25-Gbps×4 optical transmitter with adjustable asymmetric pre-emphasis in 65-nm CMOS

This paper describes the design and experiment results of 25 Gbps, 4 channels optical transmitter which consist of a vertical-cavity surface emitting laser (VCSEL) driver with an asymmetric pre-emphasis circuit and an electrical receiver. To make data transfers faster in directly modulated a VCSEL-based optical communications, the driver circuit requires an asymmetric pre-emphasis signal to compensate for the nonlinear characteristics of VCSEL. An asymmetric pre-emphasis signal can be created by the adjusting a duty ratio with a delay circuit. A test chip was fabricated in the 65-nm standard CMOS process and demonstrated. An experimental evaluation showed that this transmitter enlarged the eye opening of a 25 Gbps, PRBS=2⁹-1 test signal by 8.8% and achieve four channels fully optical link with an optical receiver at a power of 10.3 mW=Gbps=ch at 25 Gbps.