Travelling-Wave Mach-Zehnder Modulator (TW-MZM) Active Photonic Isolator

Tools Used: OptSim Circuit

Optical isolators help block unwanted optical signals and prevent undesired effect on the rest of the photonic circuit. It has remained a challenge to design an isolator that is easy to integrate with rest of the silicon and provides good isolation. Implementations using nonlinear optical elements come with power related undesired side-effects, while some active isolator implementations haven’t been successful in achieving higher degree of isolation.

In this application note, we demonstrate a design of an active isolator that uses traditional TW-MZM. The design is based on the analysis presented in Ref. [1]. The schematic is shown in Figure 1. 

The principle of operation is as follows. Instead of applying RF drive from the transmitting end of TW-MZM, it’s applied from the output end. So forward optical and RF move in the opposite directions, and the back-signal travel as co-propagation. The RF drive is a square-wave with 2*Vpi peak-to-peak. A mirror is used to send the reflected signal back through the modulator.

Figure 2 shows a comparison of the forward and backward signals. 

As seen in Fig. 2, the simulation achieves 12.3dB of isolation at 1550nm, which is close to the experimental observations reported in [1].

The application note demonstrated design of an active isolator using traditional TW-MZM. The simulation and reported experimental observations have an excellent agreement. Learn more about OptSim Circuit.

References

1.       Po Dong, “Travelling-wave Mach-Zehnder modulators functioning as optical isolators,” Optics Express, vol. 23, no. 8, April 2015, pg. 10498-10505.