Negative Frequency-Chirped 112-Gb/s PAM-4 Using an Integrated Germanium Franz-Keldysh Modulator

Integrated germanium-on-silicon Franz-Keldysh modulators (Ge-EAMs) offer a small footprint and low-energy-per-bit performance for high-speed data communications. In this letter, we study the use of Ge-EAMs for four-level pulse amplitude modulation (PAM-4), and report on the linearity of the modulator in terms of its spurious-free dynamic range. The experimental results show that the negative frequency chirp from the modulator can improve the transmission distance of the PAM-4 signal. Using pre-equalization with a 25-GHz band-width-limited arbitrary waveform generator, we obtained a data rate of 112 Gb/s with a bit-error-rate of 2.5 × 10
^-3
under the 7% overhead hard-decision forward error correction threshold. We observed an extended transmission distance in our 112-Gb/s PAM-4 experiment on a standard single-mode fiber thanks to the negative frequency chirp of the modulator.