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

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 without differential decoding, and with a record (linewidth)×(symbol duration) product of 2.3×10
^-4
.