A DAC-based Transmitter with VCSEL Bias-Current Generation enabling 180 Gbit/s PAM-8 Electrical and 100 Gbit/s PAM-4 VCSEL-based Transmission in 22nm SOI

Recent trends like AI, cloud computing, video streaming, and the internet of things create a demand for low cost, high bandwidth and highly energy efficient short-reach data links reaching data rates beyond 100 Gbit/s for the use in data centers. Transmitters (TX) utilizing a directly modulated vertical-cavity surface emitting laser (VCSEL) diodes provide adequate (BER/SNR) performance at lowest cost but have not significantly increased in speed/bandwidth in the last years [1], [3]. Utilizing PAM-4 modulation essentially doubles the data rate (in comparison to NRZ), however VCSEL drivers have not been able to exploit this, as the fastest published PAM-4 driver [2] is not outperforming the fastest NRZ driver [3]. Recent improvements have solely been achieved in the realm of power efficiency where [3], [4] were able to push the efficiency to under 1 pJ/bit at data rates of up to 80 Gbit/s. This is due to the highly nonlinear frequency behavior of the VCSEL, depicted in Fig. 1. The resonance frequency (and hence the bandwidth) changes both with modulation current (which is roughly proportional to the PAM-4 level) as well as temperature, making PAM-4 especially challenging as the ringing goes through adjacent eyes and the bandwidth change leads to eye skewing. In lab measurements, this nonlinear behavior has already successfully filtered out by the computationally expensive approach of Volterra filtering [5]. Later, equalization techniques which can be easily implemented using a lookup table (LUT) by switching the FFE coefficients data-dependent [6] were developed.