Effect of Gain Saturation on Wideband WDM Signal in PPLN-Based Optical Parametric Amplifier

The periodically poled LiNbO
₃
(PPLN) based optical parametric amplifier (OPA) is a technology that contributes to the construction of multi-band wavelength-division multiplexing (WDM) optical networks thanks to its wide amplification bandwidth and wavelength conversion function. In addition, the fast response of the OPA means it is unaffected by sudden changes of wavelength channels due to dynamic channel add-drop. However, such fast response leads to nonlinear signal distortion when operating in the gain saturation region. This paper experimentally investigates the conversion of gain saturation into the nonlinear amplitude distortion in a 64-Gbaud 64QAM WDM signal. We characterized the input power tolerance of a PPLN-based OPA in wideband WDM applications and found that the nonlinear amplitude distortion can be suppressed by increasing the number of WDM channels. For a 75-GHz-spacing 64-channel WDM configuration, the total input power tolerance was improved by ∼10 dB compared to a single-channel case. We also confirmed that there is almost no wavelength dependence of the effect of gain saturation on the signal distortion. These results show that PPLN-based OPAs have a wide total input power range for amplifying wideband WDM signals in terms of the nonlinear distortion.