Full 3D Modulators in Quadrature (TFLN)

Before reading...
This simulation is an advancement on the TFLN Modulator example. Read this page to learn how to create an electro-optic modulator in MT-FIMMPROP

This simulation is an advancement on the TFLN Modulator example; here two ‘push-pull’ modulators are combined in quadrature to double signal transmission rate, similar to what is seen in the DQPSK modulation scheme.

There are four unique states when modulator A and B are combined in quadrature, (On, On), (Off, Off), (On, Off), and (Off, On); the latter pair distinguished by the output’s phase. 

By using a thermo-optic phase shifter, the two modulators are offset to one another by a pi/2 phase shift (known as arranging the modules in quadrature). Heating a length of waveguide in one of the modulators increases the effective index of the fundamental mode. Over a length of 200 um, the light propagating in that mode picks up a pi/2 phase shift compared to the other modulator.

The presence of this phase shift results in the four output states to be arranged at equal distance in the argand plane.

The modulators have been designed in MT-FIMMPROP and by using FIMMWAVE’s thermo-optic mode solver to calculates the effective index change as a function of input power. These results are incorporated into the MT-FIMMPROP device using the same ‘surrogate material model’ routine as outlined in the TFLN Modulator example.

• Length invariant: The MZM’s size predominantly is made up by a long straight waveguide. EME simulates straight waveguides relatively instantly, describing them by the modes on the single cross section. 
   
• Size invariant: Alternate method FDTD scales with simulation volume and simulation duration. With a length over 16,000 um, designers would have to wait for light to propagate down the full length before seeing any results using FDTD.

• Component design: Y-Splitters, MMIs, and directional couplers used in MZI designs are designed effectively in EME. After simulation, the length of the components can be scanned without need for re-simulation. This allows these components to be easily tuned to a 50:50 splitting ratio.

This example shows a frequency domain optical simulation for designing the components and the layout of the full modulator. To see modulation, a time domain simulation method is needed in PICWave. 

PICWave includes:

• A traveling wave electrode model required for the highest speed modulations. Metal microstrips can be simulated with FIMMWAVE’s complex mode solvers to find the group index of microwave modes (relating to the electrode’s impedance).
   
• Rigorous simulations of components from FIMMPROP can be imported to PICWave (Y-splitters, MZIs, bends)
   
• The model of effective index as a function of voltage can also be included in PICWave (TFLN from FIMMWAVE’s electro-optic model, silicon from Harold).