Silicon PAM-4 optical modulator driven by two binary electrical signals with different peak-to-peak voltages.

We demonstrate a silicon PAM-4 optical modulator, which is based on a symmetric Mach-Zehnder interferometer. Two uncorrelated binary electrical signals with different peak-to-peak voltages are applied to the phase shifters of the silicon optical modulator. Accordingly, two different phase shifts are generated in the two arms. After the permutation, there are totally four phase differences between the two arms and the output optical power has four levels. The device can work at 32 Gbaud in the wavelength range from 1525 to 1565 nm, which is promising for the next-generation high-speed silicon optical link.