Experimental Demonstration of Optical Signal Detection Using Nonlinear Fourier Transform

The Ablowitz-Ladik-a mathematical tool for calculating the nonlinear Fourier transform (NFT) of a time-domain signal-is applied to coherently detected short 16-GBd BPSK sequences after transmission over a few spans of standard SMF fiber in lab. Decision schemes are investigated, which compare the nonlinear spectrum of received signal blocks consisting of a sequence of four BPSK symbols with a set of calculated reference spectra. Decision on the continuous part of the NFT spectrum was successfully demonstrated. At higher signal power performance, degradation was linked to peaks in the continuous amplitude spectrum. They are emerging at large signal noise and change depending on the noise seed and lead to an increased variance of the minimum distance criterion which was applied for decision. Decision based on the discrete part of the nonlinear spectrum worked successfully at high signal power. In particular, the position of one or more eigenvalues in the complex plane, referred to as eigenvalue pattern, exhibited a low variance for signal with noise and enable low error rate. However, the measurements also show that with increasing signal power, link length, and noise, the reliability of the proposed detection is limited, and further refinement of decision criteria seems necessary.