DSP-Free 25-Gbit/s PAM-4 Transmission Using 10G Transmitter and Coherent Amplification

We demonstrate 25-Gbit/s four-level pulse amplitude modulation transmission over 28-km standard single-mode fiber using an off-the-shelf 10-Gbit/s electro-absorption modulated laser and a 3 × 3 fiber splitter based simplified coherent receiver in the C-band. No digital signal processing and dispersion compensation have been used either at the transmitter or at the receiver. Furthermore, the use of an intensity modulated signal allows us to avoid the phase and frequency locking of the local oscillator. We achieved receiver sensitivities of -18.25, -17, and -13.4 dBm at a bit error rate below the hard-decision forward error correction threshold (i.e., 3.8×10
^-3
) and power budgets of 20.80, 19.55, and 15.95 dB in back-to-back, after 20.7 and 28 km, respectively. We also demonstrate polarization independent operation by slightly modifying the architecture of the simplified coherent receiver.