4 × 128-Gb/s PDM-DMT signal transmission over 1440-km SSMF with high phase noise tolerance

In this paper, we propose an intensity modulation and coherent detection scheme with high phase noise tolerance for polarization division multiplexing (PDM) discrete multi-tone (DMT) signal transmission by employing Kramers-Kronig (KK) detection and digital carrier regeneration (DCR). At the transmitter side, DMT signal is modulated by a Mach-Zehnder modulator (MZM) setting bias around the null point and transmitted with the suppressed optical carrier. At the receiver side, a directly modulated lasers (DMLs) locating at the edge of DMT signal is used as the local oscillator (LO) for coherent detection. For signal recovery, KK detection is first used to reduce the signal to signal beating noise. Digital optical carrier is then regenerated by the DCR scheme and the DMT signal could be recovered by enveloping detection with the regenerated digital carrier, which can achieve high laser line-width tolerance and mitigate the residual phase noise caused by KK detection. To verify the effectiveness of the proposed scheme, we compare the KK and DCR based receiver digital signal processing (DSP) with conventional receiver DSP by using both DMLs with larger linewidth (~10 MHz) or external cavity lasers (ECLs) with smaller linewidth (less than 100 kHz) as optical carrier and LO. The results show that KK and DCR can mutually improve the system performance with ECLs as optical carrier and LO. Moreover, it is shown that the signal using DMLs cannot be recovered without the DCR method due to the high laser line-width of DML. Finally, we successfully demonstrate 4 × 128-Gb/s KK and DCR based PDM-DMT signal transmission over 1440-km SSMF by employing DMLs as optical carrier and LO.