling wave model’s impedance values.

At high power operation there are power dependant losses simulated in PICWave including the dynamic coupled mechanisms of two-photon absorption and the resulting free-carrier absorption (FCA). The success of PICWave’s model is shown through validation against [1].

Modulator design also relies on simulation of components which can be performed in Photon Design’s FIMMPROP using the EigenMode Expansion (EME) method to solve Maxwell’s equations.

• Directional Couplers – EME allows for near instant sweeps of an MMI’s length so finding the 50/50 splitting ratio is very simple.
   
• Waveguide Bends – EME simulates waveguide cross sections not a device’s full bounding volume. This allows very fast design of bends which can also include tapers.
  FIMMPROP’s EME can also uniquely simulate bends in anisotropic materials such as X-Cut thin film lithium niobate (TFLN) though they are not used in this simulation.
   
• Tapered Waveguides – This MZM involves a transition between two waveguide cross sections. FIMMPROP can be used to confirm this transition is adiabatic for a given length and transition function (i.e linear/ parabolic).

FIMMPROP will export scattering matrix spectra of its components to PICWave for use in circuit level simulation.

The full connected layout of the waveguides of this device can be created in software MT-FIMMPROP where rigorous EME simulations of the full device can be performed.