Progresses of Pilot Tone Based Optical Performance Monitoring in Coherent Systems

Pilot tone (PT) is a low frequency, small intensity modulation applied to high speed optical channel. Traditionally PT is used for channel identification and channel power monitoring. A pilot tone scheme suitable for optical performance monitoring (OPM) in coherent optical communication systems is described. Two effects, Stimulated Raman scattering (SRS) and dispersion fading, are discussed. An important enhancement, multiband pilot tone, is introduced, which provides sub-channel monitoring capability. With the advanced PT technologies, signal spectrum, and relative frequency offset between signal and optical filter can be monitored with sub-GHz resolution. PT also enables direct OSNR and fiber nonlinear noise monitoring with high accuracy and sensitivity.