Optical predistortion enabling phase preservation in optical signal processing demonstrated in FWM-based amplitude limiter

We propose and demonstrate a technique to obtain phase preservation in optical processing which is suitable for many four-wave mixing (FWM) based processing devices, provided they may be modified to incorporate two conjugating FWM stages. It functions by using a first conjugating nonlinear stage to predistort the signal using self-phase modulation (SPM) such that when the signal undergoes further SPM in a second conjugating FWM stage, the two SPM contributions negate each other, resulting in phase preserving output. In this work, we use the second stage to perform amplitude squeezing through parametric gain saturation and characterise the scheme by regenerating a QPSK signal contaminated with broadband amplitude noise. Experimental analysis of the system with and without predistortion is provided and phase preserving operation using the proposed scheme is confirmed