Abstract
We propose a direct detection scheme for pulse amplitude modulation (PAM) signals over long-reach fiber transmission with phase noise mitigation. In this scheme, an optical carrier is transmitted as pilot together with the optical signal, whose frequency is located at the edge of the optical signal. The electrical field of the optical signal can be retrieved via the beating between the pilot and optical signal in one photodetector at the receiver side, which facilitates the digital compensation of chromatic dispersion effect. To enable real value signal processing, digital carrier regeneration is proposed to mitigate the phase noise fluctuation in the proposed scheme. To further improve the performance, the digital signal processing including Kramers-Kronig field reconstruction and Volterra equalizer are applied to reduce the signal-to-signal beat noise and nonlinear distortions after fiber transmission. The experimental results for both 56- and 64-Gb/s PAM-4 signal transmission cases show that the performances can be significantly improvement based on the proposed scheme and DSP methods. Both 56- and 64-Gb/s PAM-4 signal transmissions over 720- and 1440-km standard single mode fiber have been successfully achieved under 7% and 20% forward error correction codes, respectively.
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