High-Order Harmonic Ambiguity Exclusion for Optical Performance Monitoring Based on Subsampling Technique

As bit rates increase, monitoring signal quality in the optical channel online using bit-error-rate (BER) becomes increasingly challenging. Software synchronization can offer a low-cost solution for monitoring signal quality using eye diagrams and the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$Q$</tex> factor. However, existing software synchronization algorithms based on the Chirp-Z Transform (CZT) are susceptible to issues such as harmonic ambiguity and jitter, leading to syn-chronization failures and the inability to reuse synchronization results. In this paper, we propose a high-order harmonic ambiguity exclusion algorithm through spectrum analysis. The high-order harmonic ambiguity exclusion algorithm was validated through back-to-back (BTB) transmission experiments using 9.6961 GBaud 4-level pulse amplitude modulation (PAM-4) signal in an Intensity Modulation Direct Detection (IM/DD) optical fiber communication system. Compared to the CZT based synchro-nization algorithm, with an OSNR varying from 22dB to 30dB, our proposed algorithm shows an improvement in synchronization success rate ranging from 4.4% to 29.7%, and a reduction in the standard deviation of the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$Q$</tex> factor ranging from 0.044 to 0.307. The results indicate that it improves the synchronization success rate and provides more stable synchronization.