This example has been updated. Find the latest version at Travelling Wave Mach-Zehnder Modulator.
The effective index of the modulator arm as a function of voltage characterizes the electro-optic behaviour of the component. This data (effective index vs. voltage) can be tabulated and exported to construct a circuit model for simulation with INTERCONNECT. Open the tw_modulator.icp project in INTERCONNECT. The project contains the circuit model representing the travelling wave modulator, and each arm of the modulator is modeled by a length of waveguide and an optical modulation (phase shift and loss) segment that is specified with measured data. Run the simulation in INTERCONNECT for three different voltages applied to the top arm of the modulator: -0.5 V, 0 V, and 0.5 V. The bottom arm should be kept at a reference amplitude of 0 V. After each simulation, plot the gain recorded in the ONA in the same visualization. Note the shift in the wavelength of the notch in transmission.
Note: the component-level simulations must first be run in CHARGE to generate the data for the effective index as a function of voltage (saved in the twmod_neff_V.dat data file), which is required to model the optical modulator arms in INTERCONNECT. Alternately, the simulation data is already loaded in the INTERCONNECT project file which can be used to run the simulation.
Open the project tw_modulator.icp in INTERCONNECT. The optical circuit model for the travelling wave modulator is composed of a Y-branch feeding two lengths of waveguide (following the reference design, the length of one arm is purposefully extended by 100 um). Next, the optical modulator elements are connected to the waveguides. These elements will introduce a phase shift and loss as a function of the amplitude of the electrical signal applied to the third port. A second Y-branch recombines the two light from the two arms. The circuit model is driven by an optical network analyzer. Values are recorded as a function of wavelength over the range from 1550 nm to 1565 nm.
Initially, the upper arm is biased to -0.5 V, while the lower arm is maintained at 0 V. Run the simulation, and visualize the gain recorded by the ONA. Perform two more simulations, adjusting the amplitude of the bias on the upper arm to 0 V and 0.5 V and adding both results to the existing visualization. The optical transmission response, shown in the figure that follows, corresponds closely to that reported in the reference paper. The FSR is 6.1 nm at 0 V (6.2 nm estimated from reference). The total shift in transmission is determined to be 0.9 nm for 1 Vp-p at 0 V bias (approximately 0.8 nm in the shift of the notch can be observed in the measured data, although ripple in the data makes an accurate estimate difficult) .
In the example file, the modulation is applied to the top arm, where the waveguide is 100 nm shorter than the bottom arm. If the modulation is on the longer arm, the transmission curves will shift to the opposite directions.
See Traveling Wave Modulators-(Circuit/System Simulations) for how to use the distributed circuit model in INTERCONNECT to study the effect of index mismatch, impedance mismatch and microwave loss on the performance of the traveling wave modulator.