Digital Coherence Enhancement Enabling 6-GBd DP-64QAM Using a 1.4-MHz Linewidth Laser

Abstract — This letter demonstrates the possibility of using digital coherence enhancement—a means of estimating and digitally compensating local oscillator laser phase noise using a secondary, low speed coherent receiver—to enable the reception of high order quadrature amplitude modulation (QAM) using a monolithically integrated tunable semiconductor laser as local oscillator. We show that this coherence enhancement technique compensates both the Lorentzian and non-Lorentzian distributed phase noise of this laser, enabling reception of DP-64QAM with-out differential decoding, and with a record (linewidth)×(symbol duration) product of 2.3 × 10−4. Index Terms — Coherent receivers, quadrature amplitude modulation (QAM), laser linewidth, optical communications