A 50 Gb/s PAM-4 EAM driver in 28-nm CMOS technology

With the rapid development of Ethernet, the research of new high-speed optical communication technology is getting more and more attention. The external modulation technique is unaffected by frequency chirp makes the fiber optic system more stable. Meanwhile, the small driving voltage and efficient area of the electroabsorption optical modulator make the electro-absorption modulator(EAM) driver more widely used. The signal emission by the optical module is critically dependent on the performance of the EAM driver. This article presents a 50 Gb/s four pulse amplitude modulation(PAM-4) EAM driver in 28-nm CMOS. The proposed EAM driver consists of a continuous-time linear equalizer (CTLE), a variable gain amplifier (VGA), a preamp, and an output stage circuit. Linearity is enhanced by using inverter-based structures in the proposed CTLE, VGA, and preamp. The source-degenerated resistors are adopted by CTLE and VGA to provide equalization and improve linearity. Additionally, shunt peaking inductors are used to extend the bandwidth. To provide a large output swing, the output stage adopts the cascode structure. Measured results demonstrate that: the EAM driver has 23.12 GHz bandwidth and 2.14 V output voltage swing at 50-Gb/s. The EAM driver consumes 226 mW power while having an active area of 0.26 mm2.