Driver circuit for a PAM-4 optical transmitter using 65 nm CMOS and silicon photonic technologies

A push–pull silicon photonic Mach–Zehnder modulator (MZM) driver is presented which uses a switched capacitor approach to generate a ∼2 V peak-to-peak differential 4-level pulse amplitude modulation (PAM-4) signal. The driver chip includes a Gray encoder and retiming flip-flops. The switched capacitor approach allows driving the lumped silicon photonic MZM with reduced power consumption compared with the conventional approach of driving MZMs (with transmission line based electrodes) with a power amplifier. This is critical for upcom-ing short-reach link standards such as 400 Gbit/s 802.3 Ethernet. The chip was fabricated using a 65 nm CMOS technology and flip-chipped on top of the silicon photonic chip (fabricated using IMEC’s ISIPP25G technology) that contains the MZM. Open eyes with 4 dB extinction ratio for a 36 Gbit/s (18 Gbaud) PAM-4 signal are experimen-tally demonstrated. The electronic driver chip has a core area of 0.11 mm² and consumes 236 mW from 1.2 to 2.4 V supply voltages. This corresponds to an energy efficiency of 6.55 pJ/bit including Gray encoder and retiming, or 5.37 pJ/bit for the driver circuit only.