VCSEL driver with synthesis of 25-Gb/s PAM-4 signal in 90-nm CMOS technology

In this study, a high-speed four-level pulse amplitude modulation (PAM-4) vertical-cavity surface-emitting laser (VCSEL) diode driver circuit is implemented in Taiwan Semiconductor Manufacturing Company 90-nm complementary metal–oxide–semiconductor technology. The laser diode driver (LDD) can input directly two non-return-to-zero signals to synthesize a PAM-4 current signal. In the LDD, the broadband techniques, including a de-emphasis technique, an equalizer, and a negative capacitor are used to enhance the circuit bandwidth. Through the eye diagram measurements, the performance of the PAM-4 VCSEL LDD chip on a printed circuit board is verified to be suitable in 25-Gb/s optical transmitters for short-reach transmissions.