  We experimentally demonstrate the first field-programmable gate-array-based
real-time fiber nonlinearity compensator (NLC) using sparse K-means++ machine
learning clustering in an energy-efficient 40-Gb/s 16-quadrature amplitude
modulated self-coherent optical system. Our real-time NLC shows up to 3 dB
improvement in Q-factor compared to linear equalization at 50 km of
transmission.
