Transmission of 20 Gb/s PAM-4 signal over 20 km optical fiber using a directly modulated tunable DBR laser

We report 20 Gb/s transmission of four-level pulse amplitude modulation(PAM) signal using a directly modulated tunable distributed Bragg reflector(DBR) laser. Transmission distance over 20 km was achieved without using optical amplifiers and optical dispersion compensation modules. A wavelength tuning range of 11.5 nm and a 3 dB bandwidth greater than 10 GHz over the entire wavelength tuning range were obtained.