Quality metrics for optical signals: Eye diagram, Q-factor, OSNR, EVM and BER

TLDR

Measuring optical signal quality is essential in optical communications, with common metrics including eye diagram shape, OSNR, Q‑factor, EVM for QAM, and BER, the most definitive but often difficult to quantify. The study compares these quality metrics analytically, via simulation, and experimentally. The authors analyze BER estimates derived from OSNR, Q‑factor, and EVM, comparing them to measurements across six modulation formats at 20 GBd and 25 GBd using a software‑defined transmitter. They find that for channels with additive Gaussian noise, EVM reliably predicts quality, and non‑data‑aided BER below 0.01 can be estimated from measured EVM.

Abstract

Measuring the quality of optical signals is one of the most important tasks in optical communications. A variety of metrics are available, namely the general shape of the eye diagram, the optical signal-to-noise power ratio (OSNR), the Q-factor as a measure of the eye opening, the error vector magnitude (EVM) that is especially suited for quadrature amplitude modulation (QAM) formats, and the bit error ratio (BER). While the BER is the most conclusive quality determinant, it is sometimes difficult to quantify, especially for simulations and off-line processing. We compare various metrics analytically, by simulation, and through experiments. We further discuss BER estimates derived from OSNR, Q-factor and EVM data and compare them to measurements employing six modulation formats at symbol rates of 20 GBd and 25 GBd, which were generated by a software-defined transmitter. We conclude that for optical channels with additive Gaussian noise the EVM metric is a reliable quality measure. For nondata-aided reception, BER below 0.01 can be estimated from measured EVM.