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. 

Figure 1. Schematic of an active, TW-MZM based, isolator implementation 

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. 

Figure 2. Forward (blue) and backward (red) optical 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.


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