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This OptSim Circuit application note provides an example of how to model a Traveling Wave Mach-Zehnder modulator (TW-MZM) using discrete photonic integrated circuit (PIC) elements. The schematic is shown in Figure 1.
Figure 1: OptSim Circuit TW-MZM schematic
The layout is comprised of bidirectional PIC elements such as two traveling wave optical phase shifters, optical splitter and combiner with user-defined power ratio. Each traveling wave phase shifter has an optical waveguide and a surrounding electrical transmission line introducing change in the waveguide’s refractive index and propagation loss. The interaction between the electrical and optical signals is distributed along the propagation direction. The waveguide’s thermal behavior (and hence modulator) is modeled via derivative of effective index, parameter VπL, and propagation loss as functions of temperature1.
The performance of the TW-MZM depends on the match between the effective index of the optical waveguide and the electrical transmission line. A perfect match results in a nearly infinite bandwidth, while a 10% mismatch heavily affects performance as shown in Figure 2.
Figure 2: Impact of mismatch between the effective index of the optical waveguide and the electrical transmission line on the received eye diagram
The microwave circuit for the traveling wave electrode connections is composed of a transmission line, a first impedance between the generator and the phase shifter electrode, and a second impedance between the phase shifter electrode and the load. Any mismatch in these impedances creates electrical microwave reflections originating from the load to the circuit and adversely affects the overall bandwidth of the TW-MZM, as shown in Figure 3.
Figure 3: Bandwidth narrowing due to increasing (top left to lower right) amount of mismatch in the impedances of traveling-wave electrodes of the modulator
OptSim Circuit allows you to set up realistic and complex traveling wave MZM devices with realistic effective indices, driving microwave circuits and load impedances.
1 Hongtao Lin, Okechukwu Ogbuu, Jifeng Liu, Lin Zhang, Jurgen Michel, and Juejun Hu, “Breaking the energy-bandwidth limit of electro-optic modulators: Theory and a device proposal,” Journal Of Lightwave Technology, vol. 31, no. 24, December 15, 2013, pg. 4029-40