Enhanced Clock Tone Detection Using a Normalized Interpolated-Power Gardner Phase Detector for Bandwidth-Limited Optical Fiber Transmissions

Timing synchronization is challenging under severe bandwidth limitations due to the requisite high-frequency components are suppressed, rendering conventional clock recovery (CR) algorithms ineffective. In such cases, clock tones (CTs) become indistinguishable from noise or are entirely masked. To overcome this challenge, we propose a novel phase detector (PD), the normalized interpolated-power Gardner (NIP-Gardner), which employs nonlinear normalization of interpolated power to enhance the CT and improve timing extraction. Compared with the interpolated-power Gardner (IP-Gardner) and fourth-power Gardner (FP-Gardner) PDs, the NIP-Gardner PD achieves lower residual jitter and enhanced phase-locked-loop (PLL) bandwidth robustness. We experimentally validate its effectiveness in a 60-GBd polarization-division-multiplexed quaternary phase-shift keying (PDM-QPSK) system with a limited 3-dB transmitter bandwidth reduced to 7.5 GHz, which is 1/8 of the baud rate. Results indicate that the NIP PD outperforms existing schemes in terms of clock-to-noise ratio (CNR), residual timing jitter performance, and residual chromatic dispersion (rCD) tolerance. In back-to-back (BTB) experiments, the NIP scheme achieves optical signal-to-noise ratio (OSNR) gains of 1.2 dB and 3.6 dB relative to the IP and FP schemes, respectively. In 300 km fiber transmission experiments, it demonstrates an improvement in rCD tolerance of 125.22 ps/nm (150%) over the IP scheme and 166.96 ps/nm (400%) over the FP scheme.