Direct detection of pilot-assisted PAM-4 signals with large phase noise tolerance

A pilot-assisted direct detection scheme is experimentally demonstrated when distributed feedback lasers (DFBs) are used as transmitter carrier and pilot carrier sources. In this scheme, the complex electrical field can be constructed digitally in a Kramers-Kronig receiver. Therefore, power fading problem due to chromatic dispersion (CD) and signal-to-signal beating noise can be mitigated. Considering large laser phase noise due to large linewidth of DFB laser, phase pre-compensation (PPC) is realized by extracting the carrier phase information digitally. After PPC, finite impulse response filtering-based linear or nonlinear channel equalization can be applied to deal with the intersymbol interference induced by CD and channel nonlinear distortions. 50 Gb/s four-level pulse amplitude modulated (PAM-4) optical signal over 320-km standard single mode fiber (SSMF) transmission is experimentally verified without inline CD compensation. The experimental results show that the proposed PPC scheme can successfully recover the signal suffering from strong laser phase noise. It is also shown that complex Volterra series-based channel equalization can improve the system performance after 720 km SSMF transmission.