Group Delay Compensation By Combining 3-tap FFE With CTLE for 80Gbps-PAM4 Optical Transmitter

Group delay peak-peak value and bandwidth bottleneck from VCSEL are challenges to achieve high speed optical interconnect at 50Gbps bits rates and above. This paper presents a PAM4 VCSEL-based Optical Transmitter that employs 3-tap feedforward equalizer (FFE) and continuous time linear equalizer (CTLE) to overcome these problems. By utilizing one pre-emphasis tap and CTLE, bandwidth of this system is extended to 26GHz. Moreover, peak-peak value of group delay for this system is reduced to 6.7pspp by using one pre-fall tap when frequency is below 26GHz. To adapt to various input signal amplitude, a variable gain amplifier (VGA) is placed in the frontend of transmitter. By using 0.18μm SiGe BiCMOS technology, the operating bits rates are up to 80Gbps with 2.8dBm optical modulation amplitude.