Nonlinearity mitigation in phase-sensitively amplified optical transmission links

Licentiate thesis, 2019

The fundamental limitations in fiber-optic communication are caused by optical amplifier noise and the nonlinear response of the optical fibers. The quantum-limited noise figure of erbium-doped fiber amplifier (EDFA) or any phase-insensitive amplifier is 3 dB. However, the noise added by
the amplification can be reduced using phase-sensitive amplifier (PSA) whose quantum-limited noise figure is 0 dB. PSAs can also compensate
for the nonlinear distortions from the optical fiber with copier-PSA implementation. At the transmitter, a copier which is nothing but a
phase-insensitive amplifier is used to create a conjugated copy of the signal. The signal and idler are co-propagated in the span, experiencing
correlated nonlinear distortions. The nonlinear distortions are reduced by the all-optical coherent superposition of the signal and idler in the
PSA.
In this work, an analytical investigation is performed for the nonlinearity mitigation using the PSAs, by calculating the residual nonlinear
distortion after the coherent superposition in PSAs. The optical bandwidth and the dispersion map dependence on the nonlinearity mitigation
in the PSAs are analytically and experimentally studied. A modified Volterra nonlinear equalizer (VNLE) is used to reduce the residual nonlinear
distortions after PSAs. Experiments were performed to show that PSAs can mitigate cross-phase modulation (XPM), which was evident
by observing the constellation diagrams. The maximum allowed launch power increase was also measured to quantify the XPM mitigation. To
the best of our knowledge, this is the first experiment that showed the mitigation of XPM in a phase-sensitively amplified transmission link.
Also, the effectiveness in mitigating self-phase modulation (SPM) and XPM using a PSA is studied.